Phenyl(triazolyl)carbene revisited: Unique role of triazolyl group on carbene chemistry

Abstract

AbstractThe chemistry of (1,4‐diphenyl‐1H‐1,2,3‐triazol‐5‐yl)(phenyl)carbenes (1T), originally reported by Smith et al., was reinvestigated by using modern spectroscopic methods as well as density functional theory (DFT) calculations, and the results were compared with that observed for analogous diphenylcarbene (1H) to clarify the role of triazole group in diarylcarbene chemistry. Product analysis indicated that 1T gave two intramolecular cyclization products (2 and 3) formed as a result of a formal attack of carbene on the phenyl rings at 1‐ and 4‐positions of the triazole ring. The results were closely similar to that observed for 1H. The behaviors of triplet carbene 31T were directly monitored not only by electron spin resonance (ESR) and UV‐Vis spectroscopic methods in rigid matrix at low temperature but also by laser flash photolysis in solution at room temperature and compared with those observed for 31H. Whereas the lifetime of 31T was some 2 orders of magnitude longer than that of 31H, the reactivity of 31T toward typical triplet trapping reagents was greater than that of 31H. In order to elucidate these rather conflicting observations, geometries and relative energies of carbenes 1T and 1H as well as species involved in the reactions leading to the final products were calculated by the DFT method. The results are interpreted as indicating that 31H decays by participating in the reaction of upper‐lying 11H much more easily than 31T. The difference in the reactivity toward external trapping reagents is explicable by taking into account the difference in steric crowdedness around carbene center between the two.