Oxazol-2-ylidenes. A new class of stable carbenes?

The investigation of the stability of several imidazol-2-ylidene analogue cyclic carbenes by an isodesmic reaction has revealed that the hitherto unknown oxazol-2-ylidene exhibits only slightly smaller stability than imidazol-2-ylidene, outperforming some of the already synthesized carbenes. Selenazol-2-ylidene also shows significant stability. The contribution of aromaticity to the stabilization has been analysed for the different five-membered ring carbenes, and was found to be relatively small for the oxygen containing systems. Investigation of possible reactivity/decomposition pathways reveals that properly substituted oxazol-2-ylidene is stable against dimerization. The thermodynamically feasible cycloreversion reaction yielding isocyanate and acetylene is prevented by a significant barrier, and furthermore with proper substitution (ring annellation) the ring can be stabilized thermodynamically as well. While in the presence of water a hydrolytic ring opening occurs; this reaction can be hindered if the water content of the reaction mixture is reduced to a few equivalents. This hydrolytic behaviour as well as the electrophilicity and nucleophilicity indices of several known nucleophilic carbenes were compared, revealing that oxazol-2-ylidene exhibits a reduced nucleophilicity with respect to imidazol-2-ylidene, while its electrophilicity is only slightly increased. This unique combination might result in unexpected (organo) catalytic activities, further expanding the colourful applications of NHCs.