Rearrangements and the intervention of protonated cyclopropanes in the thermal decompositions of cycloalkyl chloroformates

Abstract

The thermal decompositions of cycloalkyl chloroformates in the liquid phase are accompanied in most cases by the formation of rearrangement products. The cycloalkyl chlorides and cycloalkenes formed are largely accounted for in terms of carbonium ion rearrangements which proceed irreversibly to more stable structures. In addition, 5–10% of the products obtained by the thermal decomposition of cyclopentylmethyl, cyclohexylmethyl, and cycloheptyl chloroformate are considered to be formed via protonated cyclopropane intermediates; supporting evidence comes from the identification of bicyclo[3,1,0]hexane and bicyclo[4,1,0]heptane as products in the six- and seven-carbon systems respectively. Preponderant retention of configuration in the unrearranged decomposition products from cis- or trans-2-methylcycloalkyl chloroformates is indicative of a mechanism involving collapse of an intermediate ion-pair. The decompositions are accelerated by catalytic quantities of pyridinium chloride or aluminium trichloride; the former almost entirely eliminates the formation of rearrangement products and olefins in the primary alkyl systems, whilst the latter causes increased rearrangement and isomerization of the products.