Reaction of trifluoromethylcarbene with cis- and trans-but-2-enes

Trifluoromethylcarbene, generated photolytically from 2,2,2-trifluorodiazoethane, reacts essentially stereo-specifically with trans- and cis-but-2-enes in the liquid phase, to give dimethyltrifluoromethylcyclopropanes, and olefinic insertion products. In the vapour phase, or in solution in an inert solvent, the degree of stereospecificity of the cyclopropane formation and the yield of insertion products are both lower, consistent with the formation of singlet trifluoromethylcarbene and its decay to a triplet state of lower energy. The vapour-phase reaction appears to involve a unimolecular singlet–triplet crossover.