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

Abstract

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.

Five‐Membered Heterocycles with Three Heteroatoms: Triazoles

“…Bicyclic 1,2,4-triazolium salts have varied synthetic utility in a host of reactions. 1,2,4-Triazolium salts 247 have been identified as a new family of stable annulated Nheterocyclic carebenes that found applications in catalytic benzoin condensations and transesterifications at ambient temperature [292]. N-Pentafluorophenyl triazolium tetrafluoroborate salts 248 were found to be useful catalysts in the macrocyclization of a,v-dialdehydes to a-hydroxyketones [293] and in the synthesis of 1,2-amino alcohols via azidation of epoxy aldehydes (where modest asymmetric induction was achieved) [294].…”

Current Developments in the Syntheses of 1,2,4-Triazole Compounds

“…This type of triazolium metal complexes was generally resistant to decomposition and could be used as catalysts without additional ligands (Scheme 6) [55][56][57]. Some other triazolium compounds such as pyridinebased triazoliums [58] could also be successfully prepared in high yields by this method and all the triazolium compounds were found to exhibit much potentiality for further utilization in coordination and organometallic chemistry. Benzene bridged bis-morpholine hydrazone 27, a condensed product resulting from m-phenylenedihydrazine and the corresponding morpholin-3-one, could successfully undergo cyclization with triethyl orthoformate to form triazole compound 28 in overall yields of 51-64%.…”

2-Methyl-1,2,4-triazolo[4,3-a]pyridin-2-ium tetrafluoroborate

“…For the synthesis of a related compound and for related structures, see: Ma et al (2008); Wei et al (2009).…”

“…The title compound was prepared according to the method of Ma et al (Ma et al, 2008) and Wei et al (Wei et al, 2009).…”