Sourav Kumar Santra

Bu4NI Catalyzed C–N Bond Formation via Cross-Dehydrogenative Coupling of Aryl Ethers (Csp3–H) and Tetrazoles (N–H)

Intermolecular C-N bond formations via cross-dehydrogenative coupling (CDC) of aryl ethers and tetrazoles have been developed under a metal-free condition. In the presence of catalytic amount of tetrabutylammonium iodide (TBAI) and aqueous TBHP, aryl ethers coupled efficiently with tetrazoles to afford hemiaminal ethers. This strategy showed high level of regioselectivity for substrates possessing multiple sp(3) C-H bonds adjacent to the ethereal oxygen.

Generation of bis-Acyl Ketals from Esters and Benzyl Amines Under Oxidative Conditions

Treatment of benzylamines with esters at an elevated temperature are expected to give amides. However, in the presence of TBAI/TBHP, esters possessing a methylene carbon α-to oxygen with benzylamines provide bis-esters rather than the expected amides. Benzylamines under oxidative conditions generate less nucleophilic carboxylates, which couples at the sp(3) C-H bonds of esters and cyclic ethers to give bis-acyl ketals and α-acyloxy ethers, respectively.

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.

Peroxide-Free Pd(II)-Catalyzed Ortho Aroylation and Ortho Halogenation of Directing Arenes

A Pd(II)-catalyzed peroxide-free ortho aroylation of directing arenes has been developed via cross dehydrogenative coupling (CDC) in the presence of the terminal oxidant Cu(OAc)2·H2O. Ortho aroylation of directing arenes proceeds via decarbonylation of the in situ generated phenyl glyoxal, which is obtained from 2-acetoxyacetophenone in the presence of the oxidant Cu(OAc)2·H2O. However, changing the oxidant to CuX2 (X = Cl, Br) provided exclusive di-ortho-halogenated 2-arylbenzothiazoles. During the halogenation, CuX2 served the dual role of a halogen source as well as a co-oxidant.