Basicity of Carbenes

Considering the abundant evidence for carbene protonation, some quantitative estimate for the base strength of carbenes is clearly desirable. The conventional spectrometric or potentiometric methods of determining the pKa in solution are not applicable, with the exception of some onium ions 1 and their conjugate bases 2 (Section V.B). In favorable cases, equilibria of carbenes with the conjugate carbenium ions have been studied in the gas phase. Proton affinities of various carbenes can be obtained from their enthalpies of formation, and by ab initio computation (Section V.A). Kinetic data have been evaluated to obtain the pKa of carbenes in solution (Section V.B). 

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Photolyses of 3-azinortricyclane (45) were conducted within CyDs and FAU zeolites to alter the selectivity of the carbene intermediate, 3-nortricyclanylidene (46). It was hypothesized that constriction of carbene 46 within each host would suppress the formation of 4-ethynylcyclopentene (47) and concomitantly enhance that of quadricyclane (48). But only 47 was observed. Since CyD innermolecular products were also formed, control experiments in relevant solvents were performed. Both triplet-state participation and the Bronsted basicity of carbene 46 were hinted at. 

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