Dearomatization of transition metal-coordinated N-heterocyclic ligands and related chemistry

Chapter Two.
2,2′-Bipyridine (bipy) and 1,10-phenanthroline (phen), employed for a long time in all areas of coordination chemistry, are archetypical auxiliary ligands that stabilize metal fragments but are not themselves the site of the reactivity. Several examples of dearomatization of one of the pyridine rings of coordinated bipy and phen are discussed, including one example of ring opening. The highly reactive species resulting from deprotonation of coordinated N-alkylimidazoles can result, besides of the mentioned dearomatization, in other intramolecular C–C coupling reactions involving monodentate pyridines, other N-alkylimidazoles, imine, nitrile, isonitrile, and carbonyl ligands. In some examples, the products of these couplings, and of those between two pyridines, can be oxidatively rearomatized, affording pyridylimidazoles and bipyridines. Similar reactions leading to C–C couplings have been demonstrated for deprotonation of dimethylsulfide and methylphosphane ligands. In some instances, the deprotonation of coordinated N-alkylimidazoles produces complexes with rare N-alkylimidazol-2-yl ligands. The protonation or alkylation of the nonsubstituted nitrogen of the latter affords coordinated N-heterocyclic carbenes.
The authors gratefully acknowledge funding from Ministerio de Economía y Competitividad and FEDER (grants CTQ2012-37379-C02-01 and CTQ2012-37379-C02-02, and FPI graduate studentships to M.E.V. and M.A.H.) and Principado de Asturias (grant FC-15-GRUPIN14-103, administered by FICYT) and Ministerio de Educación Cultura y Deporte (FPU graduate studentship to R.A.).