Hydroboration of enol acetates derived from aldehydes. Nucleophilic Reduction of α-acetoxyorganoboranes with hydroborating agent

Hydroboration of enol acetates derived from aldehydes. Nucleophilic Reduction of α-acetoxyorganoboranes with hydroborating agent

Tetrahedron Letters No.47, pp. 4937-4940, 1968. Pergamon Press. Printed in Great Britain. RTDROBORATION OF ENOL ACETATES DERIVED FROM ALDERTDES. ...

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Tetrahedron

Letters No.47,

pp. 4937-4940,

1968.

Pergamon Press. Printed in Great Britain.

RTDROBORATION OF ENOL ACETATES DERIVED FROM ALDERTDES. NUCLEOPHILIC REDUCTION OF &-ACBTOINORG~OBORAN~S WITH RTDROBORATING AGENT (1)

Akira Suzuki, Kenichi Ohmori, Hidenori Takenaka and Mitsuomi Itoh Department of Chemical Process Engineering, Rokkaido University, Sapporo, Japan (Received in Japan 17 July 1968; received in UK for publication 20 August 1968) Recently it was reported that hydroboration of isobutenyl acetate with diborane places practically all of the borane in the d-position. and that the &-substituted derivative appears to be relatively stable and are readily oxidized to the correspondingaldehyde (2) as shown in equation 1.

In this connection, we wish to report here that the reactiotiof excess

diborane with enol acetates derived from aldehydes give different products, depending upon the

Q-6 )C=CCH.z!?!_,

cy

OAC

(0) c%. Cb;CH-CH:B
Golay

capillary column of Carbowax 20 M. From the results, it is evident that an intermediate, a-acetoxyorganoborane (I) achieves

4937

No.47

4938 removal of the acetoxy the intermediate high reaction

group and its replacement

is reacted with excess diborane

a-alkoxy-(5)

reaction", groups.

as indicated

over a long reaction

in equation A, vhen

time or at relatively

temperature.

Pasto and his co-workers (4),

by hydrogen

and

a reaction

previously

reported

Q-haloorganoboranes involving

Unfortunately,

(6,9).

an intramolecular

sufficient

such a reaction

evidence

in the cases of a-alkthioxy-

They interpreted exchange

this as "Q-transfer

of hydrogen.and

has not been advanced

other functional

to support

the view that .

an intramolecular

rearrangement

was involved.

TABLE Reac.time

Product

Reac.temp.

Isobutyl

distribution,

aldehyde

Isobutyl

Material

balance

%

OC

1

0

95

5

100

2

0

95

5

98

4

0

95

5

99

8

0

91

9

98

1

20

87

13

100

alcohol

99

2

20

77

23

4

20

55

45

97

8

20

26

74

94

1

40

28

72

95

2

40

8

92

96

4

40

3

97

92

8

40

1

99

91

here that this reaction

based on the following

reasons.

was hydroborated

As depicted

proceeds

by using the excess disiamylborane

One can expect to obtain an alcohol

after oxidation,

if the reaction

proceeds

ment of siamyl group and hydrogen) proceeds

reduction

The gas chromatographic

the yields of V and diethyl not find any traces of III.

(III) and/or diethyl

by 11d-transfer"

mechanism

acetaldehyde

with another analysis

reduction,

(II) of diethyl

in tetrahydrofuran

path A-a

(IV) (rearrange-

On the other hand, if

disiamylborane,

of the reaction

at 20' and

acetaldehyde

through

and/or path A-b (non-rearrangement).

by nucleophilic

(V) should be obtained.

by means of a nucleophilic

in equation 2, the enol acetate

for 24 hrs (7).

the reaction

X

hr.

It was concluded

acetaldehyde

1

2-ethyl-1-butanol

mixture

(IV) were 27 % and 39 %, respectively,

showed that

and we could

4939

Ne.41

Y

4 1

sd

‘Ok I (0)

(0)

E*NCti-CH Et’

<$

1 Et N CH-CH20H Et’ V

Aext, an attempt was made to hydroborate 1-methyl-Z-acetoxycyclohexene (VI) by diborane in tetrahydrofuran,and then it was found that the product is essentially pure cis-Z-methylcyclohexanol (VIII). This indicates that the reduction of d-acetoxyorganoborane (VII) proceeds with com$ete inversion. In regard to this, Pasto (6,9) proposed that the reaction with overall inversion of stereochemistryproceeds through a transfer involving an "open book" transition state, whereas the reaction with retention of stereochemistryinvolves a "closed book" transition. Flowever,these mechanisms seem to require extraordinary transition, if the reaction is indeed intramolecular. On the other hand, complete inversion is readily understandable if we are dealing with a nucleophilic displacement as revealed in equation 3_.

49-D

Ko.47

Further, it was observed that in the hydroboration of the enol acetate from dimethyl acetaldehyde,the amount of isobutanol in the products increases with the increase of the concentrationof hydride used, as indicated in Table 2. This evidence also supports the view of a nucleophilic reduction mechanism.

TABLE 2* H- Concent-

yield

Product distribution,X

ration, N

%

1

100

100

3

100

63

37

5

a7

48

52

6.5

97

47

53

* Added II-,H-/olefin - 8; acetate was added to the

isobutyraldehyde

isobutanol trace

reac. temp. 20'; reac. time, 4 hr. THF

The en01

solution of borane.

Wallace (8) found that diborane in tetrahydrofuranexhibits appreciable conductivity. This avldence may also supporr rho removal of rhe acetoxy group by nucl~~philic displacement.

RFaFERl3NCF.S 1. The authors gratefully acknowledge the assistance of the Phillips Petroleum Company, Bartlesville,Oklahoma for the supply of pure 2-methyl-2-butene. 2. H. C. Brown and R. L. Sharp, 3. Am. Chem. Sot., in the press. 3.

G. Zweifel and H. C. Brown, "Organic Reaction," 13, 1 (1963). =

4.

D. J. Pasto and J. L. Miesel, J. Am. Chem. Sot., 82, 2118 (1963).

5. D. J. Pasto and C. C. Cumbo, ibid., 82, 4343 (1364). 6. D. J. Pasto and R. Snyder, J. Org. Chem.. 32, 2773 (1966). 7. This reaction was extraordinarilyslow, as compared with the case of diborane. a.

W. J. Wallace, Ph. D. Thesis, Purdue University (1962).

9.

D. J. Pasto and J. Hickman, J. Am. Chem. Sot., g,

5608 (1967).