Le radical méthylènecyclopropyle: étude théorique. Chloration par t-BuOCl et réduction par (nBu)3SnH de dérivés cyclopropéniques

Abstract

The allylic chlorination of ethyl 2,3-dialkyl 2-cyclopropenecarboxylate or primary and tertiary alcohols is highly regio- and stereoselective. The halogenated methylenecyclopropane derivative only is observed. In that compound, the halogen substituent and the functional group are in a trans position with respect to the ring. The halogenated allylic compounds of methylenecyclopropane and of cyclopropene undergo reduction by (nBu)3SnH with the same stereochemistry. Products arising from ring opening are also observed. The exocyclic migration of the double bond can be related to the formation of a transient allylic radical. A theoretical study performed within the MNDO (UHF and RHF) framework leads to a planar allylic radical, whatever the substituent. The radical abstraction reaction which takes place during the reduction process is easier for a cyclic carbon atom than for an exocyclic one. In the former case, the faster reaction is obtained for a chlorinated carbon atom.