RuII-Complexes of heteroditopic chelating NHC ligands: effective catalysts for the ß-alkylation of secondary alcohols and the synthesis of 2-alkylaminoquinoline derivatives following the dehydrogenative protocol.

RuII-Complexes of chelating heteroditopic N-heterocyclic carbene ligands featuring imidazol-2-ylidene (ImNHC) and 1,2,3-triazol-5-ylidene (tzNHC) donors connected via a CH2 spacer, 1a-c, were found to be very effective catalysts for the cross-coupling of secondary and primary alcohols with the elimination of H2O. Diverse ß-alkylated secondary alcohols were thus obtained by following this method in excellent yields of up to 95% by employing a very low catalyst (1a) loading of 0.01-0.001 mol% along with the inexpensive base KOH. Mechanistically, the present protocol follows the borrowing hydrogen strategy which was established by various control experiments including deuterium labelling experiments and importantly, 1H NMR and ESI-MS analyses validated the participation of a Ru-H species in the catalytic cycle. Remarkably, the present system displayed the highest Ru-based TON of 396 000 for the ß-benzylation of 1-phenylethanol with a catalyst loading of 1 ppm (0.0001 mol%). Additionally, diverse 2-alkylaminoquinoline derivatives were synthesized in a one-pot manner from 2-aminobenzyl alcohol, 2-arylacetonitrile, and various primary alcohols.

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

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.

Catalyst- and solvent-free efficient access to N-alkylated amines via reductive amination using HBpin.

A sustainable approach which works under catalyst- and solvent-free conditions for the synthesis of structurally diverse secondary amines has been uncovered. This one-pot protocol works efficiently at room temperature and is compatible with a wide range of sterically and electronically diverse aldehydes and primary amines. Notably, this simple process offers scalability, excellent functional group tolerance, chemoselectivity, and is also effective at the synthesis of biologically relevant molecules.

Simple silver(I)-salt catalyzed selective hydroboration of isocyanates, pyridines, and quinolines.

AgSbF6 has been established as an effective catalyst for the hydroboration of structurally and electronically diverse isocyanates under ligand- and solvent-free conditions which selectively yielded either N-boryl formamides or N-boryl methylamines under different conditions. Further, various N-heterocycles can be selectively hydroborated using this simple catalytic system; pyridine derivatives undergo preferential 1,4 hydroboration whereas the formation of tetrahydroquinoline (after hydrolysis) via complete heterocycle hydrogenation was observed for quinolines.

Silver-Catalyzed One-Pot Three-Component Synthesis of α-Aminonitriles and Biologically Relevant α-Amino-phosphonates.

A simple silver salt (AgSbF6 )-catalyzed aminophosphonylation and Strecker reaction have been developed and successfully applied to a wide range of substrates (>55 substrates). This solvent-, ligand-, and base-free one-pot three-component protocol operates effectively at room temperature to provide diversified α-aminophosphonates and α-aminonitriles, which gave access to the respective α-amino amides. Importantly, the present catalyst system is also capable to produce the rarely reported and biologically relevant aminophosphonates (having anti-leishmanial activity). Further, mechanistic studies reveal that the present phosphonylation protocol follows a radical pathway.

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

AgSbF6 was developed as an effective catalyst for the hydroboration of various unsaturated functionalities (nitriles, alkenes, and aldehydes). This atom-economic chemoselective protocol works effectively under low catalyst loading, base- and solvent-free moderate conditions. Importantly, this process shows excellent functional group tolerance and compatibility with structurally and electronically diverse substrates (>50 examples). Mechanistic investigations revealed that the reaction proceeds via a radical pathway. Further, the obtained N,N-diborylamines were showcased to be useful precursors for amide synthesis.

AgSbF6-Catalyzed anti-Markovnikov hydroboration of terminal alkynes.

AgSbF6-Catalyzed anti-Markovnikov addition of pinacolborane (HBpin) to terminal alkynes to produce the E-vinylboronates is reported. This efficient methodology is scalable, compatible with sterically and electronically diverse alkynes, and works at room temperature under solvent-free condition. The utility of this method is demonstrated in the facile synthesis of the clinically important (E)-2,4,3',5'-tetramethoxystilbene.

Catalyst-Free and Solvent-Free Facile Hydroboration of Imines.

A facile process for the catalyst-free and solvent-free hydroboration of aromatic as well as heteroaromatic imines is reported. This atom-economic methodology is scalable, compatible with sterically and electronically diverse imines, displaying excellent tolerance towards various functional groups, and works efficiently at ambient temperature in most of the cases, affording secondary amines in good to excellent yield after hydrolysis.