1,2,3-Triazolylidenes as Versatile Abnormal Carbene Ligands for Late Transition Metals

摘要

N-Heterocyclic carbenes (NHCs) have become very popular ligands in transition metal chemistry predominantly due to their efficiency in improving catalyst activities. Their impact, often superior to that of ubiquitous phosphines, has generally been rationalized by the covalent M-C_(carbene) bond and by the strong donor ability of NHCs. Arduengo-type imidazolylidenes have been used most widely (A, Chart 1), presumably because the free carbene is extensively stabilized by heteroatoms adjacent to the carbene, which makes them easy to handle. Pioneering work by Bertrand and others evidenced that singlet carbenes may be isolated also with less pronounced heteroatom stabilization. For example, carbenes B-D were analyzed by crystallography, while more reactive E and F have been analyzed in situ.In contrast, abnormal carbenes such as G have not been characterized in their free form, perhaps due to the small contribution of the carbenoic resonance form. First results indicate that such abnormal carbenes are stronger donors than carbenes A-E, which should provide new opportunities for catalyst design.