Ru(II)-Catalyzed Switchable C-H Alkylation and Spirocyclization of 2-Arylquinoxalines with Maleimides via ortho-C-H Activation.

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[1,2-b]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.

Visible light-induced photocatalytic C-H ethoxycarbonylmethylation of imidazoheterocycles with ethyl diazoacetate.

A visible light-mediated regioselective C3-ethoxycarbonylmethylation of imidazopyridines with ethyl diazoacetate (EDA) was achieved under mild reaction conditions. In contrast to the carbene precursors from α-diazoester a first C3-ethoxycarbonylmethylation of imidazopyridines via a radical intermediate has been established. The present methodology provides a concise route to access pharmacologically useful esters with wide functional group tolerance in high yields.

Organophotoredox-Catalyzed C(sp2)-H Difluoromethylenephosphonation of Imidazoheterocycles.

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 (B2pin2) 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.

Visible-Light-Induced Regioselective Cross-Dehydrogenative Coupling of 2 H-Indazoles with Ethers.

A visible-light-promoted regioselective C(sp2)-H/C(sp3)-H cross-dehydrogenative coupling between 2 H-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 2 H-indazoles have been synthesized in moderate to good yields. Mechanistic studies suggest a radical pathway of the present reaction.

Visible-Light-Induced Organophotoredox-Catalyzed Phosphonylation of 2 H-Indazoles with Diphenylphosphine Oxide.

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

(Diacetoxy)iodobenzene-Mediated Regioselective Imidation of Imidazoheterocycles with N-Fluorobenzenesulfonimide.

Metal-free (diacetoxy)iodobenzene-mediated regioselective imidation of imidazoheterocycles using commercially available N-fluorobenzenesulfonimide as an imidating reagent has been developed. This protocol exhibits broad substrate scope with good to excellent yields of the imidated imidazopyridines under mild conditions in short reaction times. The present protocol also represents an efficient way to access the imidated derivatives of imidazo[2,1-b]thiazole, benzo[d]imidazo-[2,1-b]thiazole, indoles, and indolizines. A radical mechanistic pathway has been proposed for the present protocol.

Environment-Friendly Protocol for the Chlorination of Imidazoheterocycles by Chloramine-T.

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.

Aminomethylation of Imidazoheterocycles with Morpholine.

A hitherto unreported aminomethylation occurs at C-3 of imidazopyridines with morpholine in the presence of (diacetoxyiodo)benzene at ambient temperature in short reaction times. This methodology is also applicable to indolizine, imidazo[2,1-b]thiazole, benzo[d]imidazo[2,1-b]thiazole, and indole. Interestingly, the aminomethylation involving morpholine as a source of methylene group is a new phenomenon. This protocol is of much potential for the synthesis of aminomethylated derivatives under mild reaction conditions.

Zn(OTf)2-Catalyzed Synthesis of Imidazole-Substituted Allenes.

A Zn(OTf)2-catalyzed simple and efficient method has been developed for the synthesis of imidazole-substituted allenes by the reaction of 1,1,3-triphenylprop-2-yn-1-ol and imidazoheterocycles. A library of tetrasubstituted allenes with broad functionalities have been prepared with excellent yields. The present methodology is also applicable to imidazo[1,2-a]pyrimidine, imidazo[2,1-b]thiazole and benzo[d]imidazo[2,1-b]thiazole.