Reactions of free radicals with (eta)^3-allylpalladium(II) complexes: phenyl and trityl radicals

The compounds ((eta)^3-allyl)PdCl(PPh3) and [((eta)^3-allyl)Pd(PPh3)2]Cl react with phenyl and trityl radicals generated from the thermal decomposition of phenylazotriphenylmethane (PhN=NCPh3, 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, [((eta)^3-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 1phenylpropene. 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-(eta)^3-cyclohexenyl)PdCl]2 gives a mixture of trans-3methoxy-6-phenylcyclohexene and trans-4-methoxy-3-phenylcyclohexene, consistent with initial formation of a phenyl-palladium(III) intermediate followed by phenyl migration to the (eta)^3-cyclohexenyl ligand (reductive elimination).