Cyclization of the 3-tert-Butylhex-5-enyl Radical: A Test of Transition-State Structure

Article abstract of DOI:10.1021/jo00020a019

The reaction of either of the radical precursors 5 or 8 with tributylstannane in benzene gives a mixture of open-chain and cyclized products 11 and 12, the ratio of yields of which depends, as expected, on the concentration of stannane.The magnitude of the rate constant for cyclization of the radical 9, k_c ca. 2 * 10⁸ s^-1 at 80 deg C, reflects the effect of the bulky tert-butyl substituent on the strain energy of the ground state.The ratio of yields of the diastereomers of 12 (cis/trans = 4.5) is independent of stannane concentration.The difference of free energies of activation, ΔG(excit)(trans) - ΔG(excit)(cis) of 1.0 kcal, is in agreement with the theoretical predictions based on the hypothesis that cis cyclization proceeds through a chairlike transition structure 13c while trans cyclization involves a boatlike transition structure 15c.