Organoboranes for Syntheses - American Chemical Society

Chapter 5

New Organic Synthetic Methods Using the NaBH /I System 4

2

Downloaded by UNIV OF ARIZONA on January 12, 2013 | http://pubs.acs.org Publication Date: February 26, 2001 | doi: 10.1021/bk-2001-0783.ch005

Mariappan Periasamy School of Chemistry, University of Hyderabad, Central University, P.O., Hyderabad 500046, India

Research efforts in this laboratory revealed that the readily accessible and easy to handle NaBH /I reagent system can be used in place of other hydride reagents in several synthetic applications. The diborane generated using the NaBH /I reagent system has been used for the preparation of iodoborane-amine complexes that have good synthetic potential. A n interesting catalytic process involving hydroboration with weak complexes of borane and Lewis bases followed by exchange of the alkyl group to catechol borane has been developed. The H B:THF prepared in situ in T H F using NaBH and I is useful for reduction of functional groups. 4

2

4

2

3

4

2

Introduction The sodium borohydride is one of the most widely used reagents in chemistry. It is certainly a reagent of choice for the reduction of organic functional groups, especially for the reduction of aldehydes and ketones (/). The carboxylic acids, esters, amides and nitriles are resistant towards NaBUt under

© 2001 American Chemical Society In Organoboranes for Syntheses; Ramachandran, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

65

66 ambient conditions (7). Historically, Brown and Subba Rao (2) discovered the hydroboration of carbon-carbon double bonds during their efforts to increase the reactivity of NaBH» using A1C1 (2). This discovery followed the development of various methods of generation of diborane using Lewis acids and NaBRj (J). Although borane is now commercially available in various forms (e.g. H B : T H F , H B : S M e and H B : N R (3), efforts are still continuing for developing more convenient methods of reductions using NaBH* along with additives. For example, it has been reported that the NaBH4/R SeBr combination reduces amides and nitriles to the corresponding amines (4). At the time, we have been investigating the generation of B H using I and N a B H (J). It occurred to us that a systematic investigation on the synthetic applications of the readily accessible and easy to handle NaBHj/I combination should yield fruitful results. Hence, we have undertaken efforts to explore the synthetic possibilities of this reagent system. We describe here the results of these studies. Our preliminary reports also prompted other scientists to investigate the synthetic applications of the N a B I t y ^ system. Their results are also described here. 3

3

3

2

3

3

2


2

6

2

2

4

2

Generation of B H Using NaBH and I 2

6

4

2

In 1965, it has been reported that the reaction of NaBK* with I gives diborane in >90% yield (eq 1) (6). 2

2 NaBH

4

+ I

diglyme »

2

2NaI + H + B H 2

2

(1)

6

These authors used a vacuum line technique and isolated the diborane in a series of liquid nitrogen traps (6). The diborane generated in this way was found to be free of any detectable impurities compared to the diborane generated using NaBHt and F B : O E t that contains trace amounts of B F . The advantage of diborane generated using the NaBH4/I has been also demonstrated in the reduction of dianisylketone 1 (7). Whereas the B H generated using NaBHj and I gave the desired secondary alcohol 3, the reaction of " B H " obtained from NaBHj and F B : O E t led to over reduction to the compound 2 due to the presence of B F impurity (7). Clearly, the complication due to the presence of B F is prevented when the B H generated from NaBH4/I is used. Since the I is readily accessible and more easy to handle compared to F B:OEt , it is surprising that the N a B R / h system has not received attention for a long time. This may have been due to the non-availability of a detailed procedure for the diborane generation using the NaBH4/I combination, as suggested by Lane in a review article describing synthetic applications of diborane (8). Further, the use of vacuum line technique 3

2

3

2

2

6

2

2

3

6

2

3

3

2

6

2

2

3

2

2

In Organoboranes for Syntheses; Ramachandran, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

67


by the original authors (6) might have also played a role in driving away the bench-top synthetic chemists from using this simple, convenient reagent system.

We were looking for a simple way of preparation of diborane for applications using the NaBR*/^ reagent system. We have observed that the same apparatus assembly recommended by Brown for the generation of B H from NaBH4/F B:OEt in diglyme can be used for the preparation of B H from N a B E t and I (3). Thus, I in diglyme is added slowly to a slurry of N a B H in diglyme and the evolved mixture of B H and H gases are bubbled through a solution containing appropriate Lewis bases to obtain the corresponding B H complexes (5,6). We have followed this procedure for the preparation of H B : T H F or Ph(Et) N:BH for hydroboration and other synthetic applications (Scheme 1) (5). 3

2

2

2

6

2

6

2

4

2

6

2

3

3

2

3

H B:THF 3

diglyme 2NaBH

4

+ I

,

2

» B H 2

• H B:N(Et) Ph N(Et) Ph

6

3

2

3

2NaI + H

2

2

2

Scheme 1: Generation of diborane from NaBH and l in diglyme 4

2

Synthetic Applications of Ph(Et) N:BH prepared Using NaBH and I for B H Generation 2

2

3

4

2

6

The Ph(Et) N:BH complex has been used in the hydroboration of representative olefins (5). The Itsuno-Corey oxazaborolidine-H B:THF reduction, the CBS process, is the method of choice for obtaining high level of asymmetric induction (>95% ee) in the reduction of ketones (9-11). We have shown that the C B S 2

3

3


68 oxazaborolidine can be readily prepared using Ph(Et) N:BH applications (Scheme 2) (12). 2

2NaBH

+ I

4

diglyme . » BH

2

2

2NaI + H

H

(


V

" N

Ph(Et) N: — » Ph(Et) N:BH 2

6

for synthetic

3

2

2

3

2

θ

Ph^CH

* B

HO

3

H B:THF

Η

Ph^CH 95% ee

3

Η CBS catalyst

3

Scheme 2: Insitu generation of oxazaborolidine and its utilization in catalytic asymmetric reductions

Synthetic Applications of I BH:N(C H )2Ph and I B:N(C H ) Ph Complexes 2

2

5

3

2

5

2

The iodoborane-N,N-diethylaniline complexes have been prepared through the reaction of B H : N ( C H ) P h with appropriate amount of I at 0-25° C (Scheme 3) (13,14). 0.5I IBH :N(C H ) Ph 3

2

5

2

2

2

Ph(C H ) N:BH 2

5

2

h

Benzene 3

1.5I

2

2

2

5

2

I BH:N(C H ) Ph 2

2

5

2

I B:N(C H ) Ph 3

2

5

2

Scheme 3: Preparation ofB-iodoborane-N,N-diethylaniline complexes

We have examined the applications of I B H and I B H complexes for hydroboration of alkenes to obtain dialkyl and mono alkyl boranes. Unfortunately, these species disproportionate and hence the dialkyl and trialkyl boranes were not formed cleanly. Fortunately, however, the H B I and B I complexes are found to be useful for iodinations of alcohols, reductive iodinations of carbonyl compounds and hydroiodination of alkenes and alkynes. Alcohols react with Ph(Et) N:BHI to give the corresponding iodides in good yields (Scheme 4) (13). 2

2

2

2

2


3

69 Ph(Et) N:BHI : 1 Benzene, 25°C 2

RCH OH 2

2

RCH I _ 2

g ( )

8 5 %

OH exo/endo

80-85%

Scheme 4: lodinatination of alcohols using Ph(Et) N:BHI


2

2

The aldehydes, ketones and carboxylic acids undergo reductive iodination under these conditions (Scheme 5) (14).

ο Y.

ΪI

Ph(Et) N:BHl2 2

RCHR' g2%

R R' Benzene, 25°C R' = H,alkyl,aryl HOOC(CH ) COOH 2

-

8

ICH (CH ) CH I 2

2

8

2

60% Scheme 5: Reductive iodinations using Ph(Et) N:BHI 2

2

The "HI" species can be readily generated using Ph(Et)2N:BI CH3COOH for synthetic applications (Scheme 6) (75).

3

P

H

(

E

T

)

2

N

:

B

I

_ £ H £ ° O H _

J

and

„,

Benzene, 25°C I

1

HI R-HC=CH

^(CH ) COOCH 2

8

R^CH 82-84%

2

HI 3

3

1

1 H C ^(CH ) COOCH 3

2

8

3

80% Scheme 6: In situ generation of 'HI' and its reaction with olefins


70 Shibasaki et al followed this method of "HI" generation for hydroiodination of an alkynyl ketone (eq 2) (76). I.

H

Q #

3

H c">=CH

"

3

0 ^ ? \ ^ 0 \

Ph(Et) N:BH 2

/

3|

CH COOH Benzene, 25°C 14h 3

2

O^y^O Λ

(2)

I 95%

Kabalka et al found that the Ph(Ef)2N:BI is useful for cleavage of ethers (77), lactones (7

Organoboranes for Syntheses - American Chemical Society

Recommend Documents