Cyclopropane Ring Opening by Photolytically Generated Bromine Atoms

Abstract

Bromine atoms, generated by irradiation by a mercury lamp bearing a filter to assure that only wavelengths >310 nm passed, were permitted to react with a series of 1,2-diarylcyclopropanes in carbon tetrachloride solution. The major product in all cases (>80%) was the 1,3-dibromo-1,3-diarylpropane. Kinetic data at approximately 10ˉ³M gave a rate expression dBr₂/dt = -k(cyclopropane)(bromine)¹′². A Hammett treatment of the data gave p = -0.5 for both the cis series and the trans series. Synthesis of several possible minor products and their comparison with the reaction mixture by TLC is reported. Possible reactions such as induced isomerization are thus eliminated from consideration.