In the presence of a base such as KO-t-Bu, generates the imidazol-2-ylidene, an example of an N-heterocyclic carbene (NHC). NHCs are relatively strong bases, and valuable replacements for phosphine ligands in transition metal chemistry. They are relatively stable to dimerization, but sensitive to air and moisture, so are best generated in situ. Review: Angew. Chem. Int. Ed., 36, 2163 (1997). For a review of stable carbenes, see: Chem. Rev., 100, 39 (2000).Used as a phosphine-free ligand in various metal-catalyzed coupling reactions, often with advantageous results in difficult cases. For use in the Pd-catalyzed cross-coupling of aryl Grignards with aryl chlorides (Kumada reaction), see: J. Am. Chem. Soc., 121, 9889 (1999). Many examples have been recorded of the use of NHC ligands in the Suzuki coupling reaction, for examples utilizing 1,3-dimesitylimidazol-2-ylidene, in the coupling of arylboronic acids with relatively unreactive aryl chlorides, see: J. Org. Chem., 64, 3804 (1999); J. Organomet. Chem., 595, 186 (2000); Tetrahedron Lett., 45, 3511 (2004). Has been found to give superior results to phosphines in the coupling of sulfonyl chlorides with boronic acids: Org. Lett., 6, 95 (2004).The carbene has also been reported to be an excellent nucleophilic catalyst in transesterification reactions, milder, more selective and more active than conventional systems: Org. Lett., 4, 3583, 3587 (2002).
