Thirumanavelan Gandhi

Ru(II)-Catalyzed β-Carboline Directed C–H Arylation and Isolation of Its Cycloruthenated Intermediates

A Ru(II)-catalyzed C-H arylation approach has been developed utilizing β-carboline alkaloids as the directing group. Selective formations of diarylated products from moderate to excellent yields were accomplished. Broad substrate scope with excellent functional group tolerance for C1-phenyl/thienyl/PAHs-β-carbolines was demonstrated. X-ray crystal structure of cycloruthenated complex 2cr and no arylation reaction with model substrate 13 strongly suggests that N2 is the directing group than N9 in C1-aryl-β-carbolines. Catalytic properties and stability of the cycloruthenated complexes have been explored. Library of biologically relevant new β-carboline derivatives and isolation of its cycloruthenated intermediates are the highlights of this work.

Synthesis and photophysics of extended π-conjugated systems of substituted 10-aryl-pyrenoimidazoles

The synthesis of π-extended 10-aryl-pyrenoimidazoles having different substituents was realised via Ru(ii)-catalyzed oxidative annulation of 10-aryl-pyrenoimidazole with diphenylacetylene. The single crystal X-ray structure of trifluoromethyl and carboxylate substituted annulated-10-aryl-pyrenoimidazoles confirms the near coplanarity of the pyrene and imidazole moieties and the presence of twisted conformation resulting in intermolecular C-Hπ interactions. The lowest energy absorption maximum becomes red-shifted characteristic to the nature of the substituent owing to the extended π-conjugation, and specifically the nitro substituent shows intense absorption in the visible region with the maximum at 440 nm. All the molecules were found to show intense fluorescence both in solution and solid states. Strikingly, 170 nm red-shifted fluorescence with a large Stokes shift ca. 7000 cm-1 for the nitro derivative, a value nearly two-fold higher than the parent compound despite its rigid polyaromatic skeleton was observed. The combination of electron rich π-conjugated aromatic systems with electron deficient substituents induces the intramolecular charge transfer interactions, which has been corroborated with the theoretical calculations.

Palladium(II)/N-Heterocyclic Carbene-Catalyzed Direct C–H Acylation of Heteroarenes with N-Acylsaccharins

N-Acylsaccharin represents a facile acyl group transfer agent to heteroarenes in the presence of Pd(II)/NHC complexes appended with a pyrene unit. Catalytic acylation of heteroarenes was enhanced by the noncovalent interaction between the pyrene unit and substrates. High functional group tolerance, broad substrate scope, and moderate to good yields of 2-acylated azoles are added features of this method.

Dibenzofuran and dibenzothiophene based palladium(II)/NHC catalysts – synthesis and applications in C–C bond formation

In the quest for a new ligand system for Pd(II)/NHCs, we developed new dibenzofuran and dibenzothiophene based palladium N-heterocyclic carbene catalysts D1–D6 in good yields. All the catalysts were characterized by multinuclear NMR spectroscopy and HRMS. The X-ray crystal structure of the representative dibenzothiophene based Pd(II)/NHC D4 was determined. Among the precatalysts, D1 was shown to be highly effective in the Suzuki–Miyaura cross-coupling reaction of heterocyclic bromides with boronic acids. Besides, D1 affords diverse arylated benzoxazoles via direct C–H bond functionalization with substituted bromo derivatives.

Palladium(II)/N-Heterocyclic Carbene Catalyzed One-Pot Sequential α-Arylation/Alkylation: Access to 3,3-Disubstituted Oxindoles

Rationally designed fluorene-based mono- and bimetallic Pd-PEPPSI complexes were synthesized and demonstrated to be effective for the one-pot sequential α-arylation/alkylation of oxindoles. This streamlined approach offers efficient access to functionalized 3,3-disubstituted oxindoles in excellent yields (up to 89%) under mild reaction conditions.