Arghya Banerjee

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.

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.

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.

Pd(II)-catalysed o-aroylation of directing arenes using terminal aryl alkenes and alkynes

A substrate-directed Pd-catalysed o-aroylation strategy has been demonstrated using new aroyl surrogates viz. terminal aryl alkenes and alkynes in the presence of TBHP. By a subtle change in catalyst from Cu to Pd, a differential selectivity is observed. While terminal aryl alkenes/alkynes in the presence of Cu/TBHP are reported to act as o-aryloxy (ArCOO–) sources, the use of Pd/TBHP installs an aroyl (ArCO–) group at the ortho position with respect to the directing arenes.

Palladium catalyzed ortho-halogenation of 2-arylbenzothiazole and 2,3-diarylquinoxaline

A palladium catalysed ortho-halogenation strategy has been developed using benzothiazoles and quinoxalines as the directing substrates. This method provides mono-o-halogenated product at the other available ortho site of a mono-ortho substituted 2-arylbenzothiazole. However, ortho-unsubstituted 2-arylbenzothiazole afforded di-ortho halogenated product exclusively. The preformed (or installed) ortho-group is towards the sulphur side of the benzothiazole from energy minimised calculation. Thus the selective formation of di-ortho-halogenated products is due to favourable exposure of the second ortho site for subsequent halogenation. However the phenyl ring in 2,3-diarylquinoxalines can be selectively mono ortho halogenated. A plausible reaction mechanism has been proposed for this halogenation process.

Benzylic ethers as arylcarboxy surrogates in substrate directed ortho C–H functionalisation catalysed by copper

A copper catalysed ortho-benzoxylation of 2-arylpyridines has been accomplished using benzylic ethers as the alternative arylcarboxy sources (ArCOO–) via sp2 C–H bond activation. The use of the Pd/TBHP catalytic system is reported to install an o-aroyl (ArCO–) moiety at the 2-arylpyridine while the Cu/TBHP combination fixes a benzoxy (ArCOO–) group at the ortho site.

Palladium-catalysed regioselective aroylation and acetoxylation of 3,5-diarylisoxazole via ortho C–H functionalisations

The higher directing ability of N over O in 3,5-diarylisoxazole is demonstrated during the construction of C–C and C–O bonds. Out of the four ortho sp2 C–Hs and one internal sp2 C–H in 3,5-diarylisoxazoles, regioselective aroylation and acetoxylation take place at one of the ortho-C–Hs proximal to the N atom using Pd(OAc)2 as the catalyst in the presence of suitable oxidants and solvents.

Ruthenium(II) Catalyzed Regiospecific C–H/O–H Annulations of Directing Arenes via Weak Coordination

Ruthenium(II) catalyzed oxidative C-H/O-H annulations have been demonstrated using two different directing arenes viz. 2-arylquinolinone and 2-arylbenzoxazinone with internal alkynes. Regiospecific annulations have been observed for both directing arenes via the assistance of weaker carbonyl oxygen in the presence of a stronger nitrogen-directing site. In this substrate-controlled convergent protocol the weaker directing group dictates the annulation path.

Shape and Size of Cobalt Nanoislands Formed Spontaneously on Cobalt Terraces during Fischer–Tropsch Synthesis

Cobalt-based catalysts undergo a massive and spontaneous reconstruction to form uniform triangular nanoislands under Fischer-Tropsch (FT) conditions. This reconstruction is driven by the unusual and synergistic adsorption of square-planar carbon and CO at the 4-fold edge sites of the nanoislands, driving the formation of triangular islands. The size of the nanoislands is determined by the balance between energy gain from creating C/CO-covered edges and energy penalty to create C/CO-covered corners. For carbon chemical potentials corresponding to FT conditions, triangular Co islands with 45 Co atoms (about 2 nm) are the most stable surface structure. Decreasing the carbon chemical potential and hence the stability of square-planar carbon favors the formation of larger islands, until reconstruction becomes unfavorable and CO-covered terraces are thermodynamically the most stable. The predicted structure of the islands is consistent with in situ scanning tunneling microscopy images obtained for the first time under realistic FT reaction conditions on a Co(0001) surface.