KINETICS OF 5-EXO CYCLIZATIONS OF N-ALKYL-4-PENTENAMINYL RADICALS ANDBETA-FRAGMENTATIONS OF BETA-(DIALKYLAMINO)ALKYL RADICALS

The kinetic conclusions of a recent report by Maxwell and Tsanaktsidis (J. Am. Chem. Soc. 1996, 118, 4276) were investigated. The kinetics o f ring opening of the (N-butyl-2-pyrrolidinyl)methyl radical (2) to th e N-butyl-4-pentenaminyl radical (1) and the reverse reaction, 5-exo c yclization of 1 to 2, were determined at 50 and 80 degrees C by compet itive Bu3SnH trapping. Rate constants for 5-exo cyclization of a dialk ylaminyl radical and for beta-fragmentation of a beta-(dialkylamino)et hyl radical were measured by direct laser flash photolysis (LFP) metho ds. In contrast to the conclusions of Maxwell and Tsanaktsidis, all of these radical reactions were facile with rate constants of at least 1 x 10(4)s(-1) . The claim by Maxwell and Tsanaktsidis that bis(tributy ltin oxide) catalyzes dialkylaminyl radical reactions was investigated by LFP kinetic studies of the 5-exo cyclization of the N-methyl-5,5-d iphenyl-4-pentenaminyl radical (20) in the presence of the additive wh ich demonstrated that (Bu3Sn)(2)O does not have a catalytic effect on the reaction, Computations of the energies of the N-methyl analogs of radicals 1 and 2 with a high level of theory (fourth-order Moller-Ples set perturbation theory) and by a hybrid density functional theory wit h a very large basis set indicate that the cyclization reaction is exp ected to be slightly exergonic at 298 K. This work demonstrates that t he kinetic results reported by Maxwell and Tsanaktsidis were spurious. We speculate that impurities of dichalcogens in their radical precurs or samples were reduced by Bu3SnH to highly reactive chalcogen hydride s (arylthiols and benzeneselenol) in their kinetic studies.