The development of catalysts relying on earth-abundant ^st-row transition metals is highly desirable. The unique electronic properties of 1^st-row transition metal would give rise to more unexpected and surprising reactivities for homogeneous hydrogenation reactions. In addition, novel hydrogenation mechanisms distinguished from the classical “inner-sphere” and “outer-sphere” mechanisms are needed for the hydrogenation of some challenging substrates. In this thesis, we focus on the understanding of Co chemistry and the chemoselective issues related to the hydrogenation of C=C and C=O catalyzed by Co complexes (Chapters 2-5). Furthermore, we discussed two interesting examples of hydrogenation including (NHC)Cu(I)H-guanidine bifunctional catalysis for hydrogenation of esters (Chapter 6) and...[ Read more ]

The development of catalysts relying on earth-abundant ^st-row transition metals is highly desirable. The unique electronic properties of 1^st-row transition metal would give rise to more unexpected and surprising reactivities for homogeneous hydrogenation reactions. In addition, novel hydrogenation mechanisms distinguished from the classical “inner-sphere” and “outer-sphere” mechanisms are needed for the hydrogenation of some challenging substrates. In this thesis, we focus on the understanding of Co chemistry and the chemoselective issues related to the hydrogenation of C=C and C=O catalyzed by Co complexes (Chapters 2-5). Furthermore, we discussed two interesting examples of hydrogenation including (NHC)Cu(I)H-guanidine bifunctional catalysis for hydrogenation of esters (Chapter 6) and ionic hydrogenation of oxocarbenium ions (Chapter 7).

Co(I), as one of the 1^st-row transition metals with a d⁸ electron configuration, prefers 5-coordinated Co (I) complexes due to its relatively smaller d-p gap when compared to other heavier d⁸ transition metal centers. When Co(I) combines with a carbene ligand, it was found that trans-dihydride complexes could be stabilized, which are important intermediates frequently proposed in transition metal catalyzed hydrogenation reactions.

Our theoretical studies indicate that via an “inner-sphere” mechanism, the Co(I) catalyst [Co(I)(H₂)(PR₃)] was able to selectively hydrogenate C=C but not C=O, while the Co(I)/tetraphosphine catalyst [(P₄N₂)Co(I)H] catalyzed selective reduction of C=O rather than the C=C via an “outer-sphere” mechanism.

Besides the classical “inner-sphere” and “outer-sphere” mechanisms, novel hydrogenation mechanisms are interesting and can give surprising results and have the potential for the hydrogenation of challenging substrates. Related to this aspect, this thesis also covers two novel hydrogenation mechanisms: (1) a combination of a super hydride donor NHCCu(I)H and an organic guanidine pendant in the ligand is powerful to reduce the relative inert ester group (Chapter 6) and (2) a far less explored ionic hydrogenation mechanism was reported for the enantioselective hydrogenation of oxocarbenium ions (Chapter 7).