5 Catalytic Homogeneous Hydrogenations Using Micellar and Phase Transfer Reaction Conditions
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DANIEL
L.
REGER
a n d M. M. HABIB
University of South Carolina, C o l u m b i a , S C 29208
Product distributions can be altered by the addition of neutral micelle-forming surfactants to the catalyzed
hydrogenation
K [Co(CN) H]3
5
of 2-methyl-1,3-butadiene, 1,3-
pentadiene, and 2,3-dimethyl-1,3-butadiene. Although rate accelerations are modest, the micelles significantly prevent decomposition of the catalyst and solubilize the organic substrates in the aqueous medium.
Phase transfer reaction
conditions have proved to be even more successful for these metal-catalyzed reactions.
The reactions are inexpensive,
easy to set up, generally regioselective and, because of substantial rate accelerations and stabilization of the catalyst, large amounts of substrate are rapidly converted to product.
'"phere
is a c o n s i d e r a b l e
homogeneous
catalysts
research
effort
a i m e d at d e v e l o p i n g n e w
d e r i v e d f r o m t r a n s i t i o n metals
(1,2).
Al-
t h o u g h m a n y v e r y u s e f u l systems of this t y p e a r e b e i n g d e v e l o p e d , t h e i m p a c t of n e w catalysts i n synthetic o r g a n i c c h e m i s t r y has b e e n s o m e w h a t l i m i t e d because m a n y of these c o m p o u n d s a r e either r e l a t i v e l y d i f f i c u l t to p r e p a r e (3,4) o r use expensive starting materials ( 5 ) . A n a l t e r n a t i v e a p p r o a c h f o r d e v e l o p i n g n e w c a t a l y t i c systems u s e f u l i n o r g a n i c synthesis is t o u s e k n o w n catalysts that a r e i n e x p e n s i v e a n d a r e easy t o p r e p a r e u n d e r u n u s u a l e x p e r i m e n t a l c o n d i t i o n s . T o this e n d , w e h a v e b e e n i n v e s t i g a t i n g the h y d r o g e n a t i o n of w a t e r - i n s o l u b l e substrates, u s i n g t h e w a t e r s o l u b l e catalyst K [ C o ( C N ) H ] (6). 3
5
simply mixing K C N and C o C l
2
T h i s catalyst is easily p r e p a r e d b y
i n a h y d r o g e n atmosphere.
I n hydro-
g e n a t i o n reactions, i t g e n e r a l l y o n l y h y d r o g e n a t e s c o n j u g a t e d C - C d o u b l e 0-8412-0429-2/79/33-173-043$05.00/0 © 1979 American Chemical Society King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
44
INORGANIC
bonds.
COMPOUNDS WITH UNUSUAL
T h u s , dienes are h y d r o g e n a t e d to monoenes.
extensively (6,7,8), reactions.
PROPERTIES
II
Although studied
this catalyst has not g e n e r a l l y b e e n u s e f u l f o r o r g a n i c
T h e reactions are g e n e r a l l y not regioselective.
2-methyl-l,3-butadiene
(isoprene)
F o r example,
is h y d r o g e n a t e d to a l l three p o s s i b l e
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monoenes as s h o w n b e l o w .
S o m e c o n t r o l o v e r the p r o d u c t s c a n be g a i n e d b y v a r y i n g the c y a n i d e - t o c o b a l t ratio ( 7 ) .
O t h e r p r o b l e m s are that the catalyst is i n h i b i t e d b y
excess substrate a n d that the l i f e t i m e of the catalyst is l i m i t e d , e s p e c i a l l y if a s t o i c h i o m e t r i c a m o u n t of K C N is a d d e d . T h i s s u m m a r y covers results o n the use of the K [ C o ( C N ) H ] cata3
5
lyst to h y d r o g e n a t e dienes u n d e r t w o sets of u n u s u a l r e a c t i o n c o n d i t i o n s . T h e first is to c a r r y o u t the reactions i n the presence concentrations of n e u t r a l m i c e l l e - f o r m i n g surfactants.
of f a i r l y
high
T h e s e c o n d is to
c a r r y out the reactions u n d e r phase-transfer r e a c t i o n c o n d i t i o n s . Reactions in Micellar
Solution
T h e k i n e t i c s a n d m e c h a n i s m s of m a n y o r g a n i c a n d i n o r g a n i c reactions are g e n e r a l l y a l t e r e d b y the presence of m i c e l l e s ( 9 ) .
F o r example, a
rate a c c e l e r a t i o n i n the i n o r g a n i c r e a c t i o n of H g - i n d u c e d a q u a t i o n of + 2
[Co(NH ) Cl] 3
expected
5
that
+ 2
of factors u p to 140,000 has b e e n o b s e r v e d (10).
s i m i l a r effects w o u l d
be
observed
It w a s
if t r a n s i t i o n
metal
c a t a l y z e d h y d r o g e n a t i o n s w e r e c a r r i e d out i n the presence of m i c e l l e s . T h e c o n d i t i o n s that w e r e chosen ( a f u l l a c c o u n t of this phase of this research has b e e n p u b l i s h e d i n R e f . 11)
for the h y d r o g e n a t i o n reactions,
u s i n g the K [ C o ( C N ) H ] catalyst (4.4 m m o l ) w e r e 40 m L of w a t e r , 1 3
5
a t m H , r e p e a t e d s m a l l (0.1 m L ) injections of substrate, constant i o n i c 2
strength, a n d r e a c t i o n times never >
24 h r . T h e catalyst is m o r e stable
u n d e r a h y d r o g e n atmosphere, a n d the p r o b l e m s d e s c r i b e d b y others that w e r e caused b y a g i n g reactions (12)
w e r e not e n c o u n t e r e d .
T a b l e s I, I I , a n d I I I s h o w the effect of the n e u t r a l surfactant B r i j 35 (C
1 2
H 5 ( O C H C H ) 2 3 0 H , concentration = 2
2
2
5.0 X 10" M) o n the h y d r o 2
g e n a t i o n of three dienes at v a r i o u s C N : C o ratios (11).
Rate
accelerations
m e a s u r e d b y h y d r o g e n u p t a k e w e r e m o d e r a t e , less t h a n t w o f o l d .
For
isoprene ( T a b l e I), the m i c e l l e s f a v o r the p r o d u c t i o n of 2-methyl-2-butene, m a k i n g the r e a c t i o n v i r t u a l l y regiospecific.
F o r 2,3-dimethyl-l,3-buta-
d i e n e ( T a b l e I I ) , t h e m i c e l l e s f a v o r p r o d u c t i o n of 2 , 3 - d i m e t h y l - l - b u t e n e .
King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
5.
REGER AND HABIB Table I.
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CN:Co
Micellar,
45
Phase Transfer Rxn Conditions
Product Distribution of K [Co ( C N ) H ] - C a t a l y z e d Hydrogenation of 2-Methyl-1,3-butadiene 3
Ratio
Surfactant
5
Product
—
4.3 4.3 5.2 5.2 6.0 6.0
Percentages
7 2 9 5 22 19
3 1 3 2 43 29
B r i j 35
—
B r i j 35
—
B r i j 35
90 97 88 93 35 52
T h i s m a t e r i a l c a n b e p r o d u c e d 1 0 0 % selectively at a C N : C o ratio of seven. T h e presence of m i c e l l e s o n t h e h y d r o g e n a t i o n of 1,3-pentadiene III)
(Table
causes m o r e 2-pentene to be p r o d u c e d b u t c o m p l i c a t e s the r e a c t i o n
s o m e w h a t because a l a r g e r a m o u n t of d s - 2 - p e n t e n e is f o r m e d . I n a d d i t i o n to these changes i n p r o d u c t d i s t r i b u t i o n s , t h e m i c e l l e s affect the r e a c t i o n i n other w a y s . A s m e n t i o n e d earlier, the catalyst is n o t v e r y stable, e s p e c i a l l y at C N : C o ratios
