Balamurugan Ramalingam

Highly enantioselective titanium-catalyzed cyanation of imines at room temperature.

A highly active and enantioselective titanium-catalyzed cyanation of imines at room temperature is described. The catalyst used is a partially hydrolyzed titanium alkoxide (PHTA) precatalyst together with a readily available N-salicyl-beta-aminoalcohol ligand. Up to 98% ee was obtained with quantitative yields in 15 min of reaction time using 5 mol % of the catalyst. Various N-protecting groups such as benzyl, benzhydryl, Boc, and PMP are tolerated.

Reusable Supported Ruthenium Catalysts for the Alkylation of Amines by using Primary Alcohols

Efficient and recyclable ruthenium catalysts were synthesized from readily available polystyrene‐ or silica‐supported phosphine ligands. Catalysts bound to the polymer support through an ether linkage showed good to excellent activity towards the N‐alkylation of primary and secondary amines to afford the alkylated products in 62–99 % yield at 120–140 °C. The supported phosphine ligand/ruthenium ratio was found to be crucial for higher catalytic activity and lower ruthenium leaching. The continuous flow N‐alkylation of amines was demonstrated by using the supported catalyst in a column reactor. By adopting the hydrogen‐borrowing strategy, the synthesis of the anti‐Parkinson agent Piribedil was established in 98 % yield at 140 °C.

Benzimidazolin-2-ylidene N-heterocyclic carbene complexes of ruthenium as a simple catalyst for the N-alkylation of amines using alcohols and diols

Simple air and moisture stable ruthenium complexes 1–3 and 3a were synthesized from readily available benzannulated N-heterocyclic carbene ligands (bimy = benzimidazolin-2-ylidene). These complexes were found to be efficient catalysts for the alkylation of amines using alcohols as alkylating agents. Catalysts 1, 2 and 3a gave excellent yields of up to 99% for the alkylation of various amines using benzylic and aliphatic alcohols at 130 °C for 18 h under solventless conditions. Catalyst 3a bearing both phosphine and carbene ligands gave excellent yields of up to 98% for the synthesis of heterocyclic amines by double alkylation of primary amines using linear diols. The practical utility of these catalysts was demonstrated for the synthesis of pharmaceutically important amines in a more environmentally benign way under solventless conditions.

Self‐Supported Chiral Titanium Cluster (SCTC) as a Robust Catalyst for the Asymmetric Cyanation of Imines under Batch and Continuous Flow at Room Temperature

A robust heterogeneous self-supported chiral titanium cluster (SCTC) catalyst and its application in the enantioselective imine-cyanation/Strecker reaction is described under batch and continuous processes. One of the major hurdles in the asymmetric Strecker reaction is the lack of availability of efficient and reusable heterogeneous catalysts that work at room temperature. We exploited the readily hydrolyzable nature of titanium alkoxide to synthesize a self-supported chiral titanium cluster (SCTC) catalyst by the controlled hydrolysis of a preformed chiral titanium-alkoxide complex. The isolated SCTC catalysts were remarkably stable and showed up to 98 % enantioselectivity (ee) with complete conversion of the imine within 2 h for a wide variety of imines at room temperature. The heterogeneous catalysts were recyclable more than 10 times without any loss in activity or selectivity. The robustness, high performance, and recyclability of the catalyst enabled it to be used in a packed-bed reactor to carry out the cyanation under continuous flow. Up to 97 % ee and quantitative conversion with a throughput of 45 mg h(-1) were achieved under optimized flow conditions at room temperature in the case of benzhydryl imine. Furthermore, a three-component Strecker reaction was performed under continuous flow by using the corresponding aldehydes and amines instead of the preformed imines. A good product distribution was obtained for the formation of amino nitriles with ee values of up to 98 %. Synthetically useful ee values were also obtained for challenging α-branched aliphatic aldehyde by using the three-component continuous Strecker reaction.

Towards understanding of laccase-catalysed oxidative oligomerisation of dimeric lignin model compounds

Laccase-catalysed oligomerisation of dimeric β-O-4 linked lignin model compounds was studied in detail to understand the oligomerisation process by monitoring the reaction progress using high performance liquid chromatography (HPLC) and mass spectroscopy (MS). The initial oxidation intermediates of oligomerisation were isolated for the first time and characterised by spectroscopic methods sucessfully. The experimental observations indicated that C5–C5′ biphenyl linkages, one of the most thermodynamically stable linkages present in the native lignin, are formed exclusively during the early stage of the oligomerisation process. The experimental observations were supported by density functional theory (DFT) calculations of relative free energies of possible products. The C5–C5′ biphenyl tetramer is the thermodynamically more favoured product compared to the C5–O–C4′ product by a free energy difference of 10.0 kcal mol−1 in water. Among the various linking possibilities for further formation of hexamers, the thermodynamically more stable product with a similar C–C linkage is proposed as a plausible structure based on the mass of the hexamer isolated and DFT calculations. The current study demonstrates that laccase catalyzes the oligomerisation more preferentially than oxidative bond cleavage in β-O-4 linkages and that product formation is likely controlled by the thermodynamic stability of the resultant oligomers.

Laccase-Catalyzed Synthesis of Low-Molecular-Weight Lignin-Like Oligomers and their Application as UV-Blocking Materials

The laccase-catalyzed oxidative polymerization of monomeric and dimeric lignin model compounds was carried out with oxygen as the oxidant in aqueous medium. The oligomers were characterized by using gel permeation chromatography (GPC) and matrix-assisted laser desorption ionization time-of-flight mass spectroscopy (MALDI-TOF MS) analysis. Oxidative polymerization led to the formation of oligomeric species with a number-average molecular weight (Mn ) that ranged from 700 to 2300 Da with a low polydispersity index. Spectroscopic analysis provided insight into the possible modes of linkages present in the oligomers, and the oligomerization is likely to proceed through the formation of C-C linkages between phenolic aromatic rings. The oligomers were found to show good UV light absorption characteristics with high molar extinction coefficient (5000-38 000 m-1  cm-1 ) in the UV spectral region. The oligomers were blended independently with polyvinyl chloride (PVC) by using solution blending to evaluate the compatibility and UV protection ability of the oligomers. The UV/Vis transmittance spectra of the oligomer-embedded PVC films indicated that these lignin-like oligomers possessed a notable ability to block UV light. In particular, oligomers obtained from vanillyl alcohol and the dimeric lignin model were found to show good photostability in accelerated UV weathering experiments. The UV-blocking characteristics and photostability were finally compared with the commercial low-molecular-weight UV stabilizer 2,4-dihydroxybenzophenone.