Free carbenes from complementarily paired alkynes.

Carbenes (R 1 R 2 C:) like radicals, arynes and nitrenes constitute an important family of neutral, high-energy, reactive intermediates-fleeting chemical entities that undergo rapid reactions. An alkyne (R 3 C≡CR 4 ) is a fundamental functional group that houses a high degree of potential energy; however, the substantial kinetic stability of alkynes renders them conveniently handleable as shelf-stable chemical commodities. The ability to generate metal-free carbenes directly from alkynes, fuelled by the high potential (that is, thermodynamic) energy of the latter, would constitute a considerable advance. We report here that this can be achieved simply by warming a mixture of a 2-alkynyl iminoheterocycle (a cyclic compound containing a nucleophilic nitrogen atom) with an electrophilic alkyne. We demonstrate considerable generality for the process: many shelf-stable alkyne electrophiles engage many classes of (2-alkynyl)heterocyclic nucleophiles to produce carbene intermediates that immediately undergo many types of transformations to provide facile and practical access to a diverse array of heterocyclic products. Key mechanistic aspects of the reactions are delineated.