Synthesis and Reactivity of Metal Complexes with Acyclic (Amino)-(Ylide)Carbene Ligands

摘要

The widespread use reached by N-heterocyclic carbenes (NHCs) as ancillary ligands for transition-metal catalysis is a consequence of two main factors: 1) their very strong σ-electron releasing ability and 2) their relative stability, which rehes on the effective donation of electron density from both nitrogen lone pairs to the formally empty orbital at the carbene center. However, nitrogen atoms are not unique for this task and other entities having an electron pair located in an orbital of appropriate symmetry might also serve the same purpose. Thus, formal replacement of one or even both nitrogen atoms by other main-group elements, such as O, S, or P, has proven to be a very efficient tool to modify the donor properties and reactivity of NHCs. For this particular task, ylides can also be seen as suitable nitrogen surrogates owing to the non-shared electron pair on the formally negatively charged carbon atom (Scheme la). Moreover, the lower electronegativity of carbon when compared with nitrogen should make the resulting carbenes particularly good electron-releasing ligands.