Selenoesters of All Oxidation States

Selenoesters of All Oxidation States

2.6 Selenoesters of All Oxidation States AKIYA OGAWA and NOBORU SONODA Osaka University, Japan 2.6.1 INTRODUCTION 461 2.6.2 SELENOL ESTERS 2.6.2.1...

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2.6 Selenoesters of All Oxidation States AKIYA OGAWA and NOBORU SONODA

Osaka University, Japan 2.6.1

INTRODUCTION

461

2.6.2 SELENOL ESTERS 2.6.2.1 Preparation 2.6.2 .1.1 Acylation ofselenols and their metal salts 2.6.2 .1.2 Alkylation ofselenocarboxylates 2.6.2 .1.3 Reaction of esters with aluminum selenolate 2.6.2.1.4 Reaction of carboxylic acids with selenocyanates or N-PSP in the presence oftrialkylphosphine 2.6.2 .1.5 Miscellaneous 2.6.2 .2 Reactions

2.6.3 SELENOESTERS 2.6.3.1 2.6.3 .2

462 462 462 464 466 466 467 468 472 472 474

Preparation Reactions

2.6.4 SELENOAMIDES

476 476 476 478 481

2.6.4.1 Preparation 2.6.4.1.1 N-Unsubstituted selenoamides 2.6.4.1.2 N-Substituted selenoamides 2.6.4.2 Reactions

2.6.5 REFERENCES

482

2.6.1 INTRODUCTION As well as thiol esters, selenol esters (1) frequently exhibit a high and selective reactivity toward nucleophiles, which is enhanced even further by activation with heavy ions or oxidizing agents. These properties make selenol esters valuable acyl transfer agents. This review deals with general methods for the synthesis of selenol esters and their reactivity as acyl transfer agents. Furthermore, selenoesters (2), isomeric compounds of selenol esters, and their derivatives selenoamides (3) are also described. These compounds show the characteristic reactivity based on the carbon-selenium double bond. Hitherto known reviews for the chemistry of selenol esters and their derivatives are shown in refs. 1-5.

461

462

Acylation-type Reactions

2.6.2 SELENOL ESTERS 2.6.2.1 Preparation Many methods for the preparation of selenol esters have been developed. These can be classified into the following five general methods according to the types of reaction.

2.6.2.1.1 Acylation ojselenols and their metal salts In 1962 Renson et ale reported that a variety of selenol esters can be synthesized by the reaction of the appropriate acyl chlorides with selenols in the presence of pyridine. 6,7 Selenols can easily be prepared from elemental selenium and the corresponding Grignard reagents. In addition to simple selenol esters, a,~-unsaturated selenol esters (4) and (5) and o-substituted aromatic selenol esters (6) have been obtained by this method, as shown in Scheme 1. Butyl selenol esters are generally colorless or light yellow liquids, whereas the phenyl or substituted phenyl selenol esters are white solids, which are easily purified by recrystallization. More recently other groups have used Renson' s method to synthesize similar selenol esters. 8- 10

+

R'SeH 1 equiv.

o

~ N)

(l equiv.)

benzene, overnight

= Me, R' = Bu; 83% R = Me, R' = Ph; 81% R

R = Ph, R' = Ph; 69%

x


o

~seBU (4) 87%

(6)

(5) 60%

X = CI, 67%; X = Br, 82% X = I, 76%; X = OMe, 66%

Scheme 1

It has been reported that some selenols are directly acylated by carboxylic acids to give selenol esters. 11 - 14 For example, heating of 4-hydroselenobutyric acid at about 160 °C resulted in ring closure to give the corresponding )I-lactone (equation 1).11 160°C

oJ::!

(1)

Areneselenyl magnesium bromide has been reported to react with p-substituted benzoyl halides, giving aromatic selenol esters l5- 17 as the first examples exhibiting liquid crystal properties (equation 2).15

R2

-0-

SeMgBr

D-SOC S min

28-94%

Alkali metal salts of selenol can also be used in the synthesis of selenol esters. 17-22 Axial and equatorial selenol esters of cholestane and androstanone have been prepared from the potassium salt (7) and benzoyl chloride (Scheme 2 illustrates the axial isomer). 18

Selenoesters ofAll Oxidation States

463

EtOH

TsO

H

o aq. KOH

PhACI

pyrrolidone

KSe"""

H (7)

H

Scheme 2

Reaction of acyl or aroyl halides with thallium(I) phenyl selenide, prepared by the reaction of either thallium(I) ethoxide or thallium(I) phenoxide with benzeneselenol, gave the corresponding phenyl selenol esters in yields over 97% (equation 3).23

o +

R)lBr

ether

PhSeTI

(3) r.t., 24 h >97%

R

=Ph, Me

Tetraethylammonium borohydride reduces diphenyl diselenide to phenylselenolate anion. When an acyl halide is added to the toluene solution of phenylselenolate anion, the corresponding acylation product is isolated in good yield (equation 4).24 Resin-supported phenyl selenide anion underwent acylation with acetyl chloride under mild reaction conditions. 25

o

o

,ACI

PhSe-SePh toluene, reflux

toluene, reflux, 2 h

~sePh

(4)

54%

Selenol esters have also been prepared from trimethylsilyl selenides and acyl halides, as shown in equation (5).26

o Ar)lci

o +

5-10°C

Ar)lseR'

(5)

A mild and convenient method has been attained by using the imidazolide (8; equation 6) or triazolide (9; equation 7) of carboxylic acids as a key intermediate. Treatment of an imidazolide of carboxylic acid with 1.2 equiv. of benzeneselenol and 0.02 equiv. of sodium phenoxide provided the selenol ester in a quantitative yield. 27 In the case of the triazolide of carboxylic acids, the reaction proceeded in the absence of base. 28 excess PhSeH trace PhONa 15min.,r.t., cyc10hexane

o R)lsePh

(6)

464

Acylation-type Reactions

o

ij'N

N

.

~N

)l

N~

.

N~

N

o

PhSeH

DMF, 2 h, -10°C

R)lSePh

-10 DC, 0.5 h 88-94%

(7)

Reaction of some vinyl esters (10; Scheme 3) of carboxylic acids with selenolate gave the corresponding selenol esters in high yields because of the good leaving ability of enolate. 29

o

+

-60toO °C

~

Me2N

MeOHffHF

1 equiv.

o.

PhSeM 88%

91-98%

M=Li,Na,K

Scheme 3

Reaction of carboxylic acids with phenyl dichlorophosphate, followed by the addition of benseneselenol to the mixture afforded selenol esters (Scheme 4).30 The reaction is assumed to proceed via the pentacovalent oxyphosphorane intermediates (11a) and (lib).

o PhO' ~'Cl

o II

Cl

0

II

R~O"~'OPh

PhSeH

CI

CI _ 1,0 P"

0" I ~OPh

sePh A o R (lla)

CI 1,0

O-P~OPh

-o+)ePh R

j

o-

O=P-CI \ OPh

(lIb)

Scheme 4

It has recently been disclosed that the electrophilic cleavage of zirconocene complex (12; Scheme 5) leads easily to new Se ,Se'-disubstituted derivatives of benzene-l ,2-diselenol (13) in good yields, with quantitative recovery of the organometallic moiety of the reagent. 31 ,32

2.6.2.1.2 Alkylation of selenocarboxylates As well as acylation of selenols, alkylation of selenocarboxylates with alkyl halides is one of the most straightforward methods for synthesis of selenol esters. However, known examples of selenol esters synthesized using this strategy have been limited to a few cases,33-37 due to the difficulty of the preparation of selenocarboxylates.

Selenoesters ofAll Oxidation States

465

o

~Cl

Se

VlyCI o o

a se I~

~

§

§

Se

+

(ButCp )2ZrC12

0 (13)

Scheme 5

Jensen et ale reported that selenobenzoic Se-acid (14; equation 8) is formed as the primary product of the reaction of benzoyl chloride with hydrogen selenide in pyridine, but (14) is a very unstable liquid that reversibly loses hydrogen selenide to give dibenzoyl selenide. On rapid treatment with p-nitrobenzyl bromide, the selenobenzoic Se-acid formed in situ produced (p-nitrobenzyl)selenobenzoate in high yield. 33 0

H2Se pyridine

Ph)lCI

0

°2 N

Ph)lseH

-15°C

VHf ~

0

j

°2

1M NaHC0 3 , DMF 96%

(14)

~se N

A Ph

(8)

§

Potassium selenocarboxylates, which are useful starting materials for the preparation of selenol esters,34,35 can be prepared by the reaction of diacy1selenides with methanolic potassium hydroxide (equation 9). KOH/MeOH

(9)

93--95%

Piperidinium selenocarboxylates (15; equation 10) have been isolated in high yields from the reaction between diacyl diselenides and piperidine. The salts (15) dissolved in common protic and aprotic solvents and readily reacted with alkylating agents such as phenacyl bromide to give the corresponding esters in almost quantitative yields. 36 ,37

(RJlSJ

+

2

0

R)lo

2

CNH

R)lse- CNH 0

+ Se +

2

(15)

CH2C1 2 5°C, 20 min 58-95%

°

R~Br ether, 10°C, 1 h 82-98%

(10)

0

R)lse~R' 0

Acylation-type Reactions

466

2.6.2.1.3 Reaction of esters with aluminum selenolate One of the most general methods for synthesis of selenol esters uses dimethylaluminum methylselenolate (16), which has been found to be a remarkably efficient and versatile reagent for the conversion of O-alkyl esters to their corresponding methylselenol esters under mild conditions. 38 ,39 The reagent (16) is conveniently prepared by heating a toluene solution of trimethylaluminum with powdered selenium for 2 h under reflux. The transformation of various esters to selenol esters can be completed within 1 h. Representative results are shown in Scheme 6 (17-23). This method can also be applied to cyclic esters (19) and esters (20)-(23) containing other functional groups. Reaction of methyl trans-4-t-butylcyclohexanecarboxylate (24b; equation 12) with (16) proceeded much faster than the reaction of the cis isomer (24a; equation 11), a result suggesting that this method is highly sensitive to steric factors. 4D-42 reflux in toluene

Me2AISeMe 2h

(16)

o °C to rot., 1 h

o

o

~seMe

(i'seMe

(18) 93%

(17) 99%

~seMe

o

o

SeMe

(19) 80%

~ VN)==

0

[>-COSeMe

H

(21) 94%

(20) 84%

9

COSeMe

O~

~seMe

vJ

('J"",OH

(22) 80%

(23) 96%

Scheme 6 C0 2Me

But~ (24a)

Me2AISeMe Et20, rot., 10 h 92%

COSeMe

But~

(11 )

Me2AlSeMe

But~C02Me (24b)

Et20, rot., 005 h 96%

But~COSeMe

(12)

2.6.2.1.4 Reaction of carboxylic acids with selenocyanates or N-PSP in the presence of trialkylphosphine It has been reported that aryl selenocyanate (25; Scheme 7) reacts with carboxylic acids in the presence of tributylphosphine under mild conditions, giving rise to selenol esters in good yields. 43 The process may involve the reaction of carboxylic acids with selenophosphonium salt (26), which is the key intermediate. Since phenyl selenocyanate (25; Ar = Ph) is a sensitive liquid, which slowly decomposes on storage during a few days, an improved method using N-phenylselenophthalimide (N-PSP; 27), a stable crystalline compound, has been developed. 44 ,45 In general, the yields of selenol esters prepared from NPSP/Bu3P are higher than those obtained employing ArSeCN/Bu3P. N-Acetylselenenamide (28) can also be used in place of (25) or (27).46

467

Selenoesters ofAll Oxidation States

RC0 2H

+

0

BU3P (2 equiv.)

ArSeCN

R

CH 2C1 2, r.t.

(25)

A

SeAr

1BU3P + ArSePBu3CN

RC0 2H

0

RAO

ArSe-

+1 PBU3

(26)

Phseco~ Br

o

A

+

SePh

Q-cosePh

79%

o

46%

84%

OJ

Scheme 7

o

+

~N-sePh (27)

CH2C1 2 , -20 to 25°C 56-98%

o

ce

R

A SeAr

o

osePh

75%

o

BU3P (2 equiv.)

PhSe

SPh

92%

94%

Scheme 8

Me PhSe-N

\

Ac

(28)

2.6.2.1.5 Miscellaneous The reaction of N-acylhydrazines with benzeneseleninic acid (29; Scheme 9) in the presence of triphenylphosphine afforded high yields of diverse selenol esters.47-49 Alkyl, cycloalkyl and aryl selenol esters were prepared, even in the case of the highly hindered compounds (30) and (31). The removal of sulfur and selenium atoms from compounds (32) and (33) using triphenylphosphine easily furnished the corresponding selenol esters (equation 13).50,51 Diaryl diselenides reacted with carbon monoxide (5-100 atm) at 100-200 °C, in the presence of C02(CO)S as a catalyst, to give the corresponding selenol esters of aromatic carboxylic acids in 21-96% yields (equation 14).52

468

Acylation-type Reactions

o PhSe02H

RASePh

CH 2CI 2, r.t., 0.5-1 h 81-94%

(29) 2.2 equiv.

o

+

R)lN

PhSeOH

II

N

'SePh

OMe

But

A° SePh

(30) 83%

(31) 84%

Scheme 9

o

)l X "SeAr R (32) X

R

A° SeAr

+

(13)

=S

(33) X = Se

o

CO (tOO atm)

(PhSe)2

PhAsePh

cat. Co2(CO)8' MeCN, 200 °C

(14)

+

960/0

2%

2.6.2.2 Reactions Selenol esters are expected to be a more reactive species than the corresponding thiol esters or oxo esters due to the comparatively weak bonding between carbon and selenium. The ability of the selenol esters to serve as active acyl transfer agents has been readily. demonstrated by the easy conversion of the selenol ester (34; equation 15) to its corresponding acid (35), ester (36) or amide (37).38,53 This acylselenium bond cleavage has also been promoted by CUI and Cull salts. 38 ,39 The isopropylidene derivative (38; equation 16) of ribofuranosylacetate has been converted to a lactone (40) in good yield via the selenol ester (39). ~C02H

H 20

o

~seMe (34)

97%

HgCl 2

MeOH

CaC0 3 MeCN

88%

(35) ~C02Me

lh H 2N

D 88%

(36)

~ND (37)

H

(15)

Selenoesters ofAll Oxidation States

469

o

HO~O~coseMe

H O \ 1C02Et

H

°XO

... CuC)

(16)

°XO

(38)

(39)

Single-step conversion of carboxylic acids to amides has been achieved using the N-PSP/Bu3P reaction system in the presence of amines (equation 17).44,45

o (17)

R)lNHR'

r.t., 1.8-3.5 h 82-98%

Some examples of carbon-carbon bond formation by the reaction of selenol esters with carbon nucleophiles have been reported. Se-Acylmethyl selenocarboxylates (41) readily extruded selenium by treatment with potassium t-pentoxide to form 1,3-diketones in good yields. 35 A proposed pathway is outlined in Scheme 10. 0

R)lse~R' (41)

!

0

t-C 5H tt OK

R~R'

r.t., 4 h 77-93%

0

0

+

Se

1 H+

t-CsHIlOK

0-

0

0

R~R'

R)lse~R' 0-

-Se

0

0

R~R'

Scheme 10

A mild and efficient transformation of esters into ketones via selenol esters has been developed. Selenol esters smoothly reacted with organocuprates to produce ketones in excellent yields (equation 18).40 Reaction with a vinylcopper(I) reagent gave a,(3-unsaturated ketones (equation 19).54 Butylmanganese chloride also transformed selenol esters into ketones, although 3 equiv. of the reagent are required to complete the reaction. Me2Cd and PhHgBr were not effective for this transformation. The copper or copper(I) iodide-catalyzed insertion of diazomethane into the acyl-selenium linkage of seleno] esters was reported to afford the corresponding ketones as the chiefproducts. 55 ,56 A nonconcerted mechanism involving a tetrahedral intermediate (42; Scheme 11) has been suggested.

o

~seMe

o

RM -78°C, <1 h

~R

R = Me, 98%; R = Bu, 96%; R = But, 98%

(18)

470

Acylation-type Reactions

o

~SeMe

THF-MezS, HMPA

-25°C, 2-4 h 33-96%

o

R~SeR'

(42)

Scheme 11

Moreover, the selenol esters can acylate reactive arenes and heteroaromatic compounds when copper(I) triflate is employed as the selenophilic metal cation. 39,57 The acylation of aromatics by use of the benzene complex of copper(I) triflate (43; Scheme 12) was complete within an hour at room temperature, with benzene as solvent, and the acylation products were obtained in high yields. Intramolecular acylation was examined successfully, as shown in equation (20).

o

R

)l

SeMe

+

(CUOTf)2/PhH (43)

ArH

23-100%

o

o

o

MeO

MeO

60%; para:ortho = 2: 1

85%

63%

o

Scheme 12

OJ I~ #

Mese

o

0 70%

CO

(20)

Selenol esters also reacted with isonitrile (44; Scheme 13) at room temperature for 6-20 h in the presence of 1.5 equiv. of copper(I) oxide and triethylamine (or DBU) to give oxazole (45).39,57 The reaction presumably proceeds through an intermediate J3-ketoisonitrile (46), and copper(I) oxide functions as a reagent for complexation to the selenium moiety~ Recently it has become apparent that phenyl selenides (47; Scheme 14) are efficiently reduced to alkanes (48) with tin hydride by a free radical process. 58 The reaction of selenol esters with tributyltin hydride resulted in the formation of the corresponding aldehydes (49) and alkanes (50).59,60 The formation ratio of aldehydes to alkanes depends on the reaction temperature, as exemplified in Scheme 15. When the reaction was carried out with UV irradiation at ambient temperature, the aldehyde was formed predominantly, in high yields.

471

Selenoesters ofAll Oxidation States

o

R

)l

SeMe

+

Et3N, (DBU), THF 40-92%

(44)

r

Et02C

o

X

NC

1

R

(46) Scheme 13 BUn3SnH

R-H

R-SePh AIBN

(47)

(48)

BUn3Sne

I

BUn3SnH



-Bun3SnSePh

-BUn3Sne

Scheme 14

0

SePh

{jjH {j) H

BU3SnH

+

AIBN

(49)

AcO

(50)

benzene, 80°C

82%

17%

mesitylene, 164°C

18%

80%

Scheme 15

A new method for the transformation of -C02H to -H has been attained by formation of the selenol ester from carboxylic acid, followed by its reduction. 59-64 Owing to high chemoselectivity and mild reaction conditions, this method is often utilized for the synthesis of natural products (equation 21).30

i, PhOP(O)CI 2

BuO\\\\\

ii, PhSeH, Et3N

0

86%

Et

OMe

OSiMe2But AIBN BuD\\\\ \ benzene 82%

Et

(21)

o

OMe

Intramolecular capture of the acyl radical formed in situ from a selenol ester and tin hydride has been examined successfully.65 Phenyl selenol esters (51), with an unsaturated functional group (C=C etc.) at

Acylation-type Reactions

472

the appropriate position of the molecule, reacted with 1.2 equiv. of tin hydride in the presence of 0.05 equiv. of AIBN in benzene under reflux to yield ketones (52) through radical cyclization of the acyl radical (53; Scheme 16).65,66 0

~

(

n

~

6J

BU3SnH (1.2 equiv.)

X

AIBN (0.05 equiv.) benzene, reflux

n

2.5-3 h 69-92%

(51)

(52)

0.007-0.010 M

0

(~X n

(53)

cR

0 0

C02Me

0

86%

84%

83%

Scheme 16

It was reported that UV irradiation of o-substituted aromatic selenol esters (54) led to the 5H-[I]benzoselenino[2,3-b]pyridine (55) ring system via a selenol ester-seleninone transformation. 67 ,68

o{) cCse~ N

0 hv, benzene, 20°C

~ ::::-....

Cl (54)

N

(22)

Se (55)

2.6.3 SELENOESTERS 2.6.3.1 Preparation In an earlier investigation, aryl selenoesters have been prepared by the reaction of hydrogen selenide with imido esters (56; equation 24), but the yields were very low (56; R = Ph, 5.6%; 56; R = p-tol 3%).69,70 The yield was somewhat improved by employing methyl benzimidate hydrochloride (57; equation 23) as the starting material. 71

o N

(57)

(23)

473

Selenoesters ofAll Oxidation States

A practically useful method for synthesis of selenoesters has been developed by Barton et al. A wide range of aliphatic and aromatic selenoesters have been synthesized from the appropriate N,N-disubstituted imidoyl chlorides (58) using sodium hydrogen selenide to introduce selenium. 72,73 Imidoyl chlorides (58) can be obtained by the reaction of amides with phosgene. Representative results are shown in Scheme 17 (formulae 59-61). 0

+

0

NMe2CI

CIACI

RIANMe2

R20H

RI)lCl

0

(58)

N

+

NMe2CI

Se

NaSeH

R 1 )lOR2

RI)lOR2 (59)

R 1 = Me, R 2 = Ph, 65%; R 1 = Me(CH2)16' R2 = Me, 43% R 1 = But, R 2 = Et, 46%; R 1 = Ph, R 2 = Me, 89%

R

se~o (60)

R =Ph, 89%; R R=Me, 74%

=H, 75%;

(61) 49%

Scheme 17

Cohen investigated in detail the addition of hydrogen selenide to imido esters (56), and found that selenoesters can be prepared by the reaction at -20 to -30°C in the presence ofpyridine-triethylamine.74 EtOH

RCN HC}

(24)

-15°C

-20 to -30 °C 26-90%

a-Substituted selenoesters (64) have been synthesized by the reaction of alcohols with arylethynylselenolate salts (63), prepared from 4-aryl-l,2,3-selenadiazoles (62).75 Ar

YN l'~ Se (62)

Ar-C=C-SeK (63)

ROH

Ar

4

Se

SeK

(OR OR

Ar (64)

Scheme 18

The transition metal complex pentacarbonyl(methoxyarylcarbene)chromium(O) (65), gave selenoesters on treatment with elemental selenium.76 Although alkylation of potassium selenobenzoate with alkyl halides led to the formation of selenol esters via Se-alkylation, as described in Section 2.6.2.1.1 (equation 9), the reaction of (66; equation 26) with trimethylsilyl chloride proceeded through O-silylation to give trimethylsilyl selenobenzoate (67) in 68% yield.?7 The driving force of the reaction may be due to the strong affinity of silicon for oxygen.

474

Acylation-type Reactions

=<

se=<

OMe

(CO)5Cr

OMe

+

Se

12-29%

Ar

(25)

Ar

(65)

r.t.

+

(26) petroleum ether 68%

(66)

The aliphatic selenoesters are generally yellow liquids, while the aromatic esters are deep red oils, the latter being more stable. However, after 2-3 days, these begin to decompose slowly, accompanied by the deposition of elemental selenium, although they can be stored in a refrigerator for a few months without any such precipitation. 74

2.6.3.2 Reactions Owing to the high polarizability of the carbon-selenium double bond, selenoesters can easily be attacked by nucleophiles. This property makes a selenoester a useful reagent for the synthesis of heterocycles. 78- 83 Condensation of selenoesters with bifunctional aromatic or aliphatic amines has yielded a variety of nitrogen-containing heterocyclic compounds (Scheme 19). N-N

II R~N

~N~R ~X

X=NH,O,S

ref. 78

J-- XH

I

R' ~

NH2

#

XH

O

X=O,S ref. 80

ref. 79

ref. 82

~NH2

~'T~ N Cl

/

BunLi

Scheme 19

Selenazofurin (72), a promising antitumor agent, has been synthesized from methyl 2,5-anhydroallonimidate (68), as illustrated in Scheme 20. The key step of the synthesis is the formation of the selenazole (71) by the condensation of the selenoester (69) with ethyl 2-amino-2-cyanoacetate (70).83 In order to clarify the fundamental chemical reactivity of selenoesters, Barton et ale investigated their reaction behavior towards a number of common reagents. 72 The reduction of selenoesters was examined using sodium borohydride-triphenylphosphine or Raney nickel (W-2). In both cases, the corresponding ether (73) was obtained in high yield. On the other hand, selenoesters were converted into their parent oxocarbonyl derivatives (76) in high yield on treatment with oxidizing agents such as benzeneseleninic anhydride (74)84,85 and bis(p-methoxyphenyl) telluroxide (75; Scheme 21).86 The reaction of a selenoester with the methylene Wittig reagent (77) readily produces the vinyl ether (78; equation 27).72

Selenoesters ofAll Oxidation States HO

0 ~OMe

H2Se -22°C

HO OH

1#N H2N

)=(

CN H N J---C02Et (70) 2 MeOH/silica gel 17%

(69)

C0 2Et

1#N r(

~

0

0 ~OMe

HO OH

100%

(68)

HO

HO

475

HN0 2

NH 3

H3P02

87%

56%

HO OH (71)

HO

CONH2

h

0

HO OH (72) Scheme 20

° °

Ph'" Seo' Se ph (74) or

o

~OMe

MeOY1J (76)

or Raney Ni

0(75)

R~OR' (73)

Te

°

Scheme 21

PhJlOR (77)

(27)

(78)

During these investigations, it was observed that the selenoester reacts with triphenylphosphine in the presence of atmospheric oxygen to give the parent ester. 72 Hansen investigated in detail the reactions between triethylphosphine and a number of aliphatic and aromatic selenoesters. 87 Under oxygen-free conditions, the reaction of aliphatic selenoesters with triethylphosphine gave a purple intermediate, which was quenched with atmospheric oxygen to give the corresponding esters (79). In the case of aromatic selenoesters, the same reaction under oxygen-free conditions led to deselenative coupling to afford adialkoxy-stilbenes (80) and -dibenzyls (81). When the reaction was carried out in cyclohexene, 7-alkoxy7-phenylbicyclo[4.1.0]heptanes (82) were formed. These results suggest the formation of a-alkoxycarbene (84). The presence of benzaldehyde in the reaction mixture led to a-alkoxystilbenes (83; Scheme 22). Recently, the synthesis of selenoaldehyde was examined by the reaction of a-alkyl selenoformates (85) with aryllithium. 88- 9o 2,4,6-Tri-t-butylphenyllithium reacted with selenoformates (85) at three different sites, i.e. the selenoformyl carbon, the selenoformyl hydrogen and the selenium, to give several reaction products including 6,8-di-t-butyl-3,4-dihydro-4,4-dimethyl-lH-2-benzoselenin (86). The formation of (86) is explained in terms of the intermediacy of 2,4,6-tri-t-butylselenobenzaldehyde (87; Scheme 23). Selenoesters have easily undergone exchange reactions with sulfur to give the corresponding thioesters (88), on heating with elemental sulfur (equation 28).91,92 Some other reactions concerning selenoesters are shown in refs. 93 and 94.

476

Acylation-type Reactions

o Ph)lOR (79) R =alkyl benzene

Ph

Ph

)={

+

RO OR (80) R =aryl

~Ph [Ph-C-OR]

/~:

\

-;;

+ Et3P=Se

(81)

~OR (82) Ph

( Ph

+

Ph

Ph

RO

OR

)={

+

Et3P=Se

OR

"=<

PhCHO

(84)

(83)

Ph

Scheme 22 Se

+

ArLi

(85)

(ArSe)2

+

ArH

+

o II

H~Ar

(86)

R =But, cholesteryl; Ar =2,4,6-tri-t-butylphenyl

[H~r] (87)

Scheme 23

140°C

(28)

3h 95%

2.6.4 SELENOAMIDES 2.6.4.1 Preparation

2.6.4.1.1 N-Unsubstituted selenoamides

In general, N-unsubstituted selenoamides have been prepared by the addition of hydrogen selenide to the corresponding nitriles in the presence of base. In an earlier investigation of the synthesis of aromatic selenoamides, gaseous hydrogen selenide was directly introduced into the reaction vessel. The first report95 described the synthesis of benzeneselenoamide by the reaction of benzonitrile with hydrogen selenide in ethanolic ammonia (equation 29), while its 4-methyl derivative was synthesized under similar conditions. 96

477

Selenoesters ofAll Oxidation States

Se

ROH-NH 3

Ar

A

(29)

NH 2

In order to avoid the handling of poisonous hydrogen selenide, two other useful syntheses have been devised. One method97 uses the reaction of aromatic nitriles with aluminum selenide in the presence of pyridine, triethylamine and water (Scheme 24), while the other98 is performed by the reaction of nitriles with selenium, carbon monoxide and water in the presence of triethylamine (Scheme 25). These methods are exceedingly convenient in terms of manipulation without the isolation of hydrogen selenide for the preparation of selenoamides. Using these methods, a variety of aromatic and heterocyclic selenoamides can be obtained from the corresponding nitriles in high yields.

ArCN

X

d

+

Al 2Se3

+

Se

pyridine-Et3N

H 2O

Ar

reflux, 2 h

A

NH 2

Se

Se NH2

\yNH2

Se

§

NH 2

X = H, 92%; X = CI, 82% X = Ac, 70%; X = CN, 72%

95%

78%

Scheme 24

RCN

+

Se

+

CO 5 atm

+

H 20

Et3N

Se

THF,100°C,5h"

R)lNH

Se

2

r

Se

NH 2

NH2

Ph X =CI, 99%; X X=NH 2,82%

=OMe, 91%

74%

38%

82%

Scheme 2S

The aromatic selenoamides are generally yellow solids and are stable enough under nitrogen at ordinary temperature to be kept for several weeks in high purity. On exposure to air, they gradually decompose into the starting nitriles, elemental selenium and water, at room temperature or below. These observations suggest that the selenoamides are in equilibrium with the corresponding nitriles and hydrogen selenide (equation 30).98 RCN

+

(30)

In contrast to aromatic' selenoamides, little is known about the isolation of N -unsubstituted aliphatic selenoamides, which lack the stabilizing conjugation between the aromatic ring and the selenocarbonyl group. Hitherto known procedures are Kindler's method,99-101 using the reaction of nitriles with hydrogen selenide in the presence of sodium ethoxide (equation 31), and the method using nitriles and SeCO-H20 (Scheme 25). Treatment of 2,3,5-tri-O-benzoyl-(3-o-ribofuranosyl-1-carbonitrile (89) with liquid hydrogen selenide, with 4-(dimethylamino)pyridine as the catalyst, provided 2,5-anhydro-3,4,6-tri-O-benzoyl-o-allonselenoamide (90). Compound (90) was treated with ethyl bromopyruvate to provide ethyl 2-(2,3,5-tri-O-ben-

478

Acylation-type Reactions EtONa

+

RCN

(31)

R =Me, 77%; R =PhCH2, -100%; R = H 2NC(=Se)CH2 , 36%

zoyl-D-ribofuranosyl)selenazole-4-carboxylate (91), followed by ammonolysis to give 2-f3-D-ribofuranosylselenazole-4-carboxamide (72) as a promising antitumor agent. 102 Me'N Me

BZ01t-rfN

cat.

+

0

BZo~se

r.t.,20h

BzO OBz (89)

BzO OBz

100%

(90)

o

H2N~

Et02C

II

o EtOYBr

o

MeCN, r.t., 1 h

N~

BZO~

Se

Howse

BzO OBz

HO OH

(91)

(72)

Scheme 26

2.6.4.1.2 N-Substituted selenoamides N-substituted aliphatic selenoamides have been obtained on treatment of the corresponding amides with phosphorus pentaselenide in refluxing benzene,1°I,lo3 but the yields were generally low (equation 32). Aromatic selenoamides can be obtained in moderate to high yields by the same method. 104 Improved yields of unhindered aliphatic selenoamides were obtained by heating the analogous amides with phosphorus pentaselenide in the presence of barium carbonate in boiling xylene. l05 Synthesis of selenolactams (n = 3-5, 7 and 11) was accomplished by heating a mixture of the lactam with red phosphorus and gray selenium in refluxing xylene for 24 h. 106

o R

)l

Se NR'R"

+

P2Ses

(32)

R)lNR'R"

R' = alkyl; R" = alkyl or H Ref 101, 103 104 105

R Alkyl Aryl Alkyl

Condition Benzene reflux Pyridine reflux Xylene reflux

Additive None None

BaC03

Yield <23% 52-82% <61%

The reaction of an imidoyl chloride with sodium hydrogen selenide gave high yield of the selenoamide (equation 33).107

+

NaSeH

EtOH-
Se

Ph)lNHPh

(33)

479

Selenoesters ofAll Oxidation States

Convenient one-pot syntheses of N-substituted selenoamides from nitriles, metallic selenium, carbon monoxide, water and amines have been developed on the basis of an amino group exchange reaction between the N-unsubstituted selenoamides formed in situ and the primary or secondary amines. 108 Aromatic or aliphatic, and N-mono- or N,N-di-substituted selenoamides can be obtained in good yields by this method (Scheme 27).

RCN

+

Se

+

CO

+

H 20 -NH3

5 atm

Se

R)lN~Ph I

H 74% (R' =Pri )

46% (R'

87% (R' = PhCH2)

45% (R'2N = piperidyl)

75% (R'

=Me)

=cyclohexyl)

74% (R =PtI) 71 % (R = PhCH2)

88% (R' = n-C SH 17 )

Scheme 27

In the cases of primary amines, the corresponding selenoamides have also been obtained from nitriles, selenium, carbon monoxide and excess primary amines by a single step mixing at the beginning of the reaction (Scheme 28).108

+

Se CO

Se

+

CO

+

RCN

-CO(NHR'h

RCN

Scheme 28

A method for synthesis using the reaction of selenoesters with amines or their magnesium salts has been reported. 74 The reaction of aliphatic selenoesters with various primary alkylamide magnesium bromides in diethyl ether gave N-monosubstituted selenoamides, whereas direct addition of secondary amines to selenoesters affordedN,N-disubstituted selenoamides (Scheme 29). R'NHMgBr

65-92%

Se

R)lNHR'

Se

R)lOEt R'R"NH 3{}-70%

Scheme 29

Se

R)lNR'R"

Acylation-type Reactions

480

Based on the observation that 1,2,3-selenadiazoles can be converted to selenoesters (Scheme 18), Yalpani et ale have found that reacting 5-unsubstituted 1,2,3-selenadiazoles with various amines gave, in most cases, quantitative yields of the selenoamides (equation 34).109,110

KOH

+

(34)

[ R-C=::C-SeK ] 70-100%

Selenoamides have been prepared in good yields by adding selenium powder to a solution of lithium acetylides in an excess of diethylamine (equation 35).111 R

-

Li

(35) 75-98%

A few methods for the synthesis of selenoformamides have been reported. (Mercaptomethylene)imminium salt (92) reacted with NaSeH to give the corresponding selenoformamide (equation 36).112 1-

MeS

+/\

'==N

0

"---I

+

(36)

NaSeH

(92)

An alternative synthetic route to selenoformamides has been developed by the reaction of carbon diselenide with phenylmethaneselenol to produce triselenocarbonate (93), which reacts with primary or secondary amines to give the selenoformamide (Scheme 30).113

Ph~SeH

(PhCH2Se)2C=Se

Ph/'..SeH

(93)

Se

Ph~seJlH

polymerized.

Scheme 30

The N,N-dimethylaminomethylenamino compounds (95), obtained from amines and dimethylformamide diacetal (94), react smoothly with hydrogen selenide to give the desired selenoformamides (equation 37).114 OR' RNH 2 +

R'O~NMe2 (94)

N

60% aq. EtOH

H

.R

)l

H2Se

NMe2

12-79%

Se H)lNHR

(95)

Some other methods for the synthesis of selenoamides are shown in Schemes 31 115 and 32. 116

(37)

481

Selenoesters ofAll Oxidation States

:r=.=se__.. .

RrJ

~Br R---SeLi

~

Et2NH

5-37%

Scheme 31

(CO)5 M [ Se =

C~h ]

+

M = Cr, W; R = H, Ph

se=<:

EtzO, CO (100 atm)

R---<

70°C, 10-15 h 72-95%

Ph

Scheme 32

2.6.4.2 Reactions As with selenoesters, selenoamides are excellent reagents for the synthesis of selenium-containing heterocycles.117-122 Anhydro-2,3,5-triphenyl-4-hydroxyselenazolium hydroxide (96), as the first mesoionic selenium heterocycle,107 selenophene derivatives (97)118-121 and 1,3-selenazoles (98)12Z have been synthesized as shown in Scheme 33 and equations (38) and (39). Se

Ph

II

Ph

~

Br

I

+

NHPh

Ph

~

COzH

~

Et3N

Se

COzH

benzene

Ph ~ NPh

I

Scheme 33

N-S

N'~O Br

+

~h

)

A NEt Et

Jys N=N

Se

EtOH 2

\e-Z

reflux, 4 h 95%

(38)

Ph

(97)

°

R~X

Se

+

A NHz Ar

MeOH

se Ar ~y

§-

R

N

(39)

(98)

It has recently been reported that selenoamide is a useful reagent for the stereospecific deoxygenation of epoxides under mild conditions. 123 This deoxygenation can be applied to various epoxides, i.e. mono-, di- and tri-substituted epoxides and cyclic epoxides. A suggested reaction path is illustrated in Scheme 34.

482

Acylation-type Reactions

74%

~ 75%

-Se

Rl

R3

R2

R4

>=<

51-89%

Scheme 34

Some other reactions are shown in refs. 91, 124 and 125.

2.6.5 REFERENCES 1. K. A. Jensen, in 'Organic Selenium Compounds: Their Chemistry and Biology', ed. D. L. Klayman and W. H. H. Gunther, Wiley, New York, 1973, p. 263. 2. R. J. Shine, in 'Organic Selenium Compounds: Their Chemistry and Biology', ed. D. L. Klayman and W. H. H. GUnther, Wiley, New York, 1973, p. 273. 3. F. S. Guziec, Jr., in 'The Chemistry of Organic Selenium and Tellurium Compounds', ed. S. Patai, Wiley, New York, 1987, vol. 2, p. 215. 4. C. Paulmier, in 'Selenium Reagents and Intermediates in Organic Synthesis', Pergamon Press, Oxford, 1986, p.58. 5. S. Kato, T. Murai and M. Ishida, Org. Prep. Proced. Int., 1986,18,369. 6. M. Renson and C. Draguet, Bull. Soc. Chirn. Belg., 1962,71,260. 7. M. Renson and J. L. Piette, Bull. Soc. Chirn. Belg., 1964, 73, 507. 8. R. Mayer, S. Scheithauer and D. Kunz, Chern. Ber., 1966, 99, 1393. 9. 1. Martens, K. Praefcke, U. Schulze, H. Schwarz and H. Simon, Tetrahedron, 1976,32,2467. 10. I. A. Aliev, F. K. Zeinalov and M. A. Shakhgel'diev, Azerb. Khirn. Zh., 1983, 73 (Chern. Abstr., 1984, 100, 209284y). 11. W. H. H. Gunther, J. Org. Chern., 1966,31,1202. 12. W. H. H. Gunther, J. Org. Chern., 1967,32,3929. 13. M. Evers, R. Weber, P. Thibaut, L. Christiaens, M. Renson, A. Croisy and P. Jacquignon, J. Chern. Soc., Perkin Trans. 1, 1976, 2452. 14. L. Christiaens, J. L. Piette, A. Luxen and M. Renson, J. Heterocycl. Chern., 1984,21,1281. 15. G. Heppke, J. Martens, K. Praefcke and H. Simon, Angew. Chern., Int. Ed. Engl., 1977, 16, 318. 16. H. J. Jakobsen, B. Villadsen, B. Kohne, W. Lohner and K. Praefcke, J. Organornet. Chern., 1979,166,373. 17. G. P. Mullen, N. P. Luthra, R. B. Dunlap and J. D. Odom, J. Org. Chern., 1985,50,811. 18. S. M. Hiscock, D. A. Swann and J. H. Turnbull, J. Chern. Soc. D, 1970, 1310. 19. W. H. H. Guenther and H. G. Mautner, J. Med. Chern., 1964,7,229. 20. L. Testaferri, M. Tiecco, M. Tingoli and D. Chianelli, Tetrahedron, 1986, 42, 63. 21. L. Testaferri, M. Tiecco, M. Tingoli and D. Chianelli, Tetrahedron, 1986,42,4577. 22. S. I. Kang and C. P. Spears, Synthesis, 1988, 133. 23. M. R. Detty and G. P. Wood, J. Org. Chern., 1980,45,80. 24. J. Bergman and L. Engman, Synthesis, 1980, 569. 25. J. V. Weber, P. Faller, G. Kirsch and M. Schneider, Synthesis, 1984, 1044. 26. N. Y. Derkach and N. P. Tishchenko, Zh. Org. Khirn., 1977,13, 100 (Chern. Abstr., 1977,86, 155 314e). 27. G. S. Bates, J. Diakur and S. Masamune, Tetrahedron Lett., 1976,4423. 28. H.-J. Gais, Angew. Chern., Int. Ed. Engl., 1977,16,244.

Selenoesters ofAll Oxidation States 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96.

483

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484 97. 98. 99. 100. 101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125.

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