A metal-free visible-light-induced phosphonylation of 2H-indazoles with diphenylphosphine oxide has been developed using rose bengal as an organophotoredox catalyst under ambient air at room temperature. A library of diphenyl(2phenyl-2H-indazol-3-yl)phosphine oxide with broad functionalities has been synthesized in high yields. The experimental result suggests the radical pathway of the reaction.

A mild and efficient method for the direct difluoromethylenephosphonation of imidazopyridines has been developed using rose bengal (RB) as a photoredox catalyst. Bis(pinacolato)diboron (B 2 pin 2 ) is found to be a crucial additive in the present reaction. The present methodology is also applicable to other heterocycles like imidazo[2,1-b]thiazole, benzo [d]imidazo-[2,1-b]thiazole, and indole. The reaction possibly proceeds through a single electron transfer (SET) process.

A Ru(II)-catalyzed facile and controllable protocol for C−H alkylation and spirocyclization of 2-arylquinoxalines with maleimides has been achieved under ambient air in high yields. Sequential ortho-C−H activation and C-annulation results in the formation of diverse polyheterocycles containing spiro[indeno[1,2b]quinoxaline-11,3′-pyrrolidine]-2′,5′-diones, which are of potent interest in medicinal chemistry. Mechanistic investigations suggest a reversible cleavage of the ortho-C−H bond in the turnover-limiting step.

A visible-light-promoted regioselective C­(sp2)–H/C­(sp3)–H cross-dehydrogenative coupling between 2H-indazoles and ethers has been achieved using a catalytic amount of rose bengal as an organophotoredox-catalyst and tert-butyl hydroperoxide (TBHP) as an oxidant at ambient temperature under aerobic conditions. A variety of C-3 oxyalkylated 2H-indazoles have been synthesized in moderate to good yields. Mechanistic studies suggest a radical pathway of the present reaction.

An environment-friendly method for the chlorination of imidazoheterocycles has been developed using chloramine-T, a novel chlorinating reagent. A bunch of C-3 chloro-substituted imidazo[1,2- a ]pyridines with variety of functionalities have been synthesized in good yields under neat condition at room temperature within very short time. This chlorination process is also applicable to imidazo[2,1- b ]thiazole scaffolds. The present methodology is relevant to gram-scale synthesis. The major advantages of this system such as wide applicability, easy availability of reactants, open-air and metal- and solvent-free reaction conditions, no need of work-up, and simple purification technique represent a green synthetic protocol.