Direct measurements of the kinetics of 3-exo radical cyclizations using radical reporter groups

The E and Z isomers of the 4-(2,2-diphenylcyclopropyl)-3-butenyl radical (1), produced by laser flash photolysis of the corresponding PTOC esters in THF, cyclized to the (2,2-diphenylcyclopropyl)(cyclopropyl)methyl radical (2) that rapidly opened to 1,1-diphenyl-4-cyclopropyl-3-butenyl radicals (3). Radicals 3 were monitored by UV spectroscopy, but the observed rate constants were for the initial, relatively slow cyclizations of radicals 1 to radical 2. The Arrhenius functions determined in the temperature range of 20-58 degrees C were log(k/s(-1)) = (11.46 +/- 0.38) - (9.10 +/- 0.54)/theta for (E)-1 and log(k/s(-1)) = (12.34 +/- 0.32) - (10.10 +/- 0.45)/theta for (Z)-1 where theta = 2.3RT in kcal/mol and errors are at 2 sigma. Radical (Z)-1 cyclizes somewhat faster than radical (E)-1 as a result of a more favorable entropy of activation and despite the fact that the activation energy for cyclization of(Z)-1 through the requisite syn-transition state is greater than that for cyclization of (E)-1 through an anti-transition state.