Pyrido[1,2-a][1,2,4]triazol-3-ylidenes as a new family of stable annulated N-heterocyclic carbenes: synthesis, reactivity, and their application in coordination chemistry and organocatalysis.

General synthetic avenues to the pyrido-annulated triazolium salts with different steric and electronic properties have been developed. This architecture can be readily altered with different N-alkyl or aryl substituents at the N2 position of the triazole ring and modifications to the pyridine backbone. Deprotonation of the triazolium salts 12 with NaH led to formation of stable carbenes 11 at room temperature as clearly demonstrated through ESI mass spectra and by observation of the characteristic (13)C NMR resonance for the carbene carbon at delta = 202-208 ppm. In sharp contrast, treatment of these triazolium salts with K2CO3 led to dimerization of free carbenes 11. The dimeric enetetramine (11b)2 could react with elemental sulfur to deliver the corresponding thiourea 16 in toluene at 80 degrees C in good yield. A silver complex with the pyrido[1,2-a][1,2,4]triazol-3-ylidene is described, and the molecular structure of complex 17 was established by X-ray crystallography. The triazolium salts 12 turned out to be powerful catalysts in catalytic benzoin condensations and transesterifications at 25 degrees C. The catalytic activity was largely dependent on the steric and electronic nature of the R(1) and R(2) substituents of the triazolium salt. We rationalized that this type of triazolium-catalyzed benzoin condensations should undergo the "traditional" Breslow mechanism rather than the pathway of the dimer (11)2 as the real catalytic species.