Labeling studies of some carbonylation reactions of styrene with cationic palladium complexes

The dicarbonylation reaction of E-β-deuteriostyrene to syndiotactic poly(1-oxo-2-phenyltrimethylene) as well as to dimethyl-2-phenylbutanedioate and dimethyl-2,5-diphenyl-4-oxoheptanedioate using Pd(CF₃COO)₂/2,2′-bipyridine as the catalyst precursor in the presence of 1,4-benzoquinone in methanol takes place stereospecifically in a syn-fashion with complete retention of the label. The same result was found for the dicarbonylation to dimethyl 2-phenylbutanedioate catalyzed by Pd(CF₃COO)₂(Diop)]. In the absence of the oxidant the latter catalytic system produces methyl 2- and 3-phenylpropionates for which some scrambling of deuterium is observed when using either -deuteriostyrene or CH₃OD as the labeled substrate. Pd(CH₃CN)₄]BF₄]₂ modified with different ligands catalyses the formation of E-1,5-diphenylpent-1-en-3-one or of E-1,4-diphenylpent-1-en-3-one in tetrahydrofuran as the solvent. The label distribution using E-β-deuteriostyrene as the substrate (or styrene in the presence of dideuterium) suggests that in the synthesis of ketones catalyzed by Pd(p-CH₃C₆H₄SO₃)₂(Dppp)]·2H₂O the regioselectivity of the first inserted olefin unit does not determine the ketone regioisomer; rather which regioisomeric product preferentially forms depends on the rate of carbon monoxide insertion in either the branched or linear metal-hydrocarbyl intermediate. β-Hydrogen elimination is very rapid both after the first and the second olefin insertion.