Electronic tuning of a carbene center via remote chemical induction, and relevant effects in catalysis

摘要

The present report develops the idea that an N-heterocyclic carbene incorporating a remote anionic functionality-here, a malonate group-as a backbone component of its heterocy- clic framework, can be "post-function- alized" directly from its transition- metal complexes, upon simple addition of a variety of electrophiles interacting directly with the malonate group in the outer coordination sphere. From a palette of selected electrophilic reagents, it was thus possible to modulate the electronic donor properties of the car- bene center over a rather broad range. Both the zwitterionic complex [Rh{malo-NHCj(cod)] and the cationic derivatives [Rh{malo-NHC~E}(cod)] ~+ (where "ma/o-NHC~E" represents the ligand modified by a selected electro- phile "E") were used as pre-catalysts in two types of catalytic reactions, namely, the polymerization of phenyl- acetylene and the hydroboration of styrene. The results indicate that, in both cases, the zwitterionic species is by far the best catalyst, whereas a decrease in the ligand donicity induced by the added electrophile results in a concomitant reduction of catalytic activity. Apparent deviations to such a trend in the case of the hydroboration of styrene were rationalized in terms of an interaction between the reactive catechol- borane substrate and the remote functionality of the N-heterocyclic carbene leading to an in situ modification of the nature of the active species. These observations serve as a useful basis to define the scope and limitations of the present conceptual approach in catalysis.