Radical competition reactions used to determine the rate constants of some strained cyclopropylcarbinyl radical ring openings and the rate constants of chalcogen transfer for some malononitrile derivatives

摘要

The use of competition reactions in radical chemistry has led to the determination of rate constants in unimolecular and bimolecular radical reactions. The generation of strained phenylcyclopropylcarbinyl radicals in the presence of PhSeH yielded a mixture of ring opened and trapped products. The ratio of products and the Arrhenius function for trapping alkyl radicals with PhSeH were used to determine the rate constants for ring opening. The effect of orbital overlap of the breaking bond with the $pi$-system of the aryl ring on the rate constants of ring opening was determined. A Marcus theory approach to the prediction of rate constants was investigated. Thio group transfer agents also were investigated using competition kinetics. Radical generation from a PTOC ester in the presence of a trapping agent led to a mixture of self-trapped and thio-trapped products. The ratio of products and the Arrhenius function for self-trapping of alkyl radicals by their PTOC ester precursors were used to determine the rate constants for the new trapping agents. The effect of the leaving group on the rate constant of trapping was determined and compared to other effects such as the ease of radical attack at the chalcogen center and bond strength of the breaking bond.