ba-1982-0196.ch015

15 Hydrogenation Catalysis of Olefins by a Rhodium Hydride Complex of PhP(CH CH CH PPh ) Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 14, 2018 | https://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0196.ch015

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In Situ Generation of RhH(ttp)

J A M E S N I E W A H N E R 1 and D E V O N W. M E E K Department of Chemistry, The Ohio State University, Columbus, OH 43210

The complex RhCl(ttp), where ttp = PhP(CH CH CH PPh2)2, in the presence of either triethylaluminum or diethylaluminum chloride, is an effective homogeneous catalyst for hydrogenation of 1-olefins and 1-octyne. The rates of hydrogenation of substituted olefins are considerably slower than for terminal olefins. H-1 and P-31 NMR spectra were used to identify several different chemical species [including RhH(ttp)] in these catalytically active solutions. The observed rate of hydrogenation of 1-octene to n-octane at 20 ± 0.3°C and under a constant H pressure of 750 torr is 6.4 x 104M-1 min-1, i.e. 25 times more rapid than the Wilkinson catalyst, RhCl(PPh ) , under comparable conditions. The rate expression is first order in the rhodium complex, first order in H2, and zero order in the olefin. A mechanism involving RhR(ttp), RhH(ttp) associated with an ethylaluminum species, and H is proposed to account for the observations. The analogous system containing Et AlCl and the rhodium complex of PhP(CH CH CH PCy ) , where Cy = the cyclohexyl group, also catalyzes 1-octene at a rate comparable with that of RhCl(ttp) + Et AlCl; however, in this case, the rate depends on the concentration of the olefin. 2

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3 3

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2 2

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1Current address: Department of Physical Sciences, Northern Kentucky University, Highland Heights, KY 41076. 0065-2393/82/0196-0257$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.

258

M E T A L PHOSPHINE COMPLEXES

A

great d e a l o f research has c e n t e r e d a r o u n d catalysis b y transsition-metal c o m p l e x e s (J) since W i l k i n s o n a n d co-workers re p o r t e d t h e r a p i d h o m o g e n e o u s hydrogénation o f o l e f i n s c a t a l y z e d b y R h C l ( P P h ) (2). It n o w is g e n e r a l l y a c c e p t e d t h a t t h e f o r m a t i o n o f a c o o r d i n a t i v e l y u n s a t u r a t e d m e t a l h y d r i d e is a n e c e s s a r y c o n d i t i o n for hydrogénation b y t r a n s i t i o n - m e t a l c o m p l e x e s (3, 4). D u b o i s a n d M e e k 3

(5) have s h o w n that the complex RhCl(ttp), where ttp is P h P ( C H C H C H P P h ) 2 , c a t a l y z e d t h e hydrogénation o f 1-octene w h e n N a B H a n d e t h a n o l w e r e a d d e d . T h e c o m p l e x R h C l ( t t p ) differs s i g n i f i c a n t l y from W i l k i n s o n ' s c a t a l y s t i n t h a t t h e t r i d e n t a t e l i g a n d r e m a i n s b o n d e d (6), w h e r e a s s o m e t r i p h e n y l p h o s p h i n e d i s s o c i a t e s w h e n W i l k i n s o n ' s c a t a l y s t is u s e d (7). T h e c a t a l y t i c a c t i v i t y o f R h C l ( t t p ) w a s p r e s u m e d (5) to b e d u e t o R h H ( t t p ) , w h i c h w a s g e n e r a t e d i n s i t u . I n a d d i t i o n to N a B H , c o m p o u n d s s u c h as A l E t a n d L i N ( C H ) a l s o h a v e b e e n u s e d to generate m e t a l h y d r i d e s f r o m h a l i d e c o m p l e x e s v i a the β-elimination m e c h a n i s m (8, 9 ) . A l s o , t h e effect o f L e w i s a c i d s o n t h e h o m o g e n e o u s hydrogénation o f olefins c a t a l y z e d b y t r a n s i t i o n - m e t a l c o m p l e x e s h a s b e e n e x a m i n e d (8, 10, 11). I n this chapter, w e describe the nature of the species generated i n s o l u t i o n w h e n A l E t o r A l E t C l is a d d e d to R h C l ( t t p ) i n t o l u e n e . W e a l s o d e s c r i b e t h e k i n e t i c s o f t h e c a t a l y t i c hydrogénation o f v a r i o u s organic substrates b y R h C l ( t t p ) a n d A l E t C l a n d propose a m e c h a n i s m t h a t is c o n s i s t e n t w i t h t h e o b s e r v a t i o n s . 2

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 14, 2018 | https://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0196.ch015

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2

Experimental G e n e r a l Procedure. Solutions c o n t a i n i n g the a l u m i n u m alkyls and RhCl(ttp) are air-sensitive, a n d they were h a n d l e d u n d e r a nitrogen atmo sphere u s i n g either S c h l e n k glassware or a glovebag. Toluene a n d 1-octene were d i s t i l l e d from s o d i u m benzophenoneketyl into a dry flask u n d e r an atmo sphere of nitrogen. Other olefins were stirred at least 24 h w i t h L i A l H a n d were v a c u u m d i s t i l l e d into a dry flask. Cyclohexanone a n d 1-octyne were stored over L i n d e 4 A m o l e c u l a r sieves for 1 week, transferred v i a a syringe to a flask and freeze-pump-thawed three times u s i n g dry ice a n d 1-propanol. T r i e t h y l a l u m i n u m , A l E t , was syringed from stock bottles under an atmo sphere of nitrogen i n a glovebag. D i e t h y l a l u m i n u m c h l o r i d e , A l E t C l , used i n the k i n e t i c study was transferred i n a dry box from stock bottles into vials that contained a septum port a n d valve (available from Pierce C h e m i c a l Co.). T h e reagent then was syringed into the reaction flask u n d e r an atmosphere o f nitrogen i n a glovebag. Materials. T h e c o m p l e x RhCl(ttp) was prepared as described p r e v i o u s l y (6) . Cyclohexanone was obtained from M a l l i n k r o d t ; cyclohexene a n d 1-pentene were obtained from M C B . A l l other olefins a n d 1-octyne were pur chased from C h e m S a m p C o . T h e a l u m i n u m alkyls were obtained from A l d r i c h as 2 5 % w/w solutions i n toluene. Neat l i q u i d s used i n the H - l N M R work were obtained from these solutions by r e m o v i n g the toluene under v a c u u m . L i t h i u m d i m e t h y l a m i d e , L i N ( C H ) , was obtained from Alpha/Ventron as a 1 0 % w/w slurry i n hexane. H y d r o g e n was passed through an E n g l e h a r d D e o x o purifier and activated a l u m i n a before use. 4

3

2

3

2

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

15.

NIEWAHNER A N D MEEK

Hydrogénation

Catalysis

Spectra. P-31 a n d H - l N M R spectra were o b t a i n e d w i t h a B r u k e r H X 9 0 Fourier transform spectrometer u s i n g s p i n n i n g 10-mm tubes, a d e u t e r i u m lock from the deuterated solvent, a n d external 8 5 % H P 0 as the reference. I R spectra were o b t a i n e d u s i n g a P e r k i n - E l m e r M o d e l 337 grating spectrometer that covered the range from 4000 c m to 400 c m . K i n e t i c s . M o s t hydrogénation experiments were carried out at constant hydrogen pressure u s i n g an automatic gas-measuring instrument designed by Robert F a g a n o f the D e p a r t m e n t o f C h e m i s t r y , at T h e O h i o State U n i v e r s i t y . Some experiments were carried out u s i n g s i m p l e manometric methods w h e r e i n the pressure of h y d r o g e n decreased as the reaction proceeded. S o l u tions were thermostated at 20 ± 0.3°C. T h e system v o l u m e for reactions car r i e d out at constant pressure was about 125 m L — a b o u t 0 . 5 m M i n r h o d i u m , 640 m M i n olefin, a n d 25 m M i n A l E t C l . T h e total pressure was kept at about 775 torr; p ( H ) = 750 torr, ρ (solvents) = 25 torr. For reactions carried out at constant v o l u m e , the v o l u m e of the system was about 400 m L . N o hydrogen consumption was observed for solutions c o n t a i n i n g a l l of the reagents except RhCl(ttp), thereby e x c l u d i n g any significant catalysis b y A l E t C l itself under these conditions. Solutions were prepared under a nitro gen atmosphere i n a glovebag, c l o s e d to the atmosphere, a n d then transferred to the hydrogénation apparatus a n d v a c u u m l i n e . T h e solutions then were freeze (-196°C)-pump-thawed t w i c e before the hydrogénation experiment. 3

- 1


259

of Olefins

4

- 1

2

2

2

Results and P-31 AlEt . 3

Discussion

and

The

H-l

NMR

P-31{H-1}

RhCl(ttp) and A l E t

3

Spectra. NMR

SOLUTIONS

spectrum

of

a

O F RhCl(ttp) toluene

AND

solution

of

is s h o w n i n F i g u r e 1. T h i s p a t t e r n is t y p i c a l o f

c o m p l e x e s i n w h i c h t h e R h ( t t p ) p o r t i o n is p l a n a r a n d t h e t h r e e p h o s phorus atoms o f the ttp l i g a n d are i n a m e r i d i o n a l arrangement.

Spin-

s p i n s o u p l i n g b e t w e e n the t e r m i n a l phosphorus, the central

phos

phorus, a n d the r h o d i u m atom, a l l o f w h i c h h a v e a s p i n o f for t h e o b s e r v e d s p e c t r u m (12).

accounts

T a b l e I gives the P-31{H-1} N M R data

for t h i s a n d o t h e r s o l u t i o n s c o n t a i n i n g R h C l ( t t p ) a n d t h e a l u m i n u m a l k y l s . T h e H - l N M R s p e c t r u m for t h i s s o l u t i o n , g i v e n i n F i g u r e 2 , d e f i n i t e l y s h o w s a r h o d i u m h y d r i d e r e s o n a n c e at - 5 . 2 p p m . A l s o , t h e r e a r e t w o sets o f e t h y l r e s o n a n c e s d u e to t h e e t h y l g r o u p s o n a l u m i n u m ; h o w e v e r , t h e p o s i t i o n s o f t h e s e r e s o n a n c e s a r e n o t t h e s a m e as t h o s e o f free A l E t

3

or A l E t C l . T a b l e I I s u m m a r i z e s t h e H - l N M R d a t a o f t h e s e

e t h y l resonances,

2

w h i c h a r e t h o u g h t to a r i s e f r o m

either A l E t

3

or

A l E t C l c o o r d i n a t e d to t h e R h H ( t t p ) s p e c i e s t h r o u g h t h e h y d r i d e as 2

w e l l as t h e r h o d i u m a t o m . T h e h y d r i d e is p r e s u m e d t o c o m e

from

a

/ 3 - h y d r i d e e l i m i n a t i o n o f e t h y l e n e f r o m R h E t ( t t p ) , w h i c h is f o r m e d b y a n i n i t i a l a l k y l a t i o n r e a c t i o n . S i n c e t h e u p f i e l d set o f e t h y l r e s o n a n c e s is n o t p r e s e n t after a d d i n g e t h y l e n e , i t is a s s i g n e d to a n e t h y l a l u m i n u m s p e c i e s t h a t is c o o r d i n a t e d t o t h e h y d r i d o l i g a n d . T h i s a l s o a c c o u n t s for t h e r e s o n a n c e p o s i t i o n o f t h e h y d r i d e ( - 5 . 2 p p m ) as c o m p a r e d t h a t o b s e r v e d for R h H ( P P h ) 3

3

7 report ( R h - H ) = - 7 . 9 p p m . W e observe the same value.) T h e field

with

( - 7 . 9 p p m ) . (Strauss a n d S h r i v e r i n Ref. down-

set o f e t h y l r e s o n a n c e s is a s s i g n e d to a n e t h y l a l u m i n u m c o m -


260

M E T A L PHOSPHINE

120 Hz ι

Rh-P

COMPLEXES

0

(CH 0) P0


3

+ 13.57

3

ppm

• 5.00 ppm

Figure 1. P-31, broad-band, proton-decoupled spectrum of RhCl(ttp) in the presence of a twentyfold molar ratio of Et Al in toluene. The (CH 0) PO signal is the secondary standard, which is superimposed on the spectrum. 3

3

3

p o u n d c o o r d i n a t e d to t h e r h o d i u m . T h i s e t h y l a l u m i n u m c o m p o u n d m u s t b e i n e q u i l i b r i u m w i t h A l E t s i n c e t h e p o s i t i o n o f t h e e t h y l reso n a n c e s m o v e s c l o s e r to t h a t o f free A l E t o n a d d i n g m o r e A l E t . It h a s b e e n s h o w n t h a t t h e i n t e r n a l c h e m i c a l s h i f t o f t h e e t h y l reso n a n c e s , i.e. t h e d i f f e r e n c e s i n c h e m i c a l s h i f t b e t w e e n t h e m e t h y l a n d m e t h y l e n e p r o t o n s , d e c r e a s e s u p o n c o o r d i n a t i o n to L e w i s b a s e s ( 1 3 , 14). A l i p h a t i c e t h e r s a n d t e t r a h y d r o f u r a n ( T H F ) ( 1 3 , 1 4 ) g a v e a n i n t e r n a l c h e m i c a l s h i f t o f - 1 . 2 5 p p m as c o m p a r e d w i t h - 0 . 7 8 p p m for A l E t i t s e l f at 0 . 5 M . W e c a l c u l a t e i n t e r n a l c h e m i c a l s h i f t s o f - 1 . 0 9 p p m a n d - 1 . 2 4 p p m for t h e t w o sets o f e t h y l r e s o n a n c e s o b s e r v e d for s o l u t i o n s of RhCl(ttp) and A l E t i n t o l u e n e . T h e s e a r e a s s i g n e d to t h e e t h y l a l u m i n u m s p e c i e s w h i c h is a s s o c i a t e d w i t h t h e r h o d i u m a t o m a n d t h e h y d r i d o l i g a n d , r e s p e c t i v e l y . A v a l u e o f - 0 . 8 4 p p m is o b t a i n e d for A l E t at 5 m M i n t o l u e n e . T h u s i t a p p e a r s t h a t a t o l u e n e s o l u t i o n o f R h C l ( t t p ) a n d A l E t c a n b e c h a r a c t e r i z e d b y E q u a t i o n 1. T h e e x a c t n a t u r e o f t h e e t h y l a l u m i n u m s p e c i e s is n o t k n o w n b e c a u s e o f t h e 3

3

3

3

3

3

3

E t A l[

/ \

( t t p ) R h - H - A l E t ?± E t A l +

(ttp)Rh-H-AlEt


(1)


3

2

4

2

a

RhCl(ttp) + 2 AlEta-nCl» + olefin

2

2

R h + 2 A l E t C l + 1-Octene R h -h 2 A l E t C l + 1 - P e n t e n e

3

R h -h 2 A l E t + 1 - P e n t e n e Rh + 2 AlEt Cl

3

3

Rh + 2 AlEt Rh + 2 AlEt + C H R h -h 2 A l E t + 1 - O c t e n e

RhR(ttp).

-12.8 -12.1

—

-4.6 -12.8 -12.4 -4.6 -12.6 -12.8 20.5

ppm

2

14.2 11.5 12.0 14.4 11.7 12.2 8.4 16.1 12.2 12.3

ÔP ppm 43.8 49.9 49.3 43.0 49.3 50.0 53.7 42.7 50.0 50.0

Hz

Jp1-pi

2

T a b l e I. P - 3 1 { H - 1 } N M R D a t a "

139.7 140.5

—

108.5 141.7 141.9 108.1 141.9 139.7 185.3

Hz


2

121.3 114.7 114.7 121.3 114.7 115.8 126.5 m i n o r 117.7 m i n o r 115.8 116.2

Hz

!/ΛΑ-Ρ

METAL PHOSPHINE

COMPLEXES


262


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

1.418 1.330 1.242

Triplet

RhCl(ttp) + 2 A l E t C l

2

1.363 1.268 1.187

Triplet

3

3

RhCl(ttp) + 5 A l E t i n T H F

Only

AlEt

3

1.818 1.737 1.645

4

RhCl(ttp) + 2 A l E t + C H . Set Β a n d R h - H disappeared.

3

2

1.428 1.341 1.254

RhCl(ttp) + 4 A l E t Set A increased i n intensity relative to Set Β

3

0.570 0.482 0.394 0.299

0.397 0.321 0.223 0.136 0.197 0.102 0.021

—

O OS

M M

Ο

>

M

I

Β

Not observed

Not observed Quartet

CA3

to

3

«s»

OS

O

S*

Not observed a

-5.353 -5.613

-5.321 -5.581

Quartet

Quartet

—

0.126 0.050 -0.036

N o t observed

Not distinct

1.319 1.221 L135

-5.147 -5.418

0.191 0.104 0.017 -0.069 0.147 0.060 -0.025

1.384 1.297 1.211

0.701 0.614 0.527 0.440 0.614 0.516 0.429 0.343 0.538 0.451 0.353 0.267 0.777 0.690 0.603 0.516 1.200 1.113 1.026

1.742 1.655 1.569 1.580 1.493 1.406

3

Rh-H

Quartet

Set Β Triplet

Quartet

Set A

2

Triplet

RhCl(ttp) + 3 A l E t Set A increased i n intensity relative to Set Β

RhCl(ttp) + 2 A l E t

Reactants

3

T a b l e II. H - l N M R Parameters of S e l e c t e d Peaks i n Spectra of Solutions C o n t a i n i n g RhCl(ttp) a n d A l E t or A l E t C l


METAL PHOSPHINE COMPLEXES

264 equilibrium between

c o o r d i n a t e d a n d free e t h y l a l u m i n u m

species

a n d t h e l i k e l i h o o d t h a t A l E t C l is o n e o f t h e p r o d u c t s i n t h e r e a c t i o n 2

between

R h C l ( t t p ) a n d A l E t . H e n c e , t h e e t h y l a l u m i n u m s p e c i e s is 3

\ / indicated by E t A l . A d d i n g e t h y l e n e to a s o l u t i o n o f R h C l ( t t p ) a n d A l E t

results i n a

3

n e w set o f P - 3 1 { H - 1 } N M R p a r a m e t e r s as s h o w n i n T a b l e I. F u r t h e r


more, the H - l N M R s p e c t r u m no longer exhibits the R h - H resonance at - 5 . 2 p p m , a n d t h e set o f e t h y l r e s o n a n c e s a s s i g n e d to R h - H - A l E t is

\/ n o t o b s e r v e d . T h u s , t h i s n e w r h o d i u m s p e c i e s is t h o u g h t t o b e

EtAl-

RhEt(ttp). Adding

either

RhCl(ttp) and A l E t

1 - p e n t e n e or 3

1-octene to a t o l u e n e

solution

of

r e s u l t s i n a m i x t u r e o f t w o c o m p o u n d s t h a t are

s h o w n b y t h e P - 3 1 { H - 1 } N M R s p e c t r a . O n e c o m p o u n d is a s s i g n e d as

w

\/

\/

E t A l - R h H ( t t p ) - A l E t , w h e r e a s t h e o t h e r is E t A l - R h R ( t t p ) . A s t h e a l k y l g r o u p R is v a r i e d , w e d i d n o t o b s e r v e

significant differences

i n the

P-31 N M R parameters; h e n c e , the p e n t e n e solutions m a y c o n t a i n b o t h Rh(C H )(ttp) and Rh(C H )(ttp). 2

5

5

n

A n a l o g o u s l y , the octene

may contain both Rh(C H )(ttp) and Rh(C H )(ttp). 2

cases,

association

5

8

between

the

1 7

ethylaluminum

solutions

In all of

species

r h o d i u m c o m p l e x is p r o p o s e d . T h e c h e m i c a l e q u a t i o n s

and

these the

representing

t h e s e r e a c t i o n s a r e as f o l l o w s : EtAl^

I ^ R h C l ( t t p ) + xs A l E t

3

^

\/

(ttp)Rh-H-AlEt + C H

4

(2)

+ AlEt

(3)

2

EtAl^

EtAl^

( t t p ) R h - H - A l E t + olefin ^ SOLUTIONS O F RhCl(ttp)

(ttp)Rh-R

AND AlEt Cl.

The

2

P-31{H-1}

NMR

spectrum of a solution o f RhCl(ttp) a n d A l E t C l shows three species, 2

cf. T a b l e I. T h e m a j o r c o m p o n e n t h a s t h e s a m e s p e c t r a l p a r a m e t e r s as

\ / the c o m p o u n d t h o u g h t to b e E t A l - R h E t ( t t p ) . S i n c e the H - l

NMR

s p e c t r u m of s u c h a solution does not s h o w a h y d r i d e resonance u p f i e l d from

t e t r a m e t h y l s i l a n e ( T M S ) , p r o b a b l y neither o f the other

compo

n e n t s is a r h o d i u m h y d r i d e s p e c i e s . T h e r e a s o n for n o t o b s e r v i n g t h e r h o d i u m h y d r i d e r e s o n a n c e i n t h e p r e s e n c e o f A l E t C l is n o t u n d e r 2

s t o o d at t h i s t i m e . H o w e v e r w e find t h a t R h H ( P P h ) 3

AlEt

3

shows

no

hydride

resonance

upfield

3

i n the presence of

from

TMS.

Adding

1 - p e n t e n e o r 1-octene t o s o l u t i o n s c o n t a i n i n g R h C l ( t t p ) a n d A l E t C l 2

r e s u l t s i n a c l e a n N M R s p e c t r u m t h a t c o n t a i n s o n l y o n e set o f p a r a m -


15.


Hydrogénation

Catalysis

265

of Olefins

eters, w h i c h is c o n s i s t e n t w i t h r h o d i u m - a l k y l c o m p o u n d s o f t h e t y p e

\ / E t A l - R h R ( t t p ) , v i d a supra. T h e e t h y l a l u m i n u m resonances i n the H - l N M R s p e c t r u m result i n a n i n t e r n a l c h e m i c a l shift o f - 0 . 8 9 p p m sug gesting only slight coordination between a l u m i n u m a n d r h o d i u m . This is c o n s i s t e n t w i t h g r e a t e r s t e r i c h i n d r a n c e b e t w e e n t h e a l u m i n u m s p e c i e s a n d r h o d i u m w h e n e t h y l is a t t a c h e d t o r h o d i u m t h a n w h e n


h y d r i d e is a t t a c h e d . B a s e d o n t h e P - 3 1 { H - 1 } a n d t h e H - l N M R s p e c t r a a n d t h e fact that solutions o f R h C l ( t t p )

a n d either A l E t

3

or A l E t C l catalyze the 2

hydrogénation o f o l e f i n s , t h e f o l l o w i n g c a t a l y t i c c y c l e is t h o u g h t t o b e involved.

T h e association o f the e t h y l a l u m i n u m species

w i t h the

r h o d i u m c o m p l e x is o m i t t e d for c l a r i t y . R h C l ( t t p ) + A l E t —> R h H ( t t p ) + A l E t C l + C H 3

2

RhCl(ttp) + A l E t C l

RhEt(ttp) + A l E t C l

2

RhEt(ttp) ^

alkene

RhH(ttp) + C H 2

RhH(ttp) ^ ^

2

(4)

4

(5)

2

(6)

4

>. a l k a n e

RhR(ttp)

Carbon-13 N M R Spectrum. to w h i c h C H

2

*

*~ H

2

A solution of RhCl(ttp) a n d A l E t C l 2

has been a d d e d gives a C - 1 3 N M R s p e c t r u m w i t h a

4

low-intensity, b r o a d resonance

at 7 6 . 6 1 p p m d o w n f i e l d

from T M S .

T h i s p o s i t i o n c o r r e s p o n d s t o a c o o r d i n a t e d e t h y l e n e (15). T h e b r o a d resonance

suggests

conclusion,

that the e t h y l e n e

i n conjunction

with

is u n d e r g o i n g e x c h a n g e .

the H - l N M R data,

This

suggests

an

e q u i l i b r i u m i n v o l v i n g e t h y l e n e , the r h o d i u m species, a n d A l E t C l , i.e. 2

EtÀÎ-RhEt(ttp) + C H 2

4

^± II — R h E t ( t t p ) + E t A l ^ CH

(7)

2

Kinetics

and Mechanism.

T h e kinetic study was carried out

u s i n g A l E t C l since w e h a d difficulty i n o b t a i n i n g r e p r o d u c i b l e results 2

w i t h A l E t . T h i s p r o b l e m is t h o u g h t t o a r i s e f r o m t h e g r e a t e r a i r s e n 3

sitivity o f A l E t plot

3

as c o m p a r e d w i t h A l E t C l . F i g u r e 3 s h o w s a t y p i c a l

of hydrogen

2

consumption

vs. time

for t h e hydrogénation

of

1- o c t e n e u n d e r c o n s t a n t - p r e s s u r e c o n d i t i o n s . T h e fact t h a t t h e rate d o e s n o t t a i l off t o w a r d t h e e n d o f t h e r e a c t i o n , b u t p r o c e e d s s t e a d i l y at a c o n s t a n t rate u n t i l t h e o l e f i n is h y d r o g e n a t e d c o m p l e t e l y — t h e n i m m e d i a t e l y — i s indicative o f zero-order olefin dependence. havior

was observed

for b o t h

stops

This be

1-octene a n d 1 - o c t y n e , b u t n o t for

2- o l e f i n s .



266 MMOLES

of H

2


CONSUMED

1

2

3

4

5

MINUTES Figure

3.

A plot of hydrogen consumption vs. time for the hydrogénation of 1-octene at constant pressure

E F F E C T O F HYDROGEN PRESSURE.

R e a c t i o n s c a r r i e d o u t at d i f

ferent pressures o f h y d r o g e n r e s u l t e d i n essentially the s a m e s e c o n d o r d e r rate constant, k = 6 . 2 x 1 0 M 4

- 1

min

- 1

(first o r d e r i n r h o d i u m

c o m p l e x a n d first o r d e r i n h y d r o g e n ) , as s h o w n i n T a b l e I I I . T h e s o l u b i l i t y o f h y d r o g e n i n toluene w a s calculated from data available i n the International C r i t i c a l Tables. A l l three reactions r e s u l t e d i n l i n e a r Table III. Effect o f Pressure of H y d r o g e n x 10*

[RhCUttp)]

k, M

0

min

(torr)

P(Ht) (torr)

4.9

481

240

5.9

5.4 5.4

651 767

414

7.3 5.5

f

537

1

avg = 6.2 [AlEt Cl] = 2.5 x 10" , [1-Octene] = 0.637M, 20°C. 2

2


15.

NiEWAHNER A N D M E E K

Hydrogénation

Catalysis

of Olefins

plots o f the t y p e s h o w n i n F i g u r e 4, w h i c h s h o w that the

267

hydrogen

d e p e n d e n c e is first o r d e r . T h e r a t e c o n s t a n t s c a l c u l a t e d from L n Ρ

(H ) 2

v s . t i m e p l o t s a r e t h e s a m e as t h o s e o b t a i n e d f r o m p l o t s o f o l e f i n c o n c e n t r a t i o n v s . t i m e . T h e l a t t e r m e t h o d is less c o m p l i c a t e d . EFFECT

OF OLEFIN CONCENTRATION.

Table

I V l i s t s d a t a for

r e a c t i o n s c a r r i e d o u t at d i f f e r e n t c o n c e n t r a t i o n s o f 1-octene. T h e s e d a t a a r e c o n s i s t e n t w i t h t h e z e r o - o r d e r b e h a v i o r o f o l e f i n w h i c h is i m p l i e d


i n F i g u r e 4. EFFECT OF RHODIUM CONCENTRATION.

Increasing the concen

t r a t i o n o f t h e r h o d i u m c o m p l e x , R h C l ( t t p ) , i n c r e a s e s t h e rate o f t h e r e a c t i o n as s h o w n i n T a b l e V . T h e s e d a t a (see

F i g u r e 5) g i v e a l i n e a r

p l o t t h a t passes t h r o u g h t h e o r i g i n . T h u s , t h e r e a c t i o n is first o r d e r i n total r h o d i u m concentration.

Ln P ( H ) 2

4.35L

4.00

.

,

,

,

,

I

2

3

4

5

L.

6

MINUTES

Figure

4.

A plot of Ln P(H ) vs. time for hydrogénation constant volume 2

of 1-octene at



268

Table IV. Effect of Olefin Concentration k, M "


[l-Octene]

rain"

1

x

1

0.127 0.319 0.319 0.637 0.637 0.956 0.956 1.27 1.27

I0"

0

4

7.3 5.3 7.8 6.3 5.6 7.1 7.0 7.0 5.5 a v g = 6.5

[RhCl(ttp)] = 5.0 χ ΙΟ" , 10" ; P(H ) = 750 torr, 20°C. a

[AlEt Cl] = 2.5 x

4

4

2

2

EFFECT OF AlEt Cl CONCENTRATION.

T h e air sensitivity of the

2

a l u m i n u m a l k y l s has c a u s e d difficulty i n q u a n t i t a t i v e l y d e t e r m i n i n g t h e effect o f t h e c o n c e n t r a t i o n o f t h e A l E t C l o n t h e r e a c t i o n r a t e . U s i n g 2

new

bottles

of

AlEt Cl, 2

concentrations

of

AlEt Cl

greater

2

3.7 x 1 0 ~ M , a n d a n A l : R h ratio > 3 p r o d u c e d n o o b s e r v a b l e 3

than

change

i n the rate w i t h i n c r e a s i n g concentrations o f A l E t C l . O l d e r s u p p l i e s o f 2

A l E t C l , i n t h e p r e s e n c e o f R h C l ( t t p ) a n d 1-olefins, d i d n o t c a t a l y z e 2

h y d r o g é n a t i o n o f 1-octene u n t i l t h e c o n c e n t r a t i o n o f A l E t C l w a s a b o u t 2

2 x 1 0 " M a n d the A l : R h ratio was about 1 0 : 1 . T h u s , u s i n g R h C l ( t t p ) 3

at c o n c e n t r a t i o n s o f 1 0 ~ M , t h e r a t e o f r e a c t i o n b e c a m e i n d e p e n d e n t o f 3

t h e c o n c e n t r a t i o n o f A l E t C l , as l o n g as i t s c o n c e n t r a t i o n w a s 2

than about 2 x

greater

10" M.

MECHANISM.

2

A consideration o f the N M R data a n d the k i n e t i c

d a t a l e a d s u s t o p r o p o s e t h e f o l l o w i n g m e c h a n i s m for t h e c a t a l y t i c hydrogénation o f olefins b y R h C l ( t t p ) a n d excess

AlEt Cl. 2

Table V. Effect of R h o d i u m Concentration

[RhCl(ttp)]

x JO

4

(M

Rate min~ )

x JO"

k, M- minl

l

1

5.6 7.2 5.6 6.3 5.8

0.028 0.074 0.090 0.126 0.131

1.43 3.00 4.71 5.85 6.56

a

avg =

6.1

« [l-Octene] = 0.637, [AlEt Cl] = 2.5 x ΙΟ" ; P(H,) = 750 torr, 20°C. 2

2


4

15.


Hydrogénation

Catalysis

269

of Olefins

RATE, M min" 0.13 L


0.11 0.091 0.07 0.05 0.03 0.01 1.00 Figure

5.

2.00 3.00 4.00 (jRhCllttp)] χ I04

5.00

A plot of the total rhodium concentration the reaction

6.00 vs. the rate

R h C l ( t t p ) + A l E t C l ?± R h E t ( t t p ) + A l E t C l κ. RhEt(ttp) ^ RhH(ttp) + C H κ, RhH(ttp) + alkene ^ RhR(ttp) 2

2

of

2

4

RhR(ttp) + Η -^· RhH(ttp) + alkane 2

T h e r e s u l t a n t rate l a w is: K

-d[H ]

s

t

dt

[olefin]

k [Rh] [H ]. r

[AlEtCl.MC.Hj «^.[AlEtaCl]

T h e o b s e r v e d r a t e l a w , -d[H ]/dt 2

[C H ] K 2

+

4

2

=

+

1

+

K

fc d[Rh]r[H ], obS

2

rolefinl 3[olefanj

2

requires that the

a b o v e t e r m i n b r a c k e t s b e e i t h e r o n e o r a c o n s t a n t . T h i s is c o n s i s t e n t


270


w i t h the P-31 N M R data o f solutions c o n t a i n i n g R h C l ( t t p ) ,

AlEt Cl, 2

a n d o l e f i n w h i c h w e h a v e i n t e r p r e t e d to i m p l y the p r o d u c t i o n o f o n l y RhEt(ttp); thus K

3

w o u l d b e large. A l s o , the concentration o f C H 2

k n o w n to b e e i t h e r v e r y s m a l l or z e r o b e c a u s e o n l y 1 m o l o f C H 2

b e f o r m e d p e r m o l e o f R h C l ( t t p ) (thus, 0 . 5 m M ) , a n d the

4

is

4

can

freeze-pump-

t h a w p r o c e s s w o u l d r e m o v e m o s t , i f n o t a l l , t h a t is p r o d u c e d b y t h e stoichiometric reaction.


SUBSTRATES HYDROGENATED. strates

hydrogenated

AlEt Cl

catalytically

T a b l e V I lists the various sub by

solutions

of

RhCl(ttp)

and

a n d t h e r e l a t i v e rates b a s e d o n 1-octene b e i n g a s s i g n e d a

2

v a l u e o f 1 0 0 . T h e rates i n d i c a t e , as is e x p e c t e d , t h a t i n t e r n a l a n d m o r e h i n d e r e d olefins react s l o w e r t h a n t e r m i n a l , u n h i n d e r e d olefins. After exposure

to h y d r o g e n ,

the

solutions w e r e a n a l y z e d b y

gas-liquid

c h r o m a t o g r a p h y ( G L C ) , a n d they c o n t a i n e d o n l y the saturated alkane i n a l l b u t o n e c a s e . I n t h e c a s e o f 1-octene, 9 6 % o f t h e o l e f i n

was

r e d u c e d to η-octane a n d 4 % w a s i s o m e r i z e d to a n i n t e r n a l o l e f i n . T h e η-octane w a s o b t a i n e d q u a n t i t a t i v e l y f r o m 1-octyne, e v e n t h o u g h it should

pass

through

1-octene

and

possibly

be

isomerized

like

1-octene. A l s o , t h e r a t e o f h y d r o g é n a t i o n o f 1 - o c t y n e d e c r e a s e d as t h e t i m e i n c r e a s e d b e t w e e n p r e p a r a t i o n o f t h e s o l u t i o n a n d hydrogénation. T h e s e t w o facts s u g g e s t t h a t 1 - o c t y n e f o r m s a n i n t e r m e d i a t e w i t h t h e r h o d i u m c o m p l e x t h a t is d i f f e r e n t f r o m t h e i n t e r m e d i a t e t h a t is f o r m e d b y 1-octene. LIFETIME OF T H E CATALYST.

T h e rate o f reactions u s i n g s o l u

tions o f p r e v i o u s l y u s e d catalyst d e c r e a s e d w i t h successive additions o f 1-octene. T h e s e o b s e r v a t i o n s , a l o n g w i t h t h o s e r e p o r t e d i n t h e p r e v i o u s s e c t i o n o n t h e effect o f A l E t C l c o n c e n t r a t i o n , i n d i c a t e t h a t t h e 2

l i f e o f t h e c a t a l y s t is r e l a t e d m o r e t o t h e a i r s e n s i t i v i t y o f t h e A l E t C l 2

t h a n to t h e i n s t a b i l i t y o f t h e r h o d i u m c o m p l e x . T a b l e V I . R e l a t i v e Rates of Hydrogénation of Various Substrates

Substrate l-Octene 2,3-Dimethyl-l-butene 2,3-Dimethyl-2-butene cis - 2 - O c t e n e trans - 2 - O c t e n e trans - 2 - P e n t e n e Cyclohexene 1 , 3 - O c t a d i e n e (cis + trans) 1-Octyne Cyclohexanone l - O c t e n e ( W i l k i n s o n ' s catalyst)

Relative Rate 100 5.2 no reaction 1.2 0.84 1.1 no reaction no reaction >28 no reaction 3.8


15.

NIEWAHNER A N D MEEK USE OF A MORE

cally

hindered

Hydrogénation

Catalysis

HINDERED CATALYST.

catalyst

RhCl(Cyttp),

where

271

of Olefins

U s i n g the more Cyttp

steri

represents

the

Ph tetra-substituted cyclohexyl l i g a n d Cy P(CH2)3P(CH )3PCy2, resulted 2

2

in

a calculated second-order

rate

c o n s t a n t o f 6.5 x 1 0 M 4

min

_ 1

- 1

.

H o w e v e r , t h e r e a c t i o n is d e f i n i t e l y d e p e n d e n t o n t h e c o n c e n t r a t i o n o f


the olefin, i n contrast to the R h C l ( t t p ) constant 10 M" 4

2

for

the

equation

r =

2

m i n " . T h e olefin dependence

r

is

rate

1.2 x

about

is u n d e r s t a n d a b l e i n t e r m s

of

for c a t a l y s i s b y R h C l ( t t p ) . I n t h e c a s e

of

1

the m e c h a n i s m p r o p o s e d

system. The third-order

fc[H ][Rh] [octene]

the more h i n d e r e d catalyst R h C l ( C y t t p ) , the e q u i l i b r i u m constant K

3

w o u l d n o t b e e x p e c t e d t o b e as l a r g e ; t h u s , t h e h y d r o g é n a t i o n m e c h a n i s m w i t h this catalyst m a y b e more c o m p l e x a n d m a y i n v o l v e

the

olefin. SOLID LiN(CH ) 3

2

OBTAINED OR AlEt . 3

yellow-orange

FROM

TREATING

I n either toluene

RhCl(ttp)

WITH

or T H F , R h C l ( t t p )

solid on adding L i N ( C H ) . The 3

2

EITHER gives a

solid resulting from

t o l u e n e is d a r k e r o r a n g e t h a n t h a t p r o d u c e d i n T H F ; h o w e v e r , t h e I R s p e c t r a o f these materials are i d e n t i c a l . F u r t h e r m o r e , b a s e d o n the I R spectra, the s a m e p r o d u c t c a n b e o b t a i n e d b y t r e a t i n g a n excess AlEt

3

tate a s o l i d . T h e s p e c t r a s h o w a p e a k at 1 7 9 0 c m E l e m e n t a l analyses from

of

w i t h R h C l ( t t p ) i n t o l u e n e , f o l l o w e d b y a d d i n g ether to p r e c i p i

LiN(CH ) 3

2

of several

different

- 1

a s s i g n e d to ^ ( R h - H ) .

reaction products

obtained

i n T H F gave inconsistent a n d uninterpretable results.

T h u s , w e a r e u n a b l e to p r o p o s e a s a t i s f a c t o r y f o r m u l a t h a t is c o n s i s t e n t w i t h the I R data that show the R h - H m o i e t y b u t no O - H stretching frequency. Acknowledgments W e a r e g r a t e f u l t o t h e N o r t h e r n K e n t u c k y U n i v e r s i t y for a s a b b a t i c a l l e a v e w h i c h w a s g r a n t e d to J . N i e w a h n e r d u r i n g 1 9 7 8 - 1 9 7 9 , t o P. K r e t e r , T . M a z a n e c , a n d R. W a i d for c o l l e c t i n g t h e P - 3 1

NMR

s p e c t r a , a n d to J o h n s o n M a t t h e y a n d I n g e l h a r d I n d u s t r i e s for l o a n s o f small amounts of rhodium trichloride. Literature

Cited

1. White, C. "Organometallic Chemistry"; The Chemical Society, London, 1976, 5. 2. Osborn, J. Α.; Jardine, F. H.; Young, J. F.; Wilkinson, G. J. Chem. Soc. 1966, A , 1711. 3. James, B. R. "Homogeneous Hydrogenation"; Wiley: New York, 1973. 4. Muetterties, E . L., E d . "Transition Metal Hydrides"; Marcel Dekker: New York, 1971. 5. Dubois, D. L.; Meek, D. W. Inorg. Chim. Acta. 1976, 19, L29-30.



272


6. Nappier, T. E . , Jr.; Meek, D. W.; Kirchner, R. M.; Ibers, J . A. J. Amer. Chem. Soc. 1973, 95, 4194. 7. Tolman, C. Α.; Meakin, P. Z.; Lindner, D. L.; Jesson, J. P. J. Amer. Chem. Soc. 1974, 96, 2763. 8. Strauss, S. H . ; Shriver, D. F. Inorg. Chem. 1978, 17, 3069. 9. Diamond, S. E . ; Mares, F. J. Organomet. Chem. 1977, 142, C55-57. 10. Porter, R. Α.; Shriver, D. F. J. Organomet. Chem. 1975, 90, 41. 11. Sloan, M . F.; Matlack, A. S.; Breslow, D. S. J. Am. Chem. Soc. 1963, 85, 4014. 12. Tiethof, J. Α.; Peterson, J. L.; Meek, D. W. Inorg. Chem. 1976, 15, 1365. 13. Hatada, K.; Juki, H . Tetrahedron Lett. 1968, 213-217. 14. Yamamoto, O. Bull. Chem. Soc. Japan 1963, 36, 1463. 15. Levy, G. C.; Nelson, G. L. "Carbon-13 Nuclear Magnetic Resonance for the Organic Chemist"; Wiley-Interscience: New York, 1972, p. 143. RECEIVED July 28, 1980.


ba-1982-0196.ch015

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