【文章名】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.
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.
[摘要]：General synthetic avenues to the pyrido-annulated triazolium salts with different steric and electronic properties were 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 gave stable carbenes 11 at room temp. as clearly demonstrated through ESI mass spectra and by observation of the characteristic 13C NMR resonance for the carbene carbon at d = 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?in good yield. A silver complex with the pyrido[1,2-a][1,2,4]triazol-3-ylidene is described, and the mol. structure of complex 17 was established by x-ray crystallog. The triazolium salts 12 turned out to be powerful catalysts in catalytic benzoin condensations and transesterifications at 25? The catalytic activity was largely dependent on the steric and electronic nature of the R1 and R2 substituents of the triazolium salt. The authors 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.
