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 °C were log( k /s - 1 ) = (11.46 ± 0.38) − (9.10 ± 0.54)/ θ for ( E )- 1 and log( k /s - 1 ) = (12.34 ± 0.32) − (10.10 ± 0.45)/ θ for ( Z )- 1 where θ = 2.3 RT in kcal/mol and errors are at 2σ. 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.