Regioselective Ruthenium-Catalyzed Carbonylative Direct Arylation of Five-Membered and Condensed Heterocycles

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.

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.

Microwave-assisted palladium catalysed C-H acylation with aldehydes: synthesis and diversification of 3-acylthiophenes

The use of MW allows the efficient palladium(II)-catalysed C-3 acylation of thiophenes with aldehydes via C(sp²)-H activation for the synthesis of (cyclo)alkyl/aryl thienyl ketones (43 examples). Compared to standard thermal conditions, the use of MW reduces the reaction time (15 to 30 min vs. 1 to 3 hours), leading to improved yields of the ketones (up to 92%). The control of positional selectivity is achieved by 2-pyridinyl and 2-pyrimidyl ortho-directing groups at C-2 of the thiophene scaffold. To show the synthetic applicability, selected ketones were subjected to further transformations, including intramolecular reactions to directly embed the directing group in the core structure of the new molecule.

Synthesis of Bis-heteroaryls Using Grignard Reagents and Pyridylsulfonium Salts

Herein are reported ligand-coupling reactions of Grignard reagents with pyridylsulfonium salts. The method has wide functional group tolerance and enables the formation of bis-heterocycle linkages, including 2,4′-, 2,3′-, and 2,2′-bipyridines, as well as pyridines linked to pyrimidines, pyrazines, isoxazoles, and benzothiophenes. The methodology was successfully applied to the synthesis of the natural products caerulomycin A and E.

Cp*Co(III)-Catalyzed Regioselective C2 Amidation of Indoles Using Acyl Azides

A cobalt-catalyzed C2-selective amidation of indoles using acyl azides has been accomplished. Isotope experiments suggest that C-H activation is reversible. The use of sustainable Co catalysis, functional group diversity, substrate scope, and regioselectivity are the important practical features.

Ruthenium(II)-Catalyzed Regioselective C-8 Hydroxylation of 1,2,3,4-Tetrahydroquinolines

Ru(II)-catalyzed chelation-assisted highly regioselective C8-hydroxylation of 1,2,3,4-tretrahydroquinolines has been developed. Various 1,2,3,4-tetrahydroquinolines underwent smooth C8-H hydroxylation with cheap and safe K₂S₂O₈ as the oxidant and oxygen source to furnish the corresponding products in good to excellent yields with high tolerance of the functional groups. The choice of a readily installable and removable N-pyrimidyl directing group is the key to catalysis. Mechanistic studies suggest the involvement of a six-membered ruthenacycle intermediate in the catalytic cycle. The method can also be extended to the direct hydroxylation of other (hetero)arene C-H bonds.

Rhodium-catalysed alkoxylation/acetalization of diazo compounds: one-step synthesis of highly functionalised quaternary carbon centres

An intermolecular tandem reaction for the rapid build-up of densely functionalised α-alkoxy-β-oxo-esters has been developed. This novel process applies the easy to handle trimethyl orthoformate as a C1-building block in the rhodium(II)-catalysed alkoxylation/acetalization of donor-acceptor substituted diazo compounds. The concomitant C-O/C-C bond formation reaction gives products with unique quaternary carbon centers, substituted by groups of different oxidation level (ester, protected aldehyde and alkoxide).

Ortho C-H Acylation of Aryl Iodides by Palladium/Norbornene Catalysis

Reported herein is a palladium/norbornene-catalyzed ortho-arene acylation of aryl iodides by a Catellani-type C-H functionalization. This transformation is enabled by isopropyl carbonate anhydrides, which serve as both an acyl cation equivalent and a hydride source.

Ru-catalysed C-H arylation of indoles and pyrroles with boronic acids: scope and mechanistic studies

The Ru-catalysed C2–H arylation of indoles and pyrroles by using boronic acids under oxidative conditions is reported. This reaction can be applied to tryptophan derivatives and tolerates a wide range of functional groups on both coupling partners, including bromides and iodides, which can be further derivatised selectively. New indole-based ruthenacyclic complexes are described and investigated as possible intermediates in the reaction. Mechanistic studies suggest the on-cycle intermediates do not possess a para-cymene ligand and that the on-cycle metalation occurs through an electrophilic attack by the Ru centre.

Regioselective oxidative Pd-catalysed coupling of alkylboronic acids with pyridin-2-yl-substituted heterocycles

A total of 19 alkylated heterocycles (thiophenes, benzo-thiophenes, pyrroles, furans) were prepared (36–99% yield) from the respective pyridin-2-yl-substituted precursors employing alkylboronic acids as the alkylating reagents in an oxidative (Ag₂CO₃ and 2,6-dimethyl-1,4-benzoquinone as oxidants) Pd-catalysed coupling reaction.