Phase-Transfer Catalysis - American Chemical Society

Chapter 12

Triphase Catalysis in Organometallic Anion Chemistry Robert A. Sawicki

Downloaded by CORNELL UNIV on September 3, 2016 | http://pubs.acs.org Publication Date: December 19, 1987 | doi: 10.1021/bk-1987-0326.ch012

Texaco Research Center, Beacon, NY 12508

The ability of both simple metal oxides and functionalized derivatives to assist in various chemical transformations is well documented. When these reactions are performed in the presence of a second insoluble reagent, a triphasic solid-liquid-solid reaction system can result. Immobilized polyethylene glycols have been shown to promote both displacement and carbonylation reactions. In those schemes where organometallic anions are produced, anionically activated alumina was found to be a superior reagent. Using the inorganic base/metal oxide combination, both dicobalt octacarbonyl and iron pentacarbonyl are converted to the corresponding metal anions. Reaction with carbon monoxide, alcohols and electrophiles such as benzyl halides and oxiranes produce the corresponding esters following a carboxyalkylation scheme. The advantages of this system include high selectivities, good yields, mild reaction conditions, and facile separations. Attempts have been made to identify the portion of the catalytic cycle which appears favorably influenced by the metal oxide surface.

The p r e p a r a t i o n o f n o v e l phase t r a n s f e r c a t a l y s t s and t h e i r a p p l i c a t i o n i n s o l v i n g s y n t h e t i c problems a r e w e l l d o c u m e n t e d ( l ) . Compounds such as q u a t e r n a r y ammonium and phosphonium s a l t s , phosphoramides, crown e t h e r s , c r y p t a n d s , and o p e n - c h a i n p o l y e t h e r s promote a v a r i e t y o f a n i o n i c r e a c t i o n s . These i n c l u d e a l k y l a t i o n s ( 2 ) , carbene r e a c tions (3), y l i d e reactions(4), epoxidations(5), polymerizations(6), reductions(7), oxidations(8), eliminations(9), and d i s p l a c e m e n t r e a c t i o n s ( 1 0 ) t o name o n l y a few. The unique a c t i v i t y o f a p a r t i c u l a r c a t a l y s t rests i n i t s a b i l i t y t o transport the i o n across a phase boundary. T h i s boundary i s n o r m a l l y one w h i c h s e p a r a t e s two i m m i s c i b l e l i q u i d s i n a b i p h a s i c l i q u i d - l i q u i d r e a c t i o n system.

0097-6156/87/0326-0143$06.00/0 © 1987 American Chemical Society

Starks; Phase-Transfer Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


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Recent e f f o r t s have c o n c e n t r a t e d on the i m m o b i l i z a t i o n o f these m a t e r i a l s onto b o t h o r g a n i c p o l y m e r s ( 1 1 ) and m e t a l o x i d e s ( 1 2 ) t o s i m p l i f y , by f i l t r a t i o n , the s e p a r a t i o n , r e c o v e r y and r e c y c l e proc e s s . These s u p p o r t e d c a t a l y s t s now f u n c t i o n i n a t r i p h a s i c e n v i r o n ment i n e i t h e r a l i q u i d - s o l i d - l i q u i d o r s o l i d - l i q u i d - s o l i d r e a c t i o n m i x t u r e . I n t h i s c a s e , the c a t a l y s t must t r a n s f e r the a n i o n from the s u r f a c e o f the c r y s t a l l a t t i c e t o the l i q u i d phase. Here a d s o r p t i o n phenomena o f t e n s i g n i f i c a n t l y a f f e c t the r e a c t i o n r a t e ( 1 3 ) . Simple r e f r a c t o r y o x i d e s such as a l u m i n a have a l s o been shown t o promote c h e m i c a l t r a n s f o r m a t i o n s ( 1 4 ) . D i s p l a c e m e n t and o x i d a t i o n r e a c t i o n s ( 1 5 ) as w e l l as d e s u l f u r i z a t i o n s ( 1 6 ) and o x i d a t i v e coupl i n g s (17) are enhanced through the use o f impregnated s u p p o r t s . The main advantages observed i n t h i s a r e a are a g a i n improved s e l e c t i v i t i e s , r e a c t i v i t i e s , and s i m p l i f i e d s e p a r a t i o n s . Phase t r a n s f e r c a t a l y s i s has more r e c e n t l y been a p p l i e d t o the important area of organometallic chemistry(18). Reported a p p l i c a t i o n s i n c l u d e b o t h the p r e p a r a t i o n ( 1 9 ) and the use o f t r a n s i t i o n metal catalysts i n isomerizations(20), carbonylations(21) and reductions(22). The f i r s t a p p l i c a t i o n o f phase t r a n s f e r c a t a l y s i s i n m e t a l c a r b o n y l c h e m i s t r y was r e p o r t e d by A l p e r i n 1977(23). I t was found t h a t m e t a l c a r b o n y l a n i o n s c o u l d be r e a d i l y g e n e r a t e d by this t e c h n i q u e and used t o p r e p a r e p i - a l l y l , c l u s t e r , and o r t h o - m e t a l a t e d complexes(24). Our e f f o r t s i n t h i s a r e a o f c a t a l y s i s began i n 1980. Our i n i t i a l emphasis was on the p r e p a r a t i o n o f s u p p o r t e d phase t r a n s f e r c a t a l y s t s . We l a t e r became i n t e r e s t e d i n the c h e m i s t r y o f a n i o n i c a l l y a c t i v a t e d a l u m i n a ( 2 5 ) and the r e a c t i v i t y o f m e t a l c a r b o n y l a n i o n s p r e p a r e d under these c o n d i t i o n s . A b r i e f d e s c r i p t i o n o f our work i n the p r e p a r a t i o n o f these m a t e r i a l s and t h e i r s y n t h e t i c applications follows. The p o t e n t i a l o f u s i n g phase t r a n s f e r c a t a l y s i s i n s o l v i n g l a r g e s c a l e i n d u s t r i a l chemical problems has resulted i n an e x p l o s i o n o f i d e a s i n b o t h the p r e p a r a t i o n and u t i l i z a t i o n o f new m a t e r i a l s . One such c l a s s o f c a t a l y s t i s the h i g h m o l e c u l a r w e i g h t polyethylene g l y c o l s ( 2 6 ) w h i c h have been a t t a c h e d to organic polymers t o a l l o w f o r f a c i l e s e p a r a t i o n s ( 2 7 ) . T h i s a b i l i t y i s o f c o u r s e most i m p o r t a n t i n many i n d u s t r i a l p r o c e s s e s where the c r i t e r i a s e t f o r c a t a l y s t s e l e c t i o n i n c l u d e s r e c o v e r a b i l i t y as w e l l as s t a b i l i t y , a v a i l a b i l i t y and s a f e t y ( 2 8 ) . Our i n t e r e s t i n p o l y e t h y l e n e g l y c o l s c e n t e r e d on a s i m p l e scheme t o i m m o b i l i z e these m a t e r i a l s onto m e t a l o x i d e s u r f a c e s . The s u r f a c e o f s i l i c a g e l c o n t a i n s b o t h s i l a n o l - O H groups and -0s t r a i n e d s i l o x a n e g r o u p s ( 2 9 ) . A s i m p l e s y n t h e t i c pathway t o produce c o v a l e n t l y bonded g l y c o l s was proposed where r e a c t i o n ( 3 0 ) would o c c u r between the OH group o f the g l y c o l and the s u r f a c e o f a r e f r a c t o r y oxide. R e s u l t s and D i s c u s s i o n I m m o b i l i z e d P o l v a l k y l e n e G l y c o l s . P o l y a l k y l e n e g l y c o l s and monom e t h y l e t h e r s o f v a r i o u s m o l e c u l a r w e i g h t s were h e a t e d a t t o l u e n e r e f l u x w i t h b o t h s i l i c a g e l and alumina. Carbon a n a l y s e s o f the



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p r o d u c t s i n d i c a t e d the degree o f f u n c t i o n a l i z a t i o n t o be between 0.1 and 0.4 mmoles o f g l y c o l p e r gram o f s u p p o r t w h i c h i s t y p i c a l o f the c a p a c i t y o f m e t a l o x i d e s . The powders a r e n o r m a l l y S o x h l e t e x t r a c t e d w i t h t o l u e n e and/or methanol t o remove any adsorbed g l y c o l p r i o r t o use. S e v e r a l examples are g i v e n i n T a b l e 1 ( 3 1 ) . These m a t e r i a l s were subsequently evaluated under phase t r a n s f e r c o n d i t i o n s t o determine t h e i r c a t a l y t i c a b i l i t y ( 3 2 ) . The r e s u l t s from the d i s p l a c e m e n t r e a c t i o n o f p o t a s s i u m a c e t a t e w i t h 1-bromobutane i n v a r i o u s s o l v e n t s are g i v e n i n T a b l e I I . I n t o l u e n e , the p o l y e t h y l e n e g l y c o l on s i l i c a showed the h i g h e s t a c t i v i t y . The p o l y p r o p y l e n e g l y c o l a n a l o g was however much l e s s a c t i v e . I t has been r e p o r t e d i n the l i t e r a t u r e ( 3 3 ) t h a t p o l y e t h y l e n e glycols develop a h e l i c a l conformation t h a t a l l o w s f o r t h e i r a b i l i t y to complex m e t a l c a t i o n s . P o l y p r o p y l e n e g l y c o l s , on the o t h e r hand, were found t o have a n o n - p l a n a r z i g z a g c h a i n because o f the s t e r i c e f f e c t o f the methyl group. T h e i r low e x t r a c t i n g power may be a t t r i b u t e d t o t h i s c o n f o r m a t i o n a l phenomena. Table I . P o l y a l k y l e n e G l y c o l s Glvcol(a) PEG-400 PEG-400 PEGMME-350 PEGMME-350 PPG-425 PPG-425

I m m o b i l i z e d onto M e t a l Oxide S u r f a c e s

Support Si02 A1203 Si02 A1203 Si02 A1203

%C 6.9 4.1 5.6 3.1 6.0 3.8

mmoles z l v c o l / g r a m 0.33 0.20 0.27 0.17 0.30 0.19

(a) PEG-400 - p o l y e t h y l e n e g l y c o l , avg. mol. w e i g h t 400; PEGMME-350 - p o l y e t h y l e n e g l y c o l monomethyl e t h e r , avg. mol. w e i g h t 350; PPG-425 - p o l y p r o p y l e n e g l y c o l , avg. mol. w e i g h t 425.

Table I I .

Catalyst Si02 PEG-400 PEG-400/SiO2 PEG-400/A1203 PPG-425/SÎ02 PEG-400/SÎ02 PEG-400/A12O3

R e a c t i o n o f 1-Bromobutane w i t h Potassium Acetate at Solvent R e f l u x Cone (a) -10 6.6 4.0 6.0 6.6 4.0

Time(b) 3(6) 3 3(6) 3 3(6) 3 3

Solvent PhCH3 PhCH3 PhCH3 PhCH3 PhCH3 PhCH3/H20 H20

%Yield(c) 0(37) 32 58(70) 33 7(10) 0 49

(a) C o n c e n t r a t i o n as mmoles o f g l y c o l ( c a l c ) . (b) Time i n h o u r s e i t h e r 3 o r 6. (c) Y i e l d s a r e based on gas chromatographic a n a l y s i s u s i n g decane as the i n t e r n a l s t a n d a r d . P o l y a l k y l e n e g l y c o l s i m m o b i l i z e d onto m e t a l o x i d e s appear most s u i t e d f o r r e a c t i o n s t h a t do n o t r e q u i r e the t r a n s p o r t a t i o n o f an a n i o n a c r o s s a l i q u i d - l i q u i d phase boundary. The displacement


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r e a c t i o n o c c u r s r e a d i l y i n e i t h e r an o r g a n i c o r aqueous r e a c t i o n medium b u t none i s o b s e r v e d when an a q u e o u s - o r g a n i c b i p h a s i c m i x t u r e i s employed. O v e r a l l , the r e a c t i v i t y o f t h e s e c a t a l y s t s f o l l o w s the s o l u b i l i t y o f the n u c l e o p h i l e i n pure g l y c o l . T h i s was demonstrated e x p e r i m e n t a l l y by comparing the r e a c t i o n s o f sodium i o d i d e , p o t a s sium a c e t a t e , sodium a c e t a t e and p o t a s s i u m c y a n i d e w i t h bromobutane. The yields followed the salt solubility in pure glycol (NaI>K0AONa0AOKCN) . Carbonylation of Benzyl Halides. Several organometallic reactions i n v o l v i n g a n i o n i c s p e c i e s i n an a q u e o u s - o r g a n i c two-phase r e a c t i o n system have been effectively promoted by phase transfer catalysts(34)· These i n c l u d e r e a c t i o n s o f c o b a l t and i r o n complexes. A f a v o r i t e model r e a c t i o n i s the c a r b o n y l a t i o n o f b e n z y l h a l i d e s u s i n g the c o b a l t t e t r a c a r b o n y l a n i o n c a t a l y s t . Numerous examples have appeared i n the l i t e r a t u r e ( 3 5 ) on the preparation of p h e n y l a c e t i c a c i d u s i n g aqueous sodium h y d r o x i d e as the base and t r i a l k y l a m m o n i u m s a l t s ( E q u a t i o n 1 ) . These r e a c t i o n s o c c u r a t low p r e s s u r e s o f c a r b o n monoxide and m i l d r e a c t i o n t e m p e r a t u r e s . E a r l y work on the c a r b o n y l a t i o n o f a l k y l h a l i d e s r e q u i r e d the use of sodium amalgam t o g e n e r a t e the c o b a l t t e t r a c a r b o n y l a n i o n from the c o b a l t dimer(.36) .

CH2X

0 II CH2C0R

As, f o r the most p a r t , the c o r r e s p o n d i n g e s t e r d e r i v a t i v e s are a more i m p o r t a n t s y n t h e t i c t a r g e t , r e c e n t l i t e r a t u r e has demons t r a t e d methods t o p r e p a r e the e s t e r s d i r e c t l y . Examples i n c l u d e the use o f n i c k e l c a r b o n y l i n a methanol/dimethylformamide solvent s y s t e m ( 3 7 ) ; the d i r e c t c o n v e r s i o n o f b e n z y l a l c o h o l t o m e t h y l p h e n y l a c e t a t e u s i n g c o b a l t c a r b o n y l (38.) and a r e a c t i o n system w h i c h u t i l i z e s an ammonium s a l t bound t o an o r g a n i c p o l y m e r ( 3 9 ) . As the s u p p o r t e d g l y c o l c a t a l y s t s worked b e t t e r i n p r o m o t i n g r e a c t i o n s i n a s i n g l e s o l v e n t system, we explored the d i r e c t c a r b o n y l a t i o n o f b e n z y l h a l i d e s u s i n g an a l c o h o l s o l v e n t , b a s e , and cobalt carbonyl. Our initial experiments concentrated on the r e a c t i o n o f b e n z y l bromide a t room temperature and one atmosphere c a r b o n monoxide. We chose sodium h y d r o x i d e as the b a s e , methanol as the s o l v e n t , and l o o k e d a t the p r o d u c t d i s t r i b u t i o n . We were i n t e r e s t e d i n the s e l e c t i v i t y t o e s t e r and the r e a c t i v i t y o f t h i s system. The r e s u l t s are g i v e n i n T a b l e I I I . P h e n y l a c e t i c a c i d was found t o be the main carbonylation p r o d u c t when sodium h y d r o x i d e was the base. The p r e s e n c e o f a phase t r a n s f e r c a t a l y s t , e i t h e r the c o m m e r c i a l l y a v a i l a b l e q u a t e r n a r y


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Table I I I . Base


C a r b o n y l a t i o n o f B e n z y l Bromide i n M e t h a n o l ( a ) PhCH20CH3

Catalyst

NaOH NaOH NaOH NaOH/Al203 NaOH/Al203

147


BTEAC* PEGMME-350/A1203 BTEAC*

PhCH2C02CH3

PhCH2C02H

0 0 0 3 37

17 61 75 42 3

66 29 25 21 29

(a) R e a c t i o n c o n d i t i o n s — room temperature, 24 h o u r s , 1 atm CO, base (50 mmoles), b e n z y l bromide (25 mmoles), d i c o b a l t o c t a r b o n y l (1.5 mmoles), and methanol (75 m l ) . (b) E t h e r and e s t e r y i e l d s were d e t e r m i n e d by VPC, a c i d y i e l d was o b t a i n e d f o l l o w i n g e x t r a c t i o n and i s o l a t i o n . * BTEAC - B e n z y l t r i e t h y l a m m o n i u m c h l o r i d e .

ammonium s a l t (BTEAC) o r our g l y c o l e t h e r on a l u m i n a c a t a l y s t , f a v o r e d the f o r m a t i o n o f the e t h e r v i a a W i l l i a m s o n e t h e r s y n t h e t i c pathway(40). A r e p o r t u s i n g i r o n p e n t a c a r b o n y l i n a s i m i l a r scheme s u g g e s t e d t h a t a two-phase l i q u i d system was r e q u i r e d t o p r e v e n t the f o r m a t i o n o f the e t h e r ( 4 1 ) . A n i o n i c a l l y A c t i v a t e d Alumina. A t t h i s time we had a l s o d e v e l o p e d an i n t e r e s t i n a n i o n i c a l l y a c t i v a t e d a l u m i n a . These b a s i c r e a g e n t s were active in promoting alkylation(42), condensation(43) and h y d r o l y s i s ( 4 4 ) r e a c t i o n s . Thus, we impregnated a l u m i n a w i t h sodium h y d r o x i d e and used t h i s c o m b i n a t i o n b o t h w i t h and w i t h o u t a phase t r a n s f e r c a t a l y s t ( b e n z y l t r i e t h y l ammonium c h l o r i d e ) . When BTEAC was added, the c o n v e r s i o n t o e t h e r was d e c r e a s e d and the f o r m a t i o n o f e s t e r was n o t e d . I n the absence o f a phase t r a n s f e r c a t a l y s t , the e t h e r became a minor p r o d u c t and m e t h y l p h e n y l a c e t a t e became the major p r o d u c t w i t h c o p r o d u c t i o n o f p h e n y l a c e t i c a c i d . T h i s e s t e r does n o t r e s u l t from e s t e r i f i c a t i o n o f the a c i d as s i m p l e s t i r r i n g o f p h e n y l a c e t i c a c i d w i t h Na0H/Al203 i n methanol does n o t produce methyl phenylacetate. To f u r t h e r e x p l o r e t h i s phenomenon, we p r e p a r e d a v a r i e t y o f base on a l u m i n a r e a g e n t s . T h e i r a c t i v i t y i n the c a r b o x y m e t h y l a t i o n o f b e n z y l bromide i s p r e s e n t e d i n T a b l e IV. The r e a c t i o n c o n d i t i o n s were m i l d (room t e m p e r a t u r e , 1 atm CO) and a t w o - f o l d excess o f base was used a l o n g w i t h a c a t a l y t i c amount o f c o b a l t c a r b o n y l . The p r o d u c t d i s t r i b u t i o n was q u a n t i f i e d by VPC. The m i x t u r e s c o n t a i n e d s t a r t i n g m a t e r i a l , e s t e r p r o d u c t , and v a r i o u s amounts o f m e t h y l b e n z y l e t h e r . No d e t e c t a b l e amounts o f b e n z y l a l c o h o l , k e t o n e s , o r hydrocarbons were seen. P o t a s s i u m methoxide a l o n e a f f o r d e d m o s t l y the e t h e r . A m i x t u r e o f p o t a s s i u m methoxide and a l u m i n a gave a s l i g h t improvement i n e s t e r y i e l d b u t the predomi n a n t p r o d u c t was a g a i n the e t h e r . I n c o n t r a s t , when p o t a s s i u m methoxide on a l u m i n a was used, the c a r b o x y a l k y l a t e d p r o d u c t , m e t h y l p h e n y l a c e t a t e , was p r e p a r e d i n 70 y i e l d w i t h l i t t l e e t h e r d e t e c t e d . B e n z y l c h l o r i d e r e a c t e d i n a s i m i l a r f a s h i o n under t h e s e m i l d r e a c t i o n c o n d i t i o n s . Other a l k o x i d e and c a r b o n a t e bases c o u l d be used as

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T a b l e IV.

Carboxymethylation Base


A1203 MeOK MeOK+Al203 MeOK/Al203 MeONa/Al203 EtONa/Al203 t-BuOK/Al203 Na2C03/Al203 NaHC03/Al203 (a)

(b)

o f B e n z y l Bromide(a) Y i e l d . %(b) 0 5 21 30 70(53) 74 65 45 39(23) 67

R e a c t i o n c o n d i t i o n s = room temperature, 1 atm CO, 24 h o u r s , base (50 mmoles), benzyl bromide (25 mmoles), dicobalt o c t a c a r b o n y l (1 mmole) and methanol (75 m l ) . Y i e l d s were determined by VPC using internal standard techniques, d i s t i l l e d i s o l a t e d y i e l d s are given i n parentheses.

w e l l . The a l k o x i d e s c o u l d be p r e p a r e d by r e a c t i o n o f sodium hydroxi d e w i t h t h e a l c o h o l and d e p o s i t i o n o f t h e m i x t u r e onto a l u m i n a . Using this base (Na0Me/Al203) methyl phenylaceteate was o b t a i n e d i n 74 p e r c e n t y i e l d . A l s o , o t h e r e s t e r s , such as i s o b u t y l p h e n y l a c e t a t e , c o u l d be p r e p a r e d by r e a c t i o n i n t h e a p p r o p r i a t e s o l v e n t ( i s o b u t a n o l , 67 p e r c e n t y i e l d ) u s i n g t h e sodium a l k o x i d e on alumina reagent(45). The f o r m a t i o n o f b y p r o d u c t m e t h y l b e n z y l e t h e r was t h e key r e a s o n f o r t h e low s e l e c t i v i t y t o e s t e r i n t h e absence o f a l u m i n a . A more c a r e f u l e x a m i n a t i o n o f t h e p r o d u c t d i s t r i b u t i o n s w i t h time was made u s i n g t h e a l k o x i d e , a l k o x i d e on a l u m i n a and b i c a r b o n a t e on a l u m i n a bases. The r e s u l t s from T a b l e V i n d i c a t e t h a t t h e f o r m a t i o n o f e t h e r was i n d e e d t h e predominant pathway w i t h a l k o x i d e a l o n e , w h i l e t h e presence o f a l u m i n a r e t a r d e d t h i s c o n v e r s i o n and promoted the c a r b o x y a l k y l a t i o n pathway. The b i c a r b o n a t e on a l u m i n a gave little ether product and e x c e l l e n t s e l e c t i v i t y t o t h e methyl phenylacetate. The n e x t s e r i e s o f experiments were r u n v a r y i n g t h e base c o m p o s i t i o n ( T a b l e V I ) . W i t h o n l y a s l i g h t excess o f base, t h e y i e l d o f b y p r o d u c t e t h e r was much lower u s i n g p o t a s s i u m methoxide, b e n z y l bromide, and 1 atm CO. The a l k o x i d e on a l u m i n a r e a g e n t s a g a i n gave h i g h e r c o n v e r s i o n s and s e l e c t i v i t i e s t o e s t e r . A l a r g e r enhancement was observed u s i n g sodium b i c a r b o n a t e . T h i s l a r g e l y i n s o l u b l e base was v e r y i n a c t i v e i n p r o m o t i n g the c a r b o x y a l k y l a t i o n r e a c t i o n even i n t h e presence o f a phase t r a n s f e r c a t a l y s t . S i m p l y m i x i n g w i t h alumina d i d n o t improve t h i s a c t i v i t y . However, when d e p o s i t e d onto a l u m i n a , t h e r e s u l t i n g base was much more e f f i c i e n t and good y i e l d s and h i g h s e l e c t i v i t i e s t o e s t e r were n o t e d . T h i s a b i l i t y does n o t appear t o be r e s t r i c t e d t o alumna as i t was demonstrated t h a t o t h e r m e t a l o x i d e s such as s i l i c a and t i t a n i a work e q u a l l y w e l l .


12.


SAWICKI

T a b l e V.

C a r b o x y m e t h y l a t i o n o f B e n z y l Bromide S e l e c t i v i t y to Ester(a)

Base

Time

MeOK

MeONa/Al203


NaHC03/A1203

(a)

(b)

1 4 24 1 4 24 1 4 24

27 33 36 56 74 74 31 51 67

31 48 60 12 13 13 0 0 6

C a r b o x y m e t h y l a t i o n o f B e n z y l Bromide Ester vs Ether(a)

Base MeOK Me0K/Al203 Me0Na/Al203 NaHC03 NaHC03+BTEAC NaHC03+A1203 NaHC03/Al203 NaHC03/PEGMME-350/Al203 NaHC03/Si02 NaHC03Ti02 K2C03* K2C03/A1203*

(b) *

Hr Hrs Hrs Hr Hrs Hrs Hr Hrs Hrs

Product D i s t i l l a t i o n ( b ) Ether.% Ester.%

R e a c t i o n c o n d i t i o n s — room temperature, 1 atm CO, base (50 mmoles), b e n z y l bromide (25 mmoles), d i c o b a l t o c t a c a r b o n y l (1 mmole) and methanol (75 ml) Yields were determined by VPC using internal standard t e c h n i q u e s , r e m a i n i n g m a t e r i a l was s t a r t i n g h a l i d e .

Table V I .

(a)

149

Product D i s t r i b u t i o n ( b ) Ether.% Ester.% 43 55 52 15 16 25 49 50 54 50 23 17

14 3 8 3 4 4 3 2 2 2 33 28

R e a c t i o n c o n d i t i o n s = room temperature, 1 atm CO, 4 h o u r s , base (30 mmoles), methanol (75 m l ) , b e n z y l bromide (25 mmoles), d i c o b a l t o c t a c a r b o n y l (1 mmole). Y i e l d s were d e t e r m i n e d by VPC u s i n g i n t e r n a l s t a n d a r d s . I r o n p e n t a c a r b o n y l (1 mmole) used as c a t a l y s t .

A r e c e n t r e p o r t ( 4 6 ) on t h e use o f i r o n c a r b o n y l and p o t a s s i u m c a r b o n a t e i n a s i m i l a r c a r b o x y a l k y l a t i o n scheme t o p r e p a r e m e t h y l p h e n y l a c e t a t e prompted us t o examine t h e use o f c a r b o n a t e on a l u m i n a i n a s i m i l a r manner. I t was suggested t h a t i f t h e amount o f f r e e base was l e s s t h a n the amount o f i r o n c a r b o n y l t h a n e t h e r f o r m a t i o n would n o t o c c u r b e i n g t h a t i r o n c a r b o n y l was a b e t t e r e l e c t r o p h i l e t h a n b e n z y l h a l i d e . Under our c o n d i t i o n s , t h e m e t a l c a r b o n y l a n i o n


150


was formed b u t the y i e l d s o f e s t e r and s e l e c t i v i t y was t h a n when c o b a l t c a r b o n y l was the c a t a l y s t .

much

poorer

C a t a l y t i c C y c l e . A t t e m p t s t o determine the reasons f o r the improved a c t i v i t y o f the base on a l u m i n a r e a g e n t s f o l l o w e d two p a t h s . B e i n g as s o l u b i l i t y o f base i n methanol appeared t o g r e a t l y e f f e c t the p r o d u c t i o n o f m e t h y l b e n z y l e t h e r , we compared the amount o f e x t r a c t a b l e base w i t h s e l e c t i v i t y t o e s t e r . T h i s c o n c e n t r a t i o n o f b a s e , presumably a l k o x i d e , was d e t e r m i n e d by s t i r r i n g the base i n methanol, f i l t e r i n g , and t i t r a t i n g the f i l t r a t e w i t h a c i d . T h i s c o m p a r i s o n i s g i v e n as T a b l e V I I .


Table V I I .

Base MeOK Me0K/Al203 NaHC03 NaHC03/Al203 NaHC03/Si02 (a)

(b)

E x t r a c t a b l e / T i t r a t a b l e Base vs Carboxyalkylation A c t i v i t y %Titrated(a) 67 35 0.3 2.3 2.3

Ester.% 43 55 15 60 54

Selectivitv(b) 57 71 75 82 76

The base (30 mmoles) and methanol (75 ml) were s t i r r e d a t room temperature f o r f i f t e e n m i n u t e s , f i l t e r e d , and the f i l t r a t e t i t r a t e d t o the p h e n o l p h t h a l e i n e n d p o i n t w i t h IN HC1. S e l e c t i v i t y i s d e f i n e d as the p e r c e n t o f c o n v e r t e d benzyl bromide w h i c h appears as m e t h y l p h e n y l a c e t a t e .

I n the c a s e s where s u b s t a n t i a l base can be termed e x t r a c t a b l e t i t r a t a b l e , i . e . , methoxide, the d e p o s i t i o n onto a l u m i n a r e s u l t s i n a d e c r e a s e i n c o n c e n t r a t i o n o f base and an i n c r e a s e i n b o t h y i e l d and s e l e c t i v i t y t o m e t h y l p h e n y l a c e t a t e . I n the b i c a r b o n a t e case, the d e p o s i t i o n r e s u l t s i n an i n c r e a s e i n a v a i l a b l e base and the y i e l d of ester i s increased d r a m a t i c a l l y . I t i s p o s t u l a t e d t h a t the slow s t e p i n the c a r b o x y a l k y l a t i o n o f a l k y l h a l i d e s i s the c l e a v a g e o f the a c y l t e t r a c a r b o n y l c o b a l t compound(47). These m a t e r i a l s r e a c t s l o w l y w i t h a l c o h o l s a t room temperature and more r a p i d l y w i t h a l k o x i d e i o n . I t seems l i k e l y t h a t the h i g h e r a c t i v i t y o f the base on a l u m i n a r e a g e n t s , can be r e l a t e d to t h e i r a b i l i t y to supply a s u f f i c i e n t q u a n t i t y of a l k o x i d e to f a v o r the c a r b o x y a l k y l a t i o n s t e p but not a q u a n t i t y h i g h enough t o produce s u b s t a n t i a l amounts o f e t h e r . T h i s can be f u r t h e r substant i a t e d by o b s e r v i n g the i n f r a r e d spectrum o f the r e a c t i o n i n progress. Before a d d i t i o n o f the b e n z y l h a l i d e , the o n l y carbonyl a d s o r p t i o n peak i s found a t 1900 cm"l, i n d i c a t i v e o f the c o b a l t tetracarbonyl anion. After addition, this band immediately d i s a p p e a r s and peaks a t 2000 cm~l are observed. These most l i k e l y r e p r e s e n t the c o r r e s p o n d i n g a c y l complex. R e a c t i o n w i t h methoxide y i e l d s the p r o d u c t and r e g e n e r a t e s the c o b a l t a n i o n . I n the absence o f s u f f i c i e n t methoxide, the r e a c t i o n r e q u i r e s a t t a c k by the much


12.

SAWICKI


151

p o o r e r n u c l e o p h i l e ( m e t h a n o l ) . The complete c a t a l y t i c c y c l e i s shown (Equation 2). The c o m b i n a t i o n o f base and a l u m i n a , i n e s s e n c e , r e p r e s e n t s a t r i p h a s i c s o l i d - l i q u i d - s o l i d c a t a l y t i c system p a r t i c u l a r l y when h i g h l y i n s o l u b l e bases are used. W h i l e the r e s u l t s suggest t h a t


CH2C02CH3

CH2X

(2)

the key i n g r e d i e n t i n i n c r e a s e d a c t i v i t y i s a s o l u t i o n phenomenon, the i n t e r a c t i o n o f the s u b s t r a t e , c a t a l y s t or r e a c t i o n i n t e r m e d i a t e w i t h the m e t a l o x i d e s u r f a c e cannot be downplayed i n importance. C a r b o x y a l k y l a t i o n o f P r o p y l e n e Oxide. These r e a g e n t s were a l s o used in a similar carboxyalkylation scheme to prepare methyl 3 - h y d r o x y b u t y r a t e by r e a c t i o n w i t h p r o p y l e n e o x i d e (Equation 3). T h i s might r e p r e s e n t a way t o p r e p a r e s u b s t i t u t e 1,3 d i o l s ( 4 8 ) following reduction of the ester or reactive monomers by pyrolys is/dehydration. L i t e r a t u r e examples on the r e a c t i o n o f c o b a l t carbonyl(49), cobalt carbonyl anion(50), and i r o n carbonyl anion(51.) with p r o p y l e n e o x i d e have been r e p o r t e d . However e i t h e r the r e a c t i o n c o n d i t i o n s r e q u i r e d are s e v e r e , the y i e l d s are low, o r the r e a c t i o n i s not c a t a l y t i c . The r e s u l t s from our work on the r e a c t i o n o f p r o p y l e n e o x i d e w i t h c o b a l t c a r b o n y l and base i n methanol are g i v e n i n T a b l e V I I I . S e v e r a l b a s e / m e t a l o x i d e c o m b i n a t i o n s were e v a l u a t e d under m i l d r e a c t i o n c o n d i t i o n s . The d i f f e r e n c e i n a c t i v i t y between the bases was not as pronounced as t h a t o b s e r v e d i n the r e a c t i o n w i t h b e n z y l h a l i d e s w i t h the e x c e p t i o n o f p o t a s s i u m methoxide w h i c h , when used a l o n e , gave e x c l u s i v e l y the h y d r o x y e t h e r r e s u l t i n g from methoxide a d d i t i o n t o the epoxide r i n g . However, the a c t i v i t y o f sodium



152

b i c a r b o n a t e d i d improve when the o x i d e was p r e s e n t . An e x p l a n a t i o n can be made by c l o s e r e x a m i n a t i o n o f the IR. I n t h i s system the predominant c a r b o n y l a d s o r p t i o n band observed d u r i n g r e a c t i o n was t h a t a t t r i b u t e d t o the c o b a l t a n i o n . Here the slow s t e p appears t o be the a d d i t i o n o f the c o b a l t t e t r a c a r b o n y l a n i o n t o o x i r a n e w h i c h


0

i s known t o o c c u r s l o w l y a t room temperature. There i s not a b u i l d - u p o f a c y l complex and thus l e s s a dependence on a v a i l a b l e base. T h i s system may be more r e p r e s e n t a t i v e o f a t y p i c a l r e a c t i o n a s s i s t e d by m e t a l o x i d e s where the i n c r e a s e i n a c t i v i t y c o u l d be a t t r i b u t e d t o the a d s o r p t i o n o f a s u b s t r a t e , c a t a l y s t o r r e a c t i o n intermediate. Table V I I I .

P r e p a r a t i o n of Methyl Base NAHC03 NaHC03/Al203 NaHC03/Si02 NaHC03/Ti02 MeOK/A1203 MeOK

(a)

(b)

3-Hydroxybutyrate(a) Ester. %(c) 54 74 70 62 53 0

R e a c t i o n c o n d i t i o n s = 6 h o u r s , 75°C 50 p s i g , base (18 mmoles), propylene o x i d e (86 mmoles), methanol (75 ml) and d i c o b a l t o c t a c a r b o n y l (1.5 mmoles). Y i e l d s were d e t e r m i n e d by VPC u s i n g i n t e r n a l s t a n d a r d t e c h n i ques .

Conclusions The use o f s i m p l e m e t a l o x i d e s and f u n c t i o n a l i z e d d e r i v a t i v e s t o s o l v e the problems found i n i n d u s t r i a l c h e m i c a l o p e r a t i o n s may be an important one. Any catalyst o r reagent that i s inexpensive, r e c y c l a b l e , s e p a r a b l e , and a l l o w s r e a c t i o n s t o o c c u r more s e l e c t i v e l y and under m i l d e r r e a c t i o n c o n d i t i o n s would c e r t a i n l y be o f


12.

SAWICKI

Catalyse in Organometallic Anion Chemistry

153

i n t e r e s t t o an a l r e a d y energy c o n s c i o u s i n d u s t r y . The w e a l t h o f i n f o r m a t i o n t h a t i s a l r e a d y a v a i l a b l e i n t h i s a r e a t h r o u g h the e f f o r t s o f b o t h academic and i n d u s t r i a l r e s e a r c h e r s speaks w e l l o f the p o t e n t i a l f o r t h i s t e c h n o l o g y . Acknowledgments I w o u l d l i k e t o thank Texaco I n c . f o r p e r m i s s i o n t o p u b l i s h t h i s p a p e r , J . B r o a s , J r . f o r e x p e r i m e n t a l a s s i s t a n c e , and B . Townsend for preparation of t h i s manuscript.

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Phase-Transfer Catalysis - American Chemical Society

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