Anion-Exehange-Triggered 1,3-Shift of an NH Proton to Iridium in Protic N-Heterocyclic Carbenes: Hydrogen-Bonding and Ion-Pairing Effects

摘要

The chemistry of N-heterocyclic carbenes (NHCs) has become one of the most active and exciting topics in synthesis and catalysis. NHCs bearing N-alkyl or -aryl wingtips are predominant, and there are only limited, although increasing, literature reports on NHC systems with hydrogen wingtips ligated on transition metals such as iridium, rhodium, osmium, ruthenium, rhenium, manganese, chromium, and platinum (Figure 1). In general, these NH-protic NHC complexes can be synthesized by 1) protonation of 2- or 4-pyridyl or 2-imidazolyl complexes; 2) cleavage of N-C or N-Si bonds within NHC units; 3) intramolecular attack of a ligated isonitrile by a pendent NH2 or OH group generated in situ; and 4) metal-mediated tautomerization of N-heterocycles, a process involving C-H activation. Among these methods, metal-mediated tautomerization is most intriguing in that N-heterocycle to NHC tautomerization is important not only in biological process but also in C-C bond formation, where rhodium and ruthenium protic NHC complexes are established as active catalysts.