Synthesis of azacyclic compounds by an aminium radical route: orbital overlap requirements

Abstract

Model compounds of Δ4,5-, Δ5,6-, and Δ6,7-alkenylnitrosamines were synthesized and photolyzed to generate the corresponding alkenylaminium radicals. The Δ5,6-alkenylaminium radicals underwent efficient intramolecular addition leading to the exclusive formation of pyrrolidine derivatives, the yield of which was lower if the steric strain and hindrance were severe in the cyclization transition state. The Δ6,7-alkenylaminium radical cyclized inefficiently to piperidine derivatives and the Δ4,5-alkenylaminium radicals did not appear to cyclize; in both cases other pathways having lower activation energies, such as elimination and hydrogen abstraction, predominated. Evidence that the direction of cyclization is controlled by the degree of p-orbital overlap, but not by stability of the developing products, in the transition state is presented. The stereochemical factors associated with the overlap determined the yields of the cyclization products. This is in excellent agreement with the fact that the transition state of cyclization is reached at an early stage in the reaction coordinate. Metal ion catalyzed or radical initiated decomposition of some alkenyl chloramines were found either to cyclize to a mixture of pyrrolidine and piperidine derivatives or to undergo no cyclization.