Nitration of α,β-unsaturated esters. Evidence for positive charge build-up adjacent to carbonyl carbon

Reactive intermediates formed in the nitration of certain α,β-unsaturated esters with nitronium tetrafluoroborate exhibit behaviour expected of highly reactive α-carbonyl cations. Three diagnostic reaction types have been observed which indicate the presence of these destabilised cations: (i) trapping in a Ritter reaction, (ii) cyclopropane formation from propyl cations, (iii) Wagner–Meerwein migration of alkyl groups. Semi-empirical calculations of the relative gas-phase stabilities of the proposed intermediate cations are useful in rationalising the observed chemistry.