RhCl3·3H2O-Catalyzed Regioselective C(sp2)–H Alkoxycarbonylation: Efficient Synthesis of Indole- and Pyrrole-2-carboxylic Acid Esters

Carbonylative synthesis and functionalization of indoles

Carbonylation processes have become widely recognized as a versatile, convenient, and low-cost method for the synthesis of high-value compounds. Given the great importance of heterocyclic compounds, the carbonylative approach has become increasingly important for their synthesis. In this mini-review, as a class of benzo-fused nitrogen-containing heterocyclic compounds, we summarized and discussed the recent achievements on the synthesis and functionalization of indole derivatives via carbonylative approaches.

Rhodium-catalysed additive-free alkoxycarbonylation of indoles: 2,4,6-trimethylbenzoic acid-based carbonate anhydrides as a versatile alkoxycarboxyl source

We report a CO-free approach to indole-2-carboxylic esters: rhodium-catalysed C(2)-alkoxycarbonylation of indoles with 2,4,6-trimethylbenzoic acid-based carbonate anhydrides. Selective C-O bond cleavage of the anhydrides facilitates the introduction of various alkoxycarbonyl groups. Control experiments suggest that merging a rhodium catalyst and KI promotes the in situ formation of the RhI species.

Cobalt(II)-Catalyzed Intermolecular Aminocarbonylation of Indoles with Amines

A cobalt(II)-catalyzed C2-H carbonylation of indoles with amines toward indole-2-carboxamides has been developed. By employing Co(OAc)2·4H2O as an inexpensive catalyst and using benzene-1,3,5-triyl triformate (TFBen) as the CO surrogate, a variety of indole-2-carboxamide derivatives were produced in moderate to high yields. Additionally, several bioactive-molecule-related compounds can be applied as substrates, as well.

Oxidative carbonylation of N-protected indoles by Rh(III)-zeolites

The oxidative carbonylation of N-protected indoles was investigated to directly synthesize indole-3-carboxylic acids. Using Rh(III)-zeolites as heterogeneous catalysts, the single-site Rh-species reach unprecedented activities (>100 turnovers), while the metal is readily recovered after reaction. X-ray absorption spectroscopy (XAS) provided evidence for site-isolation of Rh(III) species on the zeolite.

3-Alkyl-2-pyridyl Directing Group-Enabled C2 Selective C-H Silylation of Indoles and Pyrroles via an Iridium Catalyst

An iridium-catalyzed, directing group-enabled site selective intra- and intermolecular silylation of indoles and pyrroles with hydrosilanes has been developed under ligand-free conditions. Fine-tuning of the removable 3-alkyl-2-pyridyl directing group was found to be crucial for achieving high yields for C2-silylated indole and pyrrole products. Moreover, the scalability was demonstrated, and further transformations of the silylation products were achieved.

Ruthenium(II)-Catalyzed Highly Chemo- and Regioselective Oxidative C6 Alkenylation of Indole-7-carboxamides

We disclosed the first efficient method for highly chemo- and regioselective C6 alkenylation of indole-7-carboxamides using inexpensive Ru(II) catalyst through chelation assisted C-H bond activation. Electronically diverse indole-7-carboxamides and alkenes react efficiently to produce a wide range of C6 alkenyl indole derivatives. Further the C6 alkenyl indole-7-carboxamides modified to their derivatives through simple chemical transformations. The observed regioselectivity and kinetics has been evidenced by deuterium incorporation and intermolecular competitive studies. In addition, for mechanistic insights, the intermediates were analyzed by HRMS.

Synthesis of functionalized pyrrole derivatives via diverse cyclization of azomethine ylide and olefins

The azomethine ylides are generally used in 1,3-dipolar cycloadditions with various dipolarophiles. In this work, a new and diverse route has been developed for the azomethine ylides, for synthesis of novel pyrrole derivatives. The azomethine ylide, produced via C-H activation of unreactive C(sp³)-H bond of 2-methylquinoline, by molecular iodine, in the presence of pyridine. Herein, we represent novel pyrrole derivatives, synthesized from the reaction of pyridinium ylide with olefins, which formed via a reaction of isatin, dialkyl acetylenedicarboxylate derivatives and pyridine as a base in moderate to excellent yields. Various features of this cyclization, discussed.

Redox-Neutral 1,3-Dipolar Cycloaddition of 2H-Azirines with 2,4,6-Triarylpyrylium Salts under Visible Light Irradiation

A novel visible light mediated redox-neutral 1,3-dipolar cycloaddition of 2H-azirines with 2,4,6-triarylpyrylium tetrafluoroborate salts providing tetrasubstituted pyrroles has been developed. The 2,4,6-triarylpyrylium salt acts as dipolarophile as well as photosensitizer in the reaction, under blue light irradiation. The control experiments indicated single electron oxidation of 2H-azirines by photoexcited pyrylium salts, followed by coupling between an azaallenyl radical cation and triarylpyranyl radical as the key mechanistic feature. The mild conditions, wide substrate scope, and complete regioselectivity are the noticeable attributes of the reaction.

Pd-Catalyzed sp3 C-H alkoxycarbonylation of 8-methylquinolines using Mo(CO)6 as a CO surrogate

A Pd(ii)-catalyzed three-component sp³ C-H alkoxycarbonylation of 8-methylquinonlines (8-MQs) with alcohols is accomplished using the colorless crystalline Mo(CO)₆ as a CO source. The protocol is compatible with a wide range of 8-MQs and alcohols, furnishing the carbonylated adducts in moderate to good yields. The substrate scope, functional group tolerance and natural product mutation are the important practical features.

C4-arylation and domino C4-arylation/3,2-carbonyl migration of indoles by tuning Pd catalytic modes: Pd(i)-Pd(ii) catalysis vs. Pd(ii) catalysis

Efficient C4-arylation and domino C4-arylation/3,2-carbonyl migration of indoles have been developed. The former route enables C4-arylation in a highly efficient and mild manner and the latter route provides an alternative straightforward protocol for synthesis of C2/C4 disubstituted indoles. The mechanism studies imply that the different reaction pathways were tuned by the distinct acid additives, which led to either the Pd(i)-Pd(ii) pathway or Pd(ii) catalysis.

Palladium-catalyzed regioselective direct C–H bond alkoxycarbonylation of 2-arylimidazo[1,2-a]pyridines

A palladium catalyzed method for regioselective alkoxycarbonylation of imidazo[1,2-a]pyridines using W(CO)₆, as the CO source, and alcohols has been reported.

External oxidant-compatible phosphorus(III)-directed site-selective C-H carbonylation

The first development of an external oxidant-compatible system involving a phosphorus(III)-directed C-H functionalization has been uncovered. An efficient C-H esterification of indoles with CO and alcohols has been reported in which the high reactivity and the exclusive C7-selectivity derives from the selection of a P(III)-directing group and the utilization of benzoquinone as an external oxidant with palladium catalysis. This strategy shows many advantages, involving an easily accessible and removable directing group, the use of cheap carbonylation sources, a broad substrate scope, and excellent positional selectivity. Two cyclopalladated intermediates were confirmed by x-ray analysis, uncovering key mechanistic features of this P(III)-directed C-H metalation event.

RhCl3·3H2O-Catalyzed C7-Selective C-H Carbonylation of Indolines with CO and Alcohols

An attractive method for the synthesis of indoline-7-carboxylates through RhCl₃-catalyzed C-H carbonylation of indolines with CO and alcohols was developed. The copper-based oxidant and removable pyrimidyl directing group played important roles in achieving high-level yields of the title products. Based on control experiments, a possible catalytic cycle was proposed.