Homogeneous catalytic hydrogenation of dicarboxylic acid esters

Journal of Molecular Catalysis, 22 (1984) 353 - 362 353

HOMOGENEOUS CATALYTIC HYDROGENATION OF DICARBOXYLIC ACID ESTERS

UGO MA’M’EOLI, MARIO BIANCHI, GLORIA MENCHI, PIER0 FREDIANI and FRANC0 PIACENTI

Cattedm di Chimica Zndustriale, Uniuersitci di Fireraze, Via Gino Capponi 9, 50121 Florence (Italy)

(Received September 6,1983; accepted October 24,1983)

Summary

Esters of dicarboxylic acids are hydrogenated in the homogeneous phase in the presence of H4Ru,(C0)s(PBu&. The corresponding hydroxy esters are the main products from oxalic and malonic esters. Dimethyl succinate gives +y-butyrolactone, while glutaric esters do not react.

Only the ortho isomer of the phthalic esters reacts, giving phthalide and methyl benzoate.

Both electronic and steric factors affect the course of this reaction.

Introduction

The reduction of free carboxylic acid groups by catalytic hydrogenation in the homogeneous phase in the presence of phosphine-substituted cluster ruthenium carbonyl compounds of the type H4R~4(C0)12-xLx (L = PBus, PPhs) was reported some time ago [ 1,2]. From aliphatic monocarboxylic acids the only product formed is the corresponding alcohol, recovered as the ester of the starting acid. The esterification of the alcohol, which takes place immediately after its formation, does not allow reduction of all the acid since the ester is not reduced under the same conditions.

Dicarboxylic acids undergo hydrogenation by the same catalytic system, with the exception of the first two terms of the aliphatic series. In fact, while oxalic acid decomposes and malonic acid is decarboxylated to acetic acid, succinic acid is quantitatively reduced to the corresponding lactone. In addition to the corresponding la&one, glutaric acid gives rise to products of further reduction such as 2-hydroxytetrahydropyrane and 1,5- pentanediol [ 21 by a reaction which appears to be the first example of catalytic reduction of an ester in the homogeneous phase.

In considering the possible reduction of a somewhat activated ester by this catalytic system, we decided to test esters activated by a particular situa- tion of the carboalkoxy group in the molecule.

0304-5102/84/$3.00 @ Elsevier Sequoia/Printed in The Netherlands

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We began this investigation by choosing dicarboxylic acid esters as substrates, as the presence of two carboalkoxy groups in the same molecule might in fact provide the activation necessary to allow such a reaction.

Results and discussion

Dimethyl oxalate is selectively hydrogenated in the presence of H4R~- (CO),(PBu& at 180 “C and 130 atm hydrogen to methyl glycolate (Scheme 1, Table 1). Even at 200 “C the reaction does not proceed further.

Y00CH3 CH20H

COOCH, +H, - I

COOCH, + CH,OH

Scheme 1.

TABLE 1

Hydrogenation of dimethyl oxalate to methyl glycolate in the presence of various catalytic precursors

Catalytic precursor Reaction time Yield (h) (%)

H&U(CC)&‘Bu& 144 51.1

H4Ru4(CC)s(PBu3)4 a 39 100

H4Ru4(CC)sWh3)4 144 0.0

Ru/C b 144 3.0

Substrate 1.3 g, catalytic precursor 50 mg, benzene 15 ml, P(Ha) 130 atm at 20 “C, T 180 “C. alOO mg. bBrepared from H4Ru4(C0)r2 (60 mg) on carbon (1 g) (see Experimental).

In the presence of the tetra(tributylphosphine)-substituted derivative, satisfactory conversions were obtained (up to 100%); H4Ru4(CO)s(PPh3)4, the only triphenylphosphine analogue tested, appeared to be completely inactive.

In the crude of the reaction carried out in the presence of H4Ru4(C0)s- (PBu,)~ as catalytic precursor, H4R~4(C0)9(PB~3)3 and H4Ru4(CO),s(PBu3), were detected, indicating that dephosphination of the initial cluster does take place during the reaction. Species having a low phosphine content might decompose under the reaction conditions to active Ru metal [ 21. The inter- vention of metallic Ru as the catalyst of this reaction has been ruled out by the observation that ruthenium metal, obtained by thermal decomposition of H4Ru4(C0)i2 on carbon under the same conditions, displays only very low catalytic activity for this reaction. These data strongly suggest that this reac-

355

tion, when carried out in the presence of the tributylphosphine-substituted ruthenium carbonyl hydride, takes place in the homogeneous phase.

Diethyl malonate when hydrogenated in the presence of HaRue(C (PBu& provides a number of compounds, which are listed in Table 2. Ethyl 3-hydroxypropanoate is the prevailing product (45%) at 38% conversion of the substrate, and most likely is the first reaction product (Scheme 2). Ethyl 3-hydroxypropanoate may give ethyl propanoate by dehydration followed by hydrogenation. The malonic hemiester, formed by hydrolysis of the diester, may form, by decarboxylation, ethyl acetate. Carbon dioxide is in fact present in the gases from the crude. The formation of 2carbo- ethoxyethyl acetate and propanoate, present as minor products, may easily be rationalized by assuming transesterification reactions involving ethyl acetate, ethyl propanoate and ethyl 3-hydroxypropanoate.

TABLE 2

Hydrogenation of dicarboxylic acid esters in the presence of H4Ru4(CO)s(PBu&

Substrate Conversion

@)

Reaction products

(%)

Cb(COOGHs), 37.78 HOCH2CH2COOC2H5 44.7 CH3CH2COOC2Hs 27.0 CH3COOC2Hs 15.3 CH3CH2COOCH2CH2COOC2Hs 6.7 CH3COOCH2CH2COOC2Hs 6.3

( CH2COOCH3)2 7.0 CH2CH2CH2COC 100

CH2(CH2COOCH3)2 0.0 - -

Substrate 6 g, catalytic precursor 25 mg, reaction time 144 h, P(H2) 130 atm at 20 ‘C, T 180 “C. aCarbon dioxide is present in the residual gas.

Dimethyl succinate forms r-butyrolactone with 100% selectivity but at a very low rate (7% after 144 h). This product, which is most likely formed. (Scheme 3) through an alcohol-ester intermediate, is stable under the reaction conditions, as we found earlier when studying the reduction of free succinic acid [ 21.

Dimethyl glutarate under the same conditions does not react. Dimethyl o-phthalate (A) (Table 3, Scheme 4) gives phthalide (D) and

methyl benzoate (C) in addition to methanol. The molar ratio between D and C, rather high at the beginning of the reaction, decreases considerably as it proceeds. Such behaviour suggests that D is gradually transformed into C. The formation of C and D from A may take place according to Scheme 4. In fact, the absence among the reaction products of methyl formate and/or carbon dioxide and methane, which are the products of its decomposition under the same conditions [ 21, excludes the hydrogenolysis of the carbo-

356

N

5-O \

U-0 /

357

TABLE 3

Hydrogenation of dimethyl o-phthalate

Reaction time

(h)

Conversion (%)

Product composition (mol%)

phthalide methyl benzoate

Mol phthalidelmol methyl benzoate

20 2.1 78.7 21.3 3.7 88 11.8 63.0 37.0 1.7

144 21.1 54.5 45.5 1.2

Substrate 6 g, H~Ru~(CO)~(PBU~)~ 25 mg, P(H2) 130 atm at 20 “C, T= 180 “C.

COOCH3 CH20H

H2

COOCH - CH30H '

3 _a. _ 0

COOCH3 - CH30H

COOCH3

(A) (B) (Cl

+ CH30H

COOCH3

((3

a

0 CH2 \ H2

COQ - no reaction

(D)

CO2 + CH4

Scheme 4,

methoxy group. Phthalide (D) under the reaction conditions but in the absence of methanol remains unchanged [ 21. However, when methanol is added, C is readily formed. Therefore C may arise from hydrogenolysis of the hydroxymethyl group of the hypothetical intermediate B. The decrease of the molar ratio between D and C as the reaction proceeds may be rationalized by assuming that C is formed from A at a lower rate than D, which however, being less stable than C, is then slowly transformed into C.

Interestingly, dimethyl m-phthalate and p-phthalate do not react at all under these conditions, even at temperatures as high as 200 “C.

In general, in the reduction of esters of aliphatic or aromatic dicarboxylic acids (Table 4) the conversion rapidly decreases as the distance between the two functional groups in the substrate is increased.

The fact that higher conversions are obtained with dimethyl o-phthalate than with dimethyl succinate should probably be attributed to the fact that the reciprocal influence of the two carbomethoxy groups is greater when

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they are substituents of an aromatic ring, due to the greater efficiency of an aromatic system in transmitting electronic effects. This interpretation is confirmed by the extremely poor results obtained with dimethyl cis-cyclohexan-1,2dicarboxylate as substrate (Table 4).

TABLE 4

Hydrogenation of dicarboxylic acid esters to the corresponding hydroxy esters in the presence of H&u4(CO)s(PBua)4

Substrate

dimethyl oxalatea 51.1 diethyl malonate 28.8 dimethyl succinate 7.0 dimethyl glutarate 0.0 dimethyl o-phthalate 21.lb dimethyl m-phthalate 0.0 dimethyl p-phthalate 0.0 dimethyl cis-cyclohexan-1,2-dicarboxylate 1.0

Mol hydroxy ester/MO1 starting material (W)

Substrate 6 g, catalytic precursor 25 mg, P(H2) 130 atm at 20 “C, 2’ 180 “C, reaction time 144 h. aSubstrate 1.3 g, benzene 15 ml, catalytic precursor 50 mg. bPhthalide was the product recovered.

Apparently carboalkoxy groups may be reduced by this catalytic system when they are activated by another group of the same type at an appropriate distance. When the distance is too great, the reciprocal influence is low and the reactivity decreases to zero. Such an explanation is in keeping with the fact that esters of simple monocarboxylic acids cannot be reduced with this catalytic system [ 2]*.

Even a CHzOH group next to a carbomethoxy group, as in methyl glycolate, is not sufficient to activate the system.

The substitution of one of the hydrogen atoms of the methylenic group in methyl glycolate, however, increases the reactivity of the carbomethoxy group. In fact, under the same conditions the carbomethoxy group of methyl 2-hydroxyphenylacetate is reduced, with low selectivity, to the extent of 5%. Moreover with ethyl 2-hydroxypropanoate the reaction proceeds at a reasonable rate (35.5% conversion, Table 5) giving rise to 1,2-propanediol as the only product.

*The reduction of monocarboxylic acid esters such as CHsCOOCHs is reported to occur in the presence of anionic ruthenium hydride derivatives [3]. Significantly, using the neutral ruthenium derivative (PPhs)&uHCl, the reduction is possible only with activated esters such as CFsCOOCIFs [3]. The reduction of the carbonyl group involved in an ester also takes place in the presence of ruthenium carbonyl derivatives coupled with iodide promoters and proton suppliers [4].

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TABLE 5

Hydrogenation of o-hydroxy esters to glycols in the presence of H4Ru4( CO)s( PBu&

Substrate

Y00CH3 CHzOH

Conversion (%)

0

Selectivity (%)

-

COOCHs I

YoH a 15.5 b

COOCaHs

CHOH 35.5 100

CH3

Substrate 6 g, catalytic precursor 25 mg, P(H2) 130 atm at 20 “C, reaction time 24 h. aBenzene 15 ml, substrate 1.8 g, catalytic precursor 50 mg. bPhenylacetaldehyde (12.4%), 2phenylethanol (5.2%), bis(2-phenylethyl)ether (14.4%); methyl phenylacetate (46.1%) and benzaldehyde (21.9%) are also formed.

Experiments are in progress to evaluate the relevance of electronic and steric effects on the activation of the carbomethoxy group in connection with this reaction.

Although H4R~4(C0)s(PB~3)4 appears to be a good catalytic precursor, it is not at all sure that the true catalytic species is still a cluster with the same nuclearity .

In the crude from the reduction of dimethyl oxalate in the presence of this catalytic precursor, only H4Ru4( CO)s(PBus)s and H4Ru4( CO) 10(PB~s)2 have been detected of the Ru4 cluster derivatives. Since no free metal and no free phosphine are present in the crude, a decomposition reaction must take place with formation of simpler ruthenium species containing more than one phosphine molecule per ruthenium atom. Attempts to achieve further carbon monoxide substitution by phosphine in H4R&(C0)s(PBu& have thus far brought a collapse of the Ru4 structure. In the NMR spectrum of the hydrogenation crude, the presence of H4Ru&C0)s(PBu3)s and H4Ru4- (CO)iO(PBus), was indicated by signals at 26.7 T and 27.1 7 respectively, and also that of another hydridic species giving a signal at 17.7 7.

Our data do not yet allow an interpretation of the reaction course. We may however state that the reaction path reported in Scheme 5, involving the cleavage of the alkyl-oxygen bond, should be excluded since such a fission in our case wouldlead to-the formation of an alkane [5 - 81.

CH2

RCOO---CH2--CH=CH2 + [M] - RCOO-[M] “>CH

Scheme 5. C6

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Experimental

GLC analyses were performed on a Perkin Elmer Sigma 1 System; NMR spectra were recorded with a Perkin Elmer R32 spectrometer; GLC-mass spectra were recorded with a Perkin Elmer 270B spectrometer; IR spectra were recorded using a Perkin Elmer 580B Data System.

Materials %&~4(C0)12, WWCOW’JW4, H4R~4(C0)9(PB~3)3 and H4Ru4-

(C0)s(PPhs)4 were prepared as previously described [ 91. The Ru/C catalyst was prepared by thermal decomposition (180 “C)

under hydrogen (130 atm) of H4Ru4( CO)i2 (50 mg) in benzene (15 ml) on carbon black (1 g) in a 125 ml stainless steel autoclave.

All substrates used were commercial products except dimethyl cis- cyclohexan-1,2dicarboxylate, which was prepared according to a known procedure [lo]. Liquids were distilled and solids were crystallized before use.

Hydrogenation experiments and analytical methods Hydrogenations of esters were carried out as described in a previous

paper [2] for those of the carboxylic acids; amounts and reaction conditions adopted are reported in the Tables.

GLC analyses on crudes were carried out on 2 m columns packed either with FFAP 5% on Chromosorb G AW DMCS (dimethyl oxalate) or with polypropylene glycol (LB 550 X) 15% on Chromosorb W (diethyl malonate) or with Carbowax 20M 15% on Chromosorb W (all other substrates) .

Identification of products The residual gas of each experiment was monitored by IR spectro-

scopy. Carbon monoxide and light hydrocarbons (methane or ethane) were absent throughout. Carbon dioxide was detected in the residual gas from the reduction of diethyl malonate (bands at 2349 and 667 cm-‘).

r-Butyrolactone, phthalide, methyl benzoate, methyl glycolate, ethyl acetate, ethyl propanoate, 1,2-propanediol, benzaldehyde, methyl phenylacetate, phenylacetaldehyde and 2-phenylethanol were identified through their GLC-mass spectra [ 111.

Ethyl 3-hydroxypropanoate was identified by its GLC-mass spectrum which showed peaks at m/e 117 (M - H)+, 103 (M - CHa)‘, 100 (M - Hz0 = CH2=CHCOOC2H,)+, 99 (100 - H)+, 91 (M - 27 = HOCH2CH2C(OH)2)+, 90 (M - CH2=CH2 = HOCH,CH,COOH)+, 89 (M - C2H5)+, 88 (M - HCHO = CHsCOOC2H5)+, 73 (M - 0C2H5 = 0CCH2CH20H)+ = (COOC2HS)+, 72 (M - C2Hs0H = CH2CH2COG)+, 71 (72 - H)+, 55 (100 - OC2Hs = CH,=CHCO)+, 45 (OC2H,)+, 43 (CH,=COH = CH,CHO)+, 31 (CH,OH)+, 29 (C,H,)+.

2Carboethoxyethyl acetate was identified by its GLC-mass spectrum which showed peaks at m/e 117 (M - CH,CO)+, 115 (M - 0C2H5)+, 101

361

(M - CHsCOO)+, 89 (117 - &Ha)+, 88 (CH$OOC,H,)+, 73 (COOC2H5)+, 71 (117 - C,H,OH)+, 61 (CH,C(OH),)+, 60 (CH&OOH)+, 55 (CH,=CHCO = 101 - C2H50H)+, 45 (OC2H5)+, 43 (CH,CO)+, 29 (C,H,)+.

2-Carboethoxyethyl propanoate was identified by its GLC-mass spectrum which showed peaks at m/e 129 (M -C2H50)+, 117 (M -C2H&!O)+, 101 (M - C2H,COO)+, 75 (C2H,C(OH),)+, 74 (C2H,COOH)+, 73 (COOC2H, = C2H,COO)+, 71 (117 - C,H,OH)+, 57 (C2H,CO)+, 55 (CH*=CHCO = 101 - &H,OH)+, 45 (O&Hs)+, 29 (&Hs)+.

Hexahydrophthalide was identified by its GLC-mass spectrum which showed peaks at m/e 140 (M)+, 109 (M - H - CH,O)+, 95 (C,Hll)+, 85

(CeHsOz)+, 82 (CbHlo)+, 81 (C,H,)+, 68 (C,Hs)+, 67 (C,H,)+, 55 (C&H,)+, 54 W-U+.

Bis(2-phenylethyl)ether was identified by its GLC-mass spectrum which showed peaks at m/e 121 (M - C6H5CH2CH,)‘, 105 (M - C6H,- CH&H20)+, 91 (C,H,)+, 77 (C,H,)+, 65 (C,H,)+, 63 (C,H,)+, 51 (C4H3)+.

Recovery of the catalyst The hydrogenation crude of dimethyl oxalate in the presence of H4Ruq-

(CO)s(PBu& (Table 1, experiment with 100 mg catalyst) was carefully evaporated under reduced pressure (0.01 mmHg). The NMR spectrum taken on a sample of the residue dissolved in C&D, showed absorptions at 17.7 r (triplet, J 25 Hz), 26.7 r and 27.1 r. From another portion of the residue, by thin layer chromatography (Al,O,, pentane as eluent), two major products were separated. That with the longer retention time had IR and NMR spectra identical to those reported for H4R~4(C0)9(PB~3)3 [9). The other product had an IR spectrum with bands at 2078(sh), 2071(s), 2062(m), 205O(vs), 2040(s), 2022(vs), 2012(vs), 2000(m), 1995(s), 1989(m), 1980(m), 1968(m) and 1942(m) cm-l; the NMR spectrum (C6Ds, TMS) showed hydridic absorptions at 27.1 7; the ratio between the area of the 27.1 r absorption and the area of the tributylphosphine protons (8.00 - 9.25 T) was 2:27, indicating the presence of two hydridic hydrogens vs. one phosphine molecule, in agreement with the H4Ruq(CO) &PBu& for- mulation.

Elemental analysis for C3&15801#2R~4 found: C 36.89%, H 5.46%; calcd: C 37.27%, H 5.34%.

Acknowledgements

This research was partially supported by C.N.R. Rome.

References

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Documents

Homogeneous catalytic hydrogenation of dicarboxylic acid esters