Saikat Khamarui

CeCl3·7H2O Catalyzed C–C and C–N Bond-Forming Cascade Cyclization with Subsequent Side-Chain Functionalization and Rearrangement: A Domino Approach to Pentasubstituted Pyrrole Analogues

CeCl(3)·7H(2)O is found as an efficient catalyst for new intermolecular domino reactions of three-, four- and seven-component assemblies of common precursors under benign reaction conditions. Generation of enaminioesters from β-keto esters and primary amines, activation of their allylic sp(3) C-H, vinylic sp(2) C-H and N-H bonds, multi C-C and C-N bond-forming cascade cyclization with 1,2-diketones and subsequent side-chain alkylation have been developed to construct functionalized pentasubstituted pyrroles and their chiral analogues. The scope of the domino reaction is successfully explored toward synthesis of highly aryl-substituted pyrroles, pentasubstituted pyrroles bearing C2-olefinic side-chain and spiro-2-pyrrolinones and their chiral analogues via unusual side-chain amination, elimination and ring contraction. The new domino reaction is operationally simple, robust, substrate specific, selective and high yielding.

ArCH(OMe)2 - a PtIV-catalyst originator for diverse annulation catalysis

We discover an important property of a small molecule ArCH(OMe)2 which transforms catalytically inactive PtIIBr2 procatalyst in situ to an powerful catalyst PtIV-species for diverse annulation reaction. The powerful catalytic system enables selective activation of C2-H/N-H and C2-H/C4-H of acetoacetanilide and C = O/C≡C of substituted butyne-1,2-dione for C-C/C-N, C-C/C-C and C-O/C-O bond-forming inter- and intramolecular annulation towards direct syntheses of functionalised 2-pyridones, cyclohexenones and 3(2H)-furanones respectively. In contrast to the common ligand, herein highly labile C-OMe bond of ArCH(OMe)2 is expected to react with PtBr2 towards generation of the high-valent active catalyst. Unlike catalyst promoter or initiator, the reaction does not occur with PtBr2 in the absence of ArCH(OMe)2. In situ generation of PtIV-species and -OMe fragment of ArCH(OMe)2 were confirmed from the UV-vis characteristic peaks about 260 nm and trapping of -OMe group respectively. These observations provide new prospects and perspectives in catalysis for innovative catalyst design.

In Situ Generated AgII-Catalyzed Selective Oxo-Esterification of Alkyne with Alcohol to α-Ketoester: Photophysical Study

An expert and easy one-step catalytic method for the multi O-C coupling of alkyne is developed for the synthesis of valuable α-ketoesters and their chiral analogues, in contrast to the generation of esters by a noncatalytic method. The in situ generated powerful Ag(II) catalyst from AgOTf is the workhorse in the oxidative grafting of alkyne with PhIO and alcohol. The radical mechanism is confirmed in our controlled experiments and UV-vis study.

CuBr–ZnI2 Combo-Catalysis for Mild CuI–CuIII Switching and sp2 C–H Activated Rapid Cyclization to Quinolines and Their Sugar-Based Chiral Analogues: A UV–Vis and XPS Study

An unprecedented CuBr–ZnI2 combo-catalyzed mild Cu1–CuIII switching activation of sp2 C–H of highly electron-rich arenes is reported. Anilines, aldehydes, and terminal alkynes were rapidly coupled together at ambient temperature to construct a ubiquitous quinoline framework through cyclization of the C≡C bond. This smart solvent-free strategy was exploited for the direct synthesis of valuable 4-substituted, 2,4-disubstituted, and thermally labile sugar-based chiral quinolines in good yields. In contrast to the frequently used imine–alkyne cyclization reaction, this uncommonly mild CuI–CuIII combo-catalysis for a rapid three-component cyclization is expected to proceed through the formation of a flexible propargyl amine intermediate, which provides a CuI-procatalyst for rapid sp2 C–H activation with cyclization involving transient CuIII species. The in situ generation of transient CuIII species was confirmed through online ultraviolet–visible spectroscopy (UV–vis), electrospray ionization mass spectrometry (ESI-MS), and X-ray photoelectron spectroscopy (XPS) analyses.

Functionalised Mn VI -nanoparticles: an advanced high-valent magnetic catalyst

We discover MnVI-nanoparticles (NPs) bearing functional groups, high oxidation state, strong electron affinity, unique redox and paramagnetic nature, which opens up a new avenue to catalysis, magnetism and material application. However, its synthesis is challenging and remains unexplored because of associated serious difficulties. A simple benign synthetic strategy is devised to fabricate the high-valent NPs using mild reducing agent bromide, which transformed MnVII to valuable MnVI-species. The EELS-imaging of individual elements, ESI-MS, XPS and other techniques established its composition as Br(Me3SiO)MnVIO2. It revealed significantly improved magnetic moment (SQUID) with isotropic hyperfine splitting of six line spectrum (EPR). The high-oxidation state and incorporated-ligands of the metals present on the active surface of the NPs led to development of a general catalytic process for oxidative heterodifunctionalisation to C-C triple bond towards formation of a new O-C/N-C/S-C and C-C coupling cum cyclisation to biologically important flavones and their aza- and marcapto-analogues, and valuable enaloxy synthons.

Fabrication of tungsten nanocrystals and silver–tungsten nanonets: a potent reductive catalyst

This communication reports the first synthesis of well-defined colloidal W nanocrystals, Ag–W heterostructures and their broad plasmon absorbance in the visible to near infrared region. Strong interparticle plasmonic interactions significantly improve the photocatalytic reduction performance of silver–tungsten heterostructures.

Synthesis and diverse general oxidative cyclization catalysis of high-valent MoVIO2(HL) to ubiquitous heterocycles and their chiral analogues with high selectivity

The first synthesis and diverse oxidative cyclization catalysis properties of high-valent MoVI–triazole are demonstrated towards highly selective construction of benzimidazoles, benzothiazoles, isoxazolines, isoxazoles and their chiral analogues.

Reactant cum solvent water: generation of transient λ3-hypervalent iodine, its reactivity, mechanism and broad application

An outstanding amidation/imidation process is demonstrated under metal-free benign reaction conditions by grafting terminal alkynes with varied amines and reactant cum solvent water. We explored diverse substrate scope in the nonconventional approach for synthesis of several classes of valuable compounds bearing amide linkages such as N-tosyl, N-aminotosyl, N-oxobenzyl, N-alkylated sugar-based secondary and tertiary chiral amides and also exploiting it for synthesis of valuable cyclic imides through simultaneous activation of triple bond and sp2C–H of aldehyde. The in situ-generated powerful reagent PhI(OH)2 enables selective cleavage and functionalization of terminal C–C triple bonds through simultaneous construction of C–N and CO bonds. A measurable breakthrough in time-resolved ATR-MIR spectroscopy of the ongoing λ3-hypervalent iodine controlled reaction revealed formation of the unknown intermediates. The ATR-MIR technology is used for identifying infrared (IR) spectra of the individual intermediate/component present in the reaction mixture, which contains several compounds. The structures of the intermediates and their IR spectra were determined by DFT study. This unprecedented combination of experimental results and theoretical prediction provided useful information regarding the reaction insights such as the cleavage of triple bond, amination and amidation in the complex reaction process involving λ3-hypervalent iodine-bearing labile intermediates to the desired amide. The NMR and labelling (2H and 18O) studies supported the DFT-IR predicted reaction pathway.

Photocatalytic Generation of Nitrenes for Rapid Diaziridination

A blue LED, an organic photocatalyst (rose bengal), and the Lewis acid like oxidant PhI(OAc)2 were utilized to generate nitrene intermediates through reactions of 1,2-diols and aliphatic amines under mild reaction conditions. A versatile and rapid diaziridination strategy was established to construct functionalized 1,2-disubstituted diaziridines, diaziridines with chiral substituents, and 1,2,3-trisubstituted analogues with excellent reaction rates, yields, and stereoselectivities. Control and labeling experiments to elucidate the mechanism of this elegant metal-free photocatalyzed cyclization reaction were performed.