Asymmetric Hydrosilylation - American Chemical Society

6 Asymmetric Hydrosilylation 1

H. B. KAGAN, J. F. PEYRONEL, and T. YAMAGISHI

Downloaded by UNIV OF ARIZONA on June 7, 2017 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch006

Laboratoire de Synthèse Asymétrique, (Associé au CNRS LA n° 040255-02), Université Paris-Sud, 91405-Orsay, France A catalyst for asymmetric hydrosilylation of ketones was prepared from [Rh(COD)Cl] and DIOP. The hydrosilyla tion of acetophenone yields 1-phenylethanol after hydrolysis. The optical yield strongly depends on the nature of the silane RR'SiH that was used. Several types of ketones were asymmetrically reduced into chiral alcohols, the highest asymmetric induction being observed from some α-chloro ketones or α-ketoesters. It was remarkable that prochiral benzophenones such as p-OMeC H COC H could be re duced with up to 26% e.e. Mechanism of asymmetric hydrosilylation was discussed in relation with some spin trap experiments. Studies were made on supported rhodium-DIOP catalysts. Some rhodium leaching from support was demonstrated by a three-phase test. 2

2

6

4

6

5

H p h e s t e r e o c o n t r o l l e d synthesis of a g i v e n e n a n t i o m e r is a k e y o p e r a t i o n - " i n m a n y processes. V e r y o f t e n , o n e e n a n t i o m e r r a t h e r t h a n t h e r a c e m i c 1

m i x t u r e is n e e d e d b e c a u s e of its p r o p e r t i e s . I n fragrances, f o o d a d d i t i v e s , or p h a r m a c e u t i c a l d r u g s , m a n y s u c h cases c a n b e f o u n d . F o r e x a m p l e , a - a m i n o a c i d s w h i c h enter as c o m p o n e n t s of p o l y p e p t i d e s o r d r u g s a r e a l w a y s u s e d w i t h a specific a b s o l u t e c o n f i g u r a t i o n . T h e i m p o r t a n c e of c h i r a l substances

is u n d e r s t a n d a b l e i f i t is r e a l i z e d t h a t l i v i n g

are essentially c h i r a l themselves a n d a b l e t o differentiate

systems

enantiomeric

substrates at a c t i v e sites of e n z y m e s o r at b i o l o g i c a l receptors. O n e classic a l p r e p a r a t i o n of a n e n a n t i o m e r

( D ) is t o resolve a r a c e m i c m i x t u r e

( D, L ). T h i s is a t i m e - a n d e n e r g y - c o n s u m i n g process b e c a u s e t h e u n d e sired enantiomer ( L ) must be separated b y chemical a n d p h y s i c a l operations a n d r a c e m i z e d i f p o s s i b l e f o r r e c y c l i n g .

I t is necessary

t o use

s t o i c h i o m e t r i c a m o u n t s of a c h i r a l a u x i l i a r y c o m p o u n d Z * ( F i g u r e 1) Current address: Department of Industrial Chemistry, Tokyo Metropolitan University, Fukazawa, Setagayaku, Tokyo, 158 Japan. 1

0-8412-0429-2/79/33-173-050$05.00/0 © 1979 American Chemical Society

King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

6.

Asymmetric

KAGAN ET A L .

51

Hydrosilylation

to o b t a i n t h e r e s o l u t i o n t h r o u g h f o r m a t i o n a n d s e p a r a t i o n of diastereomeric products.

E a c h d i a s t e r e o m e r is t h e n d e s t r o y e d a n d the d e s i r e d

e n a n t i o m e r ( D ) is r e c o v e r e d .

T h e c h i r a l e c o n o m y is o b v i o u s i f Z * c a n

b e a c h i r a l r e a g e n t c o n t r o l l i n g the d i r e c t f o r m a t i o n of D ; the process is n o w a n a s y m m e t r i c synthesis. represents

A f u r t h e r i m p r o v e m e n t occurs w h e n Z *

a c h i r a l catalyst b e c a u s e a s m a l l a m o u n t of Z * s h o u l d b e

a b l e to c o n t r o l d i r e c t p r o d u c t i o n of a l a r g e a m o u n t of the d e s i r e d e n a n t i o m e r . F i g u r e 1 is a s c h e m a t i c c o m p a r i s o n of r e s o l u t i o n a n d a s y m m e t r i c synthesis i n t h e case w h e r e the r e a c t i o n g i v i n g rise to a r a c e m i c m i x t u r e


( D + L ) is t r a n s f o r m e d i n t o a n a s y m m e t r i c c a t a l y t i c process l e a d i n g to D . A s y m m e t r i c catalysis as a s y n t h e t i c t o o l is r e l a t i v e l y n e w ( i f e n z y m a t i c reactions are not c o n s i d e r e d ) ; its d e v e l o p m e n t b e g a n 10 years ago, m a i n l y b e c a u s e of the advances i n c o o r d i n a t i o n c h e m i s t r y . A s y m m e t r i c h y d r o g e n a t i o n started b y m o d i f y i n g the W i l k i n s o n catalyst e a r l y results (2,8,4) a m o u n t of r e s e a r c h

(J).

The

w e r e e n c o u r a g i n g e n o u g h to i n i t i a t e a v e r y l a r g e (5,6).

A s y m m e t r i c C - C b o n d formation i n olefin

c o - d i m e r i z a t i o n w a s o b s e r v e d f o r the first t i m e b y W i l k e a n d his cow o r k e r s (7).

Asymmetric hydroformylation (8)

as w e l l as s e v e r a l n e w

a s y m m e t r i c a l k y l a t i o n reactions a p p e a r e d i n the last five years A s y m m e t r i c e p o x i d a t i o n s w e r e d e s c r i b e d i n 1977

( C h i r a l a u x i l i a r y compound)

Z 1 eq . { D , L }

A

+

Z 0.5 e q J 0

^

Racemization 1 eq.

(9,10).

(11,12).

0.5 eq. L

(Prochiral) RESOLUTION

Reagent

Z

(Chiral Catalyst)

ASYMMETRIC SYNTHESIS

leq.

Figure 1.

Asymmetric

0

synthesis vs. resolution, an "energy saving" process


52

INORGANIC

COMPOUNDS

WITH

UNUSUAL PROPERTIES

H

I n p r i n c i p l e , a n y c a t a l y z e d r e a c t i o n w h e r e t h e l i g a n d s of t h e c o m p l e x are easily m o d i f i e d c o u l d b e i n v e s t i g a t e d f o r a s y m m e t r i c catalysis. T o date, enantioselectivities h i g h e r t h a n 9 5 % h a v e b e e n a t t a i n e d i n a f e w cases (13,14),

g i v i n g h o p e that f u r t h e r studies s h o u l d define a v a r i e t y

of specific c h i r a l catalysts f o r t h e p r o d u c t i o n o f v a r i o u s c h i r a l c o m p o u n d s . T h e c r e a t i o n o f a n a s y m m e t r i c center b y C - H b o n d f o r m a t i o n is a very

c o m m o n process

Hydrogenation

which

c a n i n v o l v e several t y p e s

of p r o c h i r a l olefins is o f t e n

used w i t h

of reactions. the rhodium

catalysts of t h e W i l k i n s o n t y p e ( 5 ) . T h e s e catalysts w e r e s h o w n t o b e


i n a c t i v e f o r k e t o n e o r i m i n e r e d u c t i o n e x c e p t i n some cases (15).

It was

t h e n i n t e r e s t i n g to d e v e l o p a n alternate m e t h o d f o r a s y m m e t r i c synthesis of c h i r a l alcohols o r amines.

S i n c e i t w a s f o u n d that R h C l ( P P h ) 3

a b l e t o c a t a l y z e silane a d d i t i o n s t o ketones

(16,17)

or imines

3

was (18),

p r e p a r a t i o n of c h i r a l alcohols o r amines b y a s y m m e t r i c h y d r o s i l y l a t i o n c o u l d b e e n v i s a g e d ( F i g u r e 2 ) . T h e 1 , 4 - a d d i t i o n of silanes to c o n j u g a t e d

R

R R -V-CH R H 1

1

C = C

2

H

R

3

2

R 2 ^ * R-C-OH 1

1

C = o

H O/H* 2

R

1

=SiH

R

R -C-OSi= H 2

R;

1

= SiH

C =N-R

R -V-N: s

C=CH-C-R

3

R

=SiH R

Examples

2

Si =

R —jCf—NHR H 2

R

1

O *-CH=:C-R C

H Figure 2.

H O/H*

2

tliOAll

3

OSi =

of creation of an asymmetric tion

3

1

R^-^-CHXR

H

center by C-H bond forma-


3

6.

KAGAN

Asymmetric

ET AL.

esters or ketones (19)

53

Hydrosilylation

is another p o s s i b i l i t y f o r a s y m m e t r i c h y d r o s i l y l a -

t i o n a n d c r e a t i o n of a n a s y m m e t r i c center i n the /? p o s i t i o n r e l a t i v e to a carbonyl group. W e chose to s t u d y a s y m m e t r i c h y d r o s i l y l a t i o n f o r t h e of alcohols (20)

a n d amines (18)

preparation

because the system p e r m i t s s t r u c t u r a l

m o d i f i c a t i o n s of b o t h the c h i r a l l i g a n d s a n d the silane.

T h e possibility

arose of a g o o d m a t c h i n g w i t h the substrate, l e a d i n g to h i g h stereoselect i v i t y i n the f o r m a t i o n of the c h i r a l p r o d u c t . W e w i l l o n l y c o n s i d e r h e r e


the a s y m m e t r i c synthesis of alcohols. Synthesis of Chiral

Ligands

P h o s p h i n e s are the u s u a l l i g a n d s of r h o d i u m catalysts

for hydro-

s i l y l a t i o n . A classification of c h i r a l p h o s p h i n e s a c c o r d i n g to their struct u r e is p r e s e n t e d i n T a b l e I. V e r y e a r l y w e e x a m i n e d the synthesis a n d b e h a v i o r of p h o s p h i n e s R*-PPh i n 1971

2

easily p r e p a r e d f r o m n a t u r a l p r o d u c t s (21).

W e then introduced,

the use of c h i r a l c h e l a t i n g d i p h o s p h i n e s

(4),

Table I.

A Structural Classification of Chiral

(phosphines

Phosphines ' a

of

b

Ri Type

I:

R

2

Type II:

~ ^ P

R — P P h

2

Rs * ^ Type

III:

PPh

2

Type I V :

^ R

(CH ) * 2

* ^.Ri

PPh,

R

* \

*

R

2

2

n

R

TypeV:

i

R

*R

Type V I :

p

R* "

2

Ri

*/ P

R

X

V

v

2

R

4

• This classification gives only the main types of chiral mono- and diphosphines. Many other structures are also possible, allowing, in principle, the synthesis of a large number of new ligands. R* and P* symbolize chiral groups. * For reviews on chiral phosphines prepared for homogeneous asymmetric catalysis, see Ret. 5, 6, and 26.



54

INORGANIC COMPOUNDS W I T H UNUSUAL PROPERTIES

II

T y p e I I I ) . Since g o o d results i n a s y m m e t r i c h y d r o g e n a t i o n w e r e o b t a i n e d (22) w i t h D I O P 1, i t w a s t h e n u s e d i n a s y m m e t r i c h y d r o s i l y l a t i o n

(20).

R e s u l t s w i t h D I O P a n d some other d i p h o s p h i n e s w i l l b e p r e s e n t e d here. Influence of the Structure

of Silanes upon Optical

Yield

T h e s t a n d a r d p r o c e d u r e that w a s a d o p t e d f o r a s y m m e t r i c h y d r o s i l y l a t i o n of ketones w a s t h e f o l l o w i n g . A s o l u t i o n of 1 m m o l o f k e t o n e a n d 1.1 m m o l of silane i n 3 m L b e n z e n e i n p r e s e n c e of R h C l ( D I O P ) (0.2% hours. DIOP

w i t h respect to t h e k e t o n e ) w a s s t i r r e d u n d e r n i t r o g e n f o r a f e w T h e catalyst i n benzene

was preformed b y mixing [ R h ( C O D ) C l ]

a n d s t i r r i n g 15 m i n u n d e r n i t r o g e n .

and

2

A l l o f these

operations w e r e p e r f o r m e d at 2 0 ° C . T h e c o m p l e x [ R h ( C O D ) ( D I O P ) ] C 1 0 " w h i c h c a n b e i s o l a t e d gives +

4

n o better results t h a n t h e n e u t r a l catalyst p r e p a r e d i n s i t u . A f t e r r e a c t i o n the solvent w a s e v a p o r a t e d a n d t h e r e s i d u e h y d r o l y z e d u n d e r

acidic

c o n d i t i o n s . T h e p r o d u c t was r e c o v e r e d b y d i s t i l l a t i o n o r c h r o m a t o g r a p h y . I n g e n e r a l , y i e l d s w e r e excellent Dihydrosilanes R R ' S i H

2

(90-100%).

w e r e selected

i n almost

a l l of o u r w o r k

b e c a u s e of t h e i r g o o d r e a c t i v i t y . I n T a b l e I I some representative drosilanes a n d t h e i r efficiencies i n t h e h y d r o s i l y l a t i o n o f are i n d i c a t e d .

dihy-

acetophenone

T h e r e are v a r i a t i o n s i n o p t i c a l y i e l d s a c c o r d i n g to t h e

s t r u c t u r e of d i h y d r o s i l a n e s . Steric considerations d o n o t seem to b e a b l e to g i v e a s i m p l e e x p l a n a t i o n . F o r e x a m p l e , P h ( c y c l o h e x y l ) S i H is m o r e 2

efficient t h a n P h S i H , b u t ( c y c l o h e x y l ) S i H is a p o o r reagent. S i m i l a r l y , 2

(a-naphthyl)PhSiH

2

2

2

is b e t t e r t h a n P h S i H 2

2

2

b u t n o t different f r o m

(a-

naphthyl) SiH . 2

2

U s e of a c h i r a l c o m p l e x i n h y d r o s i l y l a t i o n of a k e t o n e c a n p r o v i d e a r o u t e t o c h i r a l silanes.

T h i s m e t h o d w i t h R h C l ( D I O P ) as catalyst w a s

i n v e s t i g a t e d b y C o r r i u et a l . (26,31).

A p r o c h i r a l silane s u c h as ( a - N p ) -


6.

KAGAN

Asymmetric

ET AL.

55

Hydrosilylation

Table II. Hydrosilylation of P h - C O - C H by R R ' S i H Catalyzed by R h C l ( - ) D I O P (23,24) 3

2

PhCHOHCHs


Silane" R

R'

Cyclohexyl Ph Ph Ph m-Me-Ph a-Naphthyl Ph Ph a-Naphthyl Ph

cyclohexyl mesityl CH Ph m-Me-Ph cyclohexyl o-Me-Ph cyclohexyl a-naphthyl a-naphthyl

Optical Yield (%)

Absolute Configuration

V 6* 13 24 30 32 35 49 50 52

S R R R R R R R R R

3

° See "Influence of the Structure of Silanes upon Optical Y i e l d " for experimental details. The enol silylether of acetophenone is simultaneously formed in approximately equal amounts as the silylether of 1-phenylethanol. b

P h S i H , a f t e r r e a c t i o n o n a s y m m e t r i c a l ketone, is t r a n s f o r m e d i n t o ( a 2

H N p ) P h S i - ( O R ) , i n w h i c h t h e s i l i c o n a t o m is t h e o n l y source of c h i r a l i t y . O p t i c a l y i e l d s i n t h e r a n g e of 5 0 % w e r e Relation

between Optical

observed.

Yield and Structure

of the

Substrates

O f t e n a c e t o p h e n o n e is t h e p r o c h i r a l k e t o n e w h i c h is first tested w h e n a n e w a s y m m e t r i c r e d u c i n g agent has to b e e v a l u a t e d . d o n e i n a s y m m e t r i c h y d r o s i l y l a t i o n b y us (20)

This was

a n d others (26).

For-

t u n a t e l y , c a t a l y t i c h y d r o s i l y l a t i o n is n o t l i m i t e d t o this case. I n T a b l e I I I , some representative results w i t h a - n a p h t h y l p h e n y l silane are

summarized.

Substitution

(a-NpPhSiH ) 2

o n t h e m e t h y l g r o u p of a c e t o p h e n o n e

strongly influences t h e stereospecificity.

W h e n there is a free O H g r o u p ,

it is first s i l y l a t e d , f o l l o w e d b y a n i n t e r n a l h y d r o s i l y l a t i o n l e a d i n g to a c y c l i c s i l y l diether.

H y d r o l y s i s gives

the p h e n y l glycol

(Figure

3).

C o n t r a r y to expectation, this i n t r a m o l e c u l a r process decreases the enantiospecificity. a - C h l o r o o r b r o m o ketones w e r e s m o o t h l y h y d r o s i l y l a t e d w i t h g o o d o p t i c a l y i e l d s ( w i t h respect to t h e u n s u b s t i t u t e d k e t o n e ) .

Basic hydrol-

ysis of t h e h a l o g e n o s i l y l ether gave d i r e c t l y a c h i r a l e p o x i d e . a-Ketoesters g a v e a-hydroxyesters of h i g h o p t i c a l p u r i t y w h e n R h C l ( D I O P ) w a s u s e d as catalyst. O j i m a (27)

o b t a i n e d p r o p y l lactate w i t h 8 5 % e.e. b y r e d u c -

i n g p r o p y l p y r u v a t e i n presence of R h C l ( D I O P ) a n d also o b s e r v e d h i g h


56

INORGANIC COMPOUNDS W I T H UNUSUAL PROPERTIES

H 0/H *~ P h — C H — C H +

2

Ph_CO—CH

+ «NpPhSiH -^ Ph—CH—CH

3

2

8

.Ph OSiH^

2


^

Ph—C=CH

+

Ph—CO—CH

2

2

O S i H ^

3

H 0/H »

H 2

P

8

O H

«Np

"

II

3

h

aNp

- H P h — C O — C H O H + «NpPhSiH 2

Ph—CH—CH 0

Ph

Figure 3. asymmetric

.Ph • P h — C O — C H O S i H

Asymmetric Hydrosilylation - American Chemical Society

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