Development of Nickel Catalysts for Homogeneous Catalysis

Vijaykumar, Gonela (2018) Development of Nickel Catalysts for Homogeneous Catalysis. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The research projects investigated in this thesis, inexpensive nickel-catalyzed reactions to prepare valuable chemical substances. Specifically, new methods were developed for the construction of C–Si, C–B bonds and direct functionalization of heteroarenes with alkenes. In the present study, we have used well-defined inexpensive nickel based catalyst bearing an abnormal N-heterocyclic carbene (aNHC) for the hydrosilylation of aromatic nitro compounds. The NiCl₂(aNHC)₂ complex has been used as an efficient and well-defined catalyst for the reduction of nitroarenes with hydrosilanes to give aromatic amines in good to excellent yields. This catalytic protocol can tolerate functional groups such as halides, alkenes or nitriles. Furthermore, the longevity of the catalyst was tested in successive catalytic cycles, which indicates a sustained catalytic activity over multiple catalytic cycles. The mechanistic investigation by performing stoichiometric reaction of nickel complex and phenylsilane unravels the formation of a Ni–silyl complex. Next, we report for the first time that Ni(COD)₂/aNHC combination can promote alkylation of benzoxazole with vinylarenes to afford selectively corresponding 1,1-diarylalkane derivatives (Scheme 2B). The present study shows that instead of the expected Ni(0)-aNHC complex; an in situ generated Ni(II) cyclooctenyl complex plays a very important role. We have isolated two such Ni(II) cyclooctenyl complexes and characterized by elemental analysis, NMR spectroscopy and single crystal X-ray crystallography. The mechanistic investigation by performing stoichiometric reaction of nickel complex and benzoxazole unravels that a Ni(II)-aNHC complex acts as the active catalyst precursor for this reaction through an aNHC-Ni-H intermediate. Based on this, we propose a mechanistic pathway, which involves activation of the C-H bond of heteroarene, hydrometallation of vinylarene and reductive elimination as major steps. Next, the nickel catalyst bearing phenalenyl ligand has been developed for the hydrosilylation and hydroboration of aliphatic and aromatic olefins. Phenalenyl ligand can behave as an excellent reservoir for electrons and supply that when the metal requires it to perform multielectron redox. This efficacious nickel-catalyst can hydrosilylate a wide variety of olefins under very mild condition. The reaction is very clean and smooth so that the catalyst can perform with extremely low loading, 0.05 mol% condition, effectively translating the TON to 1200. Moreover, double hydrosilylation, sequential silylation, silylation with extremely cheap PMHS were successful which is clear improvement when the use of base metals in these reactions is concerned. We have investigated the exact role of the ligand in assisting the metal to execute multi-electron redox by solid state magnetometry by SQUID, CV, XPS and electronic structure analysis. The electronic structure analyses conclusively prove that ligand truly holds electrons upon reduction of the pre-catalyst and supply electron so that nickel is not promoted to an unusual oxidation state during the catalysis. This is a rare entry where tuning redox noninnocence of the PLY ligand has resulted such a powerful nickel catalyst for olefin hydrosilylation. Next, we report the well defined nickel catalyzed hydroboration reaction which exhibits both high activity and excellent regioselectivity for allyl/vinylarenes by using phenalenyl nickel complex reduced in situ with potassium. The hydroboration of aromatic olefins using pinacol borane (HBpin) gave branched boronic esters exclusively in excellent yields with Markovnikov selectivity and aliphatic olefins gave linear boronic esters with anti- Markovnikov selectivity along with minor olefin isomerized product.

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