Oxidation of cycloalkenes and methylenecycloalkanes by palladium(II) complexes. Part 2. NMR study on the mechanism of the ring expansion of methylenecyclobutane

Article abstract of DOI:10.1016/0304-5102(93)87040-F

The reaction of methylenecyclobutane (1) with bis(acetonitrile)chloronitropalladium(II) in methylene chloride yields cyclopentanone. Two intermediates - ?-olefin complex of palladium and product of β-nitritopalladation of 1 in a Markovnikov fashion - are observed by ¹H and ¹³C 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.