Cyclopropane ring opening of bicyclo[3.1.0]hexan-3-ols in fluorosulphuric acid

Abstract

Reactions of bicyclo[3.1.0]hexan-3-ols with fluorosulphuric acid give rise to cyclopentenium ions by a process involving initial opening of the cyclopropane ring. The process does not put deuterium onto any single carbon atom when carried out in deuteriated acid, and it is suggested that the first step of the reaction is opening of the cyclopropane ring to yield a mixture of isomeric olefins, which then protonate, and eliminate water from the carbocations formed to give the observed cyclopentenium ions. Reactions with antimony pentafluoride involve initial ionisation of the hydroxy-group and yield the trishomocyclopropenium ion or the substituted cyclopropenium ion, except for the 3-phenyl derivative, which behaves differently in that reaction involves ionisation of the hydroxy-group as the first step, but the product is still a cyclopentenium ion. Possible reasons for this behaviour are discussed.