Hindered organoboron groups in organic chemistry. 21. The reactions of dimesitylboron stabilised carbanions with oxiranes

00404020/93 $6.00+.00 0 1993 Pagenon Press Ltd

Temhdrm Vol. 49, No. 14. pp. 3007-3W344.1993 Printed in Great Britain

Hindered Orgmoboron Groups in Organic Chemistry. 21. The Reactkm of Dimesitylboron Stabilised Carbmiens with wallms anaF. vaughlu-williuar’

AndFew kltcr, Depment

and lti&td

M. Rmgcr

of Chemistry, University of Swansee. Singleton Park. Swansea SA2 gPP, U.K.

(Received in UK 4 December 1992)

Abstract. Dimtsitylboron oxidised to 1,3-dials.

stabilised carbanions react with oxiranes to give products that can be

The reactions ate, in general. under steric rather than electronic conm&

and proceed smoothly for all but tetrasubstituted oxiranes.

Some unusual skreoseleetive

effects have been observed. Intrtition.

Alkyldimesitylboranes

akyl halide~.~ RCHOH.

readily yield stable anions that react as nucleophiles with

Oxidation of the products yields alcohols so making iUes&‘HLiR equivalent to

We next turned our attention to the reactions of Mes,BCHLii

with oxiranes which,

we hoped, would proceed as outlined in Scheme 1.

+

MeQBCHLiR’

?

)

Ri R2

I

HMSOH

RtR2C(OH)CR3R4CHR50H

Scheme I

‘) l’mioudy

Gina F. Bug&n.

3007

HzodNaoH

R5CH(OH)CR’R2CR3R40H

A. PELTER etal.

3008

Oxiranes are important synthetic intermediates due to their ready and many macdons with nucleophiles.’ HaPm,

Such reactions can include attack by hetetoatoms such as HO- ‘, AcO- 5,

PhNHm9, Me& lo, Me,Sn’ ‘I, NC i2.

C!alboWarbonbotldt?~ybepl&llXdby

reaction with organometallics*3 such as organolithium xeagents14*1sor Grignard reagenta.16 The former have limited synthetic utility whilst the latter are subject to competing reactions arising from the Lewis acidity of RMgX and the various species present due to the usual Grignard equilibria.

Copper catalysed reactions of Chignard reagents with oxiranes proceed smoothly”

as do the reactions of a wide variety of copper nagents.‘622 L$CuRa~~*,

RCuCNLi~, R$!u(CN)Lia,n and R&uLi.BF,.n

Typical

reagents are

Complexities arising from the

regiochemistry of the reactions as well as from rearrangement to carbonyl compounds followed by further reactions are well illustrated by comparing results for the attack on phenyloxirane by a variety of reagents23J4*26 (Scheme 2). 0 PhCH\“Hz

OH

Ph

R

Pb

Ph CH$H(OH)R

MeNit

100

0

0

MeMgBr

50

0

50

MeCu(CN) Li

81

18

0

Bu”Cu(CN) Li

74

12

0

8

85

0

Bu”#IW!N) Liz

Scheme 2 Organodancsreact mainly at the carbon atom most able to stabilise a positive charge, that is the reactions am electronically conttolled2* Stabilised carbanions such as enolate anions react with oxhaneg9 and such reactions am pathways to lactones, sa butenolides”

and a-methylene-~-butyrolactones.32

found only one example of a reaction (eq. 1) that gives 13diol C-C bond formation.

However. we have

derivatives from oxhanes by a

The mechanism of that reaction is clearly different from that envisaged

in Scheme 1, and it is limited to the introduction of hydmxymethylene groups.”

AOSiMeBt2 : 6SiMeEtz

(1)

3009

Hindered organoboron groups in organic chemistry-XXI

The 1.3-dlol grouping is present ln many natural products and its synthesis has mcelved much attention. 3-Hydroxyketones may be reduced to either anfLu or the syn-1,3diolsss~ and 3-hydroxyestas.sr, 1,3dlketone~.~‘, 3-ketoeste~a’~ and arhydroxyoxlmnef141 are also mduad, Hydtoboratlon of s&able m some ln a stareoselective fashion, to 1,3dlols. alcoh~ls~~~, followed by oxidation can yield 1,3-dlols ln a selective fashion. lnvolves

C-C

bond

formation

(a-hydtoxymethyl)oxlranes~”

is

the

reglospecific

reaction

of

One method that LlCuMe,

with

readily available ln homochlral form’s

For this investigation we chose to study as typical carbanlons. Mes#Hs (1) and In the latter case there could be stereochemistry associated with the MessB&Hs (2). CZH.$HBMes, group being introduced, and as the oxidation of the B-C bond pmceeds with retention of configuration, this will be reflected in the stereochemistry of the 1,3dlol end product. In general, attack on oxiranes by nucleophiles proceeds with inversion of the oxirane To analyse the stereochemistry of our carbon atom to which the new bond is being made. products we made their phenylboronates, which have the advantage over benxilidene derivatives that no new chiral centre is introduced and nor is there an additional allphatlc carbon atom. The *H nrnr of the phenylboronates can be indicative of the stereochemistry.46 and in addition, Hoffmann46*47pointed out that the 1.3~diols exist in hydrogen bonded conformations approximating to a chair cyclohexane, and that the 1,3-anti-isomer (three in Hoffmann’s The convention) will always have one axial substituent at either the l- or 3positions. 13-syn (erythro)isomer &C-l)

has two equatorial substltuents at these positions.

This results ln

+ d,C-3)anti, and holds not only for the dlols themselves” but

+ dcC-3)ryn > (a&-l)

derivatives,46 which have the further advantage of giving

also for their phenylboronate

molecular ions in their mass spectra.

The oxiranes investigated are shown in Figure 1.

All

were purchased or made by conventional methods.4s

.P

%

Et Bu” A7 o

a (5) Me

Me

)nc

Me

0

(7)

H

Me

Me

)nc

Me

0

Me

(6)

Ph

Ph* t7

(8)

0

‘FCHs

(9)

(IO)

Ph, 0

‘6JIPh

0

(12)

(13)

Figure I

0 o! (14)

A. PELTER et al.

3010

A preliminary investigation to test Scheme 1 using the reactions of phenyloximne with anions (1) and (2) in THF showed that the condensation

reactions were exothennic

proceeded to give diols (15) and (16) in good isolated yields after oxidation experiments 1,2).

and

(Table 1.

The disubstituted oxirane (4) reacted more slowly with (1) but with a slight

excess of anion gave the ld-diol

(17) in excellent yield (experiments 3.45).

In this study no

effort was made to optimise the conditions for each reaction, but for comparison purpose

the

conditions used ln Table 1 were utilized plus, in some cases, heating at 6tPC for 6h. The latter conditions emphasise the advantages of the use of boron-stabilised carbanions. which am stable up to more than 1UO“C.

Table Pnuminary

rcllctiolls ~hfe@CH$i Time Temp (It) “c

1

(I) Md Mes2BCHGCH3 (2) with a&ants Anion: Oximne

Producti

25

1: 1

PhCHOH(CHJ,OH

2

25

4 :3

PhCHOHCH&H(CH,)OH

2

25

1: 1

PKHOHCHW~OH

(4)

18

25

1: 1

(17)

87

(4)

18

25

4 :3

(17)

95

Bxp.

Anion

Oxirane

1

(1)

(9)

2

2

(2)

(9)

3

(1)

(4)

4

(1)

5

(1)

% YW (15)

.) In this and all subsequent Tables, “product” refers to the 1,3-d&& obtdd

81 (16) (17)

81 41

on nrlahrlnn.

b, In this and all subsequent Tables, “yield” refers to isoiis&rd yield of 1,3-diol based on starting oxirane. Reactions with cyclohexene oxide (13) and I-methylcyclohexene oxide (14).

The reactions of

(1) and (2) with (13) showed that substitution by the stabilised carbanions pMKxeded with inversion at the oxirane carbon atom being attacked (Table 2, experiments 6 and 7).

The

reaction of (1) and (2) with (14) showed that trisubstituted oxiranes react readily with these The reactions were regiospecific at the least substituted carbon atom and gave bulky anions. only two products in each case, these being equal amounts of the 1.3~syn- and 1,3-w~i dials (19) and (21). Structures were assigned using 13C nmr spectra of the dials and their phenyl boronates (22) and (23) in conjunction with the Hoffmann criteria (Table 3).

Hindered organoboron groups in organic chemistry-XXI

3011

Table 2 Reacriom of carbanions (1) and (2) with oxiranes (13) and (147 Anion

hp. 6

(1)

Oxhne (13)

Time(h)

Temp (YY)

2

25

Product ,.OHc

syn:anti b

OH

0,

Y&U (W 82

(18)

7

(2)

(13)

18

25

8

(1)

(14)

2

25

I:1 ,.OH CL

68

82

OH

1201

9

(2)

(14)

IR

25

1:l

“? ,?” a) Oxiie

: Aniin ratioalways 1:l b) DcIhed as 1.3~syn M 13-anti. eg. r

is syn (19)

c) In this and all subsequentfommlac only r&five c0nfigur~iun.sale indicated.

Table 3 “C nmr valu&) for (19) and (21) atui their derived pknybmuates

(22) and (23)

Qmpound

C-l

c-3

sm-(19)

76.1

71.3

147.3

alui-( 19)

73.4

69.5

142.9

syn-(22) anti-

74.9

70.1

145.0

72.8

69.5

sm-mv anri-(rn)

77.6

74.4

142.3 152.0

77.1

72.6

149.7

m-03) a&(23)

73.2

70.7

143.9

73.1

69.3

142.4

‘1 In this and all subsequent Tables, 13Cvalues are in p.p.m.

62

A. PELTERer al.

3012

Reactions with oxiranes substituted by one or more phenyl groups. summtired

Thcscstudicsarc

in Table 4.

Table 4 Reactions of anions (1) and (2) with phenyl substituted oxiranes

Exp

TJACN

T-P f”c)

1

2

25

2

2

25

10

18

25

11

18

25

12

I8

25

syn.-and rJ

Pdlld

Ylrld (W 81

4:3

81

82

OH OH Ph--CH, ik (25) OH CWH Ph+ Ph (5‘5)

7:l

37

93

OH OH 13

6

60

14

2

25

15

18

Pil+

a3 Ph (2’1) OH CH@l CHS + Fll WI

4:3

31

94

OH OH 25

C&V

Ph

C&

9:l

72

(29)

4 Inthis“dO;suml U’litC8.a’yR, The

Tsblessyn andanti

are defined fcs therelationship of the 13diol

issyn intheTable.

reactions

of (1) and (2) with phenyloxirane

(9) a~ exothmnic

and proceed as

expected to give diols (15) and (16) in good yields.

The assignments of st~ctme to the components of (16) are made on the basis of Hoffmann’s criteria for the diols and their derived phenylboronates

(30) and (31) (Figure 2). The similar data for the diols and bomnates supports the view4’ that the diols exist in a hydrogen bonded pseudo chair form.

Hindered organoboron groups in organic chemistry-XXI

Experiment

3013

1 H ,Ph Ph k

OZB (30)

1

‘H 4.84dd, J, = 12, J, = 4

1% 72.79

‘H 5.16dd. J,=99, J,=4

‘PC 72.8

2

1.6 - l.Om

40.57

1.8 - 2.4m

35.27

3

3.72t, J=6

60.25

4.0 - 4.2m

60.98

Experiment 2 1.3 -syn -( 16) = (16s)

Major product .

OH

,Ph

OH

Ph

PhMC!H, (3 Is)

(16s)

‘H 4.94dd. J,=13, J,=3

‘3C 73.31

46.96

1.8 - 1.2m

45.33

67.77

3.8 - 4.2m

68.12

23.58

1.23d. J=6

22.97

‘H 4.5 - 4.9m

‘3C 74.07

2

1.6 - 1.9m

3

3.4 - 3.7m l.O8d, J=6

1

4

C-l + c-3 = 141.84 Minor product.

C-l + c-3 = 141.43

1,3 -anti -(I61 = (16a) OH

,Ph

!H

Ph

PhL

Wa)

(NW

‘H 5.21t, J=5

1

‘H a

‘3C 70.92

2

a

46.72

a

40.36

3

a

64.70

3.9 - 4.4m

64.33

4

a

23.28

1.23d, J=6 22.40 C-l + c-3 = 134.84

C-l + C-3 = 135.62 *) Indistinguishable

from major isomer.

UC 70.5 1

A. PELTERetal.

3014

The reactions of cis-1,2diphenyloxirane

(11) and the ~u~isomer

(12) with (1)

(experiments 10 and 12) allowed a check on whether inversion occurred during attack on an acyclic system, as both isomers (24) and (26) are known.@ The data are given in Figme 3. The H-l coupling constants together with the Hoffmann criteria show conclusively that the reactions have gone with clean inversion. l&stereochemistry

In this case the criteria are used to establish

as only (33) can have a C-l axial phenyl group.

Experiment 10

Ph Ph

‘H 4.88Q J=lO

“C! 79.28

‘H 5.04d, J=lO

1% 78.49

2

3.02m

56.64

2.9m

57.08

3

3.98dd, 3.9Odd

66.10

4.lm

1

C-l + c-3 = 145.38

c-1 + c-3

66.08 = 144.57

Experiment12 $?H

OH

Hso

Ph&’

A

H

(26)

‘El

O\\

Ph J!J (33)

1

4&-i, J=6

"C 75.18

‘H 5.2d. J=4

2 3

3.05dd, J,=16, J,=8 3.65m

54.92 65.52

3.52m 4.1-4.4m

c-1 + c-3 = 140.70

1% 72.20

47.11 62.56 C-1 + C-3 = 139.76

B’ph

3015

Hindere!dorganoboron groups in organic chemistry-XXI

In experiments centre

wodated

(11) aud (13). attack

with

the

MessBCH(CH,,)

by carbaniou group

aud

(2) sets up a uew chid then

with

the

CH(OH)CHs

Experiment (11) gave only two dials io 8 7:l do! groupptrdwed flXwitonoxMation. As the H-l amp&q eonstents of both diols weze IlHz and of the bosonates was lOHz, the teactions have gone with clean inversion of configuration, and themfore the isoudsm is associated with the nucleophile. The majar isomer is 1,3qwo (e.g., 1,2-a&2,3-anti-1,2diphenylbutan-l,3diol) and the minor isomer is @!!a), using HoffmamA criteria47 (Figure 4).

Experitnent II Major isomer, IJ-syn

W 80.44 60.12

‘H 5.08d, J=lO

78.95

2

4.96d. J=lO 2.7&, J=lO

2.57t, J=9

58.73

3

4.2 - 4 6m

72.83

4.2 - 4.6m

72.18

4

0.9d. J=6

22.41

l.l2d, J;;6

‘H 1

c-1 + c-3 = 153.27

c-1 + c-3

4!

21.27 = 151.13

Minor isomer, IJsnti

Ph Ph

Ph

H

(254

1

‘H 5.126, J=lO

2 3 4

(35a)

‘ge 78.39

‘H 5.54d. J=8

2.8m

58.33

a

53.40

4.2 - 4.6m

66.94

a

68.02

l.ld, J=6

20.62

1.24d, J=6

C-l + c-3 = 145.33

c-1 + c-3

‘) Hidden by signals of main isomer. -4

‘fc 75.47

19.03 = 143.49

3016

A. PELXZR et al.

The stemoelcctivity

exhibited in experiment 11 is both unusual and exciting.

other reactions of disubstitutcd oxiranes show a similar rtueo~&~tivity

and am dkusai

SOme latcc

ThcrmM-oxiraN (12) matted very slowly with anion (2) (ant 13) to give (27) on oxidation, as a 4:3 mixtme of (27a) and (27s) (Pigme 5). ~e~*dropr amsidcrably in the reaction of (12) with (2) as compared with the reaction of (11) with (2). a drop paralleled in the rates of teactions.

Experiment

I3

13 -and

Major product,

(Ma)

(27a)

UC

5.14d, J=4

“C 74.56

‘H

1

5.316 J=4

77.52

2

2.6 - 2.9m

60.05

3.1 - 3.35m

54.6

3

4.2 - Urn

68.28

4.4 - 4.7m

67.4

4

0.98d. J=6

22.08

2.286 J=7

‘I-l

C-l + C-3 = 142.84

Gl+G3

21.6 =144.92

Minor product, 13 -syn

p-J p .

Phw&

;h (27~)

1

‘H 5.198

‘%I 76.14

‘H 5&d, J=3

UC! 76.81

2

2.6 - 2%

59.86

3.1 - 3.35m

52.4

3

3.8 - 4.lm

68.78

4.8 - 5.Om

71.56

4

0.926, J=6

21.97

2.2Od, J=6 C-l + c-3

c-1 + c-3 = 144.92

20.60 - 148.37

3033

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