Ene Reactions and [2+2]Cycloadditions of Nitrilium Salts with Alkenes

Abstract

N-Alkylnitrilium salts (1) undergo ene reactions with electron-rich di- and higher substituted alkenes 2 to afford either 2-azoniaallene salts (3, 6, 9, 11) (the nitrilium salt reacting as ene and the alkene reacting as enophile) or 1-azonia-1,4-pentadiene salts (10, 12) (the alkene reacting as ene and the nitrilium salt reacting as enophile). Competing with ene reactions tri- and tetrasubstituted alkenes and N-alkylnitrilium salts undergo [2+2]cycloaddition to furnish azetinium salts (8, 13). In solution, alkyl substituted 2-azoniaallene salts tautomerize to 2-azonia-1,3-butadiene salts (4, 5, 7). The constitutions of the 2-azoniaallene salt 6c and the azetinium salt 8 were secured by X-ray crystallographic analyses.