Palladium-Catalyzed Methylation of Aryl and Vinyl Halides by Stabilized Methylaluminum and Methylgallium Complexes

The intramolecularly stabilized mono- and dialkylaluminum complexes 1a, 2, 3, 4a, 5a, 5c, 6a, 6c, 7, 8, and 9 in the presence of palladium catalysts, cross-alkylate aryl, vinyl, and benzyl bromides and iodides under mild standard laboratory conditions. Aryl bromides with carbonyl substituents or benzylic halides are converted partially into dialkyl compounds. Under similar conditions, the analogous stabilized dimethylgallium complexes 1b, 4b, 5b, 6b, and 10 methylate aryl and vinyl bromides and iodides in a highly selective manner. Substituted bromobenzenes XC6H4Br, where X = CHO, COPh, CO2Et, CN, NO2, Cl, CH2Br, or CHCHCOPh, are methylated by the organogallium reagents usually only at the aromatic ring halogen atom to give substituted toluenes as single products. The methylation rates were shown to depend on the nature of the chelating ligands, on the solvent, and on the type of palladium catalyst employed.