Cyclopropane chemistry. Part I. Thermal isomerisation of gem-dichlorocyclopropanes to olefins

Abstract

A series of 1,1-dichlorocyclopropanes containing various ring substituents has been prepared and the compounds have been pyrolysed in a flow system at 500–670°. This pyrolysis is a useful route to olefins. 1,1-Dichlorocyclopropane gives 2,3-dichloropropene in high yield. Cyclopropanes containing more chlorine substituents also give, in each case, a high yield of a single olefin isomeric with the cyclopropane; for instance 1,1,2-trichlorocyclopropane gives 1,1,3-trichloropropene, the 1,1,2,2-tetrachloro-compound gives 1,1,2,3-tetrachloropropene, and 1,1,2-trichloro-2-chloromethylcyclopropane gives 1,1,3-trichloro-2-chloromethylpropene. Cyclopropanes which contain other substituents give mixtures of isomeric olefins. For instance 1,1-dichloro-2-methylcyclopropane gives 2,3-dichlorobut-1-ene and 1,2-dichlorobut-2-ene. The results are interpreted in terms of a unimolecular concerted chlorine-atom migration and ring-opening mechanism.