Reactions of free radicals with η3-allylpalladium(ii) complexes: phenyl and trityl radicals

Abstract

The compounds (η³-allyl)PdCl(PPh₃) and [(η³-allyl)Pd(PPh₃)₂]Cl react with phenyl and trityl radicals generated from the thermal decomposition of phenylazotriphenylmethane (PhNNCPh₃, PAT) in benzene at 60 °C. The products are the palladium phenyl compounds, [PdPhCl(PPh₃)]₂ and trans-PdPhCl(PPh₃)₂, respectively, and 4,4,4-triphenyl-1-butene, the latter being the result of coupling of the trityl radical with the allyl ligands. In contrast, [(η³-allyl)PdCl]₂ reacts with phenyl and trityl radicals under the same conditions to form palladium metal, trityl chloride and 3-phenylpropene, which is subsequently catalytically isomerized to 1-phenylpropene. These disparate results are interpreted in all cases in terms of initial attack by phenyl radicals on the palladium(II) to give phenyl–palladium(III) intermediates, and it is the secondary reactions, influenced by the presence or absence of coordinated PPh₃ ligands, which provide variety in the products. The reaction of [(4-methoxy-1,3-η³-cyclohexenyl)PdCl]₂ gives a mixture of trans-3-methoxy-6-phenylcyclohexene and trans-4-methoxy-3-phenylcyclohexene, consistent with initial formation of a phenyl–palladium(III) intermediate followed by phenyl migration to the η³-cyclohexenyl ligand (reductive elimination).