Rhodium-catalyzed low pressure hydroformylation of vinyl esters: Solvent and phosphine effects on catalyst activity, selectivity and stability

Journal of Molecular Catalysis, 18 (1983) 381 - 390

381

RHODIUM-CATALYZED LOW PRESSURE HYDROFORMYLATION OF VINYL ESTERS: SOLVENT AND PHOSPHINE EFFECTS ON CATALYST ACTIVITY, SELECTIVITY AND STABILITY ANTHONY G. ABATJOGLOU, DAVID R. BRYANT and L. C. D'ESPOSITO

Union Carbide Corporation, Research and Development Department, South Charleston, WV 25303 (U.S.A.) (Received January 18, 1982)

Summary Rhodium/triarylphosphine complexes catalyze the low pressure hydroformylation of vinyl esters to ~- and fl-carboxypropionaldehydes. In contrast with hydrocarbon ~-olefins, the branched aldehydes predominate, and the regioselectivity of the reaction is virtually unaffected by carbon monoxide partial pressure. Substrate steric effects and solvent effects are significant on both the reaction rates and linear/branched aldehyde selectivities. Rhodium-promoted hydrogenolysis of vinyl esters to ethylene and carboxylic acid is a major side reaction of this process. The results are rationalized in terms of the ability of the vinyl esters to form chelates.

Introduction Most research on hydroformylation using phosphine-modified rhodium catalysts has been with hydrocarbon olefins [ 1 ]. The catalyst activity/selectivity patterns observed generally do not apply to functional olefins such as vinyl acetate, allyl acetate and ethyl acrylate. The functional group influences the hydroformylation reaction through changes in the double bond polarization and/or chelation to the metal, and facilitates secondary organic reactions such as aldol condensations and eliminations from the primary hydroformylation products. These side reactions generally affect the catalyst activity and selectivity. This paper describes our results with the low pressure rhodium-catalyzed hydroformylation of vinyl esters, and is a good illustration of the drastic effects which a functional group can have on rhodium hydroformylation catalysis. A report on the high pressure rhodium-catalyzed hydroformylation of vinyl acetate was published [2] while this work was in progress. 0304-5102/83/0000-0000/$03.00

© Elsevier Sequoia/Printed in The Netherlands

382 Experimental

Hydroformylation apparatus and procedure All h y d r o f o r m y l a t i o n rates were determined in a 100-ml stainless steel autoclave equipped with magnetic stirring. The autoclave was heated by a 200-watt band heater equipped with a proportional temperature controller. Internal temperature was monitored with a platinum resistance t h e r m o m e t e r of -+0.1 °C accuracy. The autoclave was connected to a gas manifold for initial pressurization with reactant gases. An external reservoir of 0.5 1 capacity containing CO:H2 was connected to the autoclave by means of a Research Control m o t o r valve. The autoclave was also equipped with a 100-135 psi pressure transmitter. During h y d r o f o r m y l a t i o n the autoclave was maintained at 120 psig via the reservoir/motor valve/pressure transmitter. Reaction rate was calculated from the rate of pressure drop in the external reservoir. In a typical reaction, 20 ml of catalyst solution containing Rh(CO)zacac at 7 × 10 -3 M concentration and the appropriate phosphine with a phosphine-to-rhodium mole ratio of 15:1 in bis(2-ethoxyethyl)ether solvent, was introduced into the autoclave and flushed with CO:H2 by means of the gas manifold. The autoclave was then pressurized to 120 psig with CO:H2, heated to 60 °C and maintained under these conditions for 30 min to equilibrate the catalyst. The gas pressure was vented to 5 psig and 5 ml of the olefin was injected by syringe into the autoclave. The autoclave was repressurized to 120 psig with 1:1 CO:H: from the gas manifold and then opened to the m o t o r valve-reservoir assembly.

NMR and IR spectroscopy The p r o t o n NMR samples were prepared in CDC13 using tetramethylsilane (TMS) as an internal reference. The data were collected on a HitachiPerkin Elmer 60 MHz NMR spectrometer. Samples for 3~p NMR analysis were prepared using toiuene as a solvent. A capillary insert containing d6-acetone for lock and H3PO 4 for external reference was used. The data were collected on a Varian FT-80A spectrometer using a 4000 Hz sweep w i d t h , a 2.047 s acquisition time, a 1 s pulse delay and a pulse width of 19 gs. Usually no more than 1000 transients were needed for sufficient data. All IR spectra were recorded on the Digilab FTS-15 Fourier Transform Spectrometer at a nominal resolution of 4 cm -1. A flow-through cell of special design (Harrick Scientific Corp.) equipped with ZnSe (IRTRAN IV) windows was m o u n t e d in a 6× beam condenser (Harrick Scientific Corp.). Connection of this cell to the reactor was made via 1/8 inch stainless steel tubing.

Preparation of Rh(CO)(OCOCH3)(Ph3P)~ by reaction of RhH(CO)(Ph3P)3 and vinyl acetate A 20 ml mixture of toluene-vinyl acetate (2:1) is introduced under nitrogen into a 50 ml r o u n d - b o t t o m flask containing 0.92 g (1 mmol) of

383

RhH(CO)(Ph3P)3 and a magnetic stirring bar. A homogeneous solution is obtained initially, and soon yellow crystals precipitate. Stirring is continued for 1 h. Hexane (10 ml) is added to precipitate the remaining solid. The solid is filtered, washed with 10 ml hexane and dried under vacuum. Yield 0.62 g (80%) of Rh(CO)(OCOCH3)(Ph3P)2.

Results and discussion General features o f the low pressure rhodium vinyl acetate hydroformylation Rhodium/triphenylphosphine complexes catalyze the vinyl acetate hydroformylation (eqn. 1). The reactivity of vinyl acetate is one fifth of that of pentene-1 under comparable reaction conditions [3]. H2 :CO

CH2=CH--OCOCH3

> CH3--CH--OCOCH3 + OCHCH2CH2OCOCH3 (1}

Rh :Ph3P

1

CHO

(I) (II) major product minor product The regioselectivity of the reaction is opposite to that of non-functionalized ~-olefins; the branched isomer, (~-acetoxypropionaldehyde (I), predominates. The low initial catalyst activity is accompanied by a rapid deactivation, as depicted in Fig. 1. .4 z~ © >_ ~

UJ

.2

oz

go_ .i

©

>-F=0 <

kul

a:

0 0

I

I

20

40

]

I

60 80 TIME, MIN.

I

I00

t

120

140

Fig. 1. Catalyst activity decline during vinyl acetate hydroformylation, using a rhodiumtriphenylphosphine complex.

The percentage of the linear product ~-acetoxypropionaldehyde (II) in the total product varies depending on the reaction conditions. This variation is primarily due to its decomposition to acrolein and acetic acid (eqn. 2). CH3COOCH2CH2CHO

A

> CH3CO2H + CH2=CH--CHO

catalyst

(2)

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Both acrolein and acetic acid are known rhodium inhibitors, and it is their formation that causes the catalyst deactivation. The rhodium-catalyzed decomposition of vinyl acetate itself is another source of acetic acid, and this reaction is detailed below. Different phosphines accelerate the decomposition of II at different rates (Table 1), and apparently the more nucleophilic ones are also the more effective. Whether the phosphines themselves or some phosphine-acrolein reaction product is the catalyst for this decomposition is not known. Under actual hydroformylation conditions, the less nucleophilic phosphines (X--C6H4)aP give higher catalyst stability than triphenylphosphine (Table 2). The more nucleophilic alkyldiarylphosphines examined give much lower reaction rates (<0.1 g mol/1 h) and cause extensive decomposition of fl-acetoxypropionaldehyde. TABLE 1 Thermal decomposition of fl-acetoxypropionaldehyde in the presence of different phosphines Phosphine

none n-Bu3P Ph3P ~-CIC6H4)3P

Percent decomposition as measured by acetic acid formed a Initial solution b

After heating at 60 °C for 1 h

After heating at 60 °C for 17 h

6 50 c 16 15

6 90 100 15

16 90 100 23

aDetermined by integration of its NMR methyl signal. The samples contained 0.1 M phosphine, 0.1 M ~-acetoxypropionaldehyde in deuterochloroform in sealed NMR tubes. b T h e initial sample of ~-acetoxypropionaldehyde was contaminated with some acetic acid. CRapid decomposition occurred at r o o m temperature.

TABLE 2 Hydroformylation of vinyl acetate: effect of substituted aryl phosphines a Phosphine

PPh3 (p-C1C6H4)3P (m-CIC6Ha)3P (p-FC6H4)~P

Reaction rate b

Reaction rate at 10% conversion

70 Rate decline

Selectivity¢

0.33 0.66 0.61 0.51

0.15 0.66 0.56 0.39

55 0 8 23

3.7 2.7 3.0 2.4

a/~

aReaction conditions: 60 °C, 120 psig CO:H2 = 1:1; catalyst precursor Rh(CO)aacac, 7 × 10 3 M or 720 ppm Rh in bis(2-ethoxyethyl)ether; 2.8% Ph3P (phosphine:rhodium mol ratio = 15:1). bInitial reaction rate (g mol/1 h) to total hydroformylation products. c(~ and ~ stand for a-acetoxypropionaldehyde and ~-acetoxypropionaidehyde, respectively.

385

Another important side reaction is the direct reaction of vinyl acetate with the rhodium hydride catalyst to form ethylene and Rh(CO)(OCOCH3)(Ph3P)2 (eqn. 3). RhH(CO)(Ph3P)3 + CH2=CH--OCOCH3 ~

Rh(CO)(OCOCH3)(Ph3P)2 +

(3)

+ CH2=CH2 + Ph3P

Ethylene was detected by mass spectral analysis, and the rhodium complex was isolated and characterized by elemental analysis, IR and NMR spectroscopy. Under hydroformylation conditions propionaldehyde (the ethylene hydroformylation product) and acetic acid are formed. Since the equivalents of propionaldehyde produced are well in excess of the molar amount of the rhodium catalyst, the hydrogenolysis of vinyl acetate under the hydroformylation reaction conditions is catalytic (eqns. 4 and 5). CO

Rh(CO)(OCOCHa)(PhaP)2 + H2 ~

RhH(CO)2(Ph3P)2 + CH3CO2H

RhH(CO)2(Ph3P)2 + CH2=CH--OCOCH3 ~

(4)

Rh(CO)(OCOCH3)(Ph3P)2 + + CH2=CH2

(5)

The eventual buildup of acetic acid reduces the concentration of the hydridic form of the catalyst and therefore its hydroformylating reactivity. High pressure FT-IR shows that in the presence of excess acetic acid the position of the equilibrium in eqn. 5 is on the side of the carboxylate rhodium complex; this equilibrium can be shifted by hydrogen pressure (Fig. 2). The FT-IR spectra of the catalyst under hydroformylation conditions {Fig. 3) show that the conversion of the RhH(CO)2(Ph3P)2 (the predominant rhodium species under H2:CO) to Rh(CO)(OCOCH3(Ph3P)2 is gradual, and that the equilibrium in eqn. 4 is slowly shifted to the left because of the buildup of acetic acid. a.

b.

I

2200

I

J

18OO

WAVE N U M B E R S

c.

I

2200

I

I

18OO

WAVENUMBERS

d.

I

2200

I

I

18OO

WAVENUMBERS

I

2200

18OO

WAVENUMBERS

Fig. 2. Conversion of Rh(CO)(OCOCH3)(Ph3P)2 (spectrum a) to RhH(CO)2(Ph3P)2 (spectra b, c, d) by application of 300 psi H2:CO = 1 pressure.

386

a.

b.

I

I

2100

I

1900

WAVENUMBERS

c,

I

2100

I

,,I

1900

WAVENUM BERS

I

d.

I

2100

I

[900

WAVEN UMBERS

I

2100

I

I

1900

WAVENUMBERS

Fig. 3. R h o d i u m catalyst transformations under actual vinyl acetate h y d r o f o r m y l a t i o n conditions: a, RhH(CO)(Ph3P)3 under 80 psi CO:H2 = 1 at a m b i e n t t e m p e r a t u r e ; b, after addition of vinyl acetate; c, difference s p e c t r u m of a and b; d, after heating at 70 °C.

The need for the hydrogenolysis step to activate the catalyst is supported by the fact that the activity o f a deactivated catalyst increases linearly and is first order in hydrogen pressure (e.g. from 0.01 g mol/1 h at 20 psia to 0.05 g mol/1 h at 100 psia), and the response is considerably greater than conventional hydroformylation catalysts.

Substrate steric and electronic effects on catalyst activity/selectivity in the hydroformylation o f vinyl esters A series of sterically different vinyl esters was examined to determine the effect of steric bulk of the ester on the rates and selectivities of the hydroformylation reaction. The results in Table 3 show that b o t h the catalyst activity and the selectivity for the branched isomer increase with increasing steric bulk of the ester substrate. The opposite result is found in the r h o d i u m phosphine-catalyzed hydroformylation of hydrocarbon olefins. Thus the more hindered internal olefins are less reactive than terminal olefins, and linear oxo products predominate from terminal olefins. These results can be explained on steric grounds alone. TABLE 3 H y d r o f o r m y l a t i o n activity and selectivity of vinyl esters a Olefin

Initial reaction rate b

Selectivity c a/~

vinyl vinyl vinyl vinyl

0.10 0.33 0.42 1.20

-3.7 4.4 16.0

formate acetate 2-ethylhexanoate pivalate

a R e a c t i o n conditions: 60 °C, 120 psig CO:H2 = 1:1 ; c a t a l y s t precursor Rh(CO)(PPh3)acac, 7 x 10 -3 M or 720 p p m Rh in b i s ( 2 - e t h o x y e t h y l ) e t h e r ; p h o s p h i n e : r h o d i u m mol ratio = 15:1. b R e a c t i o n rate in g tool/1 h to total h y d r o f o r m y l a t i o n products. ca and ~ stand for (~-acetoxypropionaldehyde and ~ - a c e t o x y p r o p i o n a l d e h y d e , respectively.

387

The reversed regioselectivity and low reactivity relative to hydrocarbon ~-olefins observed in vinyl ester hydroformylation is probably the result of the chelating effect of the ester carbonyl. Since the double bond polarization in vinyl esters is similar to that in hydrocarbon ~-olefins [5], the straight chain aldehyde should be the favored product (Scheme 1) if the Markownikoff mode of addition predominates in the rhodium hydride addition to the olefin [6]. Apparently, the ester carbonyl group influences the stability of the hydroformylation intermediates to such an extent that the antiMarkownikoff five-membered ring chelate product (III) predominates. O (Ph3P)m (CO), Rh--H

+

anti-Markownikoff/~ aaditi~,./ ~

CH2= CH--OCR Markownikoff addition

/ell\ ~

1 1

branched aldehyde

--o

,H,,

,CH2--CH2

/

)O

I 'o-oQ 1

straight-chain aldehyde

iv,

Scheme 1. Modes of addition of (PhaP)m (CO)nRhH to vinyl esters

The effect of the ester carbonyl moiety in controlling the hydroformylation regioselectivity is also seen in derivatives of allyl alcohol [7]. Thus, whereas with allyl alcohol and allyl ethers the linear product predominates, with allyl esters the branched product is favored. The coordination of the rhodium to the ester carbonyl group in the intermediates (III) and (IV) can slow carbon monoxide insertion and be responsible for the low hydroformylation reactivity of vinyl esters. A similar chelating action of aft-unsaturated carbonyl compounds could account for their inhibitory effect on rhodium oxo catalysts. Assuming that the chelate structures (III) and (IV) are important in determining the catalyst reactivity/selectivity, the steric effect of the R group becomes important in the sense that it will destabilize the puckered six-membered chelate more than the planar five-membered one, and thus the selectivity to the branched product should be higher. At the same time the equilibrium concentration of the non-chelated rhodium alkyl intermediates should be higher when R is bulkier and the overall catalyst activity will be higher.

388

Solvent effects on catalyst activity, selectivity and stability in vinyl acetate hydroformylation We discovered that dimethylformamide as solvent or co-solvent in vinyl acetate hydroformylation gave three to four times higher catalyst activity relative to an ether solvent. The coordinating ability of dimethylformamide may be sufficient to favor m o n o d e n t a t e coordination during vinyl ester hydroformylation. In contrast, hexene-1, which is unable to form a chelate during hydroformylation, does n o t show any significant rate enhancement in dimethylformamide. The effect of other dipolar aprotic and coordinating solvents was examined, and the results are shown in Table 4. The use of dipolar aprotic solvents not only enhances catalyst activity b u t also markedly improves catalyst stability. We found that the catalyst stability improvement correlates with the ability of certain solvents to block the inhibitory effect of acetic acid (last column of Table 4) on the rhodium hydroformylation catalyst. The catalyst stabilization effect is probably due to the strong hydrogen bonding character of these solvents. It has been d o c u m e n t e d [8] that the hydrogen bonding interactions of carboxylic acids with these 'basic' solvents are very strong; 2:1 and 1:1 acid-solvent complexes have been observed. Apparently these complexes tie up the acid sufficiently well that it is unavailable for reaction with the rhodium complexes. This protective effect was also demonstrated with other a-olefins. Table 5 shows the results of hexene-1 hydroformylation in dimethylformamide and bis(2-ethoxyethyl)ether. A significant rate reduction of 66% was observed in the ether solvent which contained acetic acid. A 19% reduction was observed in DMF which contained the same amount of acid. TABLE 4 Solvent effects in vinyl acetate hydroformylation a Solvent

Initial rate b

Rate b at 10% conversion

% Rate decline

% Rate decline upon addition of acetic acid (mol equivs. based on Rh)

bis(2-ethoxyethyl)ether dimethyl formamide dimethyl acetamide 1 -methyl -2 -pyrroli done hexamethylphosphoramide dimethyl sulfoxide sulfolane c

0.33 0.91 1.10 1.26 0.30 1.35 1.39

0.15 0.89 1.10 1.04 0.26 1.32 1.15

55 2 0 17 14 2 17

77 15 3 7 0 2 87

(80) (100) (60) ( 60 ) (60) (60) (60)

aConditions: 60 °C, 120 psig 1:1 H2:CO; catalyst precursor Rh(CO)(PPh3)(acac), 7 × 10 -3 M or 720 ppm Rh in the respective solvent; 2.8% Ph3P (phosphine:rhodium mol ratio = 15:1). bin g mol/1 h to total hydroformylation products. cSolvent decomposition was observed during the experiment.

389 TABLE 5 E f f e c t of solvent on catalyst reactivity in hexene-1 h y d r o f o r m y l a t i o n in the presence of acetic acid a,b Solvent

Initial rate c

Rate at 10% conversion

% Rate decline

dimethylformamide bis ( 2 - e t h o x y ethyl )ether

4.78 2.16

3.81 0.73

19 66 d

a R e a c t i o n conditions: 80 °C, 105 psig 1:1 H2:CO; catalyst precursor Rh(CO)(Ph~P)(acac), 2.4 × 10 -3 M in the respective solvent; 5% Ph3P ( p h o s p h i n e : r h o d i u m mol ratio = 80:1). b T h e acetic acid (750 mol equivs, based o n r h o d i u m ) was added w i t h t h e hexene-1 feed. c In g mol/1 h to total h y d r o f o r m y l a t i o n products. d A n 83% rate decline could be c o m p u t e d if a 4.3 initial rate is used. This rate is observed in this solvent in the absence o f acetic acid. TABLE 6 Solvent effects on selectivity in vinyl acetate h y d r o f o r m y l a t i o n a Solvent

Reaction rate I~/h

Selectivity b r a n c h e d / straight chain p r o d u c t

bis(2-ethoxyethyl)ether toluene dimethylformamide d i m e t h y l sulfoxide

1.13 1.60 1.39 1.25

3.3 4.3 5.2 6.1

aConditions: 730 p p m Rh as Rh(CO)2(acac), 3.85% tris(p-chlorophenyl)phosphine, 60 °C, 120 psig 1:1 CO:H2 in the respective solvent.

Another important solvent effect in vinyl acetate hydroformylation which is absent in the hydroformylation of non-functional a-olefins, is on the reaction selectivity. Table 6 shows the results in four solvents of different polarity and coordinating ability. The increase in the reaction selectivity to the branched product with increasing coordinating ability of the solvent is not understood. In contrast, the CO partial pressure, which is an important variable in determining the selectivity in oxo reactions of other olefins [1], has virtually no effect on the selectivity in vinyl acetate h y d r o f o r m y l a t i o n {Fig. 4).

Conclusions

The vinyl acetate hydroformylation using rhodium/phosphine complexes proceeds at lower rates relative to hydrocarbon a-olefins. The reaction regioselectivity is the reverse of that found with non-functional a-olefins and is virtually unaffected by carbon monoxide partial pressure, which is important in the hydroformylation of hydrocarbon a-olefins. The most significant

390 8 L r N E A R / B R A N C H E D ALDEHYDES IN"

0

0

PROPYLENE HYDROFORMYLATrON

rf6 F'F hi

04. LIJ C) >-.

i

L,O EZI

2

00

I

I

I

i

20 40 60 8O I00 CO PARTIAL PRESSURE (psi(])

120

Fig. 4. Effect of carbon monoxide partial pressure on isomer ratio in vinyl acetate and propylene hydroformylation. The propylene curve was adapted from ref. 9.

increases in hydroformylation rates and selectivities are obtained with vinyl esters of bulky acids {e.g. pivalic, 2-ethylhexanoic) as well as with coordinating solvents. It appears that the chelating effect of the ester carbonyl moiety dominates the hydroformylation properties of vinyl acetate and is responsible for the observed selectivity/reactivity behavior.

Acknowledgements The authors are grateful to Union Carbide Corporation for allowing the publication of this work.

References 1 For recent reviews see: a P. Pino, F. Piacenti and M. Bianchi, in I. Wender and P. Pino (eds.), Organic Synthesis via Metal Carbonyls, Vol. 2, Wiley, New York, 1977, p. 43. b R. L. Pruett, in Advances in Organometallic Chemistry, Vol. 17, Academic Press, New York, 1979, p. 1. 2 a B. Fell and M. Barl, J. Mol. Catal., 8 (1977) 301. b U.S. Patent 4 072 709 (1978) to H. B. Tinker. 3 C. K. Brown and G. Wilkinson, J. Chem. Soc. (.4), (1970) 2753. 4 S. Komiya and A. Yamamoto, J. Chem. Soc., Chem. Commun., (1974) 523. 5 A. C. Rojas and J. K. Crandall, J. Org. Chem., 40 (1975) 2225. 6 G. Henrici-Oliv~ and S. Olive, Topics in Current Chem. 67 (1976) 107. 7 Unpublished results from this laboratory. 8 H. Fujiwara et. al., Bull. Chem. Soc. Jpn, 48 (1975) 1970 and references therein. 9 K. L. Olivier and F. B. Booth, Hydrocarbon Process., 49 (1970) 112.