Ring opening and hydrogen atom transfer trapping of the bicyclo[2.1.0]pent-2-yl radical

Article abstract of DOI:10.1021/ja00003a031

Relative rate constants (k_r/k_H) for ring opening of the bicyclo[2.1.0]pent-2-yl radical (1) to the cyclopent-3-enyl radical and trapping of radical 1 with t-BuSH, PhSH, and PhSeH in solvent THF were measured at temperatures between -78 and 50 °C. The hydrogen atom donors reacted more rapidly with radical 1 than with the cyclopropylcarbinyl radical (6). Rate constants for ring opening of 1 (k_r) could be obtained by estimating the values of k_H via Marcus theory. From initial k_H values for reactions with radical 6, new k_H values were calculated for increasingly exergonic reactions until the derived k_r values from the three trapping agents agreed with one another and an extrapolated value of k_r from Tempo trapping of 1. The results suggest that hydrogen atom transfer reactions with 1 were about 3 kcal/mol more exergonic than reactions with 6. Arrhenius functions for ring opening of 1 averaged log (k_s/s^-1) = 13.0 - 5.2/2.3RT; the value of k_r at 25 °C is 1.5 × 10⁹ s^-1. Trapping studies of 1 and 6 with 2,6-dimethylthiophenol indicated that no special steric effects were present in hydrogen atom transfers to 1. However, highly stereoselective trapping of 1 was observed in reactions with ArSD with endo-bicyclo[2.1.0]pentane-2-d predominating, and the rate constant for decarboxylation of the endo-bicyclo[2.1.0]pentane-2-carboxy radical (endo-3) at -78 °C apparently was greater than that for decarboxylation of exo-3. The stereochemical results are ascribed to a stereoelectronic effect between the C1-C4 bond and endo-C2-X bonds of bicyclo[2.1.0]pentanes that weakens endo-C2-X bonds.