Srimanta Guin

Four Tandem C–H Activations: A Sequential C–C and C–O Bond Making via a Pd-Catalyzed Cross Dehydrogenative Coupling (CDC) Approach

An unprecedented aroylation at the ortho C-H bond with respect to a directing group has been accomplished via a Pd(II)-catalyzed cross dehydrogenative coupling approach using alkylbenzene as the synthetic equivalent of an aroyl moiety. The reaction proceeds through sequential C-C and C-O bond making at the expense of four consecutive C-H bond cleavages (three sp(3) benzylic C-Hs and one sp(2) arene C-H) to selectively install an aroyl functionality at the proximal site of substrates containing various directing groups.

Copper Catalyzed Oxidative Esterification of Aldehydes with Alkylbenzenes via Cross Dehydrogenative Coupling

Copper(II) as the catalyst in a cross dehydrogenative coupling (CDC) reaction has been demonstrated for the synthesis of benzylic esters using aldehydes and alkylbenzenes as coupling partners.

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.

Cu(II) catalyzed imine C-H functionalization leading to synthesis of 2,5-substituted 1,3,4-oxadiazoles.

A direct access to symmetrical and unsymmetrical 2,5-disubstituted [1,3,4]-oxadiazoles has been accomplished through an imine C-H functionalization of N-arylidenearoylhydrazide using a catalytic quantity of Cu(OTf)(2). This is the first example of amidic oxygen functioning as a nucleophile in a Cu-catalyzed oxidative coupling of an imine C-H bond. These reactions can be performed in air atmosphere and moisture making it exceptionally practical for application in organic synthesis.

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.

An “on-water” exploration of CuO nanoparticle catalysed synthesis of 2-aminobenzothiazoles

An “on-water” one-pot process has been engineered for the preparation of 2-aminobenzothiazole from ortho-halo (–F, –Cl, –Br and –I) substituted unsymmetrical thioureas. For ortho –I and –Br substrates the reactions afford 2-aminobenzothiazoles under metal free condition promoted by base. However, the relatively inert ortho –Cl and –F substrates undergo intramolecular arylthiolation only in the presence of CuO nanoparticles yielding 2-aminobenzothiazoles. This methodology provides easy access to aminobenzothiazoles utilising even the ortho –Cl and –F substrates. The catalyst is recyclable several times without loss of substantial activity. Other remarkable features include the wide range of functional group tolerance, absence of chromatographic purification (for ortho –I and –Br substrates) and providing moderate to excellent yield of the products under mild conditions, thus rendering the methodology as a highly eco-friendly alternative to the existing methods.

Regiospecific Benzoylation of Electron-Deficient N-Heterocycles with Methylbenzenes via a Minisci-Type Reaction

A regioselective cross-dehydrogenative coupling between electron-deficient N-heterocycles (isoquinoline, quinolines, and quinoxalines) and methylbenzenes leading to regiospecific C-aroylation has been accomplished using AlCl3 as the catalyst in the presence of oxidant TBHP. This protocol is a practical alternative to the classical Minisci reaction.