Mechanistic studies of alkene epoxidation catalyzed by nickel(II) cyclam complexes. Oxygen-18 labeling and substituent effects

Abstract

The oxidations of cyclohexene and various aryl-substituted alkenes are catalyzed by the cyclam (1,4,8,11-tetraazacyclotetradecane) complex of Ni(NO/sub 3/)/sub 2/ with iodosylbenzene as terminal oxidant. Epoxides are the major products; however, small amounts of ring-opened products, over-oxidation to ketones or aldehydes, and allylic oxidation of cyclohexene are also observed. E olefins are more reactive than the corresponding Z olefins in contrast to the results of iron porphyrin catalysis, and kinetic studies of para-substituted styrenes indicate that the reaction is facilitated by electron-donating substituents. Labeling studies with PhI/sup 18/O confirm that the epoxide oxygen is derived from PhIO while allylic oxidation and over-oxidation products involve both PhIO and exogenous sources of oxygen. A pericyclic mechanism for the formation of PhCHO is proposed along with the intermediacy of a high-valent nickel-oxo complex as the active oxidant. These results are discussed in light related transition-metal/PhIO oxidation mechanisms.