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Synthesis of γ-keto carboxylic acids from γ-keto-α-chloro carboxylic acids via carbonylation—decarboxylation reactions catalysed by a palladium system
of Molecular Catalysis, 78 (1993) 121-129 Elsevier Science Publishers B.V., Amsterdam
121
Jcru~l
M3062
Synthesis of y-keto carboxylic acids from y-keto-a-chloro carboxylic acids via carbonylation-decarboxylation reactions catalyzed by a palladium system G. Cavinato Department
of Inorganic
Chemistry,
University
of Paduu, Padua
(Italy]
and L. Toniolo* Department
of Chemistry,
University
of Venice,
Venice (Italy]
(Received March 6, 1992; accepted June 15, 1992)
Abstract A palladium-based catalytic system is highly active in the synthesis of y-keto acids of type ArCOCH,CH&OOH via carbonylation-decarboxylation of the corresponding cY-chloride. Typical reaction conditions are: P(C0) = 20-30 atm; substrate/H,O/Pd = l OO-400/ 800-1000/l (mol); temperature: 100-l 10 “C; [Pd] =0.25 X 10-‘-l X lo-’ M; solvent: acetone; reaction time: l-2 h. A palladium(I1) complex can be used as catalyst precursor. Under the reaction conditions above, reduction of the precursor to palladium metal occurs to a variable extent. High catalytic activity is observed when the precursor undergoes extensive decomposition to the metal. Pd/C is also highly active. Slightly higher yields are obtainable when the catalytic system is used in combination with a ligand such as PPhe. A mechanism for the catalytic cycle is proposed: (i) The starting keto chloride undergoes oxidative addition to reduced palladium with formation of a catalytic intermediate having a Pd-[CH(COOH)CH,COPh] moiety. The reduced palladium may be the metal coordinated by other atoms of palladium and/or by carbon monoxide and/or by a PPhe ligand when catalysis is carried out in the presence of this ligand. It is also proposed that the keto group in the P-position with respect to the carbon atom bonded to chlorine weakens the C-Cl bond, easing the oxidative addition step and enhancing the activity of the catalyst. (ii) Carbon monoxide ‘inserts’ into the Pd-C bond of the above intermediate to give an acyl catalytic intermediate having a Pd-[COCH(COOH)CH,COPh] moiety. (iii) Nucleophilic attack of H,O to the carbon atom of the carbonyl group bonded to the metal of the acyl intermediate yields a malonic acid derivative as product intermediate. This, upon decarboxylation, gives the final product. Alternatively, the desired product may form without the malonic acid derivative intermediate, through the following reaction pathway: the acyl intermediate undergoes decarboxylation with formation of a different acyl intermediate, having a Pd-[CO-CH&H&OPh] moiety, which, upon nucleophilic attack of Hz0 on the carbon atom of the carbonyl group bonded to the metal, yields the final product.
Introduction Carbonylation attention because
of functionalized substrates is receiving more and more it may be a basis for new and convenient routes for the
*Author to whom correspondence should be addressed.
0304-5102/93/$06.00
0 1993 - Elsevier Science Publishers B.V. All rights reserved
122
production of a broad spectrum of fine chemicals. We recently reported that levulinic acid, CH,COCH,CH,COOH, a y-keto acid of much interest as an intermediate of many fine chemicals [ 11, can be synthesized via Pd-HClcatalyzed regiospecific carbonylation of methyl vinyl ketone, of its reaction products with hydrochloric acid or an alkanol, or of a mixture of acetone with a formaldehyde precursor, such as for example trioxane [ 21. On the basis of experimental evidence it has been proposed that in every case the active substrate that begins the catalytic cycle, through oxidative addition to the metal, is the y-keto chloride that easily forms starting from any of the substrates just reported upon reaction with hydrochloric acid. More recently, we have reported that aromatic aldehydes with an electronreleasing group in thepura position (best is an -OH group) can be carbonylated to phenylacetic acid derivatives in the presence of a Pd-PPh,-HCl system. It has been proposed that catalysis occurs via incorporation of a molecule of carbon monoxide giving formation of a mandelic acid derivative, ArCHOHCOOH, as intermediate, which promptly reacts further with a second molecule of carbon monoxide to yield a phenylmalonic acid derivative as another intermediate, which gives the final product upon decarboxylation [ 31. This hypothesis has found strong experimental support because it has also been found that the same catalytic system is very active in promoting the reduction of a mandelic acid derivative to a phenylacetic acid derivative through a carbonylation-decarboxylation reaction [4, 51. In this case it was proposed that the catalytic cycle begins with the oxidative addition to the metal of the chloride that forms in situ upon reaction of the starting substrate with hydrochloric acid, with formation of an intermediate having a Pd-[CH(COOH)Ar] moiety, which, after carbon monoxide insertion followed by hydrolysis, gives a phenylmalonic acid derivative that decomposes with CO2 evolution, yielding the final product. In practice, the a-hydroxylic group of the mandelic acid derivative is replaced by hydrogen. We report here the synthesis of a y-aroyl carboxylic acid via reaction of the corresponding y-aroyl-cu-chloro derivative with carbon monoxide and water, catalyzed by a palladium system. This reaction occurs with substitution of the halogen by hydrogen, in practice through a carbonylationdecarboxylation reaction, as seen for the reduction of the mandelic acid derivative. Results and discussion The synthesis of y-keto acids from the corresponding y-keto-cu-chloro acids, through a carbonylation-decarboxylation reaction catalyzed by a Pdbased system, is schematized as follows: PhCOCH,CHClCOOH
+ CO + Hz0 -
PhCOCH,CH&OOH
+ CO2 + HCl
(1)
Typical reaction conditions are: P(C0) = 20-30 atm; substrate&O/ Pd=lOO-400/800-1000/l; temperature: 100-110 “C; [Pd]=0.25xlO-” 1 x lop2 M; solvent: acetone; reaction time: l-2 h.
Pd/C or any of the Pd(I1) complexes listed in Table 1 can be used as catalyst precursor. Under the reaction conditions reported above, extensive reduction of the palladium(II) precursors to palladium metal occurs. This is true also for the precursor PdC12(PPh&, which is not normally reduced to the metal under the usual carbonylation conditions, i.e., 106-110 “C, P(C0) = 50-l 00 atm, particularly when employed in the presence of additional amounts of PPha (Pd/P= l/3-4), even though it can give rise to other Pd(II) complexes. For example, it has been found that during the catalytic carbonylation of oleflns to esters in the presence of MeOH the above palladium(R) chloride phosphine complex is converted mainly to PdCl(COR)(PPh,), and PdCl(COOMe)(PPh&. The latter complex is less stable than the first toward reduction and decomposes to Pd(0) complexes of the type Pd(CO)(PPh,), and Pda(CO)a(PPh& and eventually to palladium metal [6]. These Pd(0) complexes form easily, even under room conditions, when PdC12(PPh& is treated in MeOH with carbon monoxide in the presence of a base such as a dialkylamine [ 71. They have successfully been employed as catalyst precursors for the carboxymethylation of organic halides even under room conditions ]3].
When the carbonylation reaction is carried out in the presence of relatively large amounts of hydrochloric acid, reduction to palladium metal occurs to various degrees, depending mainly on the temperature and the concentrations of the acid and the catalyst. Qualitative observations have shown that PdC12(PPh& is less stable at higher temperatures, higher concentrations of the acid, and lower concentrations of the precursor and of PPh3 [4]. In the present case, reaction (1) occurs with formation of a relatively large amount
TABLE
1
Synthesis of PhCOCHaCHaCOOH by carbonylation-decarboxylation using various catalyst precursors Run no.
Catalyst precursor
of PhCOCHaCHClCOOH
Yield (%)
1 2 3 4 5 6 7 8 9 10 11
PdlC PdK,
PPh, ’
Pd(PPh,)a PdCla PdCla, PPh, =
lPdoWPW1,
PdCl,(PPh,), Pd(AcO), Pd(AcO)a, PPha a ]PdCl(1,3-+GH,)lz [PdC1(1,3-q-CaH5)]a, PPh, a
88 91 92 91 93 93 94 91 92 90 92
Run conditions: [Pd] = 0.25 x lo-’ M; substrate/Pd= 400; solvent: acetone/water= S/O.5 (ml); P(CO)=30 atm at r.t.; temp. 110 “C; react. time: 2 h. “PdlP= l/2.
124
of HCl, which equilibria: PdCl,(PPh,),
can displace
+ HCl-
- PPhs
the PPha ligand according
[PdCl,(PPh,)]-
HCl
-
to the following
[PdC1,12-
which may favor the reduction to palladium metal by carbon monoxide. As a matter of fact, it has been found that on treating PdCl, and LiCl with carbon monoxide, even at atmospheric pressure and 20-40 “C, there is formation of a car-bony1 complex (v(C0) = 1980 cm-‘) which decomposes to palladium metal with concomitant formation of several products, mainly ethyl chlorocarbonate, ethyl acetate and diethyl carbonate, depending on the reaction conditions [ 91. Another cause of decomposition may be the subtraction of the stabilizing PPhB ligand due to its reactivity with the organic chloride. For example, in the case of the carbonylation of methyl vinyl ketone (MVK), catalyzed by a Pd-HCl system, it has been found that the precursor dissociates to the dimer [PdC12(PPh)a12 and PPha, which reacts with the y-keto chloride that forms in situ upon addition of HCl to MVK, to give a phosphonium salt, [Ph3P(CH2CH2COCH3)]+C1[2]. The data reported in Table 1 show that Pd/C gives practically the same results as Pd(I1) precursors and that no significant difference in catalytic activity is observed among the various precursors. These findings suggest that the active catalytic system obtained from the different precursors is practically the same, probably because of the formation of relatively large amounts of HCl, which may level off any major differences in the catalytic systems generated from the different precursors. Moreover, the fact that practically the same catalytic activity is observed whether in the presence of a ligand such as PPh, or in its absence, suggests that this ligand does not play a major role under the reported run conditions. This behaviour has also been found for the closely related carbonylation of MVK [2] and of mandelic acid derivatives catalyzed by Pd-HCl systems [ 4, 51. On the catalytic cycle of reaction (1) The proposed catalytic cycle (Scheme 1) proceeds through the following steps: (i) The starting y-keto-cY-chloro carboxylic acid (R) undergoes oxidative addition to reduced palladium with formation of the catalytic intermediate X having a Pd-[CH(COOH)CH&OPh] moiety. Oxidative addition to reduced palladium is known to occur readily with aryl, benzyl, vinyl and ally1 halides [lo-181. Reduced palladium may be the metal coordinated by other atoms of palladium, and/or by carbon monoxide, and/or by a PPh, ligand when catalysis is carried out in the presence of this ligand. An allylic structure of type A and a structure of type B may contribute to easing the oxidative addition step of the starting chloride to palladium. As a matter of fact, ally1 chlorides are known to undergo facile palladiumcatalyzed carbonylation [ 13, 19-241, and structure B favors the nucleophilic attack of palladium to the (Y carbon atom in the oxidative addition step. The
125 R
H-d-COOH (PI I H
T -co
2
R
R I HOOC-c-coon
+
Cl-k-coon
HCl
I
H
CR)
co Scheme 1. Proposed catalytic cycle for the carbonylation-decarboxylation RCHzCOOH (R = PhCOCH2).
of RCHClCOOH
to
formation of a cationic species of type B has been suggested in order to account for evidence concerning the addition of halogen to a,&unsaturated aldehydes and ketones in the presence of halogenic acids [ 5, 261. In addition, on the basis of NMR evidence it has been proposed that a,@nsaturated ketones are protonated by strong nonnucleophilic acids on the carbonyl oxygen to give relatively stable salts of type B [ 27, 281. PH Ph-C=CH-CH-Cl
OH I’.___+______ Ph-C-CH-CH Cl-
COOH A
COOH B
It is worth mentioning that also for the carbonylation of CHaCOCH&H&l catalyzed by a Pd-HCl system, because of the high activity shown by this system and because of the high reactivity of the y-keto chloride, it has been proposed that structures of type (A) and (B) contribute to the activation of the C-Cl bond and to enhancing of the catalytic activity. (ii) Carbon monoxide ‘insertion’ into the Pd-C bond of X gives the acyl catalytic intermediate Y. This ‘insertion’ is likely to occur via nucleophilic
126
attack of the carbon atom of the ligand [CH(COOH)CH,COPh] on a coordinate CO ligand [29, 301. Organopalladium complexes are known to undergo the carbon monoxide ‘insertion reaction’ very readily [ 3 11. (iii) The third step of the catalytic cycle is the nucleophilic attack of H,O at the carbon atom of the carbonyl group bonded to the metal of the catalytic intermediate Y, which yields a malonic acid derivative (I) as product intermediate, which, upon decarboxylation, gives the final product (P). After the nucleophilic attack there might occur formation of a hydridochloride-palladium(I1) complex that ultimately loses HCl and reforms the starting Pd(0) complex. It is also possible that intermediate Y undergoes reductive elimination of RCH(COOH)COCl, which upon hydrolysis yields the malonic acid derivative I. Decarboxylation of malonic acid derivatives is known to occur easily under acidic conditions. As a matter of fact, it has been reported that the malonic acid derivative I is easily decarboxylated upon heating in the presence of either a base or an acid [32]. It is interesting that palladium(O) complexes of type Pd(CO)(PPh&, Pd3(C0)3(PPh3), and Pd3(C0)a(PPh&, as already mentioned, are catalytically active in the carboalkoxylation of various organic halides under mild conditions, i.e., even under 1 atm of carbon monoxide at 20-100 “C [8, 33, 341. In this case, a base such as a secondary or tertiary amine must be present. By reacting Pd(CO)(PPh,), with iodobenzene under carbon monoxide, the palladium(I1) complex PdI(COPh)(PPh,), is obtained, which does not yield any benzoate ester when stirred in MeOH and in the absence of a base. When diethylamine is added, after removal of the dissolved carbon monoxide by bubbling nitrogen, a stoichiometric amount of methyl benzoate is obtained. On the basis of this evidence it has been proposed that the function of the base is to favor the direct attack of methoxide anion on the Pd-COPh moiety rather than to neutralize the acid that forms during the reaction [S]. Quite interestingly, in the present case catalysis occurs even in the absence of any base. Steps (i)-(m) are commonly proposed for the carbonylation of organic halides catalyzed by palladium complexes [35-371. It is relevant to note that a catalytic cycle similar to the one just described has also been proposed for the synthesis of phenylacetic acid derivatives occurring through the carbonylation-decarboxylation of mandelic acid derivatives catalyzed by the same systems [4, 51. Alternatively, intermediate Y may undergo decarboxylation with formation of intermediate Z, which upon nucleophilic attack of H,O yields the final product without the malonic acid derivative intermediate: Cl-Pd-CO-C-COOH Y
can
-
- coz
Cl-Pd-CO-CH Z
=
RCH&OOH
+ Pd(0) + HCl
P
In principle, after the initial oxidative addition step, the catalytic cycle proceed via the scheme shown below, through the y-ketoalkyl-
127
hydroxycarbonyl-Pd(I1) intermediate (Y’), derivative upon reductive elimination: Pd(0) + R-Cl -
R-Pd-Cl CO. X
which
R-Pd-COOH Y’
gives -
the malonic
acid
RCOOH + Pd(0) P
The hydroxycarbonyl complex would form through CO ‘insertion’ into a Pd-OH bond formed upon interaction of water with some kind of palladium complex [38]. However, the formation of any Pd-hydroxide complex seems rather unlikely in view of the presence of hydrochloric acid in a relatively high concentration in the reaction medium. Moreover, it has been found that the benzyl carbomethoxide complex of palladium(H), transPd(CHzPh)(COOMe)(PMe,),, which is closely related to the proposed intermediate Y’, does not yield the ester upon heating up to 80 “C [39]. Thus it is more reasonable that reaction (1) occurs through a catalytic cycle involving CO ‘insertion’ into a Pd-C bond rather than into a Pd-0 bond.
Experimental Materials Carbon monoxide, quality N37, was purchased from SIO Company. Solvents were HPLC grade. y-oxo-y-phenyl-a-chloropropanoic acid was prepared by addition of HCl to benzoyl acrylic acid [32], prepared in turn by reacting benzene with maleic anhydride in the presence of Al&, as reported in the literature [40]. The catalyst precursors PdCla(PPh&, [PdC12(PPh3)]a, [ PdCl( 1 ,3-77-C3H5)], Pd(AcO)a(PPh& and Pd(PPh& were prepared following known procedures [41-441. 10% Pd/C, type 3230 A, PdClz and Pd(OAc), were gifts of the Engelhard Company. Product ioTent@ication and analysis Products were identified and analyzed by IR (using a Perkin-Elmer model 683 spectrometer), by NMR (on a Varian FI’ 80 A instrument) and by HPLC (on a Perkin-Elmer liquid chromatograph, model HPLC 10, using a C18SIL-X-10 25 cm column; solvent: Hz0 70%, CH&N 30%, containing 2% CH,COOH). Synthesis of PhCOCH,CH,COOH by carbonylatiorwkcarboxglutti of PhCOCH, CHClCOOH The reactions were performed using a stainless-steel autoclave with a volume of ca. 70 ml. The catalyst precursor, reagent and solvent were introduced in a Pyrex@ bottle placed in the autoclave. A magnetic stirrer was provided. In a typical experiment, 0.025 mm01 of catalyst precursor, 10 mm01 of the keto chloride, 8 ml of acetone and 0.5 ml of water were employed. The autoclave was purged with carbon monoxide at room temperature and then
128
charged with 30 atm of the same gas. The autoclave was placed in a stirred oil bath, initially set at a temperature slightly higher than the run temperature (typically 110 “C). This temperature was reached in cu. 10 mm and maintained throughout the experiment (2 h). At the end of the experiment the autoclave was cooled in an ice bath and slowly depressurized. With each of the palladium(I1) precursors reported in Table 1 or Pd(PPh& a black mirror of palladium metal formed on the wall of the glass bottle. After the usual workup the reaction solution was analyzed by HPLC. The results obtained using various catalyst precursors are reported in Table 1.
Acknowledgements The authors wish to thank the Italian National Council (C.N.R.) for sponsoring this research (Progetto Pinalizzato Chimica Pine II) and the Engelhard Company of Milan for their generous gift of catalysts.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
27
R. H. Leonard, Ind. Eng. Chem., 48 (1956) 1331. G. Cavinato and L. Toniolo, J. Mol. Cutal., 58 (1990) 251. G. Cavinato and L. Toniolo, J. Mol. Cutal., 69 (1991) 283. G. Cavinato and L. Toniolo, J. Mol. Cutal., 78 (1992) 131. G. Cavinato and L. Toniolo, J. Mol. Cutal., submitted for publication. G. Cavlnato and L. Toniolo, J. Orgurwmet. Chem., 398 (1990) 187. M. Hidai, M. Kokura and Y. Uchida, J. Orgunomxt. Chem., 52 (1973) 431. M. Hidai, T. Hikita, Y. Wada, Y. Fujikura and Y. Uchlda, Bull. Chem. Sot. Jp., 48 (1975) 2075. M. Graziani, P. Uguagliati and G. Carturan, J. Orgurwmet. Chem., 27 (1971) 275. T. Mizoroki, K. Mori and A. Ozaki, Bull. Chem. Sot. Jpn., 44 (1971) 581. T. Mizoroki, K. Mori and A. Ozaki, Bull. Chem. Sot. Jpn., 46 (1973) 1505. R. F. Heck and J. P. Nolley, J. Org. Chem., 37 (1972) 2320. D. Medema, R. v. Helden and C. F. KohlI, Inorg. Chim. Actu, 3 (1969) 255. E. 0. Fischer and G. Burger, 2. Nutu@rsch., Teil B, 16 (1961) 702. A. V. Kavaliunas, A. Taylor and R. D. Rieke, Organometallics, 2 (1983) 377. R. D. Rieke and A. V. Kavaliunas, J. Org. C&m., 44 (1979) 3069. H. A. Dieck and R. F. Heck, J. Am. Chem. Sot., 96 (1974) 1133. P. Fitton and E. A. Rick, J. Organomet. Chem., 28 (1971) 287. J. Tsuji, J. Kiji and M. Morikawa, Tetrahedron L&t., 26 (1963) 1811. J. Tsyji, J. Fiji, S. Imamura and M. Morlkawa, J. Am. Cha. Sot., 86 (1964) 4350. J. Tsyji, S. Imamura and J. Fiji, J. Am. Chem. Sot., 86 (1964) 4491. W. T. Dent, R. Long and G. H. Whitfield, J. Ch,ern. Sot., (1964) 1588. J. Tsuji, K. Sato and H. Okumoto, Tetrahedron L&t., 23 (1982) 5189. J. Tsuji, K. Sato and H. Okumoto, J. Org. Chtvn., 49 (1984) 1341. V. L. Heasley, D. F. Shellhamer, T. L. Carter, D. E. Gipe, R. K. Gipe, R. C. Green, J. Nordeen, T. D. Rempel and D. W. Spaite, Tetrahedron Lett., 22 (1981) 2467. V. L. Heasley, T. J. Louie, D. K. Luttndl, M. D. MiIlar, H. B. Moore, D. F. Nogales, A. M. Sauebrey, A. B. Schevel, T. Y. Shilubya, M. S. Stanley and D. F. Scellamer, J. Org. Chem., 53 (1988) 2199. J. N. Marx, Tetrahedron I&t., 52 (1971) 4957.
129 28 N. C. Deno, H. G. Richey Jr., N. Friedman, J. D. Hodge, J. J. Houser and C. U. Pittman, Jr., J. Am. Chem. Sot., 85 (1963) 2991. 29 P. E. Garrou and R. F. Heck, J. Am. Chem. Sot., 96 (1974) 1133. 30 A. J. Chalk, Tetrahedron L&t., 37 (1984) 2627. 31 K. Kudo, M. Sato, M. Hidai and Y. Uchida, BuU. Chenz. Sot. Jpn., 46 (1973) 2820. 32 M. J. Bougauh, Ann. Chim. Phys., 15 (1908) 491. 33 A. Schoenberg, I. Bartoletti and R. F. Heck, J. Org. Chem., 39 (1974) 3318. 34 J. K. Stille and P. K. Wong, J. Org. Chem., 40 (1975) 532. 35 J. Tsuji, Act. Chem. Res., 2 (1969) 44. 36 J. K. Stille, L. F. Hines, R. W. Fries, P. K. Wong, D. E. James and K. Lan, Adv. CM. Se-r., 132 (1974) 90. 37 D. Milstein, Act. Chem. Res., 21 (1988) 428. 38 H. E. Bryndza and W. Tam, Chem. Rev., 88 (1988) 1163. 39 D. Milstein, J. Chem. Sot., Chem. Commun., (1986) 817. 40 0. Grummitt, E. I. Becker and C. Miesse, Org. Synth., 3 (1955) 109. 41 J. Chatt and L. M. Venanzi, J. Chem. Sot., (1957) 2351. 42 W. Kuran and A. Musco, Inorg. Chim. Acta, 12 (1975) 187. 43 T. A. Stephenson, S. M. Morehouse, A. R. Powell, J. P. Heffer and G. Wilkinson, J. Chum. Sot., (1965) 3632. 44 Y. Tatsuno, T. Yoshida and K. Seiotsuka, Inorg. Synth., 19 (1979) 220.
121
Jcru~l
M3062
Synthesis of y-keto carboxylic acids from y-keto-a-chloro carboxylic acids via carbonylation-decarboxylation reactions catalyzed by a palladium system G. Cavinato Department
of Inorganic
Chemistry,
University
of Paduu, Padua
(Italy]
and L. Toniolo* Department
of Chemistry,
University
of Venice,
Venice (Italy]
(Received March 6, 1992; accepted June 15, 1992)
Abstract A palladium-based catalytic system is highly active in the synthesis of y-keto acids of type ArCOCH,CH&OOH via carbonylation-decarboxylation of the corresponding cY-chloride. Typical reaction conditions are: P(C0) = 20-30 atm; substrate/H,O/Pd = l OO-400/ 800-1000/l (mol); temperature: 100-l 10 “C; [Pd] =0.25 X 10-‘-l X lo-’ M; solvent: acetone; reaction time: l-2 h. A palladium(I1) complex can be used as catalyst precursor. Under the reaction conditions above, reduction of the precursor to palladium metal occurs to a variable extent. High catalytic activity is observed when the precursor undergoes extensive decomposition to the metal. Pd/C is also highly active. Slightly higher yields are obtainable when the catalytic system is used in combination with a ligand such as PPhe. A mechanism for the catalytic cycle is proposed: (i) The starting keto chloride undergoes oxidative addition to reduced palladium with formation of a catalytic intermediate having a Pd-[CH(COOH)CH,COPh] moiety. The reduced palladium may be the metal coordinated by other atoms of palladium and/or by carbon monoxide and/or by a PPhe ligand when catalysis is carried out in the presence of this ligand. It is also proposed that the keto group in the P-position with respect to the carbon atom bonded to chlorine weakens the C-Cl bond, easing the oxidative addition step and enhancing the activity of the catalyst. (ii) Carbon monoxide ‘inserts’ into the Pd-C bond of the above intermediate to give an acyl catalytic intermediate having a Pd-[COCH(COOH)CH,COPh] moiety. (iii) Nucleophilic attack of H,O to the carbon atom of the carbonyl group bonded to the metal of the acyl intermediate yields a malonic acid derivative as product intermediate. This, upon decarboxylation, gives the final product. Alternatively, the desired product may form without the malonic acid derivative intermediate, through the following reaction pathway: the acyl intermediate undergoes decarboxylation with formation of a different acyl intermediate, having a Pd-[CO-CH&H&OPh] moiety, which, upon nucleophilic attack of Hz0 on the carbon atom of the carbonyl group bonded to the metal, yields the final product.
Introduction Carbonylation attention because
of functionalized substrates is receiving more and more it may be a basis for new and convenient routes for the
*Author to whom correspondence should be addressed.
0304-5102/93/$06.00
0 1993 - Elsevier Science Publishers B.V. All rights reserved
122
production of a broad spectrum of fine chemicals. We recently reported that levulinic acid, CH,COCH,CH,COOH, a y-keto acid of much interest as an intermediate of many fine chemicals [ 11, can be synthesized via Pd-HClcatalyzed regiospecific carbonylation of methyl vinyl ketone, of its reaction products with hydrochloric acid or an alkanol, or of a mixture of acetone with a formaldehyde precursor, such as for example trioxane [ 21. On the basis of experimental evidence it has been proposed that in every case the active substrate that begins the catalytic cycle, through oxidative addition to the metal, is the y-keto chloride that easily forms starting from any of the substrates just reported upon reaction with hydrochloric acid. More recently, we have reported that aromatic aldehydes with an electronreleasing group in thepura position (best is an -OH group) can be carbonylated to phenylacetic acid derivatives in the presence of a Pd-PPh,-HCl system. It has been proposed that catalysis occurs via incorporation of a molecule of carbon monoxide giving formation of a mandelic acid derivative, ArCHOHCOOH, as intermediate, which promptly reacts further with a second molecule of carbon monoxide to yield a phenylmalonic acid derivative as another intermediate, which gives the final product upon decarboxylation [ 31. This hypothesis has found strong experimental support because it has also been found that the same catalytic system is very active in promoting the reduction of a mandelic acid derivative to a phenylacetic acid derivative through a carbonylation-decarboxylation reaction [4, 51. In this case it was proposed that the catalytic cycle begins with the oxidative addition to the metal of the chloride that forms in situ upon reaction of the starting substrate with hydrochloric acid, with formation of an intermediate having a Pd-[CH(COOH)Ar] moiety, which, after carbon monoxide insertion followed by hydrolysis, gives a phenylmalonic acid derivative that decomposes with CO2 evolution, yielding the final product. In practice, the a-hydroxylic group of the mandelic acid derivative is replaced by hydrogen. We report here the synthesis of a y-aroyl carboxylic acid via reaction of the corresponding y-aroyl-cu-chloro derivative with carbon monoxide and water, catalyzed by a palladium system. This reaction occurs with substitution of the halogen by hydrogen, in practice through a carbonylationdecarboxylation reaction, as seen for the reduction of the mandelic acid derivative. Results and discussion The synthesis of y-keto acids from the corresponding y-keto-cu-chloro acids, through a carbonylation-decarboxylation reaction catalyzed by a Pdbased system, is schematized as follows: PhCOCH,CHClCOOH
+ CO + Hz0 -
PhCOCH,CH&OOH
+ CO2 + HCl
(1)
Typical reaction conditions are: P(C0) = 20-30 atm; substrate&O/ Pd=lOO-400/800-1000/l; temperature: 100-110 “C; [Pd]=0.25xlO-” 1 x lop2 M; solvent: acetone; reaction time: l-2 h.
Pd/C or any of the Pd(I1) complexes listed in Table 1 can be used as catalyst precursor. Under the reaction conditions reported above, extensive reduction of the palladium(II) precursors to palladium metal occurs. This is true also for the precursor PdC12(PPh&, which is not normally reduced to the metal under the usual carbonylation conditions, i.e., 106-110 “C, P(C0) = 50-l 00 atm, particularly when employed in the presence of additional amounts of PPha (Pd/P= l/3-4), even though it can give rise to other Pd(II) complexes. For example, it has been found that during the catalytic carbonylation of oleflns to esters in the presence of MeOH the above palladium(R) chloride phosphine complex is converted mainly to PdCl(COR)(PPh,), and PdCl(COOMe)(PPh&. The latter complex is less stable than the first toward reduction and decomposes to Pd(0) complexes of the type Pd(CO)(PPh,), and Pda(CO)a(PPh& and eventually to palladium metal [6]. These Pd(0) complexes form easily, even under room conditions, when PdC12(PPh& is treated in MeOH with carbon monoxide in the presence of a base such as a dialkylamine [ 71. They have successfully been employed as catalyst precursors for the carboxymethylation of organic halides even under room conditions ]3].
When the carbonylation reaction is carried out in the presence of relatively large amounts of hydrochloric acid, reduction to palladium metal occurs to various degrees, depending mainly on the temperature and the concentrations of the acid and the catalyst. Qualitative observations have shown that PdC12(PPh& is less stable at higher temperatures, higher concentrations of the acid, and lower concentrations of the precursor and of PPh3 [4]. In the present case, reaction (1) occurs with formation of a relatively large amount
TABLE
1
Synthesis of PhCOCHaCHaCOOH by carbonylation-decarboxylation using various catalyst precursors Run no.
Catalyst precursor
of PhCOCHaCHClCOOH
Yield (%)
1 2 3 4 5 6 7 8 9 10 11
PdlC PdK,
PPh, ’
Pd(PPh,)a PdCla PdCla, PPh, =
lPdoWPW1,
PdCl,(PPh,), Pd(AcO), Pd(AcO)a, PPha a ]PdCl(1,3-+GH,)lz [PdC1(1,3-q-CaH5)]a, PPh, a
88 91 92 91 93 93 94 91 92 90 92
Run conditions: [Pd] = 0.25 x lo-’ M; substrate/Pd= 400; solvent: acetone/water= S/O.5 (ml); P(CO)=30 atm at r.t.; temp. 110 “C; react. time: 2 h. “PdlP= l/2.
124
of HCl, which equilibria: PdCl,(PPh,),
can displace
+ HCl-
- PPhs
the PPha ligand according
[PdCl,(PPh,)]-
HCl
-
to the following
[PdC1,12-
which may favor the reduction to palladium metal by carbon monoxide. As a matter of fact, it has been found that on treating PdCl, and LiCl with carbon monoxide, even at atmospheric pressure and 20-40 “C, there is formation of a car-bony1 complex (v(C0) = 1980 cm-‘) which decomposes to palladium metal with concomitant formation of several products, mainly ethyl chlorocarbonate, ethyl acetate and diethyl carbonate, depending on the reaction conditions [ 91. Another cause of decomposition may be the subtraction of the stabilizing PPhB ligand due to its reactivity with the organic chloride. For example, in the case of the carbonylation of methyl vinyl ketone (MVK), catalyzed by a Pd-HCl system, it has been found that the precursor dissociates to the dimer [PdC12(PPh)a12 and PPha, which reacts with the y-keto chloride that forms in situ upon addition of HCl to MVK, to give a phosphonium salt, [Ph3P(CH2CH2COCH3)]+C1[2]. The data reported in Table 1 show that Pd/C gives practically the same results as Pd(I1) precursors and that no significant difference in catalytic activity is observed among the various precursors. These findings suggest that the active catalytic system obtained from the different precursors is practically the same, probably because of the formation of relatively large amounts of HCl, which may level off any major differences in the catalytic systems generated from the different precursors. Moreover, the fact that practically the same catalytic activity is observed whether in the presence of a ligand such as PPh, or in its absence, suggests that this ligand does not play a major role under the reported run conditions. This behaviour has also been found for the closely related carbonylation of MVK [2] and of mandelic acid derivatives catalyzed by Pd-HCl systems [ 4, 51. On the catalytic cycle of reaction (1) The proposed catalytic cycle (Scheme 1) proceeds through the following steps: (i) The starting y-keto-cY-chloro carboxylic acid (R) undergoes oxidative addition to reduced palladium with formation of the catalytic intermediate X having a Pd-[CH(COOH)CH&OPh] moiety. Oxidative addition to reduced palladium is known to occur readily with aryl, benzyl, vinyl and ally1 halides [lo-181. Reduced palladium may be the metal coordinated by other atoms of palladium, and/or by carbon monoxide, and/or by a PPh, ligand when catalysis is carried out in the presence of this ligand. An allylic structure of type A and a structure of type B may contribute to easing the oxidative addition step of the starting chloride to palladium. As a matter of fact, ally1 chlorides are known to undergo facile palladiumcatalyzed carbonylation [ 13, 19-241, and structure B favors the nucleophilic attack of palladium to the (Y carbon atom in the oxidative addition step. The
125 R
H-d-COOH (PI I H
T -co
2
R
R I HOOC-c-coon
+
Cl-k-coon
HCl
I
H
CR)
co Scheme 1. Proposed catalytic cycle for the carbonylation-decarboxylation RCHzCOOH (R = PhCOCH2).
of RCHClCOOH
to
formation of a cationic species of type B has been suggested in order to account for evidence concerning the addition of halogen to a,&unsaturated aldehydes and ketones in the presence of halogenic acids [ 5, 261. In addition, on the basis of NMR evidence it has been proposed that a,@nsaturated ketones are protonated by strong nonnucleophilic acids on the carbonyl oxygen to give relatively stable salts of type B [ 27, 281. PH Ph-C=CH-CH-Cl
OH I’.___+______ Ph-C-CH-CH Cl-
COOH A
COOH B
It is worth mentioning that also for the carbonylation of CHaCOCH&H&l catalyzed by a Pd-HCl system, because of the high activity shown by this system and because of the high reactivity of the y-keto chloride, it has been proposed that structures of type (A) and (B) contribute to the activation of the C-Cl bond and to enhancing of the catalytic activity. (ii) Carbon monoxide ‘insertion’ into the Pd-C bond of X gives the acyl catalytic intermediate Y. This ‘insertion’ is likely to occur via nucleophilic
126
attack of the carbon atom of the ligand [CH(COOH)CH,COPh] on a coordinate CO ligand [29, 301. Organopalladium complexes are known to undergo the carbon monoxide ‘insertion reaction’ very readily [ 3 11. (iii) The third step of the catalytic cycle is the nucleophilic attack of H,O at the carbon atom of the carbonyl group bonded to the metal of the catalytic intermediate Y, which yields a malonic acid derivative (I) as product intermediate, which, upon decarboxylation, gives the final product (P). After the nucleophilic attack there might occur formation of a hydridochloride-palladium(I1) complex that ultimately loses HCl and reforms the starting Pd(0) complex. It is also possible that intermediate Y undergoes reductive elimination of RCH(COOH)COCl, which upon hydrolysis yields the malonic acid derivative I. Decarboxylation of malonic acid derivatives is known to occur easily under acidic conditions. As a matter of fact, it has been reported that the malonic acid derivative I is easily decarboxylated upon heating in the presence of either a base or an acid [32]. It is interesting that palladium(O) complexes of type Pd(CO)(PPh&, Pd3(C0)3(PPh3), and Pd3(C0)a(PPh&, as already mentioned, are catalytically active in the carboalkoxylation of various organic halides under mild conditions, i.e., even under 1 atm of carbon monoxide at 20-100 “C [8, 33, 341. In this case, a base such as a secondary or tertiary amine must be present. By reacting Pd(CO)(PPh,), with iodobenzene under carbon monoxide, the palladium(I1) complex PdI(COPh)(PPh,), is obtained, which does not yield any benzoate ester when stirred in MeOH and in the absence of a base. When diethylamine is added, after removal of the dissolved carbon monoxide by bubbling nitrogen, a stoichiometric amount of methyl benzoate is obtained. On the basis of this evidence it has been proposed that the function of the base is to favor the direct attack of methoxide anion on the Pd-COPh moiety rather than to neutralize the acid that forms during the reaction [S]. Quite interestingly, in the present case catalysis occurs even in the absence of any base. Steps (i)-(m) are commonly proposed for the carbonylation of organic halides catalyzed by palladium complexes [35-371. It is relevant to note that a catalytic cycle similar to the one just described has also been proposed for the synthesis of phenylacetic acid derivatives occurring through the carbonylation-decarboxylation of mandelic acid derivatives catalyzed by the same systems [4, 51. Alternatively, intermediate Y may undergo decarboxylation with formation of intermediate Z, which upon nucleophilic attack of H,O yields the final product without the malonic acid derivative intermediate: Cl-Pd-CO-C-COOH Y
can
-
- coz
Cl-Pd-CO-CH Z
=
RCH&OOH
+ Pd(0) + HCl
P
In principle, after the initial oxidative addition step, the catalytic cycle proceed via the scheme shown below, through the y-ketoalkyl-
127
hydroxycarbonyl-Pd(I1) intermediate (Y’), derivative upon reductive elimination: Pd(0) + R-Cl -
R-Pd-Cl CO. X
which
R-Pd-COOH Y’
gives -
the malonic
acid
RCOOH + Pd(0) P
The hydroxycarbonyl complex would form through CO ‘insertion’ into a Pd-OH bond formed upon interaction of water with some kind of palladium complex [38]. However, the formation of any Pd-hydroxide complex seems rather unlikely in view of the presence of hydrochloric acid in a relatively high concentration in the reaction medium. Moreover, it has been found that the benzyl carbomethoxide complex of palladium(H), transPd(CHzPh)(COOMe)(PMe,),, which is closely related to the proposed intermediate Y’, does not yield the ester upon heating up to 80 “C [39]. Thus it is more reasonable that reaction (1) occurs through a catalytic cycle involving CO ‘insertion’ into a Pd-C bond rather than into a Pd-0 bond.
Experimental Materials Carbon monoxide, quality N37, was purchased from SIO Company. Solvents were HPLC grade. y-oxo-y-phenyl-a-chloropropanoic acid was prepared by addition of HCl to benzoyl acrylic acid [32], prepared in turn by reacting benzene with maleic anhydride in the presence of Al&, as reported in the literature [40]. The catalyst precursors PdCla(PPh&, [PdC12(PPh3)]a, [ PdCl( 1 ,3-77-C3H5)], Pd(AcO)a(PPh& and Pd(PPh& were prepared following known procedures [41-441. 10% Pd/C, type 3230 A, PdClz and Pd(OAc), were gifts of the Engelhard Company. Product ioTent@ication and analysis Products were identified and analyzed by IR (using a Perkin-Elmer model 683 spectrometer), by NMR (on a Varian FI’ 80 A instrument) and by HPLC (on a Perkin-Elmer liquid chromatograph, model HPLC 10, using a C18SIL-X-10 25 cm column; solvent: Hz0 70%, CH&N 30%, containing 2% CH,COOH). Synthesis of PhCOCH,CH,COOH by carbonylatiorwkcarboxglutti of PhCOCH, CHClCOOH The reactions were performed using a stainless-steel autoclave with a volume of ca. 70 ml. The catalyst precursor, reagent and solvent were introduced in a Pyrex@ bottle placed in the autoclave. A magnetic stirrer was provided. In a typical experiment, 0.025 mm01 of catalyst precursor, 10 mm01 of the keto chloride, 8 ml of acetone and 0.5 ml of water were employed. The autoclave was purged with carbon monoxide at room temperature and then
128
charged with 30 atm of the same gas. The autoclave was placed in a stirred oil bath, initially set at a temperature slightly higher than the run temperature (typically 110 “C). This temperature was reached in cu. 10 mm and maintained throughout the experiment (2 h). At the end of the experiment the autoclave was cooled in an ice bath and slowly depressurized. With each of the palladium(I1) precursors reported in Table 1 or Pd(PPh& a black mirror of palladium metal formed on the wall of the glass bottle. After the usual workup the reaction solution was analyzed by HPLC. The results obtained using various catalyst precursors are reported in Table 1.
Acknowledgements The authors wish to thank the Italian National Council (C.N.R.) for sponsoring this research (Progetto Pinalizzato Chimica Pine II) and the Engelhard Company of Milan for their generous gift of catalysts.
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