Access to carbon–carbon bonds

Chapter 10

Access to carbon-carbon bonds

The reactivity of tin-carbon bonds allows the creation of new carboncarbon bonds through substitution, addition or elimination reactions. The synthetic applications corresponding to these three types of reactions are discussed in turn in this chapter. It is noteworthy that organotins are challenging organosilicon derivatives16-19,375 and often provide wider possibilities, especially when mild experimental conditions are required. In consequence, during the last decade there has been a dramatic increase in their use for carbon-carbon bond formation. A few pioneering papers had been published in this field before 1976376, but two fundamental features have completed the advance of organotin reagents: 1. Transition metal catalysts (mainly Pd and Rh complexes) have been shown to be efficient for cross-coupling reactions involving main-group organometallics377'378. 2. Lewis acid catalysts have allowed efficient control of regio- and stereo-chemistry for addition reactions on carbonyl compounds. The importance of the organotin route for the creation of carboncarbon linkages was underlined in reviews379, but a tremendous amount of work has since been published, illustrating the versatility of this tool for organic chemists. Organotin reagents involving C/O or C/N metallotropy will not be considered in this chapter, because it is generally accepted that the reactive species—even sometimes in a minority at equilibrium—are heterometallated (for instance, organotin enolates rather than a-stannyl ketones). In consequence, reactions involving these reagents will be presented in Part Four.

10· 1 Via substitution reactions The relative weakness of the tin-carbon bond allows transfer of an organic substituent from the organotin reagent to the organic substrate under mild experimental conditions (Scheme 10.1). R^SnR2 + R3X

*- R2R3 + R^SnX Scheme 10.1

185

186

Access to carbon-carbon bonds

As with organotin hydrides, when the tin-carbon bond is weak enough the reaction can occur spontaneously on heating. However, more usually the reactions have to be promoted by free-radical initiators, Lewis acids or transition metal catalysts. The subsequent synthetic applications will be discussed in turn for reactions involving cross-coupling with organic halides and related compounds and for reactions involving cross-coupling with acyl halides. Finally, the special behaviour of the cross-coupling of organic halides in the presence of carbon monoxide will be examined. 10.1.1 Cross-coupling reactions with organic halides and related compounds General The free-radical substitutions and the Lewis-acid-promoted substitutions concern mainly allyltins and will be discussed when the allylation reactions are examined. Catalysis by transition metal complexes appears to be much more general and justifies early presentation for a better understanding of this chapter. The initial discovery was the allylation of aryl bromides by allyltributyltin, which occurs in the presence of a catalytic amount of tetrakis(triphenylphosphine)palladium380. These results have subsequently been extended to tetraalkyltins, aryltins, vinyltins and alkynyltins381-383. A considerable accelerating effect of oxygen on the reaction rate has been observed, while the phosphine ligands are of minor importance. Furthermore, highly polar solvents like HMPA are most suitable when the reaction is performed in the presence of oxygen381. In consequence, BnPdCl(PPh3)2, because of its low air sensitivity, is the catalyst of choice. The initial step in the reaction can be considered to be oxidative addition of the halide on a palladium complex TdL 2 \ The subsequent steps are transmetallation and reductive elimination, which can compete with ß-elimination when alkyltins with ß-hydrogen BnPdl_2X or P d L 2 X 2 or Pdl_ 4

r Λ t

R1R2

1

"PdL?"

R PdL 2 R 2

R^SnX

/ R1X

R 1PdL 2X

R 2 SnR|

Figure 10.1 Simplified scheme for cross-coupling of tetra-organotins with organic halides

Via substitution reactions

187

atoms are employed. This trend can be illustrated by the lower yields obtained in reactions involving tetrabutyltin compared with those involving tetramethyltin381'382. On the basis of careful studies with aryl and benzyl halides, a general scheme has been proposed which explains the cross-coupling of tetraorganotins with organic halides381. A very simplified scheme which can explain the catalytic efficiency of species such as Pd(PPh3)4, BnPdX(PPh3)2, PhPdI(PPh3)2, PdX2(MeCN)2 or Pd(dba)2 is shown in Figure 10.1. It should be noted that the cross-coupling reactions tolerate numerous functionalities and that benzyl, aryl, vinyl or alkynyl groups are more easily transferred than simple alkyl groups381'382. Furthermore, with allylic substrates the intervention of π-allylpalladium intermediates appears to be a reasonable alternative383'384. Allylation, propargylation and allenylation Free-radical allylation and allenylation. With a reported value385'386 of ca. 155 kJ mol -1 , the dissociation energy of the Sn-C linkage in allyltins appears comparable with the strength of the Sn-H linkage. Indeed, the free-radical processes observed for reactions involving tin hydrides {cf. Part Two) are also relevant to allyltins and lead to reactions not obtained with allylsilanes. With organic halides, initial reports have demonstrated unambiguously the occurrence of homolytic processes, on the basis of the influence of free-radical initiators or scavengers, sequence of reactivity, rearrange­ ments and side reactions387"390 {Scheme 10.2). .

R? Sn*

R|SnAU

(-R§SnX)

Scheme 10.2

These findings have been confirmed by physicochemical data including the results of radiofrequency probing and chemically induced nuclear polarization391. On the synthetic front, high yields of allylation products have been obtained from reactive halides such as polyhalomethanes, allylic halides, a-nitrohalides or a-halogenated esters, nitriles, sulphones, ketones and aldehydes389'392"394 {Scheme 10.3). AUBr.UV 60%

^

MeCHBrC02Me,100eC

3

70%

C02Me

Scheme 10.3

Reactions are also possible with substituted allyltins {Scheme 10.4). CL CL . / \ ^ i \ B u 3 S n ^ ^f ^Ph

MesSn^Nj^^SnMes

CCUCH0,200 o C 2 ^ 80% CCL4, AIBN,70°C

/ Γ \ / ^ T

^ CHO

(ref389)

Ph Cl 3 C

62%

Scheme 10.4

||

CCl 3

(rei 3 9 5 )

188

Access to carbon-carbon bonds

Side reactions may occur, especially with γ-alkyl-substituted allyltins, e.g. crotyltins389'396, which undergo hydrogen abstraction from the organotin reagent by an intermediate radical. The extent of this process depends on the substrate and on the experimental conditions (mainly dilution) and can be either moderate or almost complete (Scheme 10.5). Cl Cl3CC02Et,200 o C

^

r

y

CL "- C 0 2 Et

(ref389)

50% Bu 3 Sn'' v ClpH 2 1 Br,

t-Bu00t-Bu

<^\^

chlorobenzene 132°C

(ref 396)

52%

Scheme 10.5

Owing to this limitation, the allylation procedure has mainly been used to introduce γ-unsubstituted allyl groups. Thus the allylation of bromides, a-chloroethers and thioimidazoyl or phenyl selenides, promoted by AIBN or UV irradiation, can be achieved with high yieids389>393,394,397-40i (Scheme 10.6). Ph Ph^\Y"^B ' y 0 0

Ph

X^v^^

B^3SnCH2CR = CH2(2eq)> to.uene,A.BN,80"C

PhJ

U 0'

( r e f 3 9 7)

R = H 88%, , R = Me 97%

0C^O>A MH

CH 2 Cl2

^^^^^N

\

Cbz

\ AIBN, toluene S«* 1

V.

C zb

Cbz

(ref. 400)

76% overall yield

Scheme 10.6

An important feature is the compatibility of this allylation method with the presence of other functionalities such as alcohol, acetal, ether, epoxide, ester, ketone and even aldehyde389,397'398 (Scheme 10.7). —3 ► AIBN,80°C 58%

Π " 0

(ref 3 9 8 )

Scheme 10.7

However, addition followed by substitution is sometimes observed402 (Scheme 10,8). o >T

o if

4

(t ) AU 4 Sn,THF,65°C

((Yf

65%

|\_

vx

(réf. 402)

n

Scheme 10.8

As regards stereochemistry, the relative accessibility to each face of the intermediate free radical is responsible for the selectivity. As a

Via substitution reactions

189

consequence, a single isomer can be obtained if there is significant face differentiation399 (Scheme 10.9). Bu3SnAU(2eQ),AIBN benzene, 80°C

-H—0

88%

S c h e m e 10.9

Similarly, the ureido-allylation of an internal double bond can lead to a single product400 (Scheme 10.10). ( 1 ) N-PSP,CH 2 Cl 2 NH

( 2 ) Bu 3 SnAU,AlBN

I

82%

Cbz

/

Cbz S c h e m e 10.10

In the carbohydrate series, the use of these mild experimental conditions seems very attractive and indeed the allylation (or methallylation) of thiophenyl glycosides has been realized401 (Scheme 10.11). 0 ^ 0 ,-B«M^S.0-^~V-SPh

B

"3SnCH^CMe = CH?> 87%

o>

^ H t & O -

α·β

92=8

S c h e m e 10.11

The stereochemistry of this reaction can be strongly influenced by the skeleton and a reversed selectivity is obtained in the mannose series (α:β = 99:1). A homolytic process has also been proposed for the allylation of azetidinones which affords useful penem and carbapenem precursors, but the results have also been interpreted in terms of 1,4-additions after initial elimination of hydrogen chloride403'404 (Scheme 10.12). Br

CL AU4Sn,CH2CL2 86%

S c h e m e 10.12

Similarly, the production of rearranged coupling products in the reactions of allyltins with primary allyl and benzyl halides has been explained in terms of a six-membered transition state405 (Scheme 10.13). Ph

^ . ^

Cl

(f)-Bu3SnCrot 10kbar,CH 2Cl 2,50%,170η 88% S c h e m e 10.13

190

Access to carbon-carbon bonds

Finally, homolytic transfer of an allenyl group from propargyltins to carbon tetrachloride, chloroform, alkyl bromides or iodides has been successfully achieved153'406 (Scheme 10.14). Γ | Ί ^ ü ^ ^ ß

Ph3SnCH2CECH(4eq.) AIBN, benzene, 80°C * • 8uwu 45%

r

Γ

ί

|

/CJr-^V . Z—-c^ ^ — » —

(ref.406)

Scheme 10.14

An excess of the organotin reagent must be used because it isomerizes to the more stable allenyl tin407. However, the most important feature is the compatibility of this allenylation method with a large number of functional groups, as in the case of amino acid derivatives, with which the reaction occurs without racemization406 (Scheme 10.15). ,.NHC0 2t-Bu ά 'H

Ph3SnCH2C = CH(4eq.) AIBN,benzene,80°C

O ^ ^

COoPMB

^ ^NHCO?t-Bu γ ί Η | COoPMB

55% Scheme 10.15

Lewis-acid-promoted allylation. Allyltins are good candidates for crosscoupling reactions with activated halides40^"411, acetals412, thioketals413 or allylic alcohols, ethers and acetates122,411; however, the choice of the catalyst is of crucial importance in obtaining satisfactory yields. With prenyl and phytyl bromides, ZnCl2 is an efficient catalyst for cross-coupling without γ-substitution408 (Scheme 10.16).

Br

AU 4 Sn, 1 0 % ZnCl 2 ,THF,65°C * 76% Scheme 10.16

Thus the reaction of 2-(trimethylstannylmethyl)-l,3-butadiene with geranyl or prenyl bromide gives ß-farnesene or myrcene in high yields409 (Scheme 10.17). Me3Sn

Br

10%ZnCl 2,THF,65°C

\

94%

,

.

Scheme 10.17

Surprisingly, the efficiency of ZnCl2 seems to be limited to allyltins unsubstituted at the γ-position, but this problem has been circumvented by the use of bis(diethylaluminium) sulphate411 (Scheme 10.18). Me3Sn

Scheme 10.18

Via substitution reactions

191

This catalyst is also efficient for the allylation of allylic ethers or acetates, but unfortunately it leads to mixtures of a- and γ-substitution products (with complete rearrangement of the allyltin moiety)411. The rearrange­ ment of the allyltin moiety also occurs when crotyltributyltin is reacted with azetidinones in the presence of Lewis acid catalysts403'414 (Scheme 10.19). OSiR 2 t-Bu ^

η

OSiR2t-Bu CrotSnBu 3 ,Me 3 SiOTf orBF 3 .Et 2 0

f

0
S^^

75-80%

R = MeorPh

£- isomer erythro threo « 5 0 : 5 0

Scheme 10.19

Besides these general examples, interesting results involving functionally substituted ally kins must be noted. Thus N-bromomethylphthalimide has been functionalized with seleno-substituted allyltins410 and limonene has been obtained via an intramolecular reaction of the appropriate allyltin122 (Scheme 10.20).

oi

( 1 )Bu3SnCH2CMe=CHSePh,ZnBr2,CH2Cl2 \

(2)H202 Br

"*" 43%

T i C l 4 , PhNHMe,CH 2 Cl 2 (R = H) or (CF 3 C0 2 ) 2 AlMe.hexane (R = Ac)

0R

SnBu3 R = H 7 3 % , R = Ac 51 %

Scheme 10.20

With acetals and thioketals, allylation is also possible using, respectively, bis(diethylaluminium) sulphate and dimethyl(methylthio)sulphonium fluoroborate (DMTSF) as catalyst412'413 (Scheme 10.21). OMe

OMe Me3SnAU, ( E t 2 A l ) 2 S 0 4

OMe MeS

SMe

80°C, benzene,hexane 68%

SMe

Bu 3 SnAIL, DMTSF,CH 2 Cl 2 ,-23 0 C

►

89%

Scheme 10.21

The use of the couple allyltributyltin-DMTSF prevents the competitive eliminations usually observed with stronger nucleophiles; the reaction occurs with allylic rearrangement and shows high chemoselectivity, which allows the presence of various other functional groups. This property has been exploited in performing macrocyclization reactions (Scheme 10.22).

192

Access to carbon-carbon bonds SnBu3 0 ^

^

°

^

%

SMe

DMTSF

^^^^^J\

^0.

46% SMe

SMe

Scheme 10.22

It must be emphasized that allyltins appear to be much more efficient than allylsilanes for this type of substitution413. In a related area, the key step in the synthesis of (±)-isoretronecanol involves a similar cyclization415 (Scheme 10.23).

SnBu 3

MsCl,Et 3 N *~ 72%

Scheme 10.23

Besides the cross-coupling reactions involving substitution of hetero groups, it may be mentioned that aromatic hydrogen can be substituted in the presence of a stoichiometric amount of thallium trifluoroacetate416 (Scheme 10.24). OMe (CF 2 C0 2 ) 3 T l , PhOMe ( solvent ) B u

3

S n / ^ ^

—

-

Scheme 10.24

Similar results can be obtained with benzene416, illustrating the capacity of allyltins for behaving as allyl cation equivalents. Transition-metal-catalysed allylations and propargylations. The allylation of aryl bromides in the presence of Pd(PPh3)4 leads to allyl arènes in high yields and with a high tolerance of other functionalities380 (Scheme 10.25). '

\

Ö . /

Bu 3 SnAU,Pd(PPh 3 ) 4 .benzene

Z^NN

100-120°C

ioo-i2o°c

\ν_//

72-100%

Scheme 10.25

In similar conditions, aryl iodides and activated aryl chlorides also lead to allylation products, but in lower yields380. This reaction has been extended to a-ethoxy-substituted allyltins and is an efficient means of introducing a d3 propionaldehyde synthon on an aromatic ring300 (Scheme 10.26). R2

OEt

N

'

3

'-,

benzene 100-120°C

Scheme 10.26

L

^r^

Via substitution reactions

193

The substitution occurs with allyl-, methallyl-, crotyl- and prenyl-tins with 69-83% yields after isolation. An interesting illustration is the synthesis of (±)-flMurmerone417 (Scheme 10.27). ( 1)Bu3SnCH0EtCH=CHMe f 7 ^ | | ( 1 )(Bu3SnCH=CMe2/I2/Bul_i) i^\ Pd(PPh3)4 .benzene, * J < ^ ς Η 0 ( 2 ) H 2 0 ( 3 ) Cr0 3 . 2pyr x^r^(2)H2S04

30%

71 %

85%

Scheme 10.27

The cross-coupling of allyltins with benzyl and allyl halides occurs with preferential rearrangement of the allylic group emanating from the organotin partner379'408 (Scheme 10.28). + AU4Sn

SnBu 3

BnPdCl(PPh 3 ) 2> benzene, 65°C

BnPdCL(PPh3)2 CHa3,65°C

Scheme 10.28

In the first reaction the use of a THF solution and ZnCl2 ns co-catalyst allows regiospecific production of 6-methyl-hepta-l,5-diene in 80% yield. On the other hand, bis(T]3-allyl)palladium halide complexes can be used as catalysts for these cross-coupling reactions when HMPA is the solvent or when maleic anhydride is used as interceptor418. Esters of allylic alcohols behave similarly, but their reactions seem to be much more sensitive to steric hindrance384'408. For instance, cinnamyl acetate leads to cross-coupling products in 70% yield with tetraallyltin and methallyltributyltin, while only 32% and 4% yields, respectively, are obtained with crotyltributyltin and prenyltributyltin384. Furthermore, when prenyltriphenyltin is used, the coupling product is 1,3diphenylpropene (exclusive transfer of a phenyl group). An interesting feature may be pointed out: the reaction of cinnamyl acetate with hexabutylditin affords the coupling product via the intermediate cinnamyltributyltin384 (Scheme 10.29). ^ /^ Ph^^^OAc

+

B u

6Sn2

Pd(PPh 3 ) 4 ,THF,65°C ^ 51%

^

Scheme 10.29

The reactions of allyltins with allylic acetates can be obtained at room temperature in HMPA419, but unfortunately for all these cross-coupling reactions, increased steric hindrance favours ß-hydride transfers384' and the use of more labile substituted acetates gives poor selectivities420 (Scheme 10.30). OAc AlL 4 Sn,THF ^ \

Pd(PPh 3 ) 4 ,65°C

96%

69

Scheme 10.30

31

194

Access to carbon-carbon bonds

The same phenomenon, complicated by propargylic-allenic isomerization, has been observed in similar propargylation reactions involving allylic acetates and allenyltins421. Finally, with vinylic bromides or inflates, allylation remains possible and affords non-conjugated dienes in good yields ,422 (Scheme 10.31). Ph

^

Bu 3 SnAU,[Pd] > Br

Ph

63%

OTf Bu 3 SnAlL > Pd(PPh3) 4 ,LiCl > THF,70°C t-Bu

9o6/ o

^ ^

t-Bu

Scheme 10.31

Alkylation Except for an example of methylation of aromatic substrates via a free-radical pathway , simple alkyl groups are usually transferred under transition metal catalysis. Methyl transfer is more efficiently achieved than butyl transfer, because of the absence of reductive ß-elimination381'382 (Scheme 10.32). BnBu -<

Bu 4Sn, BnPdCl(PPh 3 ) 2

BnBr

H M PA

Me4Sn, BnPdCl (PPh 3 ) 2 HMPA

42%

► BnMe

82%

Scheme 10.32

This reaction tolerates the presence of numerous functionalities and is accelerated by electron-withdrawing substituents in the case of substituted aryl bromides381 (Scheme 10.33). / P ^

Me4Sn,BnPdCl(PPh3)2

JQ/--' afe

85-95%

/P^\

" - TJQ^^

Scheme 10.33 +

Diazonium salts, ArN2 X~ (X = BF4 or PF6), can also be methylated by this route and it is striking that the coupling is highly chemoselective even when the competition involves an aryl bromide functionality424 (Scheme 10.34).

'-CH·

Me 4 Sn,Pd(0Ac) 2 ,MeCN

B

76%

'%J~* / ^ \

-Me

Scheme 10.34

The alkylation of dichloronaphthoquinones has been achieved in the presence of a more complex catalytic system402 (Scheme 10.35). o o -Cl

R 4 Sn,PdCL 2 [Ph 2 P(CH 2 ) 3 PPh 2 ],DIBAL

dioxane , 100°C R=Bu 9 1 % , R = C 1 2 H 2 5

Scheme 10.35

25%

*·

^ i £ s ^ \ ^

R

Via substitution reactions

195

Under similar experimental conditions the more reactive tetramethyltin affords 2,3-dimethyl-l,4-naphthoquinone in 72% yield. Finally it must be noted that high yields of alkylation products can also be obtained from vinyl triflates . Arylation and heteroarylation Drastic conditions (ca. 200°C) are necessary to react perfluorophenyltrimethyltins with aryl bromides or iodides, and diaryl derivatives are obtained in poor to moderate yields425'426. Lewis-acid-promoted reactions have been rarely used, but it is worth mentioning the preparation of aryl cyanides by reaction of aryltins with cyanogen chloride*9'6 and an original route to vitamin Κχ409 (Scheme 10.36).

( 1 ) phytyl Br,10%ZnCL2,Et20,-78°C (2)PCC,CH 2 Cl 2 ,25°C SnMe3 OSiMe2t-Bu 4 0 % overall yield

*"

Scheme 10.36

A different kind of reaction involves acrylic acid427 (Scheme 10.37). Ph

\

Ph 4 Sn,HAuCl4

v

C

°2H

98%

' C02H

Scheme 10.37

On the other hand, with transition metal catalysis, interesting results have been obtained. For instance, the cross-coupling of aryl iodides with aryl- and thienyl-tin derivatives occurs with high yields383' 28_430 (Scheme 10.38).

*-©-■ £ΖΓ~""~ - © - 0 - * Z = H,Me,0Me,CL,N0 2

75-100%

Scheme 10.38

A similar reaction occurs with allylic halides and acetates without interference with carbonyl or nitrile functionalities431'432 (Scheme 10.39). T2*

J^jr

/F^K

+

[Pd(dba)2>2PPh3]

Bu3Sn-
81%

Et

, °2 C

r^^^CN

AJ^J

Scheme 10.39

This type of reactivity has been extended to vinyl thioethers, but here a thiophilic agent like mercuric chloride must be added to allow the use of catalytic amounts of palladium salt. Interestingly, intramolecular reactions lead to cyclization products with remarkable regioselectivity433 (Scheme 10.40).

196

Access to carbon-carbon bonds

Scheme 10.40

Vinylation and alkynylation Free-radical vinylation. On heating with α-bromo- and a-iodo-esters, vinyltins give moderate amounts of substitution products434 {Scheme 10.41). C0 2Et

ICH2C02Et,140 o C,20h Bu 3Sn

38%

Scheme 10.41

Subsequently, the reaction of styryltributyltin with carbon tetrachloride or butyl iodide has been carried out under UV irradiation435 (Scheme 10.42). CCl4,UV,38h

Ph

Ph v

ecu

SnBu* E Z

95

5

Scheme 10.42

However, this reaction is limited to reactive halides; an alternative for obtaining alkenes in a homolytic process appears to be the sunlampphotostimulated or the AIBN-initiated reaction of vinyltins with alkylmercury halides435 (Scheme 10.43). iPrHgCl , benzene, sunlamp, 18h SnBu,

Ph

86% E Z

92 8

Scheme 10.43

Finally, an interesting method for the introduction of a vinyl group has recently been proposed by Baldwin. The substitution is promoted by a hexaalkylditin, used as a source of trialkyltin radicals, and the overall process can be understood as halogen abstraction followed by an addition-elimination sequence (Scheme 10.44). Bu 3 Sn*

—^ »(-Bu3SnX)

Bu3SnCH = CHZ^ Bu3Sn

(-Bu3Sn')

Scheme 10.44

This procedure, which requires that Σ is an electron-withdrawing group, has been used as a crucial step in the synthesis of a fungal isonitrile antibiotic436 (Scheme 10.45).

Via substitution reactions

197

C02Et

\

V j ^ O T /)

Bu3Sn +

C02Et / (1 ) B u 6 S n 2 , toluene, 80eC

\

(2)AcOH, H20 79%

Scheme 10.45

Transition-metal-catalysed vinylation and alkynylation. With catalysis by palladium complexes, vinyl and alkynyl groups are more readily transferred than alkyl groups381'382 {Scheme 10.46). Bu,Sn CH=CH 2 t BnPdCl(PPh,) 2 ,ΗΜΡΑ

^

Pn.

-100%

Scheme 10.46

The substitution also occurs successfully with aryl iodides even when perfluorovinyltrialkyltins are used 383 ' 428AÎ9A37 (Scheme 10.47). PhC = CSnMe 3 ,PhPdI(PPh3) 2 - i + HMPA,20°C = 100% I - ^ Q ^ N 0

-c-£^

„ Ph-C = C - <

„ ))—NOo (rei 4 2 9 )

2

)Bu3SnCF=CF2,PhPdI(PPh3)2t

p JT\_

HMPA,70°C

m

V—7

2

(ref.437)

87%

Scheme 10.47

Furthermore, tolerance of other functionalities, both on the reagent and on the substrate, allows synthesis of highly functionalized organic compounds432 (Scheme 10.48). uivie Br. L X0 2 Me ^>-Bu3SnCH = CHCH2OH ^ \ < ^ ^ ^ [Pd(dba)2,2PPh3],THF,50oC 82%

Scheme 10.48

From the stereochemical point of view, the coupling reaction of allyl halides with vinyltins maintains the configuration at the vinyl unit432'438 and an instance has been reported where the coupling occurred with inversion of configuration at the allylic carbon atorn432^ (Scheme 10.49). BnO +

A L l Br

Pd

< P P h 3>4 , benzene, reflux

SnBu 3 C02Me r=, ■^ Cl

Bu 3Sn

[PdCdba^PPh^ CC^Bn

THF

'

50 C ° 87%

Scheme 10.49

^

^

(raff.438)

198

Access to carbon-carbon bonds

Extension to cross-coupling with diazonium salts424 or vinyl triflates422,439 can be successfully achieved, as exemplified by the synthesis of pleraplysillin-1 from 3-furfuryl bromide422 (Scheme 10.50). OTf

iI \\

— ^ // \\

^Qy

^SnBu3

\Qy

Pd(PPh3)4,LiCl,THF 75%

Scheme 10.50

It is noteworthy that this reaction is compatible with the presence of vinylor alkynyl-silanes, emphasizing the different behaviour of organosilicon and organotin derivatives422 (Scheme 10.51). SiMe3 .OTf + M e 3 S n — = — SiMe3 t-Bu"

^ ^

Pd(PPh 3) 4,LiCl,THF — *907ο t-Bu

Scheme 10.51

Finally, cross-coupling of imidoyl chlorides with alkenyl- or alkynyl-tins is an efficient route to the corresponding ketimines440. Miscellaneous substitution reactions As already mentioned, with transition metal catalysis benzyl groups can easily be transferred from benzyltins to organic halides; 1,2diphenylethane has been obtained by this route in 91% yield381. Organotins which are oc- and ß-heterosubstituted can also react with aryl halides441"443 but, as already stated, the reactivity of ß-heterosubstituted organotins will be discussed in Part Four. Methoxymethyltributyltin affords benzylic ethers in good yields442 (Scheme 10.52). \£=Λ

Bu3SnCH20Me1PdCl2(PPh3)2

)$Ö\\

\=J/

HMPA, 80 e C

\L^7

Σ = H, Me, σ - α , / 7 - C l , p - O M e , p - A c , ^ - C N ,

\OMe

p-H02

Scheme 10.52

Similarly, tributylstannylmethanol gives smooth hydroxymethylation of bromobenzene (60% yield). Acyltin derivatives can be used for the acylation of organic halides with palladium catalysis444 and similar acylations are possible using Cstannylimines with activation by fluoride ions445. 10.1.2 Cross-coupling reactions with acyl halides General The reaction of acyl halides with organotin compounds, although sometimes possible without a catalyst, usually necessitates the addition of

Via substitution reactions

199 446

Lewis acids or ammonium salts to afford finally ketones via acyl cations . However, in this field organosilicon derivatives are good rivals and a more interesting method of activation involves catalysis by transition metal complexes. This last can be achieved under very mild experimental conditions, offering a general and efficient route to ketones, challenging the more refined methods involving organozinc reagents and acyl halides447. The original work, by Migita and Stille, used rhodium or palladium complexes 48"452. It was found that when unsymmetrical organotins are used, the transfer of organic groups occurs selectively with benzoyl chloride, using 3% BnPdCl(PPh3)2 as catalyst, according to the following preferential sequence: Ph-C = C > PrC = C > PhCH = CH > Ph > Bn > MeOCH2 > Me > Bu453. This selectivity is almost unaffected by the nature of the solvent but is highly dependent on the concentration of catalyst454 (Scheme 10.53). 0 11

0

0

BnPdCL(PPh 3 ) 2

II Ph ► P t i ^ ^

Bu 3SnBn + P h ^ ^ C L

+

II P h ^ B u

HMPA ( 0 . 4 5 % mol. cat.)

60%

15

85

HCCL3 ( 0 . 4 5 % mol. cat.)

40%

15

85

HMPA ( 4 % mol. cat.)

84%

93

7

Scheme 10.53

Tetraorganotins are more suitable reagents than triorganotin halides, while the cross-coupling reaction fails when mono- or di-organotin halides are employed451. Furthermore, THF must be avoided as a solvent when the couple R 3 SnCl-Pd n complex is used, because of possible acylative cleavage of the heterocycle455. As regards mechanisms, the reaction has mainly been studied with BnPdCl(PPh3)2, with the following conclusions451: 1. In practice triphenylphosphine deactivates the reaction, while oxygen has a considerable activation effect. In consequence, the reactions are better achieved in the presence of air or under pure oxygen. 2. BnPdCl(PPh3)2 is an efficient catalyst even at 10~4M concentration, allowing turnover numbers >10000. BnPdClL 2 R 1C0Cl

Figure 10.2 Scheme showing mechanism of Pd-catalysed cross-coupling using acyl halides

200

Access to carbon-carbon bonds

The most plausible mechanism involves the generation of bis(triphenylphosphine)palladium as the active species and can be described by the catalytic scheme450'451'453 shown in Figure 10.2. Finally, it must be noted that the transfer of a chiral group occurs with preferential inversion of configuration in highly polar solvents such as HMPA453'456 (Scheme 10.54). D u -nH S(-)

H n D u

\f Ph^

BzMLgq SnBu3 HMPA,65 e C

R(_,

j

/

Ph

p h

o Scheme 10.54

On the other hand, when the coupling involves a vinyl group, the configuration of the double bond is initially retained, but enones can undergo isomerization to the more stable configurational isomers453. Allylation l,3-Bis(tributylstannyl)-2-methylenepropane reacts very easily with acyl halides457 (Scheme 10.55). Bu 3 Sn"^Y^SnBu 3

(1

> PhCH = CHCOCl,toluene,110X (2)HCl

59%

Scheme 10.55

However, this is not the usual behaviour of simple allyltins, and without additives the reaction must be forced under high pressure to obtain βγ-enones405 (Scheme 10.56). BzCt,10kbar SnBu 3

CH 2 Cl 2 ,170h 60%

Scheme 10.56

In contrast, allylchlorotin derivatives, in which the Lewis acidity of the tin centre is increased, react readily to afford βγ-enones with a complete allylic shift458 (Scheme 10.57). SnBu2Cl

RC0CL, 25°C R=Me 9 0 % , R = t-Bu 8 5 %

Scheme 10.57

With allyldibutylin chloride the reactions are complicated by partial isomerization of βγ-enones to αβ-enones, but the major difficulty is the subsequent addition of the allyltin reagent to the ketone initially obtained458. The key improvement in the allylation of acyl halides has been brought about by the use of transition metal complex catalysts. Thus allyl ketones

Via substitution reactions

201

have been obtained by cross-coupling in the presence of chlorotris(triphenylphosphine)rhodium(I) (Scheme 10.58). Bu 3 SnALL,RhCL(PPh 3 ) 3 COCL

^

jj^

^ ^

CH 2 CL 2 , 5 0 ° C 70%

Scheme 10.58

The reaction proceeds smoothly and without apparent allylic rearrange­ ment with crotyl- and prenyl-tributyltin379,448 (Scheme 10.59). "COCL

Bu 3 SnCH2CH=CMe 2 l RhCl(PPh 3 )3

Scheme 10.59

However, more recently, rearrangements have been observed under similar experimental conditions in the preparation of atlantones and other terpenic ketones55 (Scheme 10.60). SnMe3

RhCL(PPh 3 ) 3 ,CH 2 Cl 2 COCL

60°C,24h 51%

Scheme 10.60

Couplings can also be achieved with palladium catalysts such as BnPdCl(PPh3)2. However, in this case a further addition of the allyltin to the ketone obtained can occur and for this reason Pd(PPh3)4 is sometimes a more suitable catalyst454 (Scheme 10.61). Me 3 SnALL,Pd(PPh 3 ) 4

°zN \ C / c o a 50

Scheme 10.61

The occurrence of an allylic rearrangement has been exploited with a-alkoxyallyltins for the synthesis of 1,4-ketoaldehydes308 (Scheme 10.62). Bu3Sn

( 1 ) BzCL, BnPdCl ( PPh3)2,benzene,90°C (2)H30+

*~ 81%

Scheme 10.62

Ph

,CH0

202

Access to carbon-carbon bonds

Benzylation The benzylation of acyl halides occurs readily under catalysis by rhodium or palladium complexes to afford benzyl ketones in high yieids448,450,45i,456 (Scheme 10.63). RCOCL + BnSnBu3

R

[Rh] or [Pd] catalyst *-

v.

^ . j[

^Ph

0

Scheme 10.63

This general route has recently been employed for the synthesis of senecioyl-7-p-cymene55 and for the synthesis of ds-5-alkylproline derivatives {Scheme 10.64). 0 II

JU

tfe3 Me 2 C = CHCOa,CH 2 a2 RhCl(PPh 3 ) 3 53%

Ò X

Scheme 10.64

Alkylation The methylation of acyl chlorides is readily performed with Pd(PPh3)4 or BnPdCl(PPh3)2 as catalyst44*"451 (Scheme 10.65). RCOCL

Me 4 Sn,BnPdCl(PPh 35)g2 —-—: - -» HMPA,65°C

R

Y

Me

76-100% R = Me,t-Bu,2-furyl,vinyl, styryl, Z - C e H 4 ( Σ = / P - 0 M e , ^ - M e , H,/?-CL, /?-CH0,/?-CN, ^ - N 0 2 , o-C0 2 Me)

Scheme 10.65

As exemplified by the above list of substituents, the reaction tolerates numerous functionalities. However, with p-bromobenzoyl chloride, further methylation leads also to p-methylacetophenone451 (Scheme 10.66).

9 3 %

72

28

Scheme 10.66

This efficient methylation procedure has been used in the synthesis of (±)-quadrone460. The alkylation of acyl halides is also possible in the presence of ß-hydrogen atoms; valerophenone has been obtained in 85-90% yield from benzoyl chloride and tetrabutyltin451,454.

Via substitution reactions

203

Arylation and heteroarylation In the presence of an excess of Lewis acid, aryltins react readily with acyl halides461 {Scheme 10.67). o

o-

■

SnMe3

-

AcCl,AlCl3,CS2,-20oC

^

98%

Scheme 10.67

However, despite the regiospecificity of the reaction and the easier cleavage of the tin-carbon bond, the homologous organosilicon derivatives remain the usual intermediates for such reactions16"19'375. The use of palladium-promoted arylation is also a convenient means of preparing aryl ketones in high yields449'454 {Scheme 10.68). -SnBu3

MeO

BzCL,BnPdCL(PPh 3) 2 CHCl 3 ,65 e C 85%

MeO

^ ^

Scheme 10.68

Despite its potential, this reaction has rarely been used in organic synthesis, and the most interesting examples come in fact from the trimethylstannylazine series462. While acyl chlorides generally react spontaneously with 2-trimethylstannyl-pyridine and -quinoline or 1trimethylstannylisoquinoline in the absence of a catalyst, the reaction requires a palladium catalyst when the tin atom is not a-located to the nitrogen atom {Scheme 10.69). benzene

+ RCOCl N

34-95%*

SnMe 3 SnMe 3

+ RCOCl

PdCL2(PPh3)2i

Jpj

benzene, 80°C

N

70-82% R = Me,Et,i-Pr,t-Bu,c-Hex,Ph

Scheme 10.69

In a related area may be noted the direct reaction of acyl chlorides with other heteroaromatic organotins463 {Scheme 10.70). />—SnMe3

t-BuC0CL,20°C 72%

n

o

K

Scheme 10.70

Vinylation The vinylation of acyl halides occurs in the presence of a stoichiometric amount of A1C13, involving the loss of polymerizable aß-enones434 {Scheme 10.71).

204

Access to carbon-carbon bonds / / Bu

Ph

Bza,AlCl3

^

^ίΤ^Υ^

CH2Cl2,-10°C

N

58%

°

3Sn

i

Scheme 10.71

When propenyl or styryl organotins are used, αβ-enones are obtained with a fra/w-configuration29'434 (Scheme 10.72). /==

Y

AcCl,AlCl 3 CH2Cl2,-10oC

Bu3Sn

65%

Scheme 10.72

Acid anhydrides react similarly in terms of yields and stereochemistry 434 . Polymerization, the main drawback of the procedure, can be circumvented by using transition metal complexes as catalysts. The vinyl group transfer occurs in high yield (>90%) 4 5 1 and, because of the tolerance of the reaction to other functionalities, it has found interesting applications. For instance, an efficient synthesis of methyl 5-oxo-6heptenoate, a key intermediate in steroid synthesis, has been reported 451 (Scheme 10.73). o o

Ò

CL

Bu 3 SnCH=CH 2 ,BnPdCl(PPh 3 ) 2

C0 2Me

HMPA, 65°C,1min 92%

\ *"

^

l^/C02Me

Scheme 10.73

Similarly, simple vinyltin reagents have been used for the synthesis of natural products or interesting precursors 417 ' 464 (Scheme 10.74).

ro

Bu 3 SnCH=CMe 2 ,BnPdCl(PPh3)2

THF, 90°C

COCL

85%

CL

C0 2Et

(CH 2 =CH) 4 Sn,BnPdCL(PPh 3 ) 2 HMPA

0

,

Ac

56° C

7 0%

Scheme 10.74

The tolerance of functionalities is also valuable for the organotin moiety and, indeed, functionally substituted vinyltin derivatives offer interesting possibilities. For instance, a-alkoxyvinyltins, easily obtained by deprotonation of vinylic ethers 238 ' 465 , allow synthesis of unsymmetrical monoprotected a-diketones 466 (Scheme 10.75). =/0Μβ

\ _ .. bnMe 3

+

RCOCL

B

"PdCUPPh^

benzene, reflux

R π

/V-°

77-100%

R = M e , C 7 H 1 5 , c - H e x l t - B u , P h , 2-furyl

Scheme 10.75

Π

M e

Via substitution reactions

205

It is true that decarbonylation may occur in these cross-coupling reactions, but this drawback can easily be circumvented by the use of a carbon monoxide atmosphere. This procedure has been used successfully to obtain a key precursor of the macrolide antibiotic (±)pyrenophorin 45446/ (Scheme 10.76). l f ) _ B u 3 S n C H = CHC0 2Bn(1.6eq.) OS,Ph 2 t-Bu

.

BnPdCL(PPh 3 ) 2> CHCl 3 . C O d a t m )

_

_

^ ^

0 S i p h 2 t. B u

71 %

53%

Scheme 10.76

Senecioylcyclopentene has similarly been obtained in good yield, allowing access to a key intermediate for the synthesis of modhephene468 (Scheme 10.77). SnBu3

Ó

Me2C = CHCOCl,CO(3.4qtm.)

/ ^ ì f

[1

\J&

Jl^

PdCl2(PPh3)2, THF, 50°C*

( D SnCL4

(2) RhCl3 *

85 %

62 %

Scheme 10.77

Alkynylation The alkynylation reaction can occur on simple heating (without catalyst), but the reaction is more easily performed when activated by Lewis acids446'469 (Scheme 10.78). AcCL,150°C Et 3 Sn

=

R

*-

R

/ <;

==

R = Ph 4 5 % ; R = CH2CL 8 3 %

°

Scheme 10.78

Other examples reported with alkoxyalkynyltin derivatives470'471 show that the procedure is a valuable means of access to alkoxyethynyl ketones (Scheme 10.79). 1 1

N

9 2

R COCL + R 0

s=

SnMe*

*-

16-98%

M e C ?

R^

R1 /

==

<\

^

0

Scheme 10.79

Similarly an extensive series of aminoethynyl ketones has been obtained, often in high yield, from stannyl ynamines by reacting either acyl chlorides or anhydrides472'473 (Scheme 10.80).

1

R C0CL +

R2 \

N / R3

=

. SnRt3

R2 \ ► N 6-94% / R3 ether

Scheme 10.80

==

Xx

\ 0

206

Access to carbon-carbon bonds

With less activated alkynyltin derivatives, palladium complex catalysis affords an efficient route to 1-alkynyl ketones under mild experimental conditions453'454'474'475 {Scheme 10.81). PhCOCL + Me3Sn

=

pr

-PrCOCL + B u 3 S n — = — C H ( 0 E t 2 )

BnPdCL(PPh3)2 ^ CHCL 3 ,65°C 70%

pr

==

Ph / L \,

PdCL2(PPh3)2 — ► (EtO)2CH — = - < \ dichloroethane,84°C \ 0 70%

Scheme 10.81

Cross-coupling with oc-heterosubstituted organotins Diethylaminomethyltributyltin can lead to α-aminoketones without catalyst on reaction with acyl halides444 (Scheme 10.82). / . Bu3Sn^NEt2

BzCL 8 Q O /o

o

ύ .NEt2 P h ^ ^

'

Scheme 10.82

Similarly the reaction of a-stannyl nitrosamines with benzoyl cyanide affords the benzoylated nitrosamines476 (Scheme 10.83). t-Bu

PhCOCN

t-Bu

60%

Scheme 10.83

The presence of the a-nitrogen atom, associated with an accessible tin centre, seems to be necessary to realize uncatalysed reactions. However, with transition metal catalysis, methoxymethyltin derivatives can also give cross-coupling products454 (Scheme 10.84). ..

0 n

BzCL,BnPdCL(PPh3)2

n u.

,

75

0 II 25

Scheme 10.84

When the a-heteroatom is doubly bonded to the carbon atom a similar situation arises. C-Phenyl C-stannyl imines react directly with acyl chlorides to afford a-diketones in moderate yield445, while acyltriorganotins necessitate palladium complex catalysis but offer a versatile route to unsymmetrical a-diketones477 (Scheme 10.85). RK

y0

/SnBu3

+

R

2

./Cl

y

BnPdCL(PPh 3) 2

35-65%s

Scheme 10.85

R1\^A.

y

R

2

Via substitution reactions

207

When symmetrical a-diketones are desired, the reaction of hexaalkylditins under similar experimental conditions appears to be a convenient route477'478. However, to avoid decarbonylation, the reactions are better performed under carbon monoxide. These results must be compared with the synthesis, already described, of monoprotected a-diketones466. Finally should be mentioned the reaction of tributyltin cyanide with acyl halides, which provides a convenient route to acyl cyanides479,480 (Scheme 10.86). R CN ^Tf^ II 0 R = aryl , 2 - thienyl, 2 - f u r y l , t - B u

RCOCL +

Bu3SnCN

_Q/ » 80-99%

nrs

+

Bu3SnCL

Scheme 10.86

However, when R is a secondary or a primary alkyl group, acyl cyanides cannot be isolated480 (Scheme 10.87). 2 RCOCL 4- 2 Bu3SnCN 3

2Q C

°

72-93%

,*- R—(' p J ° C ÇN N C, N

+ 2 Bu3SnCL

0—K

Scheme 10.87

10.1.3 Cross-coupling reactions with organic halides in the presence of carbon monoxide General As already mentioned, carbon monoxide can be used to prevent decarbonylation in reactions involving cross-coupling of organotins with acyl chlorides (see 10.1.2). Indeed, the use of carbon monoxide under pressure in the presence of organic halide and transition metal complexes can lead to ketones481 (Scheme 10.88). Phi +

Bu44 Sn

CO(30atm),HMPA,120°C PhPdI(PPh 3 ) 2 ( 0 . 3 % )

►

Ph

79%

Bu \ ^ Π 0

Scheme 10.88

The reaction occurs with higher yields using tetramethyltin and is also possible with tetraphenyltin. Furthermore, it can be extended to numerous organic halides with turnover numbers >3000481'482. With secondary benzyl bromides and α-bromoesters, side reactions such as isomerization or elimination may occur, but this problem has been circumvented by using triphenylarsine ligands483 (Scheme 10.89). \

^C02Et T ^ I Br

(2eq.)

+ Me44Sn

(1eq.)

PdCL 2 (AsPh 3 ) 2 ,HMPA — C0(20atm,20°C),120°C

62%

Scheme 10.89

\

if H 0

Js. ^ C 0 2άE t

208

Access to carbon-carbon bonds

In some instances the palladium catalysts can be replaced by nickel catalysts, e.g. Ni(CO)4, Ni(CO)3PPh3, NiCl2(PPh3)2 or Ni(CO)2(PPh3)2, this last being the most efficient484. Good yields have been obtained in methylation and benzylation of aromatic iodides and activated bromides, but arylation with tetraphenyltin failed484,485. The catalytic cycle can be rationalized as carbon monoxide insertion into a carbon-transition-metal bond468,484 (Figure 10.3).

-co L=C0,PPh 3 or AsPh 3 M = Ni or Pd

Figure 10.3 Scheme of catalytic mechanism of cross-coupling with organic halides under CO

Naturally, with a lower pressure of carbon monoxide, the cross-coupling of organotins with organic halides occurs as a competing process without carbon monoxide insertion. This feature can be illustrated by the example432 in Scheme 10.90.

kAcL

PhSnBu53

3 % Pd(dba)2+6%PPh3 C0,THF,50°C 65-80%

^~

^v

-Ph

II 0

k^

CO (3.1 atm)

18

82

CO (31 atm)

89

11

Scheme 10.90

It must be noted that (£)-3-chloro-5-carbomethoxycyclohexene reacts with phenyl- and vinyl-tributyltin under palladium complex catalysis and carbon monoxide pressure to afford the expected ketones with a (Z)-configuration432. Allylative carbonylation The coupling of allylic halides with allyltins in presence of carbon monoxide offers an interesting route to unsymmetrical diallyl ketones486. The experimental conditions are mild enough to avoid isomerization to conjugated ketones as well as further addition of the allyltin reagent to the carbonyl group. However, allyl chlorides give higher yields than bromides and the homocoupling of the allyltin reagent can compete with the desired reaction486 {Scheme 10.91).

Via substitution reactions 209 1%[PdCL2,(MeCN)2,2PPh3] ^

SnMe

3 CHCL3,25°C,C0(6atm) 52%

Scheme 10.91

The general regiochemical trend seems to be the carbonylation of the ally lie halide at the less hindered position, while the coupling with allyltin occurs without ally lie rearrangement. Alkylative carbonylation Initially disclosed by Tanaka, this type of reaction has been shown to be efficient with benzyl chloride, styryl bromide and substituted aryl iodides, using PhPdI(PPh3)2, Ni(CO)2(PPh3)2 or π-allyl palladium chloride as catalysts481'48^484'485'487 {Scheme 10.92). / ^ \ Σ-\( ) ) - Ι + Me4Sn \y/

Ni(CO)2(PPh3)2,HMPA — C0(20atm),120-150°C

/^=Λ // Σ -<( »—(' \=^/ \

62 - 95 % Σ = H , Me, OMe , C02Et

Scheme 10.92

The remarkable tolerance of numerous functionalities allows acylation of highly functionalized molecules, as exemplified by the preparation of (acetylalkyl)dimethylxanthines488 (Scheme 10.93). o

»

Β Γ ( 0 Η 2) 4^ ΝΧ Μ

0

^

N

J W

Me4Sn,C0(100atm)

,

PdCL2(PPh3)2,120eC

I

Me

Scheme 10.93

Furthermore, the acylation of arenediazonium salts allows the presence of bromide or iodide as a substituent489 (Scheme 10.94).

63 - 90 % Σ = o,m,p-Me,o,p-Cl,p-Br

,^-I,/77-N02

Scheme 10.94

These reactions are mainly used for acetylation but remain possible when tetrabutyltin or tetraethyltin is used as alkylation reagent481'4^2,489. Arylative and heteroarylative carbonylation As opposed to alkylative carbonylation, arylative carbonylation using carbon monoxide does not work with nickel catalysts and necessitates palladium complex catalysis487 (Scheme 10.95).

210

Access to carbon-carbon bonds /ν/Ν0 2

Π '

\

N

VNO0+ /

A

_ ,

1 % ( C 3 H 5 PdCDo

ArSnMe3

*_2 £*HMPA,20°C,C0(1atm) 94-100%

ÎrVl .

Λ τ

LvJJ ^υ γ ^ ^

Ar = C 6 F 5 , Σ - C 6 H 4 ( Z = H , M e , 0 M e , C l , N 0 2 )

Scheme 10.95

In this example high yields of ketones are obtained, but with less activated iodides the formation of diphenyl derivatives competes with that of benzophenones. However 1,4-diiodobenzene affords 1,4bis(benzoyl)benzene in 78% yield487. The tolerance of other functional groups allows for instance the synthesis of benzoylacetates from bromoacetates481, while arenediazonium salts afford the expected benzophenones in good yields489. Similarly, vinyl triflates are good candidates for the synthesis of aryl vinyl ketones in high yields490. In the heteroaromatic series, 2-thienyltin and 3-furyltin derivatives behave similarly432'487. Thus egomaketone and 3-furanyl geranyl ketone have been obtained in good yields by the route432 in Scheme 10.96. [Pd] , C0(3.4atm) CL

75 %

/ \\

Scheme 10.96

Vinylative and alkynylative carbonylation This type of reaction was first achieved with p-nitroiodobenzene487, but its real potential in organic synthesis has recently been demonstrated by the synthesis of unsymmetrical divinyl ketones from vinyl iodides468. The ketones are obtained in good to high yields (40-93%) and the method tolerates the presence of other functional groups such as carbonyl groups, on both the vinyl halide and the vinyltin moieties468 (Scheme 10.97). BnPdCL(PPh 3 ) 2> THF,50 o C

\ ^ S n B u

3

C0(3.4atm) 71%

Scheme 10.97

Stereochemically, the (£)-geometry is retained in both partners, while the (Z)-geometry of the vinyltin unit, even maintained in the initial coupling process, is rapidly lost under the usual experimental conditions to afford finally the more stable (E)-isomer. It should be observed that cycloalkenyl vinyl ketones are notable intermediates for bicyclo-(n.3.0) systems via Nazarov cyclization468 (Scheme 10.98).

Via addition reactions

211

o (CH2)„

(CH 2 )„

-(CH2)Ä|[

Scheme 10.98

Finally, this carbonylative cross-coupling can be extended to the synthesis of a-acetylenic ketones468 (Scheme 10.99). (~\

+ Bu3Sn

=

pr

[Pd n ],THF,50°C C0(3.4atm) 54%

Scheme 10.99

Similar carbonylative couplings are also possible with vinyl triflates, as in the synthesis of (±)-A9(12^-capnellene490 shown in Scheme 10.100. 0Tf

BF3-Et20^

(£-)-Me 3 SnCH=CHSiMe 3

toluene ,100°C

Pd(PPh 3 ) 4 f C0,LiCL 87%

SiMe3

70%

( 1 ) L - selectride ( 2 ) Tf 2 NPh

76% OTf

(DBFvEtgO

■^ J Me

(F)Me 3 SnCH=CHSiMe 3 Pd(PPh 3 ) 4 ,C0,LiCl

(2)H2/Pd ( 3 ) Wittig

Me

86%

Scheme 10.100

10.2 Via addition reactions 10.2.1 Allylation of carbonyl compounds The pioneering work in this field is due to Neumann, who reacted allyltriethyhin with aldehydes to obtain homoallylic alcohols after protonolysis491 (Scheme 10.101). CHO

(1 ) E t 3 S n A U , A (2) C H 2 ( C 0 2 H ) 2

60-77% R = H,CL , C C 1 3 , N 0 2

Scheme 10.101

Subsequently it was shown that the reaction occurs with an allylic rearrangement and requires activated aldehydes when crotyltributyltin is used492 (Scheme 10.102).

212

Access to carbon-carbon bonds α

ν ^ Ν ^

/?CIC 6 H 4 CH0

^ ^ X y X ^

CCL3CHO 3

200°C 7 0%

OSnBu3

C l

3

100°C

C ^ J ^ 0 S nu B3

6 4%

Scheme 10.102

With allyltrialkyltins the addition occurs with low yields on nonactivated ketones and satisfactory results have been obtained only with perhalogenoketones 494 (Scheme 10.103). 0SnMe 3 F2CLC^/CF2CL

Me 3 SnML T

0

8 5%

^ s ^

C F 2

c

L

CF2CL

Scheme 10.103

As a consequence, the use of more reactive allyltin reagents or of appropriate experimental conditions appears to be necessary. This goal has been reached to some degree with tetraallyltin 4 " 2 ' 493 or 1,3bis(tributylstannyl)-2-methylenepropane 457 (Scheme 10.104). ^^^ Ph

ph

^ \

^^

Π 0

CHO

(1) Sn(AU.) 4 ,95°C — — * (2) protonolysis 60%

Ph

M ) (Bu 3 SnCH 2 ) 2 C = CH 2 , toluene,reflux

7 i 7 ^

OH OH

Ph

'

88%

Scheme 10.104

On the other hand, when allylstannation of aldehydes is performed under high pressure, homoallylic alcohols are obtained in reasonable to high yields, but the reaction mixtures must be maintained under pressure for long periods 495 . If the allylstannation of carbonyl compounds is considered as a nucleophilic addition to the carbonyl group, the reaction should be activated by fluoride ions or by Lewis acids. The first route has some efficiency in the addition of thioallyltin derivatives to aldehydes 353 , but the main developments are related to electrophilic activation according to two different approaches: 1. Use of allyltin halides which also play the role of Lewis acids. 2. Addition of catalysts such as BF 3 -Et 2 0 or TiCl 4 . The preparation of homoallylic alcohols necessarily implies a protonolysis step after allylstannation, even if not mentioned below. Allylation of carbonyl compounds with allyltin halides The efficiency of this approach has been demonstrated by Tagliavini496. When allyldibutyltin chloride was reacted with aldehydes, exothermic additions afforded, after hydrolysis, homoallylic alcohols in good yields 497 . Similarly, ketones have been reacted with allyltin chlorides 97 ' 498 or crotyltin chlorides 499 (Scheme 10.105).

Via addition reactions

213

OH 80°C,7h ^

76

i-Pr'

Scheme 10.105

However, it must be noted that the allylstannation occurs by a reversible process and that excessive heating of the adducts can lead back to the starting materials499'500. The electrophilic character of the tin atom seems to be the driving force, since allylstannation can be efficiently achieved even at -78°C using diallyltin dibromide501 (Scheme 10.106). o

All2SnBr2, T H F , - 7 8 ° C R = H 8 8 % ,R = Me 7 5 %

Scheme 10.106

The following orders of reactivity also corroborate this assumption: BrCl2SnAll>BuCl2SnAll>Bu2ClSnAll>Bu3SnAll497'498; BuCl2SnCrot>Bu2ClSnCrot>Bu3SnCrot499. It must be observed that allylstannation by allyltin chlorides can be performed in the presence of water to afford directly the homoallylic alcohols502 and that the allyl- or crotyl-dibutyltin chlorides are readily obtained by redistribution reactions503 (Scheme 10.107). CL

CL Bu2Sn -

Bu2SnCL2

B U 2 S

_Bu3SnCL

\ ^ ^

Bu2SnCL2

1

"

Bu2Sn

( Z -i

Scheme 10.107

The fact that the branched allyltin reagent is obtained first allows reaction of this unstable isomer when generated in the presence of aldehydes504 (Scheme 10.108). OH EtCHO

OH

Bu3Sn Crot / BuoSnCU 92% 89

Scheme 10.108

With more hindered aldehydes the reaction occurs regiospecifically and stereospecifically to give the (Z)-linear alcohols504 (Scheme 10.109). RCHO

OH

Bu 3SnCrot / Bu2SnCL2 R= Ph or t - B u 9 5 %

R

Scheme 10.109

1,2-Additions are also observed with αβ-enals, -enones and -ynones497,505 (Scheme 10.110).

214

Access to carbon-carbon bonds ^

^

Bu2SnCLAlU8h,20'Ct

^

°

y

^

OH

Scheme 10.110

Furthermore, when βγ-enones are generated in situ, addition at the carbonyl group can occur without previous isomerization to οφ-enones458 {Scheme 10.111). OAc Bu 2 SnCLAU,50°C

AcCL

85%

Scheme 10.111

Note that the reactions of crotylbutyltin dichloride with aldehydes give 4-chloro-2,6-dialkyl-3-methyltetrahydropyrans as by-products arising from condensation of the initial adducts with the aldehydes506. Homoallylic alcohols can also be obtained via in situ formation of the allyltin reagents. For instance, homoallylic alcohols are easily obtained from allylic phosphates and aldehydes using the SnF2-Et2AlCl system507 (Scheme 10.112). ^ ^ o L p h )

2 +

P h C H O

SnF 2 .Et 2 ALCL,CH 2 CL 2)

P

96%

h

^

0 H

Scheme 10.112

Other syntheses have been achieved by reacting allyl bromides or i o d i d e s with s t a n n o u s halides ( r e a c t i v e species = X3SnCH2CH=CH2)498'508'509 or with metallic tin (reactive species = X2Sn(CH2CH=CH2)2)501'510-514. In the first case, the allylation of aliphatic, aromatic or αβ-unsaturated aldehydes is readily achieved by reaction of the allyl iodide-tin difluoride couple, using l,3-dimethyl-2imidazolidone (DMI) as a solvent, and the allylation of ketones is also possible with reasonable yields508 (Scheme 10.113). CHO AU.I,SnF 2 ,DMI,20°C

/

\

\

/

AUI,SnF2,DMI,20°C 73 %

Scheme 10.113

The use of γ-chloroalkyl iodides, followed by a basic treatment, affords vinyloxiranes with good stereoselectivity by an allylstannation-elimination sequence509 (Scheme 10.114). (1)CLCH = CH-CH 2 I,SnCl 2 ,DMF RCH0 — ^ ^ ► 2 NaOMe 52%

R

^r-f>^ V ^ 89-74

R = Ph , C7H15,BnCH2,PhCH = CH

Scheme 10.114

+

R

/==■ ^—1 X) 11-26

Via addition reactions

215

When γ-bromoallyl iodides are used in the presence of excess stannous chloride, a double stannylation occurs, leading subsequently to (£)-dienes509'510. The second possibility for in situ generation of dialkyltin dihalides involves the reaction of metallic tin with allyl iodides or bromides501 (Scheme 10.115). RCHO

ALLX.Sn.THF 76 - 88 %

(X = I , B r )

Scheme 10.115

However, it has been found that aprotic solvents are not the most suitable ones. Thus the use of allyl bromide and metallic tin in heterogeneous ether-water mixtures leads to homoallylic alcohols from aldehydes or ketones in reasonable to high yields511 (Scheme 10.116).

v

AlLBr.Sn, e t h e r - H 2 0

-\^

45 - 73 %

Scheme 10.116

When crotyl bromide has to be used, the reaction must be performed in the presence of aluminium powder in THF-water or ether-water mixtures511 (Scheme 10.117). C 5 H n CHO

CrotBr, Sn , AL, THF - H 2 0 , H + , 20°C C5H1, ^ ° Ί1

97% OH

Scheme 10.117

Another possibility of achieving such allylation or crotylation reactions is the use of sonication using water-THF as solvent512. Allylation also occurs using precursors of carbonyl bonds and it is worth noting its compatibility with the presence of nitrile, ester or acid groups, though these functionalities can induce further reactions511'513 (Scheme 10.118). 2 B

/OR3

\

>

0 or X /

^OR

3

^CO H 2

CH 2 =CBrCH 2 Br, Sn,ether,H 2 0,H +

-

—

1

^

{

, R1-

/

(ref. 513)

70-97%

AlLBr,Sn,ether,H 2 0,H

+

»

(ref. 511)

47%

CH 2 = C(OAc)CH 2Br, Sn,ether,H 2 0, H + 73%

Scheme 10.118

B

" (ref. 513)

AcO

216

Access to carbon-carbon bonds

Recently an improvement has been made that involves electrochemical regeneration of the allyltin reagent and subsequently the use of a catalytic amount of tin for the synthesis of homoallylic alcohols in high yields (72-91%). The reaction performed in acidic methanol has been rationalized by the scheme514 shown in Figure 10.4.

^?

AUBr

cathode

Figure 10.4 Scheme for mechanism of synthesis of homoallylic alcohols with electrochemical regeneration of allyltin reagent

Lewis-acid-catalysed allylation of simple aldehydes and ketones The allylstannation of carbonyl compounds in the presence of BF 3 -Et 2 0, under mild experimental conditions, was first reported by Naruta and Maruyama515" . The initial reports led to other studies especially related to regio- and stereo-chemistry, but these aspects will be discussed in the next two subsections. As expected, the allylation of aldehydes is faster than that of ketones, but the rates are not always sufficiently different to allow selective reactions519. Numerous experiments have been performed in the aromatic and heteroaromatic series which demonstrate the occurrence of allylic rearrangements and a high tolerance to other functionalities519 {Scheme 10.119).

Qr

CHO

CHO

Bu 3 SnCH2CH=CMe 2 ,BF3»Et 2 0 - 7 8 ° C then 0°C 90%

Bu3SnALL,BF;j»Et 2 0,CH 2 CL 2 - 7 8 ° C then 0°C 66 - 9 9 %

Σ = H , /7-OMe, p-CH ,/77-N0 2 , o-OH

Scheme 10.119

This method has been successfully applied to the preparation of natu] natural products such as (±)-ipsenol412 and (±)-lavandulol521 {Scheme 10.120)

Via addition reactions Me3Sn

CHO

217

BF,.Et?0 CH2CL2,-78°C 40%

-SnBu3+(CH20)„

BF

3-Et20t

CH 2 CL 2 ,25°C 57%

Scheme 10.120

The allylic rearrangement is often considered as a rule in these reactions; however, the use of AlCl3,iPrOH can preferentially yield regioreversed addition but highly sensitive to the nature of the aldehydes522 ~23 (Scheme 10.121). /7-PrCHO,ALCL 3 -iPrOH

/7-Pfv

^Μβ +

72%

OH

OH

/

95 KE Z

• SnBu 3

i-PrCHO,ALCl 3 -iPrOH

= 85 = 15)

5

-Pr

80%

OH

Scheme 10.121

Furthermore, a-alkoxyallyltins and y-(alkoxymethyl)allyltins give ocadducts with benzaldehyde in the presence of Lewis acid catalysts30^308'524. These results may be explained by an initial catalysed isomerization of the allyltin derivatives to givefinallythe adducts after a double rearrangement, but the high degree of stereoselectivity with Y-(alkoxymethyl)allyltins can suggest other interpretations {Scheme 10.122). 'SnBu 3 BF 3 .Et 2 0

Bu

OH

3sn

PhCH0,BF 3 .Et 2 0 Ph

70% OEt

OEt .SnMe 3

EtO'

PhCH0,SnCL 4 67-80%

EtO

Zor E

Scheme 10.122

It should also be mentioned that the allylation occurs exclusively at the carbonyl site even in the presence of an epoxide ring516 (Scheme 10.123).

Bu 3 SnALL,BF 3 *Et 2 0

__

»>

CH 2 CL 2 ,-78°C then 0°C 77-91% R = H,Me,Et,Pr,t-Bu

Scheme 10.123

218

Access to carbon-carbon bonds

Additions are also obtained when bisidiethylaluminium) sulphate or titanium tetrachloride is used as catalyst ,525 (Scheme 10.124). CHO T1CI4 x

"SnPh 3

(ref. 525)

40%

Br

Scheme 10.124

Allyl acetates can also be directly converted to homoallylic alcohols without isolation of the organotin intermediate, as in the example526 of Scheme 10.125. (1)Et 2 ALSnBu 3 .Pd(PPh 3 ) 4 (2)EtCH0,BF3.Et20 67%

Scheme 10.125

It has been observed that palladium catalysts show similar behaviour to Lewis acid catalysts when a-halo ketones or aldehydes are employed; thus the substitution of the halogen atom does not occur and the addition is followed by elimination of organotin halide (probably catalysed by palladium) which affords epoxides in fairly good yields398'527. It must be kept in mind that allylation at the halogen position is possible with AIBN initiation398 (Scheme 10.126). ALLSnBu3, AIBN,80°C *

\ ^ - \

39% CHO

o

AUSnBu 3,Pd(PPh 3)4 CL

**^

*"

^ ^ λ — C

ALL 2 SnBu 2 ,BnPdCL(PPh 3 ) 2 8 6 %

Br

Scheme 10.126

Similarly ß-chloro- and γ-chloro-ketones afford respectively the allylsubstituted oxetanes and tetrahydrofurans in high yields527. When οφ-unsaturated carbonyl compounds are involved, enals give exclusively 1,2-addition, but in contrast to allylation by allyltin chlorides, enones lead to 1,4-addition when the reaction is promoted by (Et 2 Al) 2 S0 4 412 or AICI3528 (Scheme 10.127). r=\ \

Me 3 SnALL,(Et 2 AL) 2 S0 4 /

benzene - hexane, 8 0 ° C 46%

Scheme 10.127

Lewis-acid-catalysed allylation of quinones The allylation of benzoquinones promoted by BF 3 -Et 2 0 occurs at low temperature and gives a mixture of compounds, as in Scheme 10.128520.

Via addition reactions

219

(1 )Bu 3 SnAlL,BF3.Et 2 0(5eq.) - 9 0 ° C , 5 min 0

(2)H20

Scheme 10.128

The 1,2-adducts (I and II) appear to be the kinetic products, which isomerize easily in presence of Lewis acid catalysts to 1,4-adducts (III and IV). As a consequence, when the reaction mixture is warmed to room temperature before protonolysis, the 1,4-adducts are obtained quantita­ tively (III:IV = 79:21). It is also noteworthy that IV isomerizes to the more stable III under BF 3 Et 2 0 catalysis; this trend appears more clearly when methallyltributyltin is added to 2,5-dimethylbenzoquinone520 (Scheme 10.129). OH

IT

SnBu3

+

CH2CL2 ►

BF3.Et20

OH 45%

0.5eq.BF 3 .Et 2 0 1 eq.BF 3 .Et 2 0

trace

43% 99.5%

Scheme 10.129

The assumption that 1,2-adducts are initially formed seems reasonable, and they have been isolated in high yields with compounds such as 9,10-anthraquinone and 2-methoxy substituted paraquinones515'520. However, more information has been obtained from 3-substituted 1,2-naphthoquinones and the results obtained by trapping reaction mixtures at — 78°C are in agreement with the concept of competition between both types of addition process, depending on the substituents529 (Scheme 10.130). ( 1 ) Me3SnAU..BF 3 .E :t 2 o CH 2 CL 2 , - 7 8 ° C ( 2 ) HgO

ex

R = OMe

93%

100

0

R = N02

80%

19

81

R = COEt

89%

11

89

Scheme 10.130

Naturally, 1,2-adducts can be isomerized by warming to room temperature in the presence of BF 3 Et 2 0, and subsequent oxidation to allyl quinones can be performed529. On the other hand, when the substituent is at the 4-position, its nature has a great influence on the regioselectivity of the reaction: for instance, 4-methoxy-l,2naphthoquinone is allylated at the 1-position (91% yield), while 4-cyano-l,2-naphthoquinone reacts at the 2-position (91% yield)530.

220

Access to carbon-carbon bonds

When unsymmetrical allyltins are used, the regioselectivity of the allylation (a- or γ-adduct) depends both on the steric hindrance of the allyltin unit and on the nature of the quinone520 (Scheme 10.131). ( 1 ) ( E) - Crot SnBu 3t BF3 · Et 2 0

Tl

*-

CH 2 CL 2 ,-78°C then 20°C (2) H 2 0

75% 69%

R = H R = Me

Scheme 10.131

Furthermore, electronic factors also play a role, as exemplified by reactions of irafts-crotyltributyltin with 3-substituted 1,2-naphthoquinones. The a-adduct is obtained almost exclusively when the substituent is an electron-releasing group and the γ-adduct is obtained preferentially if it is an electron-withdrawing group529 (Scheme 10.132). (1 ) B F 3 . E t 2 0 , - 7 8 ° C thenO°C - SnBu 3

(2) H 2 0

(3) A c 2 0 , pyr

R =Me,Et,OMe,OEt

73 - 98 %

R =C02Me, A c , N 0 2

89 - 92 %

Scheme 10.132

On the other hand, a strong withdrawing group like a cyano group at the 4-position affords exclusively a-adducts (at the 2-position)530. With prenyltriorganotin derivatives, steric requirements at the allyltin unit are the major factor, and in practice the a-adducts are observed as single products even for 1,2-addition to 3-nitro-l,2-naphthoquinone520,529. The sole example of preparation of a γ-adduct with this reagent is the prenylation of 2-acetyl-l,4-naphthoquinone531. Other interesting results are obtained in the reactions of 2,4pentadienyltrimethyltin and 2,4-hexadienyltrimethyltin with p-quinones, where the additions give preferentially the ε-adducts528. It is noteworthy that the corresponding silyl derivatives lead to Diels-Alder reaction instead of 1,4-addition products. Among the numerous applications, prenylated and polyprenylated natural quinones constitute interesting targets easily obtainable by allylstannation followed by FeCl3 or Ag 2 0 oxidation. Coenzyme Qi has been successfully obtained by this route (Scheme 10.133). o

o

Μ β Ο \ ^ \

(1) Me 2C = CHCH 2 SnBu 3 ,BF 3 .Et 2 0

MeO X^X

(2) HCL

(3)FeCL3 75%

Scheme 10.133

ΜβΌ^^γ^^^\

"

ΜβθΧΑ 0

Via addition reactions 515

221

532

Subsequently plastoquinone-1 , plastoquinone-2 and coenzymes Q 2 , Q 3 , Q 9 and Q10 resulting from the regio- and stereo-controlled polyprenylation of 2,3-dimethoxy-5-methylbenzoquinone have similarly been obtained in 51-90% yield510. In the naphthoquinone series, polyprenylation allows access to vitamins K! and K 2 5l? (Scheme 10.134).

o

48 o /o

o

o

82 KE-Z 9 6 4 )

18

(vitamin K1) Scheme 10.134

In the field of antibiotics the method has been used for the synthesis of (±)-4-methoxydalbergione533 (Scheme 10.135). o

o

ΜβΟ^Λ^

( 1 ) ( / r ) _ p h C H = CHCH2snMe3,

(2) H20 0

BF3.Et20

(3) Ag20

M e

*~

50%

°>fSì I I I + other isomers ^ γ ^ γ ^ °

Ph 75

25

Scheme 10.135

It is worth mentioning also the synthesis of a key precursor of 11-deoxyanthracycline antibiotics by an allylation-cyclization process534 (Scheme 10.136). 0 / x X

( 1 ) i ^^^SnMe3,SnCL 4 f CH 2 CL 2 ,-78

ψΧ^ MeO

0

0

(2)H2O (31Ac30,yr

e

C

^

OAc Ι ^

.

OAc ,^v<^v^^s

-^9QO+Ç9ÇO

86o/o

ΟΜβ OAc 0

OMeOAcO

64

36

Scheme 10.136

This synthetic approach has also been used to obtain (±)-eleutherin and (±)-isoeleutherin^35 (Scheme 10.137). MeO

0

0

MeO OMeO

MeO 0

f f ) f j | ^

( 1 ) AllSnMe3,BF 3 .Et 2 0

[ f ^ l f ) ! ^

( D LiAtH 4 , ( 2 ) Hg(OAc)2

f f S ] « ^

^^Sf

(2) H20 (3)MeItK2COV3

"WA^k^V

(3) NaBH4, (4) Ce(IV)

^ ^

0

92%

0Me

81%

0

υ

Scheme 10.137

A similar method is successful in the synthesis of macroheterocycles, such as arnebinol, a prostaglandin biosynthesis inhibitor536. Finally should be mentioned a rapid route to the skeleton of mitomycins, compounds used in cancer chemotherapy537'538.

222

Access to carbon-carbon bonds

Stereochemistry of the allylation of aldehydes and ketones Comparison of allyltins with other allyl-metals is beyond the scope of this book, but information can be found in recent reviews354'539'540. To facilitate the discussion of diastereoselectivity, the terms erythro and threo will be employed according to Heathcock's convention541. They can be assimilated respectively to the terms syn and anti employed by Masamune542, and both nomenclatures will be used indifferently. OH

R^ Y

^

R2 "syn" or "erythro"

Diastereoselective addition of ^-substituted allyltins to enantiotopic faces. The first report in this field dealt with the uncatalysed crotylstannation of activated aldehydes such as chloral, which occurs with remarkable stereospecificity {Scheme 10.138). Bu3Sn '

osceno t

CL3c

20°C,10h OH

E Z

= 9 0 ; 10

threo ■ erythro - 9 0 = 10

E Z

= 65 ; 35

threo ■■ erythro = 6 7 : 33

Scheme 10.138

Comparable results have been obtained in the thermal crotylstannation of benzaldehyde and 4-chlorobenzaldehyde543 or when the reaction is performed under high pressure495. a-Alkoxycrotyltributyltins behave similarly and (£)-a-(methoxymethoxy)crotyltributyltin affords exclusively the threo-cis enol ether, which can easily be converted to trans-4,5disubstituted butyrolactone by hydrolysis and subsequent oxidation544 (Scheme 10.139). 0

( 1 ) Bu 3 SnLi

Scheme 10.139

All these results justify efforts at stereospecific synthesis of (Z)- or (£)-2-alkenyltins for stereospecific synthesis of homoallylic alcohols545. However, the stereochemical course of the addition may be strongly modified when crotylstannation is achieved with crotyltin halides or induced by Lewis acids. In the first route the stereoselectivity is generally poor: slightly favourable to the i/ireo-isomer when the reaction is performed with neat compounds506'546 or in apro tic solvents501'507 and more favourable to the erythro-isomer when it is carried out in the presence of water511. BF3-Et20-promoted crotylstannation is much more interesting because it gives eryfAro-selective addition to aldehydes, regardless of the geometry of the crotyl unit523'547 (Scheme 10.140).

Via addition reactions ^^SnBu

223

J^Etj^

3 + R C H 0

CH2Cl2 HO erythro · threo = 90 : 10 to 99 1

Z or E

R = Ph , i - P r , E t , Me , CH = CHMe, CH 2CH 2C0 2Me

Scheme 10.140

The reaction occurs with similar diastereoselectivity with (E)-2hexenyltributyltin, with geranyltributyltin548 or with w-Miw-generated Y-(trimethylsilylallyl)tributyltin549. This remarkable syji-stereoselectivity is slightly poorer when (£)-crotyltriphenyltin and 2,4-hexadienyltrimethyltin are used528'548 or when crotylstannation is performed on glyoxylates (syw-selectivity 75-90%)550' . It is almost absent with pyruvates552. However, more surprising is the reaction of (£)-cinnamyltriphenyltin, which affords exclusively the anti-isomer548, in contrast to organosilicon derivatives, for which $yji-selectivity seems to be the general rule553 (Scheme 10.141). SnBu3

+ RCHO

RCHO

BF3. Et 2 0 60-98%

BF3.Et20 65-84%*

R= Me, c - H e x , Ph , BnCH 2

Scheme 10.141

A further study has demonstrated the influence of experimental conditions on the stereochemistry of crotylstannation of cyclohexanecarboxaldehyde554 and led to the following conclusions: 1. Lewis acid catalysts such as MgBr2, ZnBr2, Znl 2 or SnCl4 give mixtures of stereo- and regio-isomers. 2. The ery/Aro-selectivity observed with BF 3 Et 2 0 is enhanced by an excess of stannane. 3. With TiCl4 the stereoselectivity is high and depends simply on the order in which the reagents are mixed. Thus the addition of crotyltributyltin to a solution containing the aldehyde and TiCl4 (normal addition) affords mainly the eryiAro-isomer, while the addition of crotyltributyltin and TiCl4 to the aldehyde (inverse addition) yields the iAreoisomer554 (Scheme 10.142). c-HexCHO + B u

^. Sn-^^^\ Λ

3

TiCL 4 -V

0H 1 ^ c-Hex^\^^s

OH +

c-Hex'

Me 'normal addition'

(1.05eq.)

(1.05eq)

>97%

< 3%

( erythro ■ threo - 93 : 7 ) 'inverse addition'

(2eq.)

(2.1 eq.)

>95% ( erythro ■ threo - 5 : 95 )

Scheme 10.142

< 5%

224

Access to carbon-carbon bonds

The inverse addition' procedure is also efficient with benzaldehyde, isobutyraldehyde or decanal in terms of yields (89-94%) and threoselectivity (86-96%). The dependence of selectivity on experimental conditions may be due to the intervention in 'inverse addition' of crotyltitanium species, which are known to give i/ireoselectivity when used alone555. New information has been obtained from intramolecular allylstannation556 (Scheme 10.143). /CHO /

Λ

catalyst

k ^ k ^ S n B u

CH

3

2 C L2 82-98

18-2

Scheme 10.143

A large excess of one isomer is obtained either under simple thermal conditions or with catalysts such as TiCl4, BF 3 -Et 2 0, SnCl4 or FeCl3. This selectivity necessarily implies a preference for synclinal over antiperiplanar transition-state geometry, but extrapolation to intermolecular reactions remains to be demonstrated. Despite limited understanding of the stereochemistry, the results are of great practical interest because they allow selective access to homoallyl alcohols as syn- or aniZ-isomers. The syn-stereoselective synthesis of homoallylic monoprotected a-glycols using heterosubstituted allyltin derivatives is an example300'308'5*7 (Scheme 10.144).

PhCHO +

R2 ! R V ^ k v

/SnBu3 T OEt

0 H f BF 3 .Et 2 0 I I ► P h ' " ^ / ^ 72-88% *

R1

0Et

R1 = R 2 = H ; R1 * R 2 = H or Me

syn-anti

= 93-96= 7 - 4

Scheme 10.144

This .syn-selectivity is the general rule with γ-heterosubstituted allyltins (Σ = OR, SR or SiMe3), in contrast to the corresponding titanium or zirconium derivatives, which react with anri-selectivity557. Note, however, that a-organoboron-substituted allyltins react preferentially at the boron atom to afford interesting anti homoallylic alcohols still stannylated at the double bond552'558. Another interesting application of s>w-stereoselectivity is exemplified by the synthesis of (±)-4-methylheptan-3-ol, an aggregation pheromone of the smaller European elm bark beetle559 (Scheme 10.145). I

Bu 3 Sn

1

\

^

\

(1 ) BF 3 .Et 2 0,CH 2 CL2,-78 o C + EtCHO ( 2 ) H / P d 2 74%

Scheme 10.145

OH

Via addition reactions

225

Diastereoselective addition ofallyltins to diastereotopic faces. When allyltins are allowed to react with carbonyl compounds presenting two diastereoto­ pic faces, allylation occurs at the less hindered face with good selectivity519 {Scheme 10.146). o

r

(^

Bu3SnAu.BF3.Et2O CH 2 CL 2 , - 2 0 ° C 93%

Scheme 10.146

This result is somewhat different from that obtained with other allyl-anionoids560 but, in comparison with allyl-alkali, allyl-magnesium and allyl-zinc reagents, allyltins appear as softer species which are used in poorly solvating solvents. Consequently, entry from the equatorial side (production of the (Z)-alcohol) is expected to be highly favoured561. With aldehydes, the a- and ß-alkoxy derivatives have received special attention because of their potential in the synthesis of sugars or antibiotics562"568. With such structures, the aim is to use appropriate experimental conditions to perform the reaction selectively under 'chelation control' (with subsequent attack of the nucleophile on the less hindered side of the chelate) or under 'non-chelation control' (with subsequent attack of the nucleophile governed by steric or electronic factors)569. 'Chelation control' involves Cram's cyclic model570 and 'non-chelation control' Felkin's model refined by Anh571 as shown in Figure 10.5. 0 P OR 2 R1S (a)

(b)

Figure 10.5 Models of (a) 'chelation control', (b) 'non-chelation control'

In fact, when α-alkoxy- or a-siloxy-cyclohexylacetaldehyde is allowed to react with allyltributyltin, marked differences are obtained depending on the nature of the catalyst and of the solvent565'568 (Scheme 10.147). / \ ^ c-Hex

Η

+

Bu

3

Sn^\^

R = Bn

TiCL 4 or MgBr 2 ,CH 2 CL 2

R=SiMe 2 t-Bu

BF3.Et20(2eq),CH2CL2

^250 5

1 95

Scheme 10.147

Thus highly selective synthesis of threo or erythro a-diols is possible. With ß-alkoxy aldehydes, where competition between chelation and non-chelation control is also possible, higher selectivities are obtained by a

226

Access to carbon-carbon bonds

chelated pathway when tin tetrachloride is used as catalyst (allyltin trichloride is the actual reagent)567 (Scheme 10.148). OBn

Scheme 10.148

Without the alkoxy substituent, allylation occurs preferentially accord­ ing to Felkin's model but with lower stereoselectivity568 (Scheme 10.149). Ph

CHO ALLSnBu 3 ,CH 2 CL 2 ,-78 0 C

ρ

ί?

BF 3

80

20

AICI3

84

16

Scheme 10.149

In the carbohydrate series, where this notable stereochemical behaviour is of great interest, synthesis of a precursor of 2-deoxy-D-ribose has been achieved by an organotin route562 (Scheme 10.150). \ |

\ ^ \ "

OoCCHoOPh

(1 ) ALU , S n F 2 , DMI ,DMF

"- — — — ~—

r^q H

( 2 ) Ph0CH2C0CL

*~

CHO 74%

anti'·■ syn 81 : 19

Scheme 10.150

2-Deoxy-L-galactose, 3-amino-2,3-dideoxy-L-xylohexose and L-diginose have been obtained from the allylation product of 4-0-benzyl-2,3-0isopropylidene-L-threose563 (Scheme 10.151). ALL2SnBr2 BnO^^Ì^^CHO

THF,-100°C*

0

~ioo%

^^^ i i B n 0

/

^

^^ ,#>

S s J

^ ^ ^ ^ ^ °H

° anti-syn

90 = 10

Scheme 10.151

Similar experimental conditions have been used in a key step towards the aliphatic segment of rifamicin S564 (Scheme 10.152). _

-

-

-

X X

7,%

HO 0

0

0

XX

0

syn ■ anti 2 0 : 1 Scheme 10.152

More recently, an elegant stereocontroUed synthesis of the two epimers of 2-substituted 4-methylene tetrahydrofurans has been proposed using (2-acetoxymethyl)allyltributyltin57i (Scheme 10.153).

Via addition reactions

227

Pd(0) catalyst, DBU Ac

0

OHC

y _ / " V ι θ Βηθ'

N

AcoQ^SnBu

3

u B F3, .t ET t2? 0

*~

° ^

°. ^

76%

(2)ΚΟΗ,ΜβΟΗ 42% OBn

Scheme 10.153

Double diastereoselection. At this point it is clear that judicious choice of the catalyst should lead to double control of stereochemistry, especially when crotyl tin derivatives are reacted with a- or ß-alkoxy aldehydes. Indeed, 'chelation control' of diastereofacial selectivity can be obtained with ^«-selectivity in the introduction of the crotyl unit566 (Scheme 10.154). OBn ^H

(£")-CrotSnBu 3 ,MgBr 2

BnO I

BnO I

+

-22°C,CH2Cl2

>90

R = c - Hex or Bu

<10

Scheme 10.154

With ß-siloxyaldehydes the introduction of the crotyltin unit in a complete syn fashion via a non-chelated transition state leads preferentially to the syn-syn-diaauct561 {Scheme 10.155). (£") - CrotSnBu3, BF 3 .Et 2 0(2.1 eq) 0

0R

CH2Cl2,-78oC

OH

92%

OR

5

Scheme 10.155

A comparable result is obtained with hydratropaldehyde523'568 {Scheme 10.156). Ph

CHO CrotSnBu 3 BF 3 ,CH 2 CL 2 ,-78°C 86

Scheme 10.156

Finally, 'double stereochemical control' has been used for a short and highly stereoselective synthesis of (±)-Prelog-Djerassi lactonic acid523'573 {Scheme 10.157). COOH Me0 2 C J^s*%

CHO

o^o. %

c r otSnBu 3

θ3.Η20^

BF 3 .Et 2 0,CH 2 CL 2 92%

major isomer 94-97%

Scheme 10.157

85%

o^ov

228

Access to carbon-carbon bonds

Enantioselective reactions. Extension to enantioselective reactions has been achieved using either chiral aldehydes or chiral allyltins. The former approach is exemplified by the crotylstannation of 8-phenylmenthyl glyoxylate550 {Scheme 10.158). o

M u R * 0 - V H

,_, A Ä „ (£")_CrotSnBu

o

■

> u R * O ^ V ^

3

. nC u.-7ftoC' R BFF,3 .^Ent 2r0.,M CH 2 L2,-78°C

o

n f + R * C r V ^

+ ( 2 / ? . 3 / n + (2/?.35)

80%

(25,3/?) 84

(25,35) 9

Scheme 10.158

The hydroboration-oxidation of the mixture obtained affords 1,4-diols (70% yield) which can be converted to verrucarinolactone (91% enantiomeric excess) on acid treatment550. The second approach uses optically active organotin compounds such as (-)-diallyl-bis-(2-phenylbutyl)tin574 (Scheme 10.159). BF 3 .Et 2 0

RCHO + ( - ) Et

HN /

R

2

λ X

OH

16-56% e.e.

Scheme 10.159

Another approach involves a chiral a-alkoxycrotyltributyltin, which is obtained by the sequence shown in Scheme 10.160. -SnBu 3

3

C

v^(V^

+(.}

^

V " l

.

^^/SnBu3 O^^OHMen

Scheme 10.160

Each diastereomer, after separation, is reacted with benzaldehyde under thermal conditions to afford the anti homoallylic alcohol with remarkable enantioselectivity575 (Scheme 10.161). \^_/SnBu

3

0^0(-)Men

fH

PhCHO 130 e C,15h

Ph

T ^

75%

(1/?)

( 1 ) 0 3 (2)Me2S (3)Ag20 (4)CH2N2 \^0(-)Men

i Ph

^

kt

J

4 0 %

(35,45)

(25,35)

>98% e e

> 9 0 % e.e.

Scheme 10.161

From the (15) diastereomer the (2i?,3i?)-
Via addition reactions R2*

1

À

^ \

Lewis acid

I

*9

75-99%

9H

.. Y

^

R

dioxane 1

229

/—\

H+100-C

R^Q^O

65-99% Scheme 10.162

With isobutyraldehyde the most suitable experimental conditions involve the use of TiCl4 (in dichloromethane) and asymmetric induction by (R)- or (5)-(methoxymethyl)phenethyl groups. Thus the (5)-(-)organotin amide affords the (5)-(-)-lactone in 49% overall yield and 79% optical yield, while the (i?)-(+) organotin amide yields the (R)-(+)enantiomer (chemical yield 96%, optical yield 78%). Similar results have been obtained with other aldehydes, with enantiomeric excess >78% 576 .

10.2.2 Allylation of miscellaneous substrates Although epoxides or activated double or triple bonds492'493'577'578 can undergo allylstannation, only moderate yields are obtained, often under drastic conditions, and only one application should be mentioned579 (Scheme 10.163). /OH AqSnMe, v

^ . Ε , , Ο , Ο Η ^

^

>

^

- 7 8 ° C then -10°C 80%

Scheme 10.163

Allyltin reagents react readily with aldimines, giving homoallyl amines in high yield with Lewis acid catalysis572'580'581 (Scheme 10.164). 1ST

. y R1

AU.SnBu 3 ,CH 2 Cl 2 ,-78°C

II

B F 3 . E t 2 0 or TiCL 4 48 - 88 % R1 = alkyl, aryl, heteroaryl ; R 2 = aryl, alkyl, Bn

Scheme 10.164

The use of (2-acetoxymethyl) allyltributyltin provides 2-substituted 4methylene pyrrolidines in good yields572 (Scheme 10.165). Pr N p h

^

p,.

AcO\>^/SnBu3 BF 3 .Et 2 0,CH 2 Cl 2 ,-78 e C

^NH ρ ή

,

Pd(0)catalyst,DBU

/ ^ ^ Α ^ Ο Α ο

?2%

P

%

h

\ ^ \ ^ k y

88%

Scheme 10.165

On the other hand, with crotyltributyltin, remarkable ^«-selectivity is observed, particularly when the reactions are performed at -78°C in the presence of TiCl4580'*81 (Scheme 10.166).

230

Access to carbon-carbon bonds j H Bn

CrotSnBu3,TiCL4,CH 2 Cl 2 ,-78 0 C

.j c-Ηβχ^

78%

*

c - Η β χ ^ γ ^ . syn ■■ anti - 9 6 : 4

Scheme 10.166

However, it should be noted that the stereochemistry is determined by the 'history' of the TiCl4-aldimine complex, which must be formed, kept and reacted at -78°C to obtain the high syn-selectivity580. As well as with aldimines, the addition of allyltriphenyltin to carbon-nitrogen multiple bonds has been observed with phenyl isocyanate, phenyl isothiocyanate and benzonitrile582. With nitriles, allylation has been reported with a normal or an 'Umpolung' reactivity. Thus perfluoroalkyl nitriles afford fluoroalkyl enamines583 (Scheme 10.167). NR!R2

mn°r AUSnMe 3 + C 8 F 17 CN —

I »

- ^ ^ ^ c F

R1 = R 2 = H 2 3 % ; R1 = H , R 2 = SnMe 3 70% ; R* = R2= SnMe 3 7%

Scheme 10.167

When the reaction is performed with triallyltin fluoride, a double allylation is observed583. Allyltributyhin can react in moderate yield with acetonitrile in the presence of thallium trifluoroacetate to give N-allyl amines, with an 'Umpolung' reactivity584. The addition of allyltins on immonium salts leads to a-allyl amines. For instance, the synthesis of 2-vinyl-substituted 1,3-diamines has been achieved as in Scheme 10.168410. ^

Bn i Μβ

I I S e Ph

Me3SnLi

.

Rn

'HF -100%

1

+

ΟΗ 2 = Ν Μ β 2 , Γ

0 kt

Rn

I

I

Μθ

Μθ

60%

^ Ν - Μ I Me

β

Scheme 10.168

Similarly, 7V-(alkoxycarbonyl)pyridinium salts are regioselectively allylated mainly at the a-position, as exemplified by the synthesis of (±)-coniine585 (Scheme 10.169). ^ ^ CLI C0 2Me

Bu3SnALL

j

8 7 %

^

^

^

Ν ^ — I C0 2 Me

(1)H2,Pd ^ (2)Me3SiI

^

N

I H

Scheme 10.169

Various substituted pyridinium salts react similarly, with >90% aselectivity585. 10.2.3 Miscellaneous addition reactions at carbon-heteroatom multiple bonds Propargylation and allenylation Like allyltins, propargyl- and allenyl-tin reagents can add to carbonyl compounds. However, a problem is the possible equilibrium between

Via addition reactions

231

propargyltins and allenyltins which is induced by electron-donating solvents or even by carbonyl compounds586'587. This phenomenon appears essentially with unsubstituted reagents, whereas addition occurs cleanly with complete rearrangement in the case of terminally substituted derivatives587 (Scheme 10.170). OH Me3SnCH2C = CMe,25°C CCI3CHO — *-

90%

^ .

^

Cl3C OH

Me3SnCH = C=CHMe,60°C 1 : —^ , ^ CCI3CHO — 80% Cl 3C

<^ '

Scheme 10.170

When propargyl- or allenyl-tin halides are produced in situ from propargyl iodides and stannous chloride aprotic solvents, addition to aldehydes affords preferentially a-allenic alcohols588, while the generation of the reagent from propargylic halides and metallic tin in the presence of aluminium gives results depending on both the halide and the solvent589, as in Scheme 10.171. OH H

=

/

(1 ) Sn.Al.THF /11 orun ( 2 ) RCHO

I a^

^ ^~^

72-94% OH RCHO

M e , S i - C = C - C H 92 I , S n , A l I *-

R = Hex,Ph,MeCH = CH

OH

SiMe*

R

SiMe 3 90-95

MeCN.DMSO diglyme

8-11

10-5 92-89

Scheme 10.171

It must be noted that allenyldibutyltin chloride reacts with aldehydes and acetone to give homopropargylic alcohols as major compounds whenever the reaction is performed in the presence of water502 (Scheme 10.172). RCHO

CH2 ==C==CHSnBu 2Cl,H 20,25°C 98-100%

R

OH 90-95

OH 10-5

Scheme 10.172

On the other hand, trapping of the alkoxytin adducts with acyl chlorides gives the corresponding esters directly590. A related reaction is the addition of l,4-bis(trimethylstannyl)-2-butyne to Eschenmoser's salt, giving 2,3-bis-(dimethylamino)methyl-l,3butadiene in 72% yield117. Alkynylation Additions occur particularly with Lewis acid catalysis491 (Scheme 10.173).

232

Access to carbon-carbon bonds Ph

^/ ZnCL2 0 -HPh-Ξ—SnEt^ " ' " S

/

V ^

^ OSnEt3

Scheme 10.173

This possibility has since been used for alkoxyethynylation of ketenes470 and more extensively for aminoethynylation of isocyanates, isothiocyanates, ketenes and carbodiimides, which affords highly functionalized ynamines591'592 (Scheme 10.174). R'

R' N—C=C—SnRl

+ X=C= Y

*- Y = C — C = C — N

'

I

R2

,

\

X —SnRg

R2

X = 0 , S o r NR 4 Y = NR 5 ,CR 5 CNorCPh 2

Scheme 10.174

Reactions of heterosubstituted organotin compounds Besides aldol-type reactions involving C-stannylated forms of organotin enolates, which will be described in Chapter 12, this type of addition concerns a-heterosubstituted organotins. Thus the addition of trihalogenomethyltributyltins to aldehydes has been performed by a nucleophilic addition process (as demonstrated by additions on cyclopropane carboxaldehyde or 5-hexenal). This affords an original route to trichloromethyl- and tribromomethylcarbinols593 (Scheme 10.175). RCHO

(1 ) B u 3 S n C X 3 ( 6 0 - 8 0 ° C ) «(2)CH 2 (COOH) 2

R

20-70%

CX 3 ^γ^ | OH

R = alkyl,Ph,PhCH = CH; X = BrorCl

Scheme 10.175

a-Stannyl nitrosamines can also add to aromatic aldehydes476 (Scheme 10.176). R& N

^SnMe 3

NO

70-80-C. t-Bu 55-72%

j O [ ^

^ N / ^ X ^ ^ - R I

I NO

OH

R1 = R2 = H.OMeor Cl R\R

2

= 0 C H 2 0 , R1 ,R 2 = H,OMe

Scheme 10.176

10.2.4 Addition of unactivated tetraorganotins to carbon-carbon multiple bonds Like allyltins, dialkylaminoethynyltin derivatives can add to the triple bond of dimethyl acetylenedicarboxylate. Although potentially valuable

Via addition reactions

233

for the synthesis of triazoles and pyrazoles, by subsequent reaction of the adducts with azides, this reaction has not yet actually been applied594. More complex reactions occur with activated double bonds ([2+2]addition followed by electrocyclic rearrangements), but here again no applications have been described595. Unactivated tin-carbon bonds can be involved in Lewis-acid-promoted alkylation. For instance, 1,1-dimethylcyclohexane has been obtained by Scheme 10.177596. ι-^Ύ

Me4Sn,CF3C02H,AlX3

Γ^Ί^

Scheme 10.177

The catalytic activity of Lewis acids follows the sequence MoCl5 < WC16 < A1C13 < T1CI4 < AlBr3, and alkyl transfer is also possible from tetrapropyltin and tetrabutyltin596. Interesting results have been obtained from internal reactions of tetraalkyltins with carbon-centred electrophiles597"599. Initial results have b e e n a c h i e v e d with ô - ( t r i m e t h y l s t a n n y l ) b u t y l - or γ(trimethylstannyl)propyl-substituted enones obtained as in the example597 of Scheme 10.178. EtO / V

R = =

>

\ °

(DLDA ^ ^ Ϋ ° (2)I(CH2)4SnMe3Ll ^ ^

(DLAHorMeLi ΤζΰΓ* "

/ = ( 0^-(CH

2

)

4

SnMe

3

(CH2)4SnMe3 R=H 60%,R=Me 85%

Scheme 10.178

Carbocyclization promoted by TiCl4 can readily be achieved, to afford eis or trans decalones as kinetic control products, depending on the temperature597 (Scheme 10.179). SnMe3 H

TiCl4(1%),CH2Cl2 92%

" Υ ^ ^ Ι ^ *"

4 0 ° C ( 2 min) - 7 8 ° C ( 30min )

\ / K ^

H eis·■trans

eis ■■ trans

93: 7 33 : 67

Scheme 10.179

It should be observed that the formation of the six-membered ring is highly sensitive to steric hindrance. Thus when a tertiary carbon centre is involved, ß-hydride abstraction occurs instead of the expected cyclization597 (Scheme 10.180). SnMe3

^ ^

^

TiCt 4 ,CH 2 Cl 2 94% Scheme 10.180

234

Access to carbon-carbon bonds

On the other hand, when the formation of a five-membered ring is expected, the ß-hydride shift does not occur and the cychzation products are cleanly obtained irrespective of the class of the electrophilic centre597 (Scheme 10.181).

°1

R

D

S

SnMe3 TiCl4,CH2Cl2

S

R = H

^ ^

y

53%

i

R

° ^ ^

R = Me 6 8 %

H

SnMe*

L

i

\

TiCl4|CH2Cl2

\ ^

82%

S

ί*

k

Scheme 10.181

The problem of 1,2- versus 1,4-addition has been examined: while unhindered systems give only 1,4-adducts, a large strain drives the reaction towards 1,2-addition, the hydroxyl function being transformed into chloride under the experimental conditions597 (Scheme 10.182). TiCl4tCH2Cl2 73% SnMe3

Scheme 10.182

The results obtained with cyclohexenones can easily be extended to other functionalities598'599. Thus, starting from allylic alcohols, Δ4'5hydrindene and Δ1,2-octalin ring systems have been synthesized, with the limitations already mentioned relating to the size of the ring and the class of the electrophilic centre598 (Scheme 10.183). TiCl4,CH 2 Cl2 ^R

R = H

88%

H (CH2)3SnMe3

R = Me

81

a

.R

TiCl4,CH2Cl2l40oC

, (OH 2 ) 4 SnMe 3 R "- H

Me

° /o

I^^Y^^l

78%

R = Me ( 0 % ; /3-hydrogen transfer )

Scheme 10.183

MM

§ | ! t*ns

Via elimination reactions

235

The same factors are also involved when the reaction is extended to conveniently substituted acetals599 (Scheme 10.184). SnMe3 ( 1 ) SnCl 4 ,CH 2 Cl 2 / V

0

\

( 2 ) S O C l 2 , LiBr.MeCN

0

/

H0

R = H 7 9 % , R = Me57%

H

Scheme 10.184

In this example tin tetrachloride was used instead of titanium tetrachloride, but the choice of the catalyst can be of crucial importance599 (Scheme 10.185).

10.3 Via elimination reactions 10.3.1 ^-Elimination reactions The a-elimination of organotin halides from halomethyltriorganotins affords carbene-type species which can be trapped by olefins. The first example reported was the nearly quantitative preparation of hexafluorocyclopropane600 (Scheme 10.186). 150-C Me3SnCF3

+

CF2 = CF2

F2C — C F 2 \ / -H Me3SnF CF2

*-

Scheme 10.186

Subsequently this decomposition was performed at lower temperature (80°C) using sodium iodide as catalyst and DME as solvent601. Applications have included difluorocyclopropanation of steroids602 and even difluorocyclopropenation of perfluoroalkyl acetylenes603. The reac­ tion has been extended to other dihalocyclopropanations with triorganostannyl trihaloacetates as in situ trihalomethyltin precursors604"606. For instance, 9,9-dichlorobicyclo[6.1.0]nonane has been synthesized in high yield606 (Scheme 10.187).

+ Cl 3 CC0 2 SnMe 3

diglyme,140°C

( *- I *-

γ/ yI/

+ Me33SnCl + (C0 2

Scheme 10.187

When mixed trihalomethyltin precursors are involved, the more labile halogen is preferentially eliminated as organotin halide604'607 (Scheme 10.188).

236

Access to carbon-carbon bonds ■Cl MesSnCClpBr

—-

-—*- i

1/

+

71 % 93

10.188 necessitate higher reaction Mono- or di-halomethyltinScheme derivatives temperatures and lead to mixtures in moderate to low yields607. The α-bromocyclopropyl derivatives, however, form a more favourable system, especially in the norcarane series, where the sy n-isomer is decomposed fairly easily, while the anft'-isomer is quite stable608. This behaviour has been utilized in the synthesis of spirocyclopropyl derivatives321,608 (Scheme 10.189). Me3Sn -Br 76%

Scheme 10.189

The occurrence of carbene species, which is the simplest explanation, now seems questionable from the pyrolysis of syn and anti norcarene derivatives609^ (Scheme 10.190). Me3Sn '

Rr

C*

benzene, 101 °C 76%

+ Me3SnBr

Br

^V-Sn M e ,3 —-SnMe benzene, 101°C

+ Me3SnBr

60%

Scheme 10.190

These results, together with those obtained in methanol, agree much better with initial dissociation into a cationic species than with a carbene mechanism609. The unexpected reductive etherification of aromatic aldehydes by a-chloro-a-ethoxymethyltributyltin, which is a fast and clean route to benzylic ethers even in the presence of a phenolic functional group, could be explained on this basis610 (Scheme 10.191). " k e n ,

",BU3SnCHCt0Et|

rX^y R*

( 2 ) KF,H 20,acetone

"S^X g

R

) M /

47-69% R1 =H;R 2 =H,0-OH,/77-OMe,/?-OMe, p - M e , o - C l R1 = m - 0 M e , R 2 = o - 0 H , p-OH or p-OMe

Scheme 10.191

+5> +

\ OEt

Bu 3SnF

♦

Via elimination reactions

237

10.3.2 ß-EIimination reactions Deoxystannylation by a ß-elimination process can be an efficient route to olefins. For instance, ß-stannyl alcohols, obtained from epoxides, yield stereospecifically olefins on acid treatment611,612 (Scheme 10.192). SnPh 3

(1)Ph3SnNa

\

(2)H20

Z E Z

H+

JL j OH

95-100%

threo 77=23

Z

erythro threo 11-20

E Z 77 23

Scheme 10.192

In fact, stannylation of epoxides follows an aftfi-addition pathway613'614 and accordingly the acid-promoted elimination is a clean £wi/-elimination. A different mechanism is involved under thermal conditions and elimination occurs by a clean ^«-elimination pathway189'369 {Scheme 10.193). H+

Ph3Sn \

100% PhS

Ph

H

/ PhS

ph

/

SPh /

110°C

\

93%

/

OH

Ph

Scheme 10.193

The ease of the deoxystannylation reaction has mainly been used for olefination of carbonyl compounds (see 9.3.3, under Miscellaneous a-heterosubstituted organolithiums, and 9.6), but other examples such as oxidation of allyltins or vinyltins can also involve a ß-elimination step (see 8.3.2 and 8.3.4). Besides these applications already described, ßdeoxystannylation has been used to obtain acids under mild conditions from the corresponding ß-bromoethyl esters615 (Scheme 10.194). ^ B r

(1)Me3SnLi

^

( 2 ) Bu 4 N + F" ( 3 ) H 2 0

RCÛ2

Z

78-86%

R = Ph,/?-ClC 6 H 4 , /?-MeOC 6 H 4 , C n H 2 3

Scheme 10.194

Furthermore, ß-elimination can occur directly during stannylation of ß-heterosubstituted halides616 and can be directed to stereospecific synthesis of olefins via an anti- or a syrc-elimination pathway, depending on the substrate and the experimental conditions (see 17.2). It can also be used for the conversion of vinyltins to alkynes by an addition-elimination reaction35'617 (Scheme 10.195). C5HH

Bu

W

J\

SnBu3

l2,A

C5H11

55% Scheme 10.195

^=

Bu

238

Access to carbon-carbon bonds

Similarly, ß-stannylthio derivatives are also subject to ßelimination148'618 and some interesting applications have been described. For instance, conjugated dienes and aliènes have been obtained in good yields from ß-stannyl sulphoxides, sulphones, imidazoyl sulphides or pyridyl sulphides619 (Scheme 10.196).

R=Ph, 3 , 4 - 0 1 . 2 ( ^ 3 , y£?-MeOC6H4 Scheme 10.196

This reaction can successfully be extended to terpenic substrates and is also valuable for the synthesis of styrene derivatives, a-substituted allylsilanes or methylenecycloalkanes619. Furthermore, it is possible to methylenate nitriles with minor modification of the experimental conditions620 (Scheme 10.197).

/VY^*

(peau

<°YY^

%

(2) ( 2 ) Bu 3 SnCH 2 I

Ό - ^ ^

V

"^

"^

(ref.619)

80-90%

I = S02PhorS^O> N-

CN ( 1 ) R 2 NLi (2)Bu 3 SnCH 2 J (3) MeLi

_ I _ J ^ J

(ref.620)

65% S c h e m e 10.197

Desulphonylation can also be readily achieved via conjugate addition of tributylstannyl-lithium to αβ-unsaturated sulphones ' 622 (Scheme 10.198). R1 \

R2 / ~"~\

R1 \ ^

Bu3SnLi THF S0 2Ph

Bu3Sn

R2

R2

/

SiQ2 CH 2CL 2orCHCL 3

\ S0 2 Ph

/ = ^

(ref.621)

RI

6 4 - 9 8 % overall yield S c h e m e 10.198

When R2 = H, the second substituent on the olefin can be introduced by trapping the intermediate anion with an electrophile such as methyl iodide, 3-phenylpropanal or chlorotrimethylsilane621' . With the last of these, vinylsilanes can be obtained selectively in good yields (47-100%) as eis- or trans-isomevs depending on the experimental conditions622 (Scheme 10.199). R

R

(1 )Bu3Snl_i (2)Me 3 SiCl* S02Ph Bu 3Sn

SiMe,

SiQ 2 ,CHa 3 ,25°C ■ — — ^

S0 2 Ph

SiQ 2 ,CHCi 3 , reflux orC6D6,850C

R = C8H] 7,C,0H2 1,BnCH2 S c h e m e 10.199

R *

SiMe53 '

96-98%Z 7SiMe3 / ^ R 8 8 - 9 8 % £"

Via elimination reactions

239

When R = Cl and with two equivalents of tributylstannyl-lithium, the reaction proceeds similarly to afford vinylstannanes225, but with αβdisubstituted sulphones a different reaction occurs which leads to the corresponding sulphoxides622. Finally may be noted the possibility of ß-hydride transfer when organotin reagents are reacted with triphenylmethyl cations623'624, but this reaction can also occur with other electrophilic species {see 10.2.4). 10.3.3 γ-Ëlimination reactions This type of elimination provides a route to cyclopropyl rings and has been mainly studied for 1,3-deoxystannylation. Two initial reports demons­ trated the ease of access to cyclopropane from 3-tosyloxypropyltrimethyltin625 or from 3-(trimethylstannyl)propanol626 and the reaction is also valuable for the synthesis of substituted cyclopropanes. However, in strained systems the production of the cyclopropyl ring is subject to limitations, as with 7-(trimethylstannyl)norbornyl mesylates, where only one of the four possible isomers affords the expected cyclopropyl derivative627'628 {Scheme 10.200). Me3Sn

OMes

«100%

Scheme 10.200

This result suggests concerted 1,3-elimination via a double inversion process. This hypothesis has been corroborated by kinetic studies629 and the reaction has been well established as a concerted process involving inversion at both reaction sites630'631 {Scheme 10.201). Ph

(1 )Bu3SnLi Ph

0

(2)PhMgBr* Bu3Sn !n

OH

Me

* AJk'

OH

Bu3Sn

43%

BF3,2AcOH 9 0 % Ph » Ph

M

Me

BF3,2AcOH 95% Ph

T

Me

M PY

Scheme 10.201

Cyclopropanation under BF 3 -AcOH catalysis appears to be very easy for tertiary alcohols or secondary benzylic alcohols, but it has limitations with other secondary alcohols and for the synthesis of fused-ring systems630'631. However, this last difficulty has been circumvented by using appropriate experimental conditions632 {Scheme 10.202).

240

Access to carbon-carbon bonds

*y

γ

^SnBu3

-*Ό

(1)PhLi,(2)S0Cl 2 62%

OH (1)Bu3SnLi

(CH2)„ il \ZJ*

+ (OO,

(2)t-BuOK-t-BuOH (3)PhLi

SnBu3

SnBu3

n =3

92%

n = 4

85%

29 65

n =5

80%

41

71 35 59

Scheme 10.202

The trans relation of the substituents is the key parameter and it must be noted that the alcohol obtained from the eis stannyl ketone affords only the olefinic elimination products. Further synthetic applications of 1,3-deoxystannylation have already been mentioned (see 9.3.3, under Miscellaneous a-heterosubstituted organolithiums, and 9.6) and this type of reaction has been successfully extended to other γ-heterosubstituted systems373'374'633-638. However, the more interesting improvements have been provided by electrophilic addition-elimination sequences which lead to cyclopropylcarbinyl halides633'634 (Scheme 10.203). ^ Bu3SrT

^

^

„x >r

CH 2 Cl 2 Bu3Sn^ ^ ^

^^

X = Cl, Br, I

^

(-Bu 3 SnX) 7 2 - 8 6 % overall yield

Scheme 10.203

Tributyltin derivatives are more suitable than the trimethyl- or triphenyltin homologues in these reactions because there is little contamination due to the cleavage of butyl groups. This type of reaction can easily be extended to the synthesis of cyclopropylcarbinyl sulphonates, sulphides or selenides, using respectively sulphur trioxide, sulphenyl chlorides or iV-phenylselenosuccinimide as electrophiles634"636 (Scheme 10.204). 2,4-(N0 2) 2-C 6H 3-SCl,AcOH,100°C Bu3Sn

75%

^

^

N02

Scheme 10.204

Similarly, on heating, 3,4-epoxybutyltributyltin cyclopropylmethanol634 (Scheme 10.205). 0SnBu3 Bu3SrT

75%

Scheme 10.205

JHgO

is converted to

Via elimination reactions

241

In some circumstances the intermediate adducts have been obtained, but elimination occurs readily on heating or solvolysis634'637 (Scheme 10.206).

a

Br l

SnBu3

/^^^SnBu» /^^^Γ^

Br 2

160°C

V " \

Br

Scheme 10.206

The above syntheses of cyclopropyl rings generally involve simple organostannanes and a useful improvement has been provided by the use of ß-stannylpropionaldehyde, which affords numerous possibilities638 (Scheme 10.207). OH

(1)Bu 3 SnH

(2)NCS-Me 2 S(3)Et 3 N

Β υ

^

81% R = Ac, Bz, C0 2 Me, p- N 0 2 C 6 H 4

Scheme 10.207

From these new organotin reagents, convenient syntheses of cyclopropyl derivatives can be achieved in high yields638 (Scheme 10.208). CF 3 C0 2 H,25°C Bu3Sn

^

^

^

^

R

IN.

R = Me90%,R = Ph89%

?\

^

^

\

R

Scheme 10.208

Similarly, ß-stannylpropionaldehyde can easily be converted to γhydroxypropyltributyltin derivatives, opening up an efficient route to functionally substituted cyclopropanes63 (Scheme 10.209). OH RLior B u

3

S n - ^ ^ ^ 0

™gX

I B u

3

S n - ^ ^

R = Bn, CHCNPh,CH2Ts

S0C1 2 , pyr R

fN—R

™For CCl 4

^

"

77-81%

Scheme 10.209

Finally, 1,3-halodestannylation or 1,3-deoxystannylation can yield olefinic compounds, after rearrangement, in the case of highly strained systems639'640 (Scheme 10.210). Me3Sn^

150°C 56% CL

OPNB

Me 2 C0,H 2 0,100°C *~ 91%

Scheme 10.210

ci

242

Access to carbon-carbon bonds

10.3.4 Miscellaneous elimination reactions While 1,2- or 1,3-deoxystannylations occur very easily, solvolysis of longer tosyloxyalkyltrimethyltins, although possible, appears to be much more difficult and of poor selectivity, leading to cycloalkanes together with the corresponding alkenes625'641. This tendency appears to be fairly general, and efficient electrophilic cyclization of organotin derivatives to afford fiveor six-membered rings has been limited to a few examples already discussed {see 10.2.4). Some elimination reactions have been considered as intramolecular substitutions and have already been presented {see 10.1). Nevertheless it is worth mentioning some recent results related to chloroalkylstannanes, which afford cyclization products or olefins depending on the relative positions of the tin and chlorine atoms642 {Scheme 10.211). _^

cyclization

χ

CL |

H-transfer

SnMe3

Scheme 10.211

When the tin centre is borne by a secondary carbon atom, ß-hydrogen transfer occurs regio- and stereo-selectively to yield trans internal olefins642 {Scheme 10.212). ci PhS^

(CHaC^^R

' „ = 1,2

PhS^

(CHjiCr

^

R

TiCU^ „=3,4

PhS

(CH2)/,

SnMe3 R = Me,Et

90%

95%

R = Me

Scheme 10.212

It is noteworthy that 1,5-hydride transfer {n = 1,3) can be performed at -78°C with high stereoselectivity {E:Z ^ 98:2), while 1,6-hydride transfer {n = 2,4) requires higher temperatures (0°C) which decrease the stereoselectivity. Finally, an interesting 1,4-elimination provides a convenient route to 2-substituted 1,3-dienes (including those in the terpenic series) from a-(hydroxymethyl)allyl sulphones643 {Scheme 10.213). R 1 ^ . ArS02

(DBuLi

^

(2)(CH 2 0)„

H0

W

=

Bu 3SnH )

/ \ ArS02 R

r j / ^ S n B u 3 160«C o^^0H R ^ ^

R

r^ 0

3 6 - 8 8 % overall yield

Scheme 10.213

^ ^

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