Beyond C2 and C3: Transition-Metal-Catalyzed C–H Functionalization of Indole

C4-H indole functionalisation: precedent and prospects

C4-decorated indoles feature in a plethora of bioactive and functional compounds of importance to natural product synthesis, material sciences, as well as crop protection and pharmaceutical industries. Traditionally, their syntheses largely involved harsh stoichiometric metalations and radical reactions. However, transition metal catalysed C-H activation has recently evolved into a powerful strategy for the late-stage diversification of indoles at the C4-H position. Modern photoredox, enzymatic and precious transition metal catalysis represent the key stimuli for developing challenging C-C and C-Het bond forming transformations under mild reaction conditions. Herein, we discuss the evolution and application of these methods for the step-economical transformations of otherwise inert C4-H bonds up to December 2017.

Copper-mediated regioselective C-H etherification of naphthylamides with arylboronic acids using water as an oxygen source

The copper-mediated regioselective C-H activation and C-O bond formation of naphthylamides with arylboronic acids has been developed using water as an oxygen source. The kinetic isotope study suggests that C-H bond activation is the rate-determining step. The H2O18 labelling experiment reveals the incorporation of oxygen from water. The substrate scope, functional group diversity and post synthetic utilities are the important practical features.

Expedient cobalt(ii)-catalyzed site-selective C7-arylation of indolines with arylboronic acids

Cobalt(ii)-catalyzed pyrimidyl directing group-assisted C7 arylation of indolines with arylboronic acids has been developed using Mn(OAc)₂·4H₂O as an oxidant. The use of cobalt(ii)-PCy₃ as a catalyst and broad substrate scope are the important practical features.

Iron promoted C3–H nitration of 2H-indazole: direct access to 3-nitro-2H-indazoles

An efficient C3–H functionalization of indazole has been demonstrated. Notably, this method involves chelation-free radical C–H nitration on 2H-indazole. The radical mechanism was confirmed by control experiments and quantum chemical calculations. The synthetic utility has been proven by the synthesis of bio-relevant benzimidazoindazoles via reductive cyclization.

Synthesis of sulfonated naphthols via C–H bond functionalization with the insertion of sulfur dioxide

A three-component reaction of naphthols, sulfur dioxide, and aryldiazonium tetrafluoroborates catalyzed by iron(iii) chloride through a radical process via direct C–H functionalization is developed, leading to diverse sulfonated naphthols in good yields.

Rhodium(iii)-catalyzed C–H activation at the C4-position of indole: switchable hydroarylation and oxidative Heck-type reactions of maleimides

A Rh(iii)-catalyzed C–H activation of indole at the C4-position leading to novel and switchable functionalization has been reported by employing a weakly co-ordinating COCF₃ group as a directing group.

Oxidative coupling strategies for the synthesis of indole alkaloids

Direct functionalization of indole through oxidative coupling reactions with enolates or phenols provides a powerful tool for assembling indole alkaloids.

Ruthenium(ii)-catalyzed selective C–H difluoroalkylation of aniline derivatives with pyrimidyl auxiliaries

A ruthenium-catalyzed alternative para- and meta-difluoroalkylation of anilines is herein reported.

Modular synthesis of heptaarylindole

The first synthesis of heptaarylindole (HAI) has been accomplished using a coupling/ring transformation strategy.

Palladium-Catalyzed Direct C-H Functionalization of Indoles with the Insertion of Sulfur Dioxide: Synthesis of 2-Sulfonated Indoles

A palladium-catalyzed direct C-H bond sulfonylation of indoles with the insertion of sulfur dioxide is achieved through a three-component reaction of 1-(pyridin-2-yl)indoles, DABCO·(SO₂)₂, and aryldiazonium tetrafluoroborates under mild conditions. Diverse 2-sulfonated indoles are generated by using 10 mol % of palladium(II) bromide as the catalyst at room temperature. This synthetic approach is efficient by merging palladium catalysis and insertion of sulfur dioxide via a radical process. 2-Pyrimidinyl can be used as the directing group well as for the C-H bond sulfonylation. Additionally, the directing group can be easily removed.

Palladium-catalyzed direct sulfonylation of C–H bonds with the insertion of sulfur dioxide

A palladium-catalyzed direct sulfonylation of C–H bonds with the insertion of sulfur dioxide under mild conditions is reported. The sulfonylative couplings with the insertion of sulfur dioxide into C–H bonds are effective, and two classes of sulfonylative products are formed in moderate to good yields by the combination of radical chemistry and palladium-catalyzed C–H activation.

Ruthenium catalyzed remote C4-selective C–H functionalisation of carbazoles via σ-activation

We report the C4-selective C–H alkylation of carbazole derivatives furnished with a pyrimidine directing group at C1.

Regiocontrolled direct C4 and C2-methyl thiolation of indoles under rhodium-catalyzed mild conditions

A straightforward and mild Rh(iii)-catalyzed remote C4 (sp²) and C2 (sp³)-methyl thiolation of an indole core was developed.

Ruthenium-catalysed σ-activation for remote meta-selective C–H functionalisation

Ruthenium-catalysed σ-activation has become a major asset in accessing remote meta-C–H-functionalisation of a variety of arenes.