Cycloadditions of 1-aza-2-azoniaallene ions to alkenes

Abstract

1-Aza-2-azoniaallene salts 3 react with ethene, and mono- to trisubstituted electron-rich alkenes under mild conditions to afford 4,5-dihydro-3H-pyrazolium salts 4. These cycloadditions proceed with complete Markovnikov regioselectivity and retention of the configuration of the alkene. Reactions of salts 3 with norbornene afford cycloadducts without rearrangement of the norbornane moiety. According to these observations, reactions of heteroallenes 3 with alkenes are mechanistically concerted ‘reverse electron-demand 1,3-dipolar cycloadditions’. In solution 4,5-dihydro-3H-pyrazolium salts 4 with a hydrogen atom in the 5-position tautomerize to 4,5-dihydro-1H-pyrazolium salts 6. A ‘general Wagner–Meerwein rearrangement’ to the 1H-isomer 5y is observed for compound 4y. The mechanistic proposals are supported by AM1 calculations. For the salt 4r and the twofold cycloadduct 4z X-ray stuctural analyses have been performed.