The effect of non-bonded interactions on the regioselectivity of cyclization of the hex-5-enyl radical

Abstract

Treatment of 1-bromo-2,2,5-trimethylhex-5-ene (9a) with tributylstannane affords 2,5,5-trimethylhex-1-ene (13a), 1,1,3,3-tetramethylcyclopentane (12a), and 1,1,4-trimethylcyclohexane (15a). Accurate determination of the yields allows the rate constants for the endo- and exo-modes of cyclization of the 2,2,5-trimethylhex-5-enyl radical (10a) to be determined. Comparison of the kinetic parameters with those for the 2,2-dimethylhex-5-enyl radical (10b) and related species indicates that interaction of the pseudo-axial 2-methyl substituent with the syn-proton at C-6 disfavours the formation from (10b) of the chair-like six-membered transition state (5). The effect is, however, too small to account for the high regioselectivity of ring closure of the parent hex-5-enyl radical.