Recyclable Pd(OAc)₂/Ligand/Al(OTf)₃ Catalyst for the Homogeneous Methoxycarbonylation and Hydrocarboxylation Reactions of Phenylacetylene

Abstract

INTRODUCTIONThe carbonylation of alkynes allows for the synthesis of R,βunsaturated carboxylic esters or acids in an easy one-step synthesis (Scheme 1). The reaction occurs much in the same manner as for alkenes, a reaction that has been quite extensively studied. 1 The reaction requires the presence of a catalyst (usually Pd), carbon monoxide under pressure, methanol or water, and an acidic co-promoter. This reaction is of interest since methyl atropate (methyl 2-phenylacrylate) and its derivatives are potential precursors of nonsteroidal anti-inflammatories such as ibuprofen and naproxen. 2 While several ligands have been applied to the catalytic conversion of phenylacetylene into its unsaturated ester derivatives, N-containing phosphine ligands have been shown to provide good catalysts in terms of stability and activity. Specifically, 2-pyridyldiphenylphosphine has earned a reputation for being particularly suited to this transformation. 3 Alternative systems have included iminophosphine-type analogues 4 and a PPh 3 /pyridine carboxylic acid system. 5 Supported catalysts have also been applied with a fair degree of success. 6 When ligands do not possess the pyridyl moiety, poor results are generally noted. 7 As with other alkoxycarbonylation-type chemistry with alkenes, the application of alkynes as substrates also calls for an acidic co-catalyst, typically a Brønsted acid. Recent work in our laboratories has involved the use of Al(OTf) 3 as a co-catalyst in the methoxycarbonylation of styrene and 1-pentene. 8 We hoped that this robust, recyclable co-catalyst would be useful in the methoxycarbonylation and hydrocarboxylation reactions of phenylacetylene and investigated its application in this reaction, discovering along the way that PÀP bidentate ligands produce excellent catalysts in the presence of this co-promoter. We also demonstrate that the Pd-based catalyst derived from BINAP is stable and capable of being recycled a number of times.' RESULTS AND DISCUSSION Initial reactions using phenylacetylene were carried out under the conditions we earlier reported, 8 making use of Al(OTf) 3 as the co-catalyst in Pd-catalyzed reactions supported by PPh 3 as ligand. The system showed a complete lack of conversion of the phenylacetylene, a result also noted by others when using monodentate phosphine ligands. 9 Several changes to the reaction conditions, while retaining the monodentate ligand, were met with failure. Previous studies have shown 1,4-bis(diphenylphosphino)butane (dppb) to be active as a ligand in the methoxycarbonylation reaction of phenylacetylene. 10 We accordingly investigated a few bidentatetype ligands with varying bite angles (Table 1), including 1,2bis(diphenylphosphino)benzene, 2,2 0 -bis(diphenylphosphino)-1,1 0binaphthyl (BINAP), and SiXantphos, in quite long-duration reactions (24 h).While 1,2-bis(diphenylphosphino)benzene provided no conversion to the desired product, BINAP-and SiXantphos-based catalysts afforded 100% conversion of the starting material, but in reactions in which less than...

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