Anupal Gogoi

Easy access to benzylic esters directly from alkyl benzenes under metal-free conditions

An efficient metal free protocol has been developed for the synthesis of benzylic esters via a cross dehydrogenative coupling (CDC) involving alkylbenzene(s) as a self- or as a cross-coupling partner(s) via the intermediacy of Ar-COOH and the benzylic carbocation obtained from the other half of the alkylbenzene; both symmetrical as well as unsymmetrical esters can be prepared using Bu4NI and TBHP.

Copper-Catalyzed Esterification of Alkylbenzenes with Cyclic Ethers and Cycloalkanes via C(sp3)–H Activation Following Cross-Dehydrogenative Coupling

A copper-catalyzed cross-dehydrogenative coupling strategy has been developed for the synthesis of two classes of esters from simple solvents. The reaction of methylarenes with cyclic ethers resulted in α-acyloxy ethers involving four sp(3) C-H cleavages, while treatment of methylarenes with cycloalkanes led to the formation of allyl esters at the expense of six consecutive sp(3) C-H bonds.

Directing Group Assisted Copper-Catalyzed Chemoselective O-Aroylation of Phenols and Enols Using Alkylbenzenes

By using alkylbenzenes as aroyl surrogates, copper(II) catalyzed chemoselective O-aroylations of 1,3-dicarbonyl compounds and phenolic -OH ortho to carbonyl (-CHO, -COR) groups have been achieved. A dual mechanism operating in tandem for these transformations has been supported by a crossover experiment.

A copper-catalyzed synthesis of 3-aroylindoles via a sp3 C-H bond activation followed by C-C and C-O bond formation.

An efficient Cu(I)-catalyzed synthesis of 3-aroylindoles has been achieved from o-alkynylated N,N-dimethylamines via a sp(3) C-H bond activation α to the nitrogen atom followed by an intramolecular nucleophilic attack with the alkyne using an aqueous solution of tert-butyl hydroperoxide (TBHP) as the oxidant. In this tandem catalytic synthesis of 3-aroylindoles both C-C and C-O bonds are installed at the expense of two sp(3) C-H bond cleavages.

Terminal aryl alkenes and alkynes as arylcarboxy surrogates toward o-benzoxylation of 2-phenylpyridine catalyzed by copper.

A variety of styrenes and phenylacetylenes serve as excellent arylcarboxy sources in bringing about substrate directed o-benzoxylation of 2-phenylpyridine derivatives catalyzed by Cu(II) in the presence of TBHP. This reaction proceeds via formation of phenylglyoxal followed by decarbonylation to benzoyl radical/benzaldehyde which acts as the arylcarboxy source.

Iodine-catalysed oxidative cyclisation of acylhydrazones to 2,5-substituted 1,3,4-oxadiazoles

An environmentally benign synthesis of 2,5-disubstituted 1,3,4-oxadiazoles has been developed starting from N-aroylhydrazones and N-acetylhydrazones at room or ambient temperature using a catalytic quantity of iodine in the presence of an aqueous hydrogen peroxide oxidant.

CuO nanoparticle catalysed synthesis of 2H-indazoles under ligand free conditions

A CuO nano catalysed one-pot synthesis of 2H-indazoles has been devised from easily accessible starting materials viz. 2-bromobenzaldehydes, primary amines and sodium azide under ligand free conditions. The nano CuO catalyst played an important role during the formation of the intermolecular C–N bond followed by the intramolecular N–N bond, providing 2H-indazoles. This method has a broad substrate scope with a high tolerance of a variety of functional groups. The catalyst can be recycled up to three times, however with slight decreases in the yields each time.

Synthesis of 1,2,4-Triazoles via Oxidative Heterocyclization: Selective C-N Bond Over C-S Bond Formation.

The higher propensity of C-N over C-S bond forming ability was demonstrated, through formal C-H functionalization during the construction of 4,5-disubstituted 1,2,4-triazole-3-thiones from arylidenearylthiosemicarbazides catalyzed by Cu(II). However, steric factors imparted by the o-disubstituted substrates tend to change the reaction path giving thiodiazole as the major or an exclusive product. Upon prolonging the reaction time, the in situ generated thiones are transformed to 4,5-disubstituted 1,2,4-triazoles via a desulfurization process. Two classes of heterocycles viz. 4,5-disubstituted 1,2,4-triazole-3-thiones and 4,5-disubstituted 1,2,4-triazoles can be synthesized from arylidenearylthiosemicarbazides by simply adjusting the reaction time. Desulfurization of 1,2,4-triazole-3-thiones is assisted by thiophilic Cu to provide 1,2,4-triazoles with concomitant formation of CuS and polynuclear sulfur anions as confirmed from scanning electron microscope and energy dispersive X-ray spectroscopy measurements. A one-pot synthesis of an antimicrobial compound has been successfully achieved following this strategy.

A Cu-catalysed synthesis of substituted 3-methyleneisoindolin-1-one

A Cu(I)-catalysed synthesis of substituted 3-methyleneisoindolin-1-ones using alkynyl acids as an alkyne source has been developed. The reaction involves the decarboxylative cross-coupling of 2-halobenzamides with aryl alkynyl acids, followed by 5-exo-dig heteroannulation. While reactions of 2-iodo benzamides proceeded without a ligand, for 2-bromo substrates, the assistance of a ligand is essential.

Cu(II) catalysed chemoselective oxidative transformation of thiourea to thioamidoguanidine/2-aminobenzothiazole

2-Haloaryl-sec-alkyl unsymmetrical thioureas (Tu) (halo = –F, –Cl) with a catalytic amount of Cu(II) salt get oxidised in situ to their disulfide intermediates followed by an imine-disulfide rearrangement to give thioamidoguanidino (Tag) moieties at room temperature. During this process Cu(II) gets reduced to Cu(I) and forms a complex with the Tag moiety from which Tag moiety can be isolated upon treatment with ammonia. However, when the same reaction was performed at an elevated temperature with a catalytic quantity of Cu(II) salt, Tu bearing o-halogens (–F, –Cl) gave 2-aminobenzothiazoles via a dehalogenative heteroarylation path and not by the Hugerschoff path involving an electrophilic substitution reaction. For thioureas containing reactive ortho halogens (such as –Br, –I) the reaction proceeds at room temperature giving 2-aminobenzothiazoles via a dehalogenative path requiring a catalytic quantity of Cu(II). No transformation of thiourea (Tu) to Tag was observed with Cu(I) salts suggesting the requirement of an oxidising Cu(II) salt for this oxidative transformation. Mild reaction conditions, environmentally benign reagents and solvent, high yields, tolerance of various functional groups are some of the essential features of this methodology.