OXIDATION OF CYCLOALKENES AND METHYLENECYCLOALKANES BY PALLADIUM(II) COMPLEXES .2. NMR-STUDY ON THE MECHANISM OF THE RING EXPANSION OF METHYLENECYCLOBUTANE

The reaction of methylenecyclobutane (1) with bis(acetonitrile)chloronitropalladium(II) in methylene chloride yields cyclopentanone. Two intermediates -pi-olefin complex of palladium and product of beta-nitritopalladation of 1 in a Markovnikov fashion - are observed by H-1 and C-13 NMR. Palladium-catalyzed ring expansion of 1 is suggested to involve heterolysis of the palladium-carbon bond and rearrangement of the resulting cyclobutyl to cyclopentyl cations. The selective formation of ring-expanded and ring-contracted carbonyl products from methylenecyclobutane and 1-methylcyclobutene, respectively, is discussed in terms of proposed mechanism.