Bhaskar Paul

Bifunctional Ru(II) complex catalysed carbon–carbon bond formation: an eco-friendly hydrogen borrowing strategy

The atom economical borrowing hydrogen methodology enables the use of alcohols as alkylating agents for selective C-C bond formation. A bifunctional 2-(2-pyridyl-2-ol)-1,10-phenanthroline (phenpy-OH) based Ru(ii) complex (2) was found to be a highly efficient catalyst for the one-pot β-alkylation of secondary alcohols with primary alcohols and double alkylation of cyclopentanol with different primary alcohols. Exploiting the metal-ligand cooperativity in complex 2, several aromatic, aliphatic and heteroatom substituted alcohols were selectively cross-coupled in high yields using significantly low catalyst loading (0.1 mol%). An outer-sphere mechanism is proposed for this system as exogenous PPh3 has no significant effect on the rate of the reaction. Notably, this is a rare one-pot strategy for β-alkylation of secondary alcohols using a bifunctional Ru(ii)-complex. Moreover, this atom-economical methodology displayed the highest cumulative turn over frequency (TOF) among all the reported transition metal complexes in cross coupling of alcohols.

A simple and efficient in situ generated ruthenium catalyst for chemoselective transfer hydrogenation of nitroarenes: kinetic and mechanistic studies and comparison with iridium systems

The catalytic activities of a series of in situ generated homogeneous ruthenium systems based on commercially available [RuCl2(p-cymene)]2 and various ligands in transfer hydrogenation of nitroarenes to anilines were investigated. Combination of [RuCl2(p-cymene)]2 and tridentate phenanthroline based ligand 2-(6-methoxypyridin-2-yl)-1,10-phenanthroline (phenpy-OMe) exhibited the highest catalytic activity for this reaction using 2-propanol as hydrogen source. This protocol provides a facile route to access aromatic amines under mild conditions in excellent yields. Notably, this system chemoselectively reduced the nitro groups over an array of other reactive functionalities such as ketone, alkene, amide, nitrile, and aryl halide. Operational simplicity, high yields, mild reaction conditions and short reaction times make this an attractive methodology for accessing various functionalized anilines. A series of controlled experiments and careful mechanistic investigation with the possible intermediates suggested that transformation of nitrobenzene to aniline with ruthenium and iridium system proceeded via direct route and condensation route respectively.

Tandem Transformation of Nitro Compounds into N-Methylated Amines: Greener Strategy for the Utilization of Methanol as a Methylating Agent

A simple air- and moisture-stable, highly efficient ruthenium NNN pincer complex is reported for the first time to catalyze the tandem transformation of various aromatic and aliphatic nitro compounds into the corresponding N-methylated amines in up to 98 % yield by using methanol as a green methylating agent. Gram-scale reactions of challenging nitro substrates demonstrated the practical application aspects of this catalytic system. Importantly, the N-methylamine moiety could be smoothly introduced to various complex molecular structures without using any expensive palladium/phosphine/amine-based cross-coupling reactions.

Utilization of MeOH as a C1 Building Block in Tandem Three-Component Coupling Reaction

Ru(II) catalyzed tandem synthesis of α-branched methylated ketones via multicomponent reactions following the hydrogen borrowing process is described. This nonphosphine-based air and moisture stable catalyst efficiently produced various methylated ketones using methanol as a methylating agent. This system was found to be highly effective in three-component coupling between methanol, primary alcohols, and methyl ketones. A proposed catalytic cycle for the α-methylation is supported by DFT calculations as well as kinetic experiments.

ortho-Amino group functionalized 2,2′-bipyridine based Ru(II) complex catalysed alkylation of secondary alcohols, nitriles and amines using alcohols

Various Ru(II) complexes bearing functionalized 2,2′-bipyridine ligands were synthesized and fully characterized. Among them, a new N6,N6′-dimethyl-2,2′-bipyridine-6,6′-diamine ligand was found to be the most electron-rich ligand as its corresponding Ru(II) complex (1a) displayed the lowest νco value and the highest efficiency in the β-alkylation of secondary alcohols with primary alcohols (TON = 98 860). Complex 1a also exhibited a greater reactivity in the monoalkylation of acetonitrile, α-alkylation as well as α-methylation of arylacetonitriles. Compared to the other reported systems, in α-methylation of nitriles complex 1a presented superior catalytic activity. The potential of complex 1a was extended further in N-methylation of amines using methanol as a green methylating agent.

Selective synthesis of mono- and di-methylated amines using methanol and sodium azide as C1 and N1 sources

A Ru(II) complex mediated synthesis of various N,N-dimethyl and N-monomethyl amines from organic azides using methanol as a methylating agent is reported. This methodology was successfully applied for a one-pot reaction of bromide derivatives and sodium azide in methanol. Notably, by controlling the reaction time several N-monomethylated and N,N-dimethylated amines were synthesized selectively. The practical applicability of this tandem process was revealed by preparative scale reactions with different organic azides and synthesis of an anti-vertigo drug betahistine. Several kinetic experiments and DFT studies were carried out to understand the mechanism of this transformation.

CCDC 1501387: Experimental Crystal Structure Determination

Related Article: Sujan Shee, Bhaskar Paul, Dr. Sabuj Kundu|2017|Chem. Sel.|2|1705|doi:10.1002/slct.201601929

CCDC 1501385: Experimental Crystal Structure Determination

Related Article: Sujan Shee, Bhaskar Paul, Dr. Sabuj Kundu|2017|Chem. Sel.|2|1705|doi:10.1002/slct.201601929

CCDC 1501386: Experimental Crystal Structure Determination

Related Article: Sujan Shee, Bhaskar Paul, Dr. Sabuj Kundu|2017|Chem. Sel.|2|1705|doi:10.1002/slct.201601929