The use of transition metals to catalyse the addition of hydridoboranes to unsaturated organic molecules was initially realised several decades ago. Although this area of chemistry received considerable attention at the time, interest in this reaction and its use in organic synthesis waned for several years. Like a phoenix rising from the ashes, this amazing catalytic reaction has grown to include the use of earth-abundant metal catalysts and a much wider range of organic substrates. Indeed, it is now commonly utilised as a diagnostic tool to assess the reactivity and catalytic ability of newly generated transition metal and main group complexes. As this field is progressing so rapidly, this review highlights some important advances up to the end of 2021 and into early 2022. Excluded from this review are 'hydroboration' reactions using diboron sources.

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The transition metal-catalysed hydroboration reaction.

Commercially available [(PPh3)2NiCl2] was found to be an efficient catalyst for the mono-N-alkylation of (hetero)aromatic amines, employing alcohols to deliver diverse secondary amines, including the drug intermediates chloropyramine (5b) and mepyramine (5c), in excellent yields (up to 97%) via the borrowing hydrogen strategy. This method shows a superior activity (TON up to 10000) with a broad substrate scope at a low catalyst loading of 1 mol % and a short reaction time. Further, this strategy is also successful in accessing various quinoline derivatives following the acceptorless dehydrogenation pathway.

[(PPh3)2NiCl2]-Catalyzed C–N Bond Formation Reaction via Borrowing Hydrogen Strategy: Access to Diverse Secondary Amines and Quinolines

Herein we report nickel-catalyzed sustainable synthesis of a few chosen five-membered fused nitrogen heterocycles such as benzimidazole, purine, benzothiazole, and benzoxazole via acceptorless dehydrogenative functionalization of alcohols. Using a bench stable, easy to prepare, and inexpensive Ni(II)-catalyst, [Ni(MeTAA)] (1a), featuring a tetraaza macrocyclic ligand (tetramethyltetraaza[14]annulene (MeTAA)), a wide variety of polysubstituted benzimidazole, purine, benzothiazole, and benzoxazole derivatives were prepared via dehydrogenative coupling of alcohols with 1,2-diaminobenzene, 4,5-diaminopyrimidine, 2-aminothiphenol, and 2-aminophenol, respectively. A wide array of benzimidazoles were also prepared via a borrowing hydrogen approach involving alcohols as hydrogen donors and 2-nitroanilines as hydrogen acceptors. A few control experiments were performed to understand the reaction mechanism.

Nickel catalyzed sustainable synthesis of benzazoles and purines via acceptorless dehydrogenative coupling and borrowing hydrogen approach

New monomeric O,N-heterocyclic germylene AdAPGe (1) based on 4,6-di-tert-butyl-N-adamantyl-o-aminophenol is synthesized and structurally characterized. Germylene 1 in the crystal demonstrates weak Ge⋯Ge interactions between adjacent molecules. The redox activity of compound 1 and its known analogs PhAPGe (2), t-BuAPGe (3) was studied using cyclic voltammetry and chemical oxidation by the phenoxy radical. Germylene 1, as opposed to 2 and 3, was found to form a relatively stable o-iminosemiquinonate moiety during oxidation. The last one was detected by EPR spectroscopy in toluene solution at low temperatures. The catalytic activity of germylenes 1–3 and dippAPGe (4) (dipp – 2,6-di-isopropylphenyl) towards hydroboration and cyanosilylation of benzaldehyde was examined. Germylene 1 demonstrates the most potent promoter properties for such reactions, which allowed cyanosilylation and hydroboration of aldehyde under mild conditions with excellent conversion quickly. Theoretical studies were performed to establish the mechanism, and different reaction routes were examined. According to the DFT (B3LYP/def2-SVP) calculation, cyanosilylation and hydroboration processes are initiated by coordinating TMSCN or HBpin with the catalyst. The followed reaction with aldehyde leads to the resulting products.

O,N-Heterocyclic germylenes as efficient catalysts for hydroboration and cyanosilylation of benzaldehyde

Electronically tuneable orthometalated Ru<sup>II</sup>–NHC complexes as efficient catalysts for C–C and C–N bond formations <i>via</i> borrowing hydrogen strategy

Silver-Catalyzed Hydroboration of C-X (X = C, O, N) Multiple Bonds.

Atom-Economic Alk(en)ylations of Esters, Amides, and Methyl Heteroarenes Utilizing Alcohols Following Dehydrogenative Strategies

In the past few decades, chemistry of cyclometalated species has gained momentum with increased applications in several areas of scientific developments. Cyclometalation reactions result in the formation of stable metallacycles through the generation of metal-carbon covalent bonds by activating the unreactive Csp2 -H or Csp3 -H bonds. The extra stability gained by the formation of metallacycles enhances their applicability scopes especially in the area of homogeneous catalysis. In the recent research development in this area, NHC ligands (strong σ-donor and generally, weak π-acceptor) have been found to be one of the most suitable candidates for the intramolecular C-H activation process which leads to the cyclometalated species. The growth in the area of cyclometalation chemistry that started in the late 20th century is still continuing and in the past few decades, various examples of NHC derived transition metal-based cyclometalated complexes came into the picture. As covering all the reported literatures in this area (includes mainly late transition metals) will exceed the limits of minireview, we restricted ourselves to the recent (2015 - May 2021) examples of the most common Ru-, Rh-, and Ir-based CNHC ^C cyclometalated complexes and their applications in various homogeneous catalytic conversions such as transfer hydrogenation, amidation, oxidation of alcohols, annulations, and so forth.

Chandra Shekhar Tiwari

Papers

Silver-Catalyzed Hydroboration of C-X (X = C, O, N) Multiple Bonds.

Atom-Economic Alk(en)ylations of Esters, Amides, and Methyl Heteroarenes Utilizing Alcohols Following Dehydrogenative Strategies

Recent Advances in the Syntheses and Catalytic Applications of Homonuclear Ru-, Rh-, and Ir-Complexes of CNHC^C Cyclometalated Ligands