Generation and trapping of 1,2 dibromo 3 methyl but 2 en 1 ylidenes

1,2-Dibromo-3,3-dimethylcyclopropene, generated by debromination of 1,1,2,2tetrabromo-3, 3-dimethylcyclopropane on reaction with methyllithium, ring-opens at 0–20°C to produce 1,2-dibromo-3-methylbut-2-en-1-ylidene. This is trapped by alkenes to give vinylcyclopropanes. In the case of electron rich alkenes, the stereochemistry is retained in the product, suggesting that a singlet carbene is involved. In the case of α,β-unsaturated esters, ketones or nitriles, the major cyclopropane produced has the vinyl and electron-withdrawing groups cisrelated. 3-(2-Methoxy-ethyl)-3-methyl- and 3-(2-bromoethyl)-3-methyl-1,2-dibromocyclopropenes also ring-open at ambient temperature but little stereoselectivity is observed in the trapping of the derived vinyl-carbenes by alkenes. The reaction of 3-(2-hydroxyethyl)-3-methyl-11,2,2tetrabromocyclopropane with methyl lithium or diethylphosphite and triethylamine does not lead to an isolable cyclo-propene, but instead to Z-1,2-dibromo-3-methylbuta-1,3-diene; quenching with deuterium oxide at low temperature leads to the incorporation of deuterium at C-I.