Synthesis and C2-functionalization of indoles with allylic acetates under rhodium catalysis

Transition metal-catalyzed C–H functionalizations of indoles

This review summarises a wide range of transformations on the indole skeleton, including arylation, alkenylation, alkynylation, acylation, nitration, borylation, and amidation, using transition-metal catalyzed C–H functionalization as the key step.

Polarclean/Water as a Safe and Recoverable Medium for Selective C2-Arylation of Indoles Catalyzed by Pd/C

Herein, we report the use of nontoxic, water-miscible Polarclean as a safe dipolar aprotic solvent for the metal-catalyzed direct C2-H arylation of indoles using Pd/C as a catalyst. The developed method allows reaching excellent yields and regioselectivities, and it tolerates various substituents on both indole and diaryliodonium salt scaffolds. Polarclean is fully recoverable and reusable; it shows a very low leaching of the metal catalyst, allowing its complete recovery and reuse for at least six representative reaction runs.

Synthesis of Functionalized Indoles via Palladium-Catalyzed Cyclization of N-(2-allylphenyl) Benzamide: A Method for Synthesis of Indomethacin Precursor

We developed an efficient method for synthesis of substituted N-benzoylindole via Pd(II)-catalyzed C–H functionalization of substituted N-(2-allylphenyl)benzamide. The reaction showed a broad substrate scope (including N-acetyl and N-Ts substrates) and substituted indoles were obtained in good to excellent yields. The most distinctive feature of this method lies in the high selectivity for N-benzoylindole over benzoxazine, and this is the first example of Pd(II)-catalyzed synthesis of substituted N-benzoylindole. Notably, this new method was applied for the synthesis of key intermediate of indomethacin.

Mild and Practical Indole C2 Allylation by Allylboration of in situ Generated 3-Chloroindolenines

C2 allylation of indole derivatives is a challenging but important transformation given the biological relevance of the products. Herein we report a selective C2 allylation strategy that proceeds via allylboration of in situ-generated 3-chloroindolenines. The reaction is mild, practical, and compatible with a wide range of C3-substituted indoles. As allylboronates are readily accessible from commercial precursors, various substituted allyl moieties can be introduced using the same protocol. To showcase the utility of this method we applied it to the synthesis of the natural product, tryprostatin B.

Rhodium-catalyzed regioselective ortho-allylation of 2,4-diarylquinazolines with allylic acetate

In this paper, a rhodium-catalyzed regioselective reaction of 2,4-diarylquinazolines with allylic acetate is described, leading to allylated quinazoline derivatives with high efficiency and good tolerance of functional groups. By switching to 2-arylquinazolin-4-ones, tandem C–H activation and aza-Michael addition are involved, leading to 1,6-dimethyl-5H-isoquinolino[1,2-b]quinazolin-8(6H)-one derivatives with high efficiency.

A novel Pd-catalysed sequential carbonylation/cyclization approach toward bis-N-heterocycles: rationalization by electronic structure calculations

An unprecedented palladium-catalysed sequential aminocarbonylation/cyclization synthetic strategy, using carbon monoxide and structurally different aliphatic diamines as N-nucleophiles, gives access, in one pot, to a new family of indole-based N-heterocyclic derivatives (hydropyrazinones, benzodiazepinones and hydroquinoxalines). Optimization of the reaction conditions towards double carbonylation (P _CO = 30 bar, T = 80°C, iodoindole/diamine ratio = 1 : 1.5, toluene as solvent) allowed the target cyclic products, which are formed in situ via intramolecular cyclization of the ketocarboxamide intermediates, to be obtained through a nucleophilic addition/elimination reaction with the pendant terminal amine groups. The structure of the diamine nucleophile was revealed to affect the reaction's selectivity, with the best yields for the cyclic products being obtained in the presence of (1S,2S)-(+)-cyclohexane-1,2-diamine (a) as the nucleophile, using either 5- or 7-iodoindole as the substrate. The reaction's selectivity was rationalized based on electronic structure calculations, which explain the effect of the diamine structure on the predominant formation of the cyclic products.

Regioselective indole C2-alkylation using β-CF3-substituted enones under redox neutral Rh(iii) catalysis

Rhodium(iii) catalyzed C–H alkylation at the C2-position of indole with β-trifluoromethyl-α,β-unsaturated ketones with a broad range of substrates and excellent functional group compatibility is described.

Regioselective synthesis of multisubstituted isoquinolones and pyridones via Rh(iii)-catalyzed annulation reactions

A mild and efficient Rh(iii)-catalyzed regioselective synthesis of isoquinolones and pyridones from N-methoxybenzamide or N-methoxymethacrylamide and diazo compounds has been developed, through tandem C–H activation, cyclization, and condensation.

Rh(III)-catalyzed oxidative coupling of 1,2-disubstituted arylhydrazines and olefins: a new strategy for 2,3-dihydro-1H-indazoles

A rhodium(III)-catalyzed oxidative olefination of 1,2-disubstituted arylhydrazines with alkenes via sp(2) C-H bond activation followed by an intramolecular aza-Michael reaction is described. This strategy allows the direct and efficient construction of highly substituted 2,3-dihydro-1H-indazole scaffolds.

Direct access to isoindolines through tandem Rh(III)-catalyzed alkenylation and cyclization of N-benzyltriflamides

The rhodium-catalyzed oxidative alkenylation of N-benzyltriflamides with olefins followed by an intramolecular cyclization via C-H bond activation is described. This method results in the direct and efficient synthesis of highly substituted isoindoline frameworks.