A concise synthesis of quinolinium, and biquinolinium salts and biquinolines from benzylic azides and alkenes promoted by copper(ii) species

Abstract

A novel copper-promoted multiple aza-[4 + 2] cycloaddition reaction of N-methyleneanilines in situ generated from benzylic azide and alkenes afforded quinolinium salts, biquinolinium salts, biquinolines or substituted quinolines depending on the substitution on the phenyl ring of benzylic azide. The reaction of para substituted benzylic azides and 2 equivalents of alkenes afforded the corresponding substituted quinolinium salts, while benzylic azides without a para substituent provided biquinolinium salts. The copper-promoted cycloaddition reaction also allows biquinoline products to be obtained from ortho-substituted benzylic azides. These reactions work well with both terminal and internal alkenes. Unsymmetrical internal alkene reactions proceed with high regioselectivity. The reaction is likely started by Lewis acidic Cu^II-assisted rearrangement of benzylic azide to N-methyleneaniline, followed by a [4 + 2] cycloaddition with alkene. Detailed mechanistic studies suggest that the biquinoline and biquinolinium salts are likely formed via radical processes.