Asymmetric Reactions and Processes in Chemistry - American

Jpn. (Pure Chem. Sec.) 1960, 81, 643. 16. Harada,K. In "The Chemistry of the Carbon-nitrogen. Double Bond"; Patai,S., Ed.; Interscience. Publishers: L...
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11 Stereochemistry of Heterogeneous Asymmetric Catalytic Hydrogenation KAORU HARADA

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University of Tsukuba, Department of Chemistry, Ibaraki 305, Japan

In this paper, the stereochemistry of heterogeneous catalytic hydrogenation of C=N- and C=O double bonds of the derivatives of α-keto acids, keto alco­ hols and diketones is described. The steric course could be explained by the chelation hypothesis. In 1961, H i s k e y e t a l . ( 1 ) r e p o r t e d t h e s u c c e s s f u l asymmetric s y n t h e s e s o f α-amïho a c i d s . They demons t r a t e d t h e s y n t h e s i s o f amino a c i d s i n 45-70% e n a n t i o m e r i c p u r i t y by c a t a l y t i c hydrogénation o f t h e S c h i f f bases p r e p a r e d from α-keto a c i d s and o p t i c a l l y a c t i v e α-methylbenzylamine f o l l o w e d by h y d r o g e n o l y s i s (Scheme 1 ) . When (S)-amine was used, (S)-α-amino a c i d r e s u l t e d . This i s a highly stereoselective reaction. However, the a u t h o r s d i d n o t d i s c u s s t h e s t e r i c c o u r s e o f t h e asymmetric hydrogénation p r o c e s s . Scheme 1 R-C-COOH " Ν C H -CH-CH (S) 6

5

3

n

R-CH-COOH L NH

H 2

Pd/C ' C H - C H - C H 6

5

3

n

Pd

(S) R-CH-COOH I

H 2

(OH)'

2

L a t e r M i t s u i e t al.(2) r e p o r t e d t h e asymmetric s y n t h e s e s o f p h e n y l g l y c i n e by t h e H i s k e y t y p e r e a c t i o n and p r o p o s e d a s t e r i c c o u r s e f o r t h e asymmetric s y n ­ t h e s i s as shown i n Scheme 2. I f i t i s a p p l i c a b l e t o a l l o f t h e H i s k e y t y p e r e a c t i o n s , t h e f o l l o w i n g may be e x p e c t e d : (a) an i n c r e a s e i n o p t i c a l y i e l d upon s u b s t i ­ t u t i o n o f α-methylbenzylamine by α-ethylbenzylamine and (b) a comparable o p t i c a l y i e l d upon s u b s t i t u t i o n o f α-methylbenzylamine by a - ( 1 - n a p h t h y 1 ) e t h y l a m i n e . 0097-6156/82/0185-0169$05.00/0 © 1982 American Chemical Society

Eliel and Otsuka; Asymmetric Reactions and Processes in Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

ASYMMETRIC

170

REACTIONS

A N D PROCESSES

IN

CHEMISTRY

Scheme 2 C,H 6 5

COOH /

C

X

CH

3

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N

0

.

(S) C

C.H . COOH 6 5 / C H I NH ^

N

C if Ν

;C

H

C.H,. COOH 6 5 / C H II 2 Ν >^(S)

H

C

6 5

H

6 5

C H

3

9 2 r

H

C

H

6 5

C H

COOH I Η,Ν-j-H

3

In o r d e r t o examine t h e s t e r i c c o u r s e proposed by M i t s u i e t a l . , we have performed asymmetric s y n t h e s e s o f a l a n i n e , α-aminobutyric a c i d , p h e n y l g l y c i n e , p h e n y l ­ a l a n i n e and g l u t a m i c a c i d from t h e c o r r e s p o n d i n g a-keto a c i d s u s i n g (S) -α-methylbenzylamine [Me (-) ] , (S) - a ethylbenzylamine [ E t ( - ) ] and ( R ) - a - ( 1 - n a p h t h y l ) e t h y l amine [Naph(-)] as t h e c h i r a l a d j u v a n t . The r e s u l t s a r e shown i n T a b l e 1(3^,4). The r e s u l t s i n d i c a t e t h a t a) t h e o p t i c a l p u r i t y o f amino a c i d s o b t a i n e d w i t h α-methylbenzylamine i s always h i g h e r than when α-ethylbenzylamine i s used, b) t h e o p t i c a l p u r i t y o f t h e amino a c i d s d e c r e a s e s s t e a d i l y as t h e b u l k o f t h e a l k y l group o f t h e α-keto a c i d s i n ­ c r e a s e s , and c) t h e o p t i c a l p u r i t y i n c r e a s e s when ( R ) a-(1-naphthyl)ethylamine i s used(£) . These f i n d i n g s show c l e a r l y t h a t t h e s t e r i c c o u r s e proposed p r e v i o u s l y does n o t e x p l a i n any o f t h e e x p e r ­ imental r e s u l t s . Based on m o l e c u l a r models we p r o ­ posed a d i f f e r e n t s t e r i c c o u r s e c o n s i s t e n t w i t h t h e experiments. S t r u c t u r e I (Scheme 3) r e p r e s e n t s a con­ f o r m a t i o n o f t h e s u b s t r a t e which s a t i s f i e s a l l o f t h e Table I

Asymmetric S y n t h e s i s o f Amino A c i d s

α-Keto a c i d R-CO-COOH R= C H

2

C

5

H

6 5

C H

C

H

2" 6 5

(CH ) COOH 2

2

Solvent:EtOH

Amino

acid

Optical purity(%)

Alanine Alanine Alanine

67 52 83

Butyrine Butyrine

44 33

Me (-) E t (-)

(S) (S) (R) (S) (S)

Me (-) E t (-)

(S) P h e n y l g l y c i n e (S) P h e n y l g l y c i n e

30 24

Me (-) E t (-)

(S) P h e n y l a l a n i n e (S.) P h e n y l a l a n i n e

14 10

Me (-) E t (-)

(S) G l u t a m i c (S.) G l u t a m i c

12 6

Me (-) E t (-) Naph(+)

3

C H

Optically a c t i v e amine

acid acid

Eliel and Otsuka; Asymmetric Reactions and Processes in Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

HARADA

11.

Heterogeneous

Asymmetric

Catalytic

Hydrogénation

Scheme 3 H R (S) ι \ I

C

H

R I

0

6 5

HN

( I )

C

0

0

H

2

^(S)

2

Scheme 4

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H

R

C

C C

H

6 5

N

^

CH

0

y° (Pd)'n

H

(ID

-C

3

R

Ρ

C

C

V "(Pdfn

(III



c o n d i t i o n s r e q u i r e d by the e x p e r i m e n t a l f i n d i n g s (_3) · The s t r u c t u r e I might be c o n s i d e r e d t o form a sub­ s t r a t e - c a t a l y s t complex as shown i n s t r u c t u r e I I (Scheme 4 ) . A m o l e c u l a r model o f s t r u c t u r e I I f i t s v e r y w e l l on the s u r f a c e o f the p a l l a d i u m c a t a l y s t . The p l a n e c o m p r i s i n g the S c h i f f base o f the α-keto a c i d i s assumed t o be p e r p e n d i c u l a r t o the p a l l a d i u m s u r ­ f a c e , w i t h the p h e n y l group l y i n g on the p a l l a d i u m s u r ­ f a c e as shown i n s t r u c t u r e I I . I f the p h e n y l group was p l a c e d as shown i n s t r u c t u r e I I I (Scheme 4 ) , the a l k y l group o f the asymmetric m o i e t y and t h a t o f the k e t o a c i d would i n t e r f e r e w i t h each o t h e r , and the s t r u c t u r e I I I would be u n s t a b l e . Thus we assume (A) the sub­ s t r a t e i n i t i a l l y i n t e r a c t s w i t h the c a t a l y s t , t o form a s u b s t r a t e - c a t a l y s t complex as shown i n s t r u c t u r e I I b e f o r e the c a t a l y t i c hydrogénation t a k e s p l a c e , and t h e n (B) the s t r u c t u r e I I i s adsorbed on the c a t a l y s t from the l e s s b u l k y s i d e o f the m o l e c u l e , and c a t a l y t i c hydrogénation t a k e s p l a c e . We have c a l l e d t h i s hydrogénation p r o c e s s "the c h e l a t i o n h y p o t h e s i s " ( 3 ) / and f u r t h e r s t u d i e s were u n d e r t a k e n t o t e s t t h i s hypothesis. T a b l e I I shows s o l v e n t e f f e c t s i n t h e asymmetric s y n t h e s i s o f a l a n i n e from p y r u v i c a c i d and ( S ) - a methylbenzylamine(A). The o p t i c a l p u r i t y o f a l a n i n e decreases with i n c r e a s i n g p o l a r i t y of the s o l v e n t . In the c a s e o f t h e asymmetric s y n t h e s i s o f g l u t a m i c a c i d from α-keto-glutaric a c i d and (S)-α-methylbenzylamine, the c o n f i g u r a t i o n o f t h e r e s u l t i n g g l u t a m i c a c i d was a c t u a l l y i n v e r t e d by the use o f p o l a r s o l v e n t s . The s u b s t r a t e appears t o i n t e r a c t w i t h the c a t a l y s t more s t r o n g l y i n a l e s s p o l a r t h a n i n a more p o l a r s o l v e n t . Thus, the p o p u l a t i o n o f the c h e l a t e d s u b s t r a t e i s

Eliel and Otsuka; Asymmetric Reactions and Processes in Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

ASYMMETRIC

172 Table

II

A N D PROCESSES

S o l v e n t E f f e c t i n t h e Asymmetric of Alanine

Solvent

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REACTIONS

Yield(%)

CHEMISTRY

Synthesis

Optical purity(%)

Hexane

75

75

AcOEt

49

60

DMFA

47

50

i-PrOH

56

46

MeOH

61

38

MeOH:H 0(l:2) 2

75

35

MeOH:H 0(l:4)

76

29

2

IN

l a r g e r i n t h e l e s s p o l a r s o l v e n t than i n t h e more p o l a r one. The u n c h e l a t e d s p e c i e s , which has a s t a b l e conf o r m a t i o n IV (Scheme 5 ) i n s o l u t i o n , c o u l d be adsorbed on t h e l e s s b u l k y f r o n t -C=N- f a c e o f t h e m o l e c u l e (Scheme 5 ) r e s u l t i n g i n an a l a n i n e d e r i v a t i v e which has the c o n f i g u r a t i o n o p p o s i t e t o t h a t o b t a i n e d from t h e chelated species. Asymmetric c a t a l y t i c hydrogénation o f t h e S c h i f f base p r e p a r e d from e t h y l p y r u v a t e and an o p t i c a l l y a c t i v e amine i n d i f f e r e n t s o l v e n t s was c a r r i e d o u t and supports the c h e l a t i o n hypothesis. F i g u r e 1 shows s o l v e n t e f f e c t s i n t h e s y n t h e s i s o f a l a n i n e and a a m i n o b u t y r i c a c i d (5^) . When (£) - b e n z y l i c amine was used as t h e asymmetric m o i e t y , t h e o p t i c a l p u r i t y o f t h e r e s u l t i n g amino a c i d i n c r e a s e d w i t h a d e c r e a s e i n s o l vent p o l a r i t y ( £ , 7 J · temperature e f f e c t was a l s o o b s e r v e d , and t h e o p t i c a l p u r i t y o f t h e amino a c i d s i n c r e a s e d upon l o w e r i n g t h e r e a c t i o n t e m p e r a t u r e ( £ , 9 ^ A

10 ) . The c h e l a t i o n h y p o t h e s i s c o u l d a l s o be a p p l i e d t o the c a t a l y t i c hydrogénation o f α-keto a c i d amides c a r r i e d o u t i n i t i a l l y by H i s k e y e t a l . , who e x p l a i n e d the s t e r i c c o u r s e assuming i n t e r m e d i a t e s t r u c t u r e V ( 1 1 ) (Scheme 6 ) . Scheme 5 H 1

C H> 6

5

R \ N

%

Ο polar solvent

c

^0 (Pd)n—

(S) amino a c i d

less polar solvent

υ R'^ C H 6 5

R £ N

,0 0~

(IV)

(R) amino a c i d

Eliel and Otsuka; Asymmetric Reactions and Processes in Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

HARADA

11.

Figure

Heterogeneous

Asymmetric

Catalytic

Hydrogénation

173

S o l v e n t E f f e c t i n t h e Asymmetric S y n t h e s i s o f Amino A c i d s from E t h y l P y r u v a t e ( 5 )

1

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Amine (R)-(+) - a - M e t h y l b e n z y l (SM-) -a-Methylbenzyl (RM+) - a - E t h y l b e n z y l (R)-(+) -1- (a-Naphthyl) ethyl (S)-(-) - a - M e t h y l b e n z y l (S)-(-) - a - M e t h y l b e n z y l (a)-(e), Alanine ( f ) , a-Aminobutyric acid

Product

Catalyst:

(a)-(d) , P d ( 0 H ) / C (e) (f) , Pd/C 9

z

Dielectric of Solvent

Constant

Scheme 6 R-N CH

34Ο

Η

R-N

\

*C/

NH

„„f

NH

i C 3

0

J Η CH3

(V)

Pd/C

-o-cCH

/

CHo

N 2

/ \

•c

COOH /

c-..„ 0

CH^

3

The change i n c o n f i g u r a t i o n o f t h e r e s u l t i n g amino a c i d w i t h t h e use o f d i f f e r e n t asymmetric m o i e t i e s i s a l s o e x p l a i n e d by the c h e l a t i o n h y p o t h e s i s ( 1 2 ) (Scheme 7 ) . I n t h e s e q u e l , we w i l l d i s c u s s a g e n e r a l i z a t i o n o f the c h e l a t i o n h y p o t h e s i s as i t a p p l i e s t o r e a c t i o n s o t h e r t h a n hydrogénation o f S c h i f f bases o f α-keto a c i d s w i t h c h i r a l amines. The c a t a l y t i c hydrogénation o f p y r u v i c a c i d amide r e s u l t e d i n the f o r m a t i o n o f l a c t a m i d e i n h i g h o p t i c a l p u r i t y (75-99% d i a s t e r e o m e r i c excess)(13). T h i s might be e x p l a i n e d by the c h e l a t e c o n f o r m a t i o n o f the s u b s t r a t e - c a t a l y s t complex shown i n Scheme 8. The c a t a l y t i c hydrogénation o f o p t i c a l l y a c t i v e b e n z o i n oxime r e s u l t e d i n t h e s t e r e o s e l e c t i v e f o r m a t i o n of o p t i c a l l y a c t i v e erythro diphenylethanolamine in

Eliel and Otsuka; Asymmetric Reactions and Processes in Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

ASYMMETRIC

174

REACTIONS

Scheme 7 ?3 /? .COOiBu

CH

C

,

C

/~ \/\-CH

Bzl-N^ ,NH \ * 'n~"

— H

3

A N D PROCESSES

C

N - f "

Η

N

λ

Ν

(pdi

3

2

3

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Z

2

H

t

N

H

2

CH (R)64%ee 3

Scheme 8 CH\ _ • 0 R I (§)

2N

H

H

H

3

5

c

2

n

Ο

H

CH Ο ,C H ÇOOH 2 \ c _ ' /-icSoiBu H I — «g V \ — - 3 (S)41%ee

5

C 0 0 l B u H

C

COOH

A - C H 3 —

H

CH Ο CH >- f c^B 1-/ > \ — (Pd) —

CHEMISTRY

Ο COOiBu

3

2

IN

3

CHo

2

^Ο R, (,„ S) x

.NH \ H / (S) V \ H (Pd),η 75 - 99%(§-S) high optical and diastereomeric purity(14) (Scheme 9). Benzil monoxime was similarly hydrogenated using a palladium catalyst to form erythro diphenylethanolamine (15). If the conformation of the substrate in the reaction is planar, as expected from steric and elec­ tric considerations, the resulting hydrogénation product should be the threo isomer(16) (Scheme 10). In Scheme 9 C H^ 6

E

χ

#

6

( § )

C *5

/

\

6

2

H

/N-OH

(Pd) n

H 0

Ç H

6

""/~\ H 0

P d / C

5

Ρα/C'

6

5

5

H-C-OH (S) erythro H-C-NH (R) 85 - 90 % (S-R) I 2

65 H

Scheme 10 Ç H 6

5

HO-C-H + I H-C-NH CH racemic threo isomer 2

6

Eliel and Otsuka; Asymmetric Reactions and Processes in Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

5

HARADA

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11.

Heterogeneous

Asymmetric

Catalytic

Hydrogénation

175

f a c t , however, the r e s u l t i n g d i p h e n y l e t h a n o l a m i n e was found t o be p r e d o m i n a n t l y the e r y t h r o form. S i m i l a r r e s u l t s have been r e p o r t e d i n the s y n t h e s i s o f t h r e o n i n e , p h e n y l s e r i n e and ephedrine(17-21) (Scheme 11). I n 1953 Chang and Hartung p r o p o s e d a mechanism f o r the hydrogénation o f d i k e t o n e monoxime which e x p l a i n s the f o r m a t i o n o f a s i n g l e racemic m o d i f i c a t i o n ( e r y t h r o form) by the f o r m a t i o n o f a r i g i d r i n g - l i k e s t r u c t u r e w i t h the c a t a l y s t (22) . The e x p l a n a t i o n c o u l d be r e g a r d e d as a c h e l a t i o n h y p o t h e s i s p r e c e d i n g t h e present generalized c h e l a t i o n hypothesis i n c a t a l y t i c hydrogénation. R e c e n t l y , s u p p o r t f o r the c h e l a t i o n h y p o t h e s i s was o b t a i n e d by examining the i n f r a r e d d i c h r o i s m o f t h e s u b s t r a t e a d s o r b e d on a m e t a l s u r f a c e u s i n g t h e h i g h s e n s i t i v i t y r e f l e c t i o n method(23-26). I n t h e s e i n v e s t i g a t i o n s the o r i e n t a t i o n o f the s u b s t r a t e on the m e t a l s u r f a c e i s j u s t as assumed by the c h e l a t i o n h y p o t h e s i s . I t was found t h a t the OH, C=0, NH~, =NOH groups i n t e r a c t w i t h the m e t a l s u r f a c e and the s u b s t r a t e s s t a n d on the c a t a l y s t s u r f a c e v e r t i c a l l y . The o b s e r v a t i o n o f the i n f r a r e d d i c h r o i s m i s c o n s i d e r e d t o be a d i r e c t p h y s i c a l e v i d e n c e f o r the c h e l a t i o n h y p o t h e s i s (Scheme 12). Scheme 11 C*H 6 5 rt

C

CH

H

, 6 5 •C

COOEt /

3

HO-N

•c w

C//

w

6

6

N-OH

,0 M

C H

n

65

C »5

M

M

CH

5

CH-

3

F^COOEt

->c—c'

C H5

.COOEt -C \ NH-Ac

6

H

HO

/

\

0

N-CH,

NH-Ac M

M

M Scheme 12 C

H 6

5 S

CH, CH

OH

2

H-^C-C

7

,0

H.V,•C—C J +/ H N^

\ .0'

3

HO

7

\ (Ni) C Hc "COOEt fi

(Ni), CH-

C—c

C—C

// 0

OEt

% ,ΝΟΗ *(Pd)'

//

% ,0

HON^ N

(Pdf'-

Eliel and Otsuka; Asymmetric Reactions and Processes in Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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176

ASYMMETRIC REACTIONS AND PROCESSES IN CHEMISTRY

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Eliel and Otsuka; Asymmetric Reactions and Processes in Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1982.