Double C-H Functionalization to Construct Polycyclic Heteroarenes Catalyzed by an Ionic Salt of a Pd Complex with an N-Heterocyclic Carbene Ligand

A Versatile Pd-Catalyzed Alkyne Annulation Process for Benzo[a]carbazoles and their Anticancer Analogues

A Pd-catalyzed, simple, and divergent approach for the direct synthesis of benzo[a]carbazoles from internal alkynes and N-tosyl-iodoindoles has been demonstrated. This methodology highlights the influences of reaction media and temperature for the synthesis of either N-protected or N-deprotected benzo[a]carbazoles. This cascade strategy provides a series of electronically different benzo[a]carbazoles with good yields. The synthesized benzo[a]carbazoles were evaluated for in vitro anticancer activity against human lung cancer A549 cells and human breast cancer MDA-MB-231 cells. Notably, two of the representative analogues displayed potent anticancer activity against both cancer cell lines.

Accessing Polycyclic Heteroarenes Enabled by Copper-Catalyzed Aerobic Oxidative C-H/C-H [4 + 2] Annulation of 3-Arylindole Derivatives

Several polycyclic aromatic hydrocarbons are delivered at room temperature by copper-catalyzed aerobic oxidative C-H/C-H [4 + 2] annulation of alkyl-substituted 3-arylindole derivatives. Specifically, dual aryl C-H functionalization is furnished under mild conditions through the 1,2-migration of copper catalyst and regioselective alkyne insertion. Mechanistic experiments demonstrate that the C-H bond cleavage on the indole and phenyl rings is not involved in the rate-limiting step.

Indolizines and Their Hetero/Benzo Derivatives in Reactions of [8+2] Cycloaddition

Peculiarities of [8+2] cycloaddition of acetylenes to indolizines are reviewed. Especially mentioned are indolizines with leaving groups at positions 3 and 5. Cycloaddition to aza- and benzo derivatives are reviewed, as well as 1,10-cyclizations and processes leading to cyclazines where indolizines are intermediates. Mechanistic features (adducts and cycloadducts) and theoretical aspects (one- or two-steps mechanism) are reviewed.

Oxidative C-H/C-H Annulation of Imidazopyridines and Indazoles through Rhodium-Catalyzed Vinylene Transfer

Transition-metal-catalyzed C-H activation followed by oxidative annulation with alkynes has been an efficient synthetic tool for the assembly of various polyaromatic scaffolds. Despite the substantial progress in this field, it is still a significant challenge to achieve the synthesis of nonsubstituted vinylene-fused compounds. In this contribution, we report a Rh-catalyzed C-H/C-H vinylene cyclization adopting vinylene carbonate as a "vinylene transfer" agent. This protocol achieves the direct annulative π-extension of imidazole- and pyrazole-fused aromatics.

Rh(III)-Catalyzed Tandem Bicyclization of 2-Arylimidazo[1,2-a]pyridines with Cyclic Enones for the Construction of Bridged Scaffolds

An efficient Rh(III)-catalyzed bicyclization of 2-arylimidazo[1,2-a]pyridine with cyclic enones has been developed for the synthesis of bridged imidazopyridine derivatives in excellent yields up to 95%. The reaction proceeds through a sequential conjugate addition of ortho-C-H bond of aryl group followed by an intramolecular C3-alkylation of imidazopyridine ring in a highly regioselective manner.

Synthesis of fused imidazo[1,2-a]pyridines derivatives through cascade C(sp2)-H functionalizations

An efficient and convenient synthesis of diversely substituted naphtho[1',2':4,5]imidazo[1,2-a]-pyridine derivatives from the cascade reactions of 2-arylimidazo[1,2-a]pyridines with a-diazo carbonyl compounds via Rh(iii)-catalyzed regioselective C(sp2)-H alkylation followed by intramolecular annulation is presented. Interestingly, when simple 2-arylimidazo[1,2-a]pyridines were used as the substrates, 5,6-disubstituted naphtho[1',2':4,5]imidazo[1,2-a]pyridines were efficiently obtained, whereas using 2-arylimidazo[1,2-a]pyridine-3-carbaldehydes as the substrates afforded naphtho[1',2':4,5]imidazo[1,2-a]-pyridine-5-carboxylates as the dominating products. Compared with literature methods for the synthesis of naphtho[1',2':4,5]imidazo[1,2-a]pyridine derivatives, the protocol presented herein has advantages such as easily obtainable substrates, simple operational procedure, high efficiency and excellent regio- and chemoselectivity.

Direct synthesis of aryl-annulated [c]carbazoles by gold(i)-catalysed cascade reaction of azide-diynes and arenes

The gold-catalysed annulation of conjugated alkynes bearing an azido group with arenes gave annulated [c]carbazoles. Using benzene, pyrrole, and indole derivatives as the nucleophiles, benzo[c]-, pyrrolo[2,3-c]-, and indolo[2,3-c]carbazoles were produced, respectively. The reaction proceeded through pyrrole and benzene ring construction accompanied by the formation of two carbon-carbon and one carbon-nitrogen bond and the cleavage of two aromatic C-H bonds. The mechanism of the reaction with pyrrole was investigated by density functional theory calculations. N,N'-dimethylated indolo[2,3-c]carbazole showed dual ultraviolet-visible-near-infrared and fluorescence spectral changes upon electrolysis.

Rhodium-catalyzed C2 and C4 C–H activation/annulation of 3-(1H-indol-3-yl)-3-oxopropanenitriles with internal alkynes: a facile access to substituted and fused carbazoles

Rhodium-catalyzed C2 and C4 C–H activation/annulation of 3-(1H-indol-3-yl)-3-oxopropanenitriles with internal alkynes has been developed. Substituted and fused carbazole derivatives were obtained through formal rhodium-catalyzed (4+2) or tandem (4+2) and (5+2) cycloaddition reactions under mild reaction conditions in good yields.

Enhancement of N-heterocyclic carbenes on rhodium catalyzed olefination of triazoles

A few rhodium complexes of N-heterocyclic carbenes were prepared through carbene transfer reactions and their structures were characterized by X-ray diffraction analysis.

Recent advances in the synthesis of indolizines and their π-expanded analogues

Synthesis of indolizines developed during the last decade is reviewed, with special emphasis given to densely functionalized architectures, breakthrough strategies, compounds bearing electron-donating and electron-withdrawing substituents and π-expanded systems.