Intramolecular 2H Group-Transfer (Dyotropic Rearrangements) in Alicyclic and Heterocyclic Bridged-Ring Systems

Examples of alicyclic frameworks containing a cyclohexa-1,3-diene ring as 2H donor held proximate to a variously-substituted acceptor Π-bond are compared for their reactivity in thermal 2H group-transfer reactions. Kinetic analysis reveals marked effects on rearrangement-rate as the acceptor Π-bond substieuents are varied. Attempted synthesis of exact structural analogues of these compounds but with a 2,3-diazacyclohexadiene component replacing the cyclohexadiene element [by (4 + 2)Π cycloadditions of 3,6-diaryl-1,2:4,5-tetrazines to dienophiles of the isodrin type] usually gives instead products of concomitant 2H group-transfer. These results are compared with the similar but generally slower 2H group-transfer rearrangement of pentacyclic 2-pyrazoline derivatives having relevant critical stereochemical features which result from 1,3-diarylnitrilioine (4 + 2)Π adductions with dipolarophiles of the isodrin type. Alkoxy and chlorine substitution of the acceptor Π-bond is rate-retarding for 2H group-transfer in both the alicyclic and heterocyclic representative systems examined, indicating truly pericyclic behaviour.