Mechanistic study of substituted 5-Hexen-1-YL radicals : evaluation of capto-dative stabilization effects

Typescript (photocopy).Three aliphatic bromides, 6-bromo-1-cyano-1-methoxy-1-hexene, 6-bromo-1-cyano-1-hexene and 6-bromo-1-methoxy-1-hexene, designed for the introduction of capto-dative stabilization effect into the well known 5-hexen-1-yl radical system, were synthesized and were investigated for their utility as fast cyclizing radical probes. Rate constants and activation parameters for the cyclizations of all three probe radicals were measured and compared to those of the parent 5-hexen-1-yl radical system. At 50 °C the rate constants for cyclization of the probe radicals to the corresponding cyclopentylcarbinyl radicals were 2.49 x 10^8 s^-1, 1.65 x 10^8 s^-1 and 1.45 x 10^6 s^-1, respectively. The efficient cyclizations of these radicals relative to hydrogen atom abstraction from tributylstannane suggest that related cyclizations may have some synthetic utility. Comparison of the Ea's of cyclization of the three radicals to the corresponding cyclopentylcarbinyl radicals also gave some evidence of a special capto-dative stabilization effect in the transition state for cyclization of the capto-dative (cyano and methoxy) substituted 5-hexen-1-yl radical. That is, the difference in the energy of activation, ΔEa, for the capto-dative substituted 5-hexen-1-yl radical relative to the parent 5-hexen-1-yl (1.73 Kcal/mol) is slightly greater (0.4 Kcal/mol) than the sums of those for capto substituted radical (1.04 Kcal/mol) and dative substituted radical (0.29 Kcal/mol) relative to the parent 5-hexen-1-yl radical. In the reactions of the bromide probes with strong electron donors such as sodium benzophenone ketyl, sodium naphthalenide and sodium metal, some evidence for electron transfer mechanisms was observed. However, due to the low yield of the cyclized products (usually less than 10%), kinetic information about the rate of cyclization was not obtained. Therefore, other possibilities, such as cyclization by anionic species which might be generated from the fast reductions of probe radicals by the electron donors, cannot be ruled out in these particular reactions. No evidence for an electron transfer mechanism was observed in the reactions of the bromide probes with various metal hydrides