The Effect of Chelating Diphosphine Ligands on Homogeneous

portant since it is a model for the discreet processes in other catalytic reactions that use ..... Heptanal (1.85). 2-Ethylbutanal (60.8). °0.02 to 0...
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The Effect of Chelating Diphosphine Ligands on Homogeneous Catalytic Decarbonylation Reactions Using Cationic Rhodium Catalysts D. H. D O U G H T Y , M. P. A N D E R S O N , A. L. C A S A L N U O V O , M. F. M c G U I G G A N , C. C. T S O , H . H . WANG, and L. H. P I G N O L E T Department of Chemistry, University of Minnesota, Minneapolis, M N 55455

Catalytic decarbonylation of aldehydes has been studied using mono- and bisdiphosphine complexes of Rh(I). The catalyst [Rh(dppp) ]BF , where dppp = 1,3bis(diphenylphosphino)propane, decarbonylates aldehydes homogeneously with rates that are more than two orders of magnitude faster than with Rh(PPh ) Cl. Additionally, good catalytic activities were measured under mild thermal conditions (100 turnovers hr-1 for benzaldehyde at 150°C) and the reactions are highly selective. The catalysts are stable for days and turnovers in excess of 100,000 have been achieved. Experiments on a variety of aldehydes have established the general usefulness of this reaction in organic synthesis. Kinetic measurements, P-31 NMR spectroscopy, and X-ray diffraction studies have permitted some general mechanistic conclusions. For the catalytic decarbonylation of benzaldehyde, a rapid pre-equilibrium is established (PhCHO)Rh(dppp) +) prior to (Rh(dppp)2+ + PhCHO 2

4

3 3

2

oxidative addition, which is the rate-determining step. A mechanistic understanding of this reaction is important since it is a model for the discreet processes in other catalytic reactions that use organometallic catalysts.

D

ecarbonylation of aldehydes and acid halides is an important syn­ thetic reaction (1, 2) and using various transition-metal c o m ­ plexes as stoichiometric or catalytic reagents for this process has 0065-2393/82/0196-0065$05.00/0 © 1982 American Chemical Society Alyea and Meek; Catalytic Aspects of Metal Phosphine Complexes Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

METAL PHOSPHINE COMPLEXES

66

r e c e i v e d considerable attention (3-8). I n this regard, tris(triphenylphosp h i n e ) c h l o r o r h o d i u m ( I ) , R h C l ( P P h ) , has r e c e i v e d the most s t u d y a n d 3

3

h a s b e e n p r o v e n u s e f u l as a d e c a r b o n y l a t i o n r e a g e n t i n o r g a n i c s y n t h e ­ sis (1, 8, 9 ) . T h i s c o m p l e x d e c a r b o n y l a t e s a l d e h y d e s a n d a c i d c h l o r i d e s stoichiometrically under m i l d conditions (

X = C l or H

Using

R X + RhCl(CO)(PPh ) 3

(1)

2

RhCl(PPh )s 3

T h e r e is g e n e r a l a g r e e m e n t o n t h e m e c h a n i s m for t h e s t o i c h i o m e t ­ ric decarbonylation of acid chlorides (9,14,15,16). T h e overall m e c h ­ a n i s m is s h o w n b y E q u a t i o n set 2 w h e r e X = C l . T h e

stoichiometric

d e c a r b o n y l a t i o n r e a c t i o n r e s u l t s from i n i t i a l o x i d a t i v e a d d i t i o n o f t h e a c i d c h l o r i d e to R h C l ( P P h ) 3

(Equation 2b, X = Cl). R h C l ( P P h )

2

3

is a

2

v e r y r e a c t i v e , l o w - c o n c e n t r a t i o n i n t e r m e d i a t e w h i c h is l i k e l y to s o l v a t e d (see

E q u a t i o n 2a)

be

(17). (2a)

RhCl(PPh ) 3

2

+ RCOX « = *

R C O ( X )RhCl(PPh ) 3

R(X )RhCl(CO)(PPh ) 3

2

* = ±

2

R C O ( X )RhCl(PPh ) 3

R(X )RhCl(CO)(PPh ) 3

(2b)

2

(2c)

2

> R X + RhCl(CO)(PPh ) 3

E q u a t i o n s 2c a n d 2 d s h o w the a c y l - a l k y l m i g r a t i o n a n d

(2d)

2

reductive

e l i m i n a t i o n s t e p s , r e s p e c t i v e l y . T h e r e is g o o d e v i d e n c e t h a t t h i s s a m e m e c h a n i s t i c s c h e m e a p p l i e s to the d e c a r b o n y l a t i o n o f a l d e h y d e s

(see

E q u a t i o n set 2 , X = H ) , a l t h o u g h i n t h i s c a s e r e a c t i o n i n t e r m e d i a t e s h a v e n o t b e e n i s o l a t e d (3, 5, 9, 18).

A d d i t i o n a l l y , e v i d e n c e exists that

t h e r a t e - d e t e r m i n i n g s t e p is o x i d a t i v e a d d i t i o n for a l d e h y d e b o n y l a t i o n (see

E q u a t i o n 2 b , X = H ) (3, 9, 18).

decar­

Several recent reports

h a v e s h o w n t h a t for s o m e s p e c i a l a l d e h y d e s , o x i d a t i v e a d d i t i o n o f t h e c a r b o n y l - h y d r o g e n b o n d i n d e e d does occur u s i n g rhodium(I) p l e x e s (8,19).

com­

I n t h e s e s t u d i e s a s t a b l e c h e l a t e w a s f o r m e d after o x i d a ­

tive a d d i t i o n that e n a b l e d isolation a n d characterization o f the p r o d ­ ucts

(8,19).

Alyea and Meek; Catalytic Aspects of Metal Phosphine Complexes Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

4.

DOUGHTY ET AL.

Decarbonylation

67

Reactions

A t t e m p e r a t u r e s a b o v e c a . 200°C, t h e d e c a r b o n y l a t i o n r e a c t i o n c a n b e d r i v e n c a t a l y t i c a l l y ( i , 4,14,

20). S c h e m e I illustrates the p r o p o s e d

c a t a l y t i c r e a c t i o n s c h e m e (15,16). i t y for b e n z a l d e h y d e presumably

because

RhCl(CO)(PPh ) 3

the

oxidative

addition

of

hr" ) 1

RCOX

to

is d i f f i c u l t (7, 21, 22). C o n s i s t e n t w i t h t h i s , t h e rate is

2

significandy greater (benzaldehyde,

T h i s c a t a l y t i c r e a c t i o n is s l o w ( a c t i v ­

d e c a r b o n y l a t i o n at 178°C is 10 t u r n o v e r s

when

IrCl(CO)(PPh ) 3

2

is u s e d as t h e

178°C, a c t i v i t y is 6 6 t u r n o v e r s h r " ) (23). 1

catalyst

Oxidative

a d d i t i o n to i r i d i u m c o m p l e x e s is w e l l k n o w n to b e m o r e f a c i l e t h a n

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w i t h their r h o d i u m analogues.

S c h e m e I.

R(X)RhCl(CO)(PPh ) 3

RCO(X)RhCl(CO)(PPh )

2

3

2

X = C l or H

Decarbonylation

of Benzaldehyde

Complexes of Rhodium

Using Cationic

Diphosphine

(I)

I n o r d e r to a c h i e v e h i g h e r c a t a l y t i c a c t i v i t i e s for d e c a r b o n y l a t i o n u n d e r m i l d t h e r m a l c o n d i t i o n s , i t is n e c e s s a r y t o u s e a m e t a l c o m p l e x o f s u f f i c i e n t b a s i c i t y to p r o m o t e f a c i l e o x i d a t i v e a d d i t i o n , b u t a l s o w i t h t h e p r o p e r s t e r e o c h e m i c a l a n d e l e c t r o n i c p r o p e r t i e s so t h a t C O l o s s a n d c a t a l y s t r e g e n e r a t i o n is r a p i d . W i t h R h C l ( P P h ) , t h e p r o d u c t c o m ­ 3

plex frans-RhCl(CO)(PPh ) 3

2

(see

t i o n e v e n at h i g h t e m p e r a t u r e therefore

Scheme

3

E q u a t i o n 2 d ) is i n e r t t o C O d i s s o c i a ­ (24)

a n d u n d e r U V irradiation (25);

I is r e q u i r e d for c a t a l y t i c d e c a r b o n y l a t i o n . W i t h

this i n m i n d , w e s y n t h e s i z e d a series o f m o n o - a n d complexes:

2

P-P = Ph P(CH ) PPh 2

bis(diphosphine)

[ R h ( P - P ) ] X a n d [ R h ( P - P ) ] X , w h e r e X = C l or B F , a n d 2

n

2

4

(hereafter n a m e d d p p m , d p p e , d p p p , a n d d p p b

for η = 1, 2, 3 , a n d 4, r e s p e c t i v e l y )

(7, 21, 22).

These complexes

are

k n o w n to u n d e r g o o x i d a t i v e a d d i t i o n r e a c t i o n s as w e l l as to b i n d C O

Alyea and Meek; Catalytic Aspects of Metal Phosphine Complexes Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

68

METAL PHOSPHINE COMPLEXES

w e a k l y a n d r e v e r s i b l y (26,

Indeed

27, 28, 29).

reactive towards C O ; however,

[Rh(dppe) ] 2

is u n -

+

i t d o e s f u n c t i o n as a r e a s o n a b l e

de­

c a r b o n y l a t i o n c a t a l y s t for a l d e h y d e s ( v i d e i n f r a ) . These diphosphine complexes

w e r e a l l o w e d to react w i t h neat

b e n z a l d e h y d e at s e v e r a l t e m p e r a t u r e s u n d e r a p u r i f i e d n i t r o g e n p u r g e i n o r d e r to d e t e r m i n e t h e i r c a t a l y t i c a c t i v i t i e s for d e c a r b o n y l a t i o n . T h e rate o f b e n z e n e p r o d u c t i o n w a s m o n i t o r e d b y g a s - l i q u i d c h r o m a t o g ­ r a p h y ( G L C ) as d e s c r i b e d p r e v i o u s l y (7,

21, 22).

The

experimental

p r o c e d u r e for t h e s e r e a c t i o n s as w e l l as for t h e s y n t h e s e s o f t h e c o m ­

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p l e x e s h a s b e e n p u b l i s h e d (7, 21, 22). Table

I.

It

using

diphosphine

RhCl(PPh ) 3

is a p p a r e n t

from

complexes

these are

T h e results are p r e s e n t e d i n

results that catalytic significantly

greater

activities than

with

o r i r o n s - R h C l ( C O ) ( P P h ) . I n d e e d , [ R h ( d p p p ) ] X (X = C l

3

3

2

2

o r B F ) c a t a l y t i c a l l y c o n v e r t s b e n z a l d e h y d e i n t o b e n z e n e at a r a t e t h a t 4

is m o r e t h a n a f a c t o r o f 1 0 f a s t e r t h a n w i t h R h C l ( P P h ) . I m p o r t a n t l y , 2

3

3

t h e b i s ( d i p h o s p h i n e ) c o m p l e x e s s h o w c o n s t a n t c a t a l y t i c a c t i v i t y for at least several days i n homogeneous

solution, a n d total turnover n u m ­

b e r s o f 1 0 0 , 0 0 0 h a v e b e e n a c h i e v e d . T h e r e a c t i o n is a l s o h i g h l y s e l e c ­ t i v e s i n c e t h e y i e l d for b e n z e n e p r o d u c t i o n f r o m b e n z a l d e h y d e u s i n g [Rh(dppp) ] 2

is 1 0 0 % . T h e s e r e s u l t s c l e a r l y d e m o n s t r a t e t h a t t h e b i s -

+

T a b l e I. C a t a l y t i c D e c a r b o n y l a t i o n o f B e n z a l d e h y d e i n t o Benzene and C O

Catalyst" RhCKPPhs)/ [Rh(dppp) ] [Rh(dppe) ] [Rh(dppm) ] [Rh(dppe) ] [Rh(dppp) ] [Rh(dppb) ] [Rh(dppp) ] [Rh(dppp)]BF RhCl(dppp) RhCl(PPh ) < tRhidppp),]^ [Rh(dppe) ] [RMdppm),]*" + ( i

2

2

+ d

+ <