Acceptor, Donor, and Captodative Stabilization in Transition States of 5-hexen-1-yl Radical Cyclizations

Article abstract of DOI:10.1021/ja00262a009

Rate constants and activation parameters for cyclization of the 6-substituted hex-5-en-1-yl radicals (6-cyano (1b), 6-methoxy (1c), and 6-cyano-6-methoxy (1d)) have been determined.At 50 deg C, the rate constants for cyclization to the corresponding cyclopentylmethyl radicals are 1.65 x 10E8, 1.45 x 10E6, and 2.49 x 10E8 s^-1, respectively.The rate accelerations for the cyclizations of 1b-d relative to that of the parent radical, hex-5-en-1-yl (1a), are discussed in terms of the substituents' perturbations of the highest occupied (HOMO) and lowest unoccuped molecular orbital (LUMO) of the alkene moiety.The large rate accelerations of 1b and 1d (275-fold and 415-fold, respectively, at 50 deg C) result primarily from increased interaction of the semioccupied molecular orbital (SOMO) with the alkene LUMO, whereas the small rate acceleration of 1c results from increased SOMO-HOMO interaction.Radicals 1b and 1d were found to have looser, hence earlier, transition states for cyclization than do 1a and 1c.Comparison of the E_a's for 1b-d relative to that for 1a indicates that there is a slight extra stabilization (captodative effect) in the transition state for cyclization of 1d.