Investigations on the Pincer-Ruthenium and Pincer-Nickel Catalyzed Organic Transformations
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The contents of the present thesis entitled “Investigations on the Pincer-Ruthenium and Pincer Nickel Catalyzed Organic Transformations” have been divided into five chapters based on the results achieved from the experimental and computational work carried out during the entire course of the PhD research programme. Chapter 1 contains a brief introduction and the literature review on the chemistry of pincer-metal complexes and their versatile applications in catalytic organic transformations. The chapter winds up with a discussion on the scope of the current thesis. Chapter 2 demonstrates an efficient and atom-economical Kharasch addition of CCl4 to styrene that is catalyzed by a series of pincer-ruthenium complexes (R2NNN)RuCl2(PPh3) (R = Cy, tBu, iPr, and Ph). Gratifyingly, very high turnovers (ca. 5670) have been observed for the (Cy2NNN)RuCl2(PPh3) catalyzed Kharasch addition of CCl4 to styrene. The reaction has also been probed to obtain a detailed mechanistic understanding. Chapter 3 sheds light on the utility of NNN pincer-ruthenium complexes based on sterically less hindered 2,6-bis(benzimidazole-2-yl) pyridine ligands in catalyzing high yield (92%) transformation of glycerol selectively (98%) to lactic acid. Systematic mechanistic studies provide a clear understanding to the role of Ru-P bond and steric crowding around the Ru centre in favoring catalysis. Chapter 4 illustrates the application of a new and well-defined pincer-nickel complex (iPr2NNN)NiCl2(NCCH3) for catalytic N-alkylation that proceeds with very high turnover (34000 TON). Detailed insight on the operative mechanism points to the involvement of both hydrogenation and alcoholysis steps in the N-alkylation reaction. The (iPr2NNN)NiCl2(NCCH3) catalyzed reactions have been extended to the dehydrogenative coupling of benzene-1,2-diamines with alcohols to yield the corresponding benzimidazoles. Chapter 5 reports a computational study on cyanomethylation reaction. Here the energetics of the cyanomethylation catalytic cycle for a series of ten pincer-nickel systems with varying electronic demands have been compared with the corresponding Miller’s catalyst (iPr2POCCNEt2)NiOtBu that is reported to be active at room temperature. The study culminates with the identification of imine based pincers with C central atom as potential contenders for the Miller’s catalyst. Presence of strong s-donors in the flanking groups and weak trans influencing pincer central atoms were found to lead to unfavorable energetics in the cyanomethylation catalysis.
Supervisor: Akshai Kumar A S