Tandem Polar/Radical Crossover Sequences for the Formation of Fused and Bridged Bicyclic Nitrogen Heterocycles Involving Radical Ionic Chain Reactions, and Alkene Radical Cation Intermediates, Performed under Reducing Conditions:  Scope and Limitations

It is demonstrated that phosphorylated forms of β-nitro alcohols provide an excellent means of entry into β-(phosphatoxy)alkyl radicals on exposure to tributyltin hydride and AIBN in benzene at reflux. These radicals then undergo heterolytic cleavage of the phosphate group to yield alkene radical cation/phosphate anion contact ion pairs which are trapped intramolecularly in a tandem polar/radical crossover sequence involving radical ionic chain reactions by allylic and propargylic amines. The substitution pattern of the alkene radical cation dictates the cyclization mode, and this may be engineered to form fused ring systems by an initial exo-mode nucleophilic cyclization or bridged bicyclic systems when the nucleophilic attack takes place in the endo-mode.