Cycloaddition of heteroatom-substituted 2-azaallyl anions with alkenes. Synthesis of 1-pyrrolines and bridged azabicyclic compounds

Nonstabilized 2-azaallyl anions bearing heteroatom substituents [(RCHNC)-C-1(X)R-2(-)Li(+), where R-1 and R-2 are hydrogen or alkyl groups and X = OMe, SPh, or NR2] were generated and found to undergo efficient [3 + 2] cycloadditions with alkenes to provide 1-pyrrolines after loss of LiX. The 2-azaallyl anions were generated by tin-lithium exchange on stannyl imidates, thioimidates, or amidines (RCH)-C-1(SnBu3)N=C(X)R-2 with n-butyllithium. The initially formed 1-pyrrolines were found to be deprotonated under the reaction conditions to afford 1-metalloenamines, which could be quenched with alkyl halides, carbonyl compounds, or MeSSMe to provide further functionalized 1-pyrrolines. Cyclic methoxy-substituted 2-azaallyl anions were generated and were found to undergo cycloadditions with alkenes to afford bridged azabicyclic compounds (1-methoxy-7-azabicyclo[2.2.1]heptanes and 1-methoxy-8-azabicyclo[3.2.1]octanes). These are the first examples of cyclic nonstabilized 2-azaallyl anions.