Reactions of Group 6 Metal Carbene Complexes with Ylides and Related Dipolar Species

Among the amazing diversity of reactions discovered on group 6 metal carbene complexes, including now fully synthetically useful reactions, such as D6tz's benzannulation and Hegedus' photochemical ketene generation, the behavior of these complexes towards ylides has been much less studied. In this article, following a brief résumé on the common preparation methods and general reactivity of group 6 carbene complexes, their reactions with ylides and ylide-like derivatives will be reviewed. Three major reaction modes of metal carbenoids with ylides will be discussed: reactions producing new carbene complexes, reactions leading to ketenimine complexes and, finally, those reactions in which the metallic moiety is not present in the final products. The first type is exemplified by the reaction of 1,3-dipoles and a,p-unsaturated complexes. The second class is exclusive of the reactivity of these complexes and isocyanides. The remaining group of reactions is more diverse, and their outcome is determined by the nature of the ylide. So far, SO, NO, NS, NC, and PC ylides have been reported to react with Fischer carbenes. Organic products such as esters, imidates, vinyl ethers, cyclopropanes, and captodative allenes have been isolated in these processes, and their formation, a_s consequence of different reactivity patterns, will be commented through the paper.