Oxygenation of 1-t-Butylcyclohexa-1,3-diene and Cholesta-2,4-diene in the Presence of Trityl Tetrafluoroborate

Abstract

Whereas the oxygenation of 1-t-butylcyclohexa-1,3-diene (4) in the presence of catalytic amounts of trityl tetrafluoroborate in dichloromethane at -78° under irradiation from a tungsten lamp gives two dimeric epidioxides as major products, cholesta-2,4-diene (5) gives under the same conditions a monomeric endoperoxide. The structures of the dimeric epidioxides have been established by a combination of chemical degradation, high-field 1H n.m.r. and an X-ray crystallographic study on the diketone (12) derived from one of the epidioxides. A mechanism involving the generation of intermediate monomer and dimer cation radicals is used to account for the formation of the dimeric epidioxides. The dimer cation radicals are expected to be more reactive than the monomer cation radicals towards oxygen. The dimerization is assumed to take place when the diene (4) and its monomer cation radical are aligned so that one reactant lies above the other whereby π-orbital overlap between the rings is maximized; oxygen is constrained to attack the one exposed face of the allylic radical system in the dimer cation radical. This accounts for the unique stereochemistry of the epidioxide products. Crystal data for diketone (12): a 29.975(4), b 5.767(1), c 11.758(2)Ǻ, β 111.67(2)°; C2/c; Z4.