Potential Energy Surface of Olefin Hydrogenation by Wilkinson Catalyst

Chapter 6 Potential

Energy

Hydrogenation

by

Surface

of

Wilkinson

Olefin Catalyst

Comparison Between trans and cis Intermediates

Downloaded by COLUMBIA UNIV on July 23, 2013 | http://pubs.acs.org Publication Date: June 8, 1989 | doi: 10.1021/bk-1989-0394.ch006

Nobuaki Koga and Keiji Morokuma Institute for Molecular Science, Myodaiji, Okazaki 444, Japan The results of recent theoretical investigation with the ab initioMOmethod on the full catalytic cycle is presented. The catalytic cycle studied is for olefin hydrogenation by the Wilkinson catalyst. We have determined with the ab initio energy gradient method the structures of the transition states as well as the intermediates of the Halpern mechanism in which all the intermediates have trans phosphines. The potential energy profile thus obtained supports the Halpern mechanism and gives evidence on the effectiveness of the Wilkinson system as a catalyst. A new mechanism, more recently proposed, considers that intermediates with cis phosphines, in contrast to trans in the Halpern mechanism, play an essential role. Our calculation indicates that energies of cis inter mediates are high, and does not support the cis mechanism for sterically unhindered olefins. However, when steric effects inhibit reactions of trans intermediates, the cis mechanism may become possible and would exhibit the kinetics which is quite dif ferent from that of the Halpern mechanism. Recent progress of methodology of quantum chemistry and technology of electronic computer is making it possible for quantum chemists to challenge the chemistry of d and/or f electrons. Now, such efforts have covered a full catalytic cycle as well as structures of complexes and intermediates and elementary organometallic reactions (I). It has been our goal to design a catalytic system theoreti cally. To the end of this goal, we have so far analyzed the organometallic reactions by using the ab initio MO calculations. Recently, we have completed the theoretical study of the catalytic cycle of hydrogénation by the Wilkinson catalyst (2), of which mechanism has been proposed by Halpem (3). This catalytic cycle shown in Scheme 1 consists of oxidative addition of H~, coordination of olefin, olefin insertion, isomerization, and reductive élimina0097-6156/89/0394-0077$06.00/0 o 1989 American Chemical Society

In The Challenge of d and f Electrons; Salahub, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


78

THE CHALLENGE OF d AND f ELECTRONS

U. c |

C

>SL

L

H

W C I A

l

U„

2

^

c

1

H

I , *H %

A

l

C = C H

c-c c=c

Η Rh'

^Rrt

=

""Rh Cl^ >L

Rh Cl^ I >L S

-Ho

1

fast / - x c

Η

rate determing step

c

Η H

[RhCI(H)L (Ç-ÇH)]

'Rh

3

cr I s

L

n

ι H

dominant catalytic cycle Scheme 1



6.

KOGA & MOROKUMA

Potential Energy Surface of Olefin Hydrogénation

t i o n o f a l k a n e . The r a t e - d e t e r m i n i n g s t e p has been b e l i e v e d t o be the o l e f i n i n s e r t i o n s t e p . We u s e d PH^ i n s t e a d o f PR~ and C^H^ as a model o f o l e f i n . The model c a t a l y t i c c y c l e we s t u d i e d i s shown i n Scheme 2. A l l the e q u i l i b r i u m and t r a n s i t i o n s t a t e s t r u c t u r e s were o p t i m i z e d by the RHF energy g r a d i e n t method. W h i l e two p h o s p h i n e s a r e always t r a n s t o each o t h e r i n a l l the i n t e r m e d i a t e s i n the H a l p e m mechanism, Brown e t a l . have v e r y r e c e n t l y p r o p o s e d a d i f f e r e n t mechanism, i n which the c i s b i p h o s p h i n e i n t e r m e d i a t e s p l a y an e s s e n t i a l r o l e ( 4 ) . T h e i r m o l e c u l a r m o d e l i n g c a l c u l a t i o n s where the van der Waals energy i s c a l c u l a t e d between s u b s t i t u t e d o l e f i n s and the Rh fragment w i t h b u l k y t r a n s p h o s p h i n e s have s u g g e s t e d t h a t when the s u b s t i t u e n t s on the o l e f i n a r e b u l k y , the s t e r i c r e p u l s i o n i s too l a r g e f o r the o l e f i n t o c o o r d i n a t e . T h e i r NMR e x p e r i m e n t s have shown the e x i s t e n c e o f the f o l l o w i n g e q u i l i b r i u m ( E q u a t i o n 1) i n w h i c h an i n t e r m e d i a t e w i t h a p a i r o f c i s phosphines can be formed. I n f a c t , ^ R h C l C P R ^ ) ^ has been d e t e c t e d i n the c a t a l y t i c system.

H

H

Phi

Rh-

•pPh, b

PPh

I

.ΛΝ

ΡΡΠ

'

a

(1)

a

- Rh ' .

3

+ PPh

j ^ pph CI

3

b

Cl

3

Based on these r e s u l t s , t h e y have s u g g e s t e d t h a t the i n t e r m e d i a t e s o f the c a t a l y t i c system have two c i s p h o s p h i n e s . I n Scheme 3 i s shown the new mechanism. I n t h i s Scheme, the key s t e p i s i s o m e r i z a t i o n (Equation 2), t r a n s - H R h C l ( P R ) t o c i s - H R h C l ( P R > presum a b l y through p s e u d o r o t a t i o n . 2

PPh

Rri j PPh

2

2

I

Η

.

3

3

2

H

3

I .,Λ CI

3

Η —

,Λ ™3 Ρ

Rh. j ^PPh CI

(2) 3

Then, the o l e f i n i n s e r t i o n and the r e d u c t i v e e l i m i n a t i o n take p l a c e from the r e s u l t a n t c i s b i p h o s p h i n e complex. I n t h i s a r t i c l e , we w i l l compare the e n e r g e t i c s o f the 'conven t i o n a l ' H a l p e m mechanism w i t h t h a t o f the Brown mechanism. The b a s i s f u n c t i o n s u s e d a r e the 3-21G f o r e t h y l e n e and h y d r i d e s , the ST0-2G f o r ' s p e c t a t o r ' l i g a n d s , PH^ and CI, and v a l e n c e double z e t a b a s i s f u n c t i o n s f o r Rh w i t h e f f e c t i v e c o r e p o t e n t i a l r e p l a c i n g the c o r e e l e c t r o n s (up t o 4p) (5a.b.6). I n a d d i t i o n , we c a r r i e d out the MP2 c a l c u l a t i o n s a t s e l e c t e d , R H F - o p t i m i z e d s t r u c t u r e s w i t h a l a r g e r b a s i s s e t , which c o n s i s t s o f u n c o n t r a c t e d (3s,3p,4d) f u n c t i o n s from the above v a l e n c e DZ s e t f o r Rh, 4-31G f o r the e t h y l group, (10s,7p)/[3s2p] f o r Ρ and CI, and ( 4 s ) / [ 3 s ] f o r the h y d r i d e s (5ce ) . The b a s i s s e t u s e d i s l i m i t e d and the e l e c t r o n c o r r e l a t i o n t a k e n i n t o account f o r a few c r i t i c a l s t e p s i s m i n i m a l . T h e r e f o r e ,


79


C H 2

6

A*-* Reductive e l i m i n a t i o n ^ / ^

J. Rh—Cl ^

H

2

I Oxidati

CH -Rh-Cl 2

H-CH

2

L


t

H 5b

l^-CH^CHz

Isomerization

Olefin coordination

g

L u Olefin insertion

Scheme 2

Scheme 3


6.

KOGA & MOROKUMA

Potential Energy Surjhce of Olefin Hydrogénation

the r e s u l t s p r e s e n t e d quantitative .

h e r e s h o u l d be

considered

t o be

semi-


H a l p e m mechanism S i n c e our c a l c u l a t i o n s on the H a l p e m mechanism have been p u b l i s h e d (2)» we w i l l g i v e a b r i e f summary f o r comparison i n a s u c c e e d i n g s e c t i o n . The p o t e n t i a l energy p r o f i l e shown i n F i g u r e 1 i s cons t r u c t e d from the e n e r g e t i c s o f the e l e m e n t a r y r e a c t i o n s i n v o l v e d i n the H a l p e m mechanism. The o p t i m i z e d s t r u c t u r e s a r e shown i n F i g u r e 2. The f i r s t s t e p o f the H^ o x i d a t i v e a d d i t i o n i s e x o t h e r m i c and l e a d s t o the d i h y d r i d e complex 3 . D u r i n g t h i s s t e p , t h e r e may be an H^ complex 2 from which o x i d a t i v e a d d i t i o n t a k e s p l a c e w i t h a l m o s t no a c t i v a t i o n b a r r i e r . The ethylene c o o r d i n a t i o n that follows r e q u i r e s no a c t i v a t i o n energy. The r e s u l t a n t e t h y l e n e d i h y d r i d e complex 4 i s i n the v a l l e y o f the p o t e n t i a l energy s u r f a c e o f the c a t a l y t i c c y c l e . E t h y l e n e i n s e r t i o n r e q u i r e s a much h i g h e r a c t i v a t i o n energy o f 18 k c a l / m o l and i s endothermic by 16 k c a l / m o l a t the RHF l e v e l . The t r a n s e t h y l h y d r i d e complex, the d i r e c t p r o d u c t o f e t h y l e n e i n s e r t i o n , i s u n s t a b l e due t o the c i s e f f e c t o f CI t o be mentioned below and the t r a n s e f f e c t o f H and o 5 ' T h e r e f o r e i s o m e r i z a t i o n takes p l a c e t o g i v e more s t a b l e e t h y l h y d r i d e comp l e x e s , w h i c h have e t h y l and h y d r i d e c i s t o each o t h e r and a r e the s t a r t i n g p o i n t o f the f i n a l reductive e l i m i n a t i o n step. This i s o m e r i z a t i o n p r o c e e d s t h r o u g h h y d r i d e and c h l o r i d e m i g r a t i o n . The f i n a l r e d u c t i v e e l i m i n a t i o n step r e q u i r e s a s u b s t a n t i a l energy b a r r i e r o f 15 k c a l / m o l . The p o t e n t i a l energy p r o f i l e i s smooth w i t h o u t e x c e s s i v e b a r r i e r s and too s t a b l e i n t e r m e d i a t e s which would b r e a k the sequence o f s t e p s . The r a t e - d e t e r m i n i n g s t e p i s found t o be o l e f i n i n s e r t i o n f o l l o w e d by i s o m e r i z a t i o n , s u p p o r t i n g the H a l p e m mechanism. I s o m e r i z a t i o n o f the e t h y l h y d r i d e complex i s an i m p o r t a n t p a r t o f the r a t e - d e t e r m i n i n g s t e p . These two r e a c t i o n s , e x o t h e r m i c o v e r a l l , has an o v e r a l l b a r r i e r h e i g h t o f about 20 k c a l / m o l . The t r a n s e t h y l h y d r i d e complex, the p r o d u c t o f e t h y l e n e i n s e r t i o n , may n o t be a l o c a l minimum (per MP2 c a l c u l a t i o n ) and t h e s e two s t e p s may w e l l be a combined s i n g l e s t e p . The a c t i v a t i o n b a r r i e r o f r e d u c t i v e e l i m i n a t i o n , though s u b s t a n t i a l , i s s m a l l e r t h a n t h a t o f the r e v e r s e o f the r a t e d e t e r m i n i n g s t e p ( i s o m e r i z a t i o n and 0-hydrogen e l i m i n a t i o n ) . T h i s i s a v e r y i m p o r t a n t r e q u i r e m e n t o f a good o l e f i n hydrogénation catalyst. I f t h i s r e v e r s e r e a c t i o n i s easy, i t would l e a d t o u n d e s i r a b l e o l e f i n i s o m e r i z a t i o n . F o r i n s t a n c e , i n the same hydrogén a t i o n c y c l e c a t a l y z e d by a Pt system, we f o u n d t h a t the r a t e determining step i s r e d u c t i v e e l i m i n a t i o n r a t h e r than o l e f i n i n s e r t i o n . T h i s p o t e n t i a l energy p r o f i l e i s e x p e c t e d t o g i v e o l e f i n i s o m e r i z a t i o n through s u c c e s s i v e o l e f i n i n s e r t i o n / ^ - h y d r o g e n e l i m i n a t i o n . Thus the Pt complex i s n o t a good c a t a l y s t f o r o l e f i n hydrogénation. On the o t h e r hand, the p o t e n t i a l p r o f i l e o f the W i l k i n s o n c a t a l y s t i n d i c a t e s an e f f i c i e n t hydrogénation w i t h o u t isomerization. c

H

I t i s i m p o r t a n t , as mentioned above, t h a t the o l e f i n i n s e r t i o n i s r a t e - d e t e r m i n i n g . T h e r e f o r e , we have compared the r e a c t i o n e n e r g e t i c s between H R h C l ( P H ) ( C H ) and H R h H ( P H ) ( C H ) , and 2

3

2

2

4

2

3

2

2

4


81


-40

-30

-20 -

-10

0

kcal/mole|

Olefin insertion

Isomerization

Reductive elimination

F i g u r e 1. P o t e n t i a l energy p r o f i l e o f the e n t i r e c a t a l y t i c c y c l e i n the H a l p e m mechanism f o r o l e f i n h y d r o g é n a t i o n , i n k c a l / m o l a t the RHF l e v e l , r e l a t i v e t o l+C^H^+H^. Numbers i n p a r e n t h e s e s a r e the MP2 energy a t the RHF o p t i m i z e d g e o m e t r i e s , r e l a t i v e t o 4 .

coordination

Oxidative addition Olefin

TS(5b+1)


ο

00

KOGA & MOROKUMA


H-? _. 2-280 I P s t-tuw 0-8631

H ;

A6

1

164-8

1.514 \ 2.302 ^Rhs—CI \-i ^— 145.5 M

H


Π

^ C = C ^ H

H ,

2.600 χ 1 2-575

n

2-282

5 7 2

^/.:R[>—Cl 160-8 TS(2-3)

W 2 C

/

« * ;

^ f L ^ R f t — CI 83.ΐΊ^925 2·3Α2 Η,1.504

/

^ t e o

Π

H1-587

TS(4-5)

H 1.081

1 1 8 1

Ha Hi

'l547

| 79-9\ 2.137

H

91>/W1-621

2 7 2

N

H

1026* 2.O6I 17^17^

' · / 115.3 H 751

H

H

2-247

V 2.095^-:

2

87-3 Α - Ο 0 2 Ό u^i-Rh^—a n

1.558

1.553/Η^λ®:- α VH

HU72 5

b

2.313

5a

1 H

Η

Λ

"7.2^\ .OOS Îoœ

TS(5+5a)

5

H

Γ1539

C 2-185 _165.1

4 ? ^ ^ —

CI

1-524 TS(5b+1)

F i g u r e 2. O p t i m i z e d s t r u c t u r e s ( i n À and deg) o f some i m p o r t a n t s p e c i e s . TS(2->3), f o r i n s t a n c e , denotes t h e t r a n s i t i o n s t a t e c o n n e c t i n g 2 and 3. Though p r a c t i c a l l y a l l t h e g e o m e t r i c a l p a r a m e t e r s were o p t i m i z e d , o n l y e s s e n t i a l v a l u e s a r e shown. Two PH3's, one above and one below the p l a n e o f paper, a r e o m i t t e d for c l a r i t y .



84

f o u n d t h a t , i n the former complex, the weak t r a n s i n f l u e n c e o f CI makes the Rh-H bond t o be b r o k e n s t r o n g e r . I n a d d i t i o n the s t r o n g CI c i s e f f e c t makes the Rh-C bond t o be formed weaker, resulting in e n d o t h e r m i c e t h y l e n e i n s e r t i o n . These two e f f e c t s o f CI combined appears t o be e s s e n t i a l t o make e t h y l e n e i n s e r t i o n the r a t e determining step. firown, mechanism The f i r s t p o i n t o f d i f f e r e n c e o f the Brown mechanism from the H a l p e r n mechanism i s i s o m e r i z a t i o n o f H R h C l ( P H ~ ) . T h e r e f o r e , we have i n v e s t i g a t e d the s t a b i l i t y o f isomers o f R R h C l ( P H ) , 3. The o p t i m i z e d s t r u c t u r e s o f H R h C l ( P H ~ ) a r e shown i n F i g u r e 3. The most s t a b l e isomer i s found t o be 3, the t r a n s i n t e r m e d i a t e o f the H a l p e r n mechanism. A l l the o p t i m i z e d s t r u c t u r e s b u t 3 a r e n e a r l y square p y r a m i d a l (though o p t i m i z a t i o n was done w i t h o u t symmetry restriction). The most s t a b l e square p y r a m i d a l isomer i s 3a w i t h a p i c a l H and b a s a l c i s p h o s p h i n e s . 3b w i t h a p i c a l p h o s p h i n e and b a s a l c i s h y d r i d e s i s n e x t . The r e m a i n i n g t h r e e i s o m e r s , 3c, 3d, and 3e a r e much more u n s t a b l e ; the e n e r g i e s r e l a t i v e t o 3 a r e 33, 37 and 39 k c a l / m o l , r e s p e c t i v e l y . Two h y d r i d e s w i t h s t r o n g t r a n s i n f l u e n c e a r e l o c a t e d t r a n s t o each o t h e r i n 3c. T h i s makes 3c 12 k c a l / m o l l e s s s t a b l e t h a n 3b i n which two h y d r i d e s a r e c i s . The l e a s t s t a b l e i s o m e r s , 3d and 3e, have a p i c a l CI. Comparison o f the s t a b i l i t y among the isomers o f 3 l e a d s t o the o r d e r o f a p i c a l p r e f e r e n c e : H>PH^>C1. H w i t h the s t r o n g e s t t r a n s i n f l u e n c e p r e f e r s the a p i c a l p o s i t i o n t h a t i s t r a n s t o the v a c a n t s i t e , and the most weakly t r a n s - i n f l u e n c i n g CI a t the a p i c a l position g i v e s the most u n s t a b l e isomers o f 3d and 3e. 3b and 3c a r e inbetween, i n a c c o r d w i t h the s t r e n g t h o f PH~ t r a n s i n f l u e n c e . S i n c e 3a and 3b a r e low i n energy, we have i n v e s t i g a t e d i s o m e r i z a t i o n from 3 t o 3a and 3b. Brown e t a l . have p r o p o s e d t h a t the c i s i n t e r m e d i a t e o f the c a t a l y t i c c y c l e i s 3b i n w h i c h one o f the b u l k y p h o s p h i n e s i s t r a n s t o o l e f i n and thus the v a c a n t c o o r d i n a t i o n s i t e i s l e s s crowded. There a r e two p o s s i b l e pathways f o r i s o m e r i z a t i o n o f 3 as shown i n Scheme 4. The i n t e r m e d i a t e s o f the second pathway a r e u n s t a b l e 3d and 3c, and i t i s u n l i k e l y t h a t i s o m e r i z a t i o n t a k e s p l a c e t h r o u g h them. The e a s i e r i s o m e r i z a t i o n pathway i s through 3b t o 3a. The t r a n s i t i o n s t a t e f o r PH^ m i g r a t i o n c o n n e c t i n g 3 and 3b have been l o c a t e d , as shown i n F i g u r e 3, w i t h the a c t i v a t i o n b a r r i e r f o r i s o m e r i z a t i o n from 3 t o 3b o f 27 k c a l / m o l (See Scheme 4 ) . T h e r e f o r e , one can c o n c l u d e i s o m e r i z a t i o n from 3 t o 3b o r 3a i s r a t h e r d i f f i c u l t ( c f . 18 k c a l / m o l , the a c t i v a t i o n energy o f the r a t e - d e t e r m i n i n g s t e p i n the H a l p e r n mechanism a t the same l e v e l o f c a l c u l a t i o n ) . S e t t i n g a s i d e t h i s h i g h a c t i v a t i o n b a r r i e r f o r a moment, the r e m a i n i n g s t e p s o f the c a t a l y t i c c y c l e i n the Brown mechanism w i l l be d i s c u s s e d . As shown i n the energy p r o f i l e o f the H a l p e r n mechanism, the e l e m e n t a r y r e a c t i o n s i n v o l v e d h e r e a r e e x p e c t e d t o be v e r y easy, and thus we have j u s t d e t e r m i n e d the s t r u c t u r e s and energies of intermediates but not of t r a n s i t i o n s t a t e s (Figure 5). The r e l a t i v e e n e r g i e s o f the i n t e r m e d i a t e s , shown i n F i g u r e 4, would be a good i n d i c a t o r o f the b a r r i e r o f each e l e m e n t a r y r e a c t i o n c o n n e c t i n g them; an endothermic r e a c t i o n r e q u i r e s a l a r g e a c t i v a t i o n energy and the a c t i v a t i o n b a r r i e r o f an e x o t h e r m i c s t e p i s low. 2

2

2


2

3

2

2




KOGA & MOROKUMA




F i g u r e 3. O p t i m i z e d s t r u c t u r e s ( i n Â and deg) o f isomers o f H ^ R h C l i P H ^ ) ^ and the 3-*3b t r a n s i t i o n s t a t e , and t h e i r e n e r g i e s ( i n k c a l / m o l ) r e l a t i v e t o 3.



H

2

9

0

complex isomerization

Olefin coordination

Dihydride

olefin insertion

Isomerization

complex

Ethyl hydride

Reductive elimination

F i g u r e 4. P o t e n t i a l energy p r o f i l e o f t h e c a t a l y t i c c y c l e i n t h e Brown mechanism f o r o l e f i n hydrogénation, i n k c a l / m o l a t t h e RHF l e v e l , r e l a t i v e t o 1+C H.+H .

Oxidative addition




F i g u r e 5. O p t i m i z e d s t r u c t u r e s ( i n Â and deg) o f some i m p o r t a n t s p e c i e s i n the Brown mechanism.


6. KOGA & MOROKUMA


E t h y l e n e c o o r d i n a t i o n t o 3b, t h e Brown's i n t e r m e d i a t e , g i v e s 4a w h i c h i s h i g h e r i n energy t h a n 4 by 6 k c a l / m o l . Olefin insertion o f 4a c a n l e a d t o 5c o r 5d. S i n c e 5c i s much more s t a b l e t h a n 5d, o l e f i n i n s e r t i o n g i v i n g 5c would take p l a c e e x c l u s i v e l y . The i n s t a b i l i t y o f 5d w i t h an a p i c a l CI i s s i m i l a r t o t h a t o f 3d and 3e d i s c u s s e d above. The e t h y l group and t h e h y d r i d e i n 5c a r e c i s t o each o t h e r and thus r e d u c t i v e e l i m i n a t i o n might take p l a c e d i r e c t l y w i t h o u t i s o m e r i z a t i o n . However, r e d u c t i v e e l i m i n a t i o n from a d fivec o o r d i n a t e complex would f a v o r a t r a n s i t i o n s t a t e where t h r e e l i g a n d s b u t C H,. and H a r e i n the same p l a n e , as shown below.


9

\

L

The r e a s o n f o r t h i s p r e f e r e n c e i s t h a t t h e d o n a t i o n and back d o n a t i o n between a deformed a l k a n e and a m e t a l fragment shown below i s e x p e c t e d t o f a c i l i t a t e easy bond exchange.

C (

^DËO ® ®

c l

donation

back-donation

Therefore, p r i o r to reductive e l i m i n a t i o n , i s o m e r i z a t i o n should t a k e p l a c e from 5c t o e t h y l h y d r i d e complexes w h i c h have H o r C^H, as an a p i c a l group, as shown below.

Et

Rh-

H

Rh-

y Et E t h y l m i g r a t i o n from 5c l e a d s t o 5c i t s e l f , and h y d r i d e m i g r a t i o n g i v e s 5d, an u n s t a b l e i n t e r m e d i a t e ; n e i t h e r o f t h e s e g i v e s a p i c a l H o r C^H^. The two r e m a i n i n g m i g r a t i o n s g i v e a more s t a b l e e t h y l h y d r i d e complexes and they have e i t h e r an a p i c a l H o r C^H^; CI m i g r a t i o n l e a d s t o s t a b l e 5e and PH~ m i g r a t i o n r e s u l t s i n 5a w i t h t r a n s p h o s p h i n e s , the i n t e r m e d i a t e or t h e H a l p e r n mechanism. The t r a n s i t i o n s t a t e f o r r e d u c t i v e e l i m i n a t i o n o f 5e t o g i v e l a has been determined, as shown i n F i g u r e 5, and i t has an a c t i v a t i o n energy o f 14.8 k c a l / m o l . 5a i s more s t a b l e t h a n 5e by 5 k c a l / m o l and the a c t i v a t i o n energy f o r r e d u c t i v e e l i m i n a t i o n from 5a t o r e g e n e r a t e 1 i s c a l c u l a t e d t o be about 15 k c a l / m o l , comparable w i t h t h e 5e-+la b a r r i e r . T h e r e f o r e , the system i s e x p e c t e d t o r e t u r n t o t h e


89

90


H a l p e r n mechanism, i f i t i s n o t p r e v e n t e d f o r some r e a s o n steric).


Comparison between two

(eg.

mechanisms

I n the Brown mechanism, s e t t i n g a s i d e the h i g h energy r e q u i r e d f o r i s o m e r i z a t i o n from 3 t o 3b, the f i n a l s t e p o f the r e d u c t i v e e l i m i n a t i o n would r e q u i r e the h i g h e s t a c t i v a t i o n energy. O l e f i n i n s e r t i o n , the r a t e - d e t e r m i n i n g s t e p i n the H a l p e r n mechanism, i s an easy p r o c e s s i n t h i s mechanism. The k i n e t i c s o f the Brown mechanism i s thus e x p e c t e d t o be c o m p l e t e l y d i f f e r e n t from t h a t o f t h e H a l p e r n mechanism. T h e r e f o r e , i n the c a s e s i n which o l e f i n i n s e r t i o n has been f o u n d t o be r a t e - d e t e r m i n i n g , the H a l p e r n mechanism i s c l e a r l y more c o n s i s t e n t and a c c e p t a b l e . The above f e a t u r e o f the Brown mechanism t h a t r e d u c t i v e e l i m i n a t i o n i s more d i f f i c u l t t h a n o l e f i n i n s e r t i o n may be r e l a t e d t o the n a t u r e o f c a t a l y s t h a v i n g a c h e l a t i n g b i d e n t a t e l i g a n d such as DIPHOS. H a l p e r n have a l s o i n v e s t i g a t e d the hydrogénation ( E q u a t i o n 3) ( 2 ) , where i s o m e r i z a t i o n from the t r a n s - t o c i s b i p h o s p h i n e complex i s n o t n e c e s s a r y .

( s )

Rh' *

S ( S )

ï + COOCH, > PnCH,CH

(3)

NHCOCHj

They have f o u n d t h a t a t the low temperature, r e d u c t i v e e l i m i n a t i o n i s r a t e - l i m i t i n g (ΔΗ -17.0 k c a l / m o l - 4 0 ° C ) . The p r e s e n t c a l c u l a t i o n s u s i n g PH^ as the phosphine and e t h y l e n e as the o l e f i n , however, does n o t e x c l u d e the p o s s i b i l i t y o f the c i s mechanism c o m p l e t e l y , s i n c e the s t e r i c f a c t o r has n o t been t a k e n i n t o a c c o u n t . The c i s mechanism might be a c c e s s i b l e i n the c a s e where o l e f i n i s too b u l k y t o c o o r d i n a t e t o the t r a n s phosphine complex. U s i n g the above c a l c u l a t i o n s as a g u i d e , h e r e we c o n s i d e r q u a l i t a t i v e l y what i s e x p e c t e d t o take p l a c e when a v e r y b u l k y o l e f i n i s h y d r o g e n a t e d . L e t us assume t h a t i n such a c a s e the e q u i l i b r i u m ( E q u a t i o n 1) g e n e r a t e s the c i s i n t e r m e d i a t e . Then, the r e a c t i o n r o u t e w i l l pass t h r o u g h 3b-+4a-»5c, each o f w h i c h i s s t e r i c a l l y n o t too crowded. I s o m e r i z a t i o n o f 5c i s n o t a l l o w e d t o l e a d t o 5a, w h i c h has the b u l k y a l k y l group c i s t o two p h o s p h i n e s and i s overcrowded. Thus i s o m e r i z a t i o n o f 5c has t o l e a d t o 5e, w h i c h i s i n t r i n s i c a l l y (without s t e r i c e f f e c t ) only s l i g h t l y l e s s s t a b l e but s t e r i c a l l y l e s s crowded t h a n 5a. R e d u c t i v e e l i m i n a t i o n o f 5e w i l l r e q u i r e an a c t i v a t i o n energy comparable t o t h a t o f 5a->l and g e n e r a t e cis-RhCl(PH ) , l a . 3

2

There a r e two p o s s i b i l i t i e s i n the r e a c t i o n s o f l a . The f i r s t i s t h a t l a i s o m e r i z e s t o 1 due t o the s t e r i c r e p u l s i o n and t h a t the same r e a c t i o n p a t h l-*2-*3->3b i s f o l l o w e d . The second p o s s i b i l i t y i s t h a t H^ o x i d a t i v e a d d i t i o n t o l a t a k e s p l a c e t o g i v e d i r e c t l y 3a and thus i n the subsequent c a t a l y t i c c y c l e s i n t e r m e d i a t e s always have c i s p h o s p h i n e s . O l e f i n c o o r d i n a t i o n t o 3a i s p r o h i b i t e d b e c a u s e o f the s t e r i c r e p u l s i o n between the b u l k y o l e f i n and two b u l k y phos p h i n e s c i s t o the o l e f i n . Thus 3a-»3b i s o m e r i z a t i o n has t o take p l a c e b e f o r e the c a t a l y t i c c y c l e p r o c e e d s .


6. KOGA & MOROKUMA


91


One can consider that the Halpern mechanism and the cis mech anism are two extremes. The Halpern mechanism is most widely accepted and our ab initio MO calculations support this from the point of view of intrinsic electronic energy. However, there may be cases where the steric effect overshadows the electronic effect. There may also be cases where both effects are important. It may be, as Collman et al. said, that "this multistep reaction is very complicated. Like a chameleon, the dominant reaction mechanism changes when the nature of the catalyst, the ligands, or the substrate is altered" (8). Conclusions In this work, we have compared the potential energy profiles of the model catalytic cycle of olefin hydrogénation by the Wilkinson catalyst between the Halpern and the Brown mechanisms. The former is a well-accepted mechanism in which all the intermediates have trans phosphines, while in the latter, proposed very recently, phosphines are located cis to each other to reduce the steric repulsion between bulky olefin and phosphines. Our ab initio calculations on a sterically unhindered model catalytic cycle have shown that the profile for the Halpern mechanism is smooth without too stable intermediates and too high activation barrier. On the other hand, the key cis dihydride intermediate in the cis mechanism is electron ically unstable and normally the sequence of elementary reactions would be broken. Possible sequences of reactions can be proposed from our calculation, if one assumes that steric effects of bulky olefin substituents prohibits some intermediates or reactions to be realized. Literature Cited. 1. Koga, N.; Morokuma, K. Tod.Phys.Organomet.Chem., in press. 2. (a) Koga, N.; Daniel, C.; Han, J.; Fu, X.Y.; Morokuma, K. J.Am.Chem.Soc., 1987, 109, 3455. (b) Daniel, C.; Koga, N.; Han, J.; Fu, X.Y.; Morokuma, K. J.Am.Chem.Soc., 1988, 110, 3773. 3. (a) Halpern, J.; Wong, C.S. J.Chem.Soc.Chem.Commun., 1973, 629. (b) Halpern, J. In Organotransition Metal Chemistry: Ishii, Y.; Tsutsui, M., Eds.; Plenum: New York, 1975; p109. (c) Halpern, J.; Okamoto, T.; Zakhariev, A. J.Mol.Catal., 1976, 2, 65. 4. Brown, J.M.; Evans, P.L.; Lucy, A.R. J . Chem. Soc. Perkin Trans. II, 1987, 1589. 5. (a) Binkley, J.S.; Pople, J.A.; Hehre, W.J. J.Am.Chem.Soc., 1980, 102, 939. (b) Hehre, W.J.; Stewart, R.F.; Pople, J.A. J.Chem.Phys., 1969, 51, 2657. (c) Ditchfield, R.; Hehre, W.J.; Pople, J.A. J.Chem. Phys., 1971, 54, 724. (d) Huzinaga, S.; Andzelm, J.; Kłobukowski, M.; Radzio-Andzelm, E.; Sakai, Y.; Tatewaki, H. Gaussian Basis Sets for Molecular Calculations: Elsevier: Amsterdam, 1984. 6. Hay, P.J.; Wadt, W.R. J . Chem. Phys., 1985, 82, 270. 7. Chan, A.S.C.; Halpern, J . J.Am.Chem.Soc.. 1980, 102, 838. 8. Collman, J.P.; Hegedus, L.S.; Norton J.R.; Finke, R.G. Principles and Applications of Organotransition Metal Chemistry: University Science Books: Mill Valley, 1987; p.535. RECEIVED

January 18, 1989


Potential Energy Surface of Olefin Hydrogenation by Wilkinson Catalyst

Recommend Documents