Lewis Acid Activation and Catalysis of Dialkylaminyl Radical Reactions

Lewis acid activation and catalysis of dialkylaminyl radical reactions is demonstrated both qualitatively and quantitatively. Cyclization of the N-butyl-4-pentenaminyl radical (1) in the presence of a wide range of Lewis acids was shown to be efficient with good to excellent yields of cyclic products obtained in reactions conducted even at -78 degrees C. Rate constants for fragmentation of the N-ethyl-2,2-diphenylethylaminyl radical (6), 6-exo cyclization of the N-methyl-6,6-diphenyl-5-hexenaminyl radical (7), and 5-exo cyclization of the N-methyl-5,5-diphenyl-4-pentenaminyl radical (8) in the presence of the Lewis acids LiBF(4), MgBr(2), and BF(3) were measured by laser flash photolysis (LFP) methods. The LFP studies demonstrated saturation kinetic behavior with respect to the Lewis acids. Equilibrium binding constants for the Lewis acids with the dialkylaminyl radicals and rate constants for reactions of the Lewis acid complexed dialkylaminyl radicals were obtained from nonlinear regression analysis of the observed rate constants.