Mono- and multinuclear pincer-type Ru(II) complex catalysts and their catalytic applications

Various pincer-type ruthenium(II) complexes were developed from our laboratories and herein the relevant advance is summarized from a viewpoint of their synthesis and organic synthetic applications as homogeneous catalysts. The investigated Ru(II)-NNN and Ru(II)-NNNN-type complexes exhibited highly catalytic activities in the transfer hydrogenation of ketones, C-C bond formation from alcohols and ketones, and dehydrogenation. RuH complexes were identified or considered as the catalytically active intermediates for these transformations. This review deals with the following subjects: (i) mononuclear ruthenium(II) pincer complexes with high catalytic activity in (asymmetric) transfer hydrogenation of ketones; (ii) multinuclear ruthenium(II) complexes for transfer hydrogenation of ketones; (iii) mononuclear ruthenium(II) complexes for other reactions such as Oppenauer-type oxidation, β-alkylation of secondary alcohols, synthesis of multisubstituted heterocycles, acceptorless dehydrogenation of N-heterocycles and secondary alcohols.