Kinetics and Mechanism of the Exo Cyclizations of ω-Formylalkyl Radicals

Article abstract of DOI:10.1021/ja00189a049

Kinetic analysis of the reactions of 5-bromopentanal (1) and 6-bromohexanal (7) indicates that both the formylbutyl radical (2) and the formylpentyl radical (8) undergo fast exo cyclization.In each case the reaction is reversible and the equilibrium lies in favor of the open-chain form.The reactions are complicated by the propensity of aldehydes to undergo abstraction of formyl hydrogen atoms.Rate constants were measured by competition against reactions of the formylalkyl radicals, 2 and 8, and the cycloalkoxy radicals, 3 and 9, with tributylstannane.Values obtainedat 80 deg C include 8.7E5 s^-1 and 1.0E6 s^-1 for cyclization of 2 and 8, respectively, and 4.7E8 s^-1 and 1.1E7 s^-1 for the reverse reactions.The Arrhenius parameters for cyclization of 2 and its alkenyl analogue, 5-hexenyl radical, are very similar, but the activation energy for cyclization of 8 is unexpectedly low.Molecualar orbital (AM1) and molecular mechanics (MM2) calculations of transition structure strain energies are consistent with these observations.