[4+1] cyclization of α-diazo esters and mesoionic N-heterocyclic olefins

Prompted by the recent stepwise mechanistic proposal for the Huisgen [3+2] cycloaddition reaction between enamine and α-diazo ester, where the nucleophilic addition of the enamine carbon onto the terminal nitrogen of diazo ester is crucial, we examined the possible use of N-heterocyclic olefins (NHOs) as highly electron-rich dipolarophiles in these reactions. The mesoionic NHOs derived from 1,2,3-triazoles undergo fast [4+1] cycloaddition to give 3-(triazolium-4-yl)-(3H)-pyrazol-4-olates at room temperature. The reaction mechanism has been explored through experimental and DFT computational studies.