Remarkable, Sterically Induced Rate Enhancement in the Insertion of Allenes into Palladium−Methyl Bonds

The rate of insertion of methyl-substituted allenes into the Pd−Me bond in chelate pyridine−thioether complexes [PdCl(Me)(R‘N-SR)] (R‘N-SR = 6-R‘-C5H3N-2-CH2SR) to give 2-methylallyl derivatives is remarkably enhanced by the presence of a methyl group in position 6 of the pyridine ring, which induces distortion on the main coordination plane, resulting in a metal substrate more prone to allene insertion. The flexibility of the sulfur-donor chelate ligand appears to be a paramount requisite.