Free carbenes from complementarily paired alkynes

Carbenes (R1R2C:) like radicals, arynes and nitrenes constitute an important family of neutral, high-energy, reactive intermediates—fleeting chemical entities that undergo rapid reactions. An alkyne (R3C≡CR4) 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. Thermal generation of free carbenes with 100% atom economy is rare. Now, it has been shown that this can be achieved by the net [3+2] cycloaddition between many classes of 2-alkynyl iminoheterocycles and electrophilic alkynes. Together with a host of carbene trapping reactions, this method provides facile and versatile access to diverse heterocycle scaffolds.