Formation of Polycyclic Dimers from Vinyl Nitroxides and Their Dissociation and Isomerization

Article abstract of DOI:10.1021/jo00256a017

The ambiphilic vinyl nitroxides (VN) 6 dimerize, affording either tricyclic dimers (TRI-DI) 8 or bicyclic dimers (BI-DI) 7.Formation of 8 is rationalized by C-C bond formation followed by an intramolecular (3 + 2 + 2) cycloaddition involving one nitrone and two carbonyl groups, whereas 7 must arise from O-C bond formation followed by an intramolecular (3 + 2) cycloaddition of nitrone and alkene moiety.Since the BI-DIs 7 are the thermodynamically more stable dimers, the course of the reaction depends on the question of whether it is possible to overcome the activation barrier for its formation.The crucial reaction step, the 1,3 dipolar cycloaddition involving the nitrone and alkene moiety, is favored by N-phenyl substitution and retarded by N-tert-butyl substitution of the nitrone group.Thus, BI-DIs 7 arise from N-phenyl-substituted VNs, whereas VNs with R¹ = tert-butyl yield TRI-DIs 8 at room temperature.Most of the TRI-DIs 8 dissociate at room temperature or slightly elevated temperatures.Dissociation is favored either by steric congestion of 8 or by enhanced stabilization of the acyl group R³CO of the VNs 6.Tetracyclic dimers (TET-DI) 12 arise from TRI-DIs 8 in refluxing methanol or in the presence of acids at room temperature.Isomerization in aprotic solvents in the absence of acids requires higher temperatures.Under these conditions formation of TET-DIs 12 is frequently accompanied by formation of BI-DIs 7 which must be formed via the VNs 6.