Rate constants for 6-exo cyclization of the 7,7-diphenyl-6-heptenyl radical (3) in benzotrifluoride (BTF, PhCF3) were determined by laser flash photolysis, and radical clock 3 was used in competition kinetic studies for determination of rate constants of reactions of the fluorous tin hydride reagent tris(2-(perfluorohexyl)ethyl)tin hydride with primary alkyl radicals. (C) 1997 Elsevier Science Ltd.