Prasanta Kumar Bera

Manganese complexes with non-porphyrin N4 ligands as recyclable catalyst for the asymmetric epoxidation of olefins

New chiral manganese complexes of N4 ligands derived from 2-acetylpyridine were prepared and used as catalysts in the enantioselective epoxidation of olefins, using H2O2 as an oxidant to give epoxides, with excellent conversions (up to 99%) and enantiomeric excess (up to 88%) within 1 h at 0 °C. A detailed mechanistic study was undertaken based on the information obtained by single crystal X-ray, optical rotation, UV-Vis, CD spectra and kinetic studies, and revealed that the reaction is first order with respect to the concentration of catalyst and oxidant and independent of substrate concentration. The complex (0.1 mol%) was successfully subjected to recyclability experiments over 3 cycles in the epoxidation of styrene with H2O2 as an oxidant and acetic acid as an additive at 0 °C with retention of performance.

Asymmetric Friedel–Crafts addition of indoles to N-sulfonyl aldimines catalyzed by Cu(II) chiral amino alcohol based Schiff base complexes

Recyclable copper(II) chiral amino alcohol based Schiff base complexes smoothly catalysed the Friedel–Crafts alkylation of indole with aryl aldimine in good yields (98%) and with enantioselectivities up to 97%. The effects of ligand structure, solvent, metal source and temperature on the reaction were also studied. The catalytic system worked very well several times retaining its performance. To understand the mechanism of the catalytic Friedel–Crafts addition reaction, a kinetic investigation was carried out with different concentrations of the catalyst Cu(II)–L2, indole and N-(3-nitrobenzylidene)-4-methylbenzene sulfonamide as a model substrate. The Friedel–Crafts alkylation reaction of N-(3-nitrobenzylidene)-4-methylbenzene sulfonamide was first order with respect to the concentration of the catalyst and the nucleophile but did not depend on the initial concentration of the substrate (aryl aldimine). An appropriate mechanism of the Friedel–Crafts alkylation reaction is proposed.

Catalysis of Enantioselective Strecker Reaction in the Synthesis of D‐Homophenylalanine Using Recyclable, Chiral, Macrocyclic MnIII–Salen Complexes

A convenient approach to the asymmetric Strecker reaction was established by synthesizing chiral, mono‐ and dinuclear, macrocyclic MnIII–salen complexes possessing achiral and chiral linkers (trigol, piperazine, and diethyl tartarate). This group of macrocyclic complexes has emerged as improved metal‐based catalysts for the enantioselective Strecker reaction of aldimines, giving high enantioselectivity (up to 99 %) for a wide range of substrates. The macrocylic complex 6 a with trigol linker works very well with TMSCN (trimethylsilyl cyanide) as a source of cyanide, using 4‐phenyl pyridine N‐oxide (4‐PPyNO) as a co‐catalyst in toluene at −40 °C. However, the macrocyclic complex 6 b with diethyl tartarate as a linker affected excellent chiral induction for both aromatic and aliphatic imines with a safer cyanide source (ethyl cyanoformate) in toluene at −20 °C in the presence of N,N‐diisopropylimine as a co‐catalyst. Complexes 6 a and 6 b used in the present study were recoverable and recyclable (five times) with retention of their performance at gram level. The kinetic study with complex 6 a for the enantioselective Strecker reaction of N‐benzyl benzylimine revealed a first‐order dependence on catalyst, substrate, and TMSCN concentration. This protocol with catalyst 6 b was further extended for the synthesis of D‐homophenyl alanine, an important analogue in factor Xa inhibitors.

In situ-generated chiral iron complex as efficient catalyst for enantioselective sulfoxidation using aqueous H2O2 as oxidant

A series of amino alcohol-derived, Schiff-base ligands L1–L4 were synthesised and characterized. Iron complexes of these ligands [FeL1(acac)], [FeL2(acac)], [FeL3(acac)] and [FeL4(acac)] were generated in situ to catalyze the asymmetric oxidation of prochiral sulfides using aqueous H2O2 as a terminal oxidant. One of these complexes [FeL1(acac)] was identified as a very efficient catalyst for the enantioselective oxidation of a series of alkyl aryl sulfides with excellent enantioselectivity (75–96% ee), conversion (up to 92%) and chemo selectivity (up to 98%). During the optimization process, a series of electron-donating benzoic acid derivatives were found to favour both conversion and enantioselectivity.

Asymmetric allylation of sulfonyl imines catalyzed by in situ generated Cu(II) complexes of chiral amino alcohol based Schiff bases

A catalytic route for enantioselective synthesis of homoallyl amines through Cu(II)-Schiff base catalyzed reaction of allyltin with aryl, alkenyl-substituted N-sulfonylimines is described. The allylation reaction is promoted by a simple in situ generated Cu(II)-amino alcohol based Schiff base complex. The addition of allyltin to aldimines delivers the desired products up to 90% yield and 98% enantiomeric excess (ee). Based on experimental observations a probable mechanism was proposed for this reaction. The current methodology was extended to the synthesis of β-phenylalanine in good yield and very good enantioselectivity.

Bimetallic titanium complex catalyzed enantioselective oxidation of thioethers using aqueous H2O2 as a terminal oxidant

A series of dimeric amino alcohol derived Schiff bases with various chiral amino alcohols and their corresponding bimetallic titanium complex were generated in situ. Thereafter with the in situ generated complexes, the asymmetric oxidation of prochiral aryl alkyl sulfides has been investigated using aqueous H2O2 as a terminal oxidant. During the study we found that the use of methanol or tert-butanol as an additive improved the catalytic activity in terms of both conversion and enantioselectivity. Moreover we observed a co-operative effect of the two reactive units of the bimetallic complex, which results in high reactivity as well as enantioselectivity compared to the corresponding monomeric complex. With this improved catalytic system several aryl alkyl sulfides and 1,3-dithianes were oxidised to the corresponding sulfoxides with good to high enantioselectivity (ee 78–99%) and conversion (70–99%). Unlike the monomer, oxidation of substrates like benzyl phenyl sulfide was achieved with high enantioselectivity as well as high yield.