An expedient synthesis of 3-substituted indoles via reductive alkylation with ketones

Catalytic and Chemodivergent Synthesis of 1-Substituted 9H-Pyrrolo[1,2-a]indoles via Annulation of β-CF3 Enones with 3-Substituted Indoles

Chemodivergent reactions are more advantageous in organic synthesis that yield diversely functionalized scaffolds from common starting materials. Herein, we report an efficient metal-free chemodivergent protocol for the synthesis of 1-substituted 9H-pyrrolo[1,2-a]indole derivatives in the presence of catalytic amounts of Lewis acid/Brønsted acid conditions using 3-substituted indoles and β-trifluoromethyl-α,β-unsaturated ketones. Fine-tuning of the catalyst and solvent system in the reaction conditions deliver the trifluoromethyl, trifluoroethylcarboxylate, or carboxylic acid substituents on the C1-position of 9H-pyrrolo[1,2-a]indole derivatives in situ. It is postulated that the solvent and LA/BA catalyst interaction was found to be crucial for the catalytic C-F activation in these transformations.

Palladium-Catalyzed Asymmetric Hydrogenation of Unprotected 3-Substituted Indoles

A palladium-catalyzed asymmetric hydrogenation of unprotected 3-substituted indoles was developed, providing a series of 3-substituted indolines in excellent yields with ≤94.4:5.6 er. The large sterically hindered bisphosphine ligand played a crucial role in the enantioselective control. In addition, the gram-scale hydrogenation experiment and product derivatizations were performed successfully.

A Metal-Free Reductive N-Alkylation of Indoles with Aldehydes

A simple metal-free method has been developed for the reductive N-alkylation of indoles employing aldehydes as the alkylating agent and inexpensive Et₃SiH as the reductant. A wide range of aromatic and aliphatic aldehydes are viable substrates along with a variety of substituted indoles. In addition, the method was applied to a one-pot sequential 1,3-alkylation of a substituted indole and successfully demonstrated on a 100 mmol scale.

Pd-Catalyzed Alkene Carboheteroarylation Reactions for the Synthesis of 3-Cyclopentylindole Derivatives

The Pd-catalyzed alkene carboheteroarylation of aryl and alkenyl triflate electrophiles bearing pendant alkenes with heteroaromatic nucleophiles affords substituted carbocycles with 3-indolyl or 3-pyrrolyl groups. The products are obtained in moderate to good yields, and the use of alkenyl triflate substrates produces products with high diastereoselectivities. The transformation is believed to proceed via a Friedel-Crafts-like reaction between the heteroaromatic nucleophile and an intermediate electrophilic palladium complex.

Cobalt-catalysed reductive C-H alkylation of indoles using carboxylic acids and molecular hydrogen

The direct CH-alkylation of indoles using carboxylic acids is presented for the first time. The catalytic system based on the combination of Co(acac)3 and 1,1,1-tris(diphenylphosphinomethyl)-ethane (Triphos, L1), in the presence of Al(OTf)3 as co-catalyst, is able to perform the reductive alkylation of 2-methyl-1H-indole with a wide range of carboxylic acids. The utility of the protocol was further demonstrated through the C3 alkylation of several substituted indole derivatives using acetic, phenylacetic or diphenylacetic acids. In addition, a careful selection of the reaction conditions allowed to perform the selective C3 alkenylation of some indole derivatives. Moreover, the alkenylation of C2 position of 3-methyl-1H-indole was also possible. Control experiments indicate that the aldehyde, in situ formed from the carboxylic acid hydrogenation, plays a central role in the overall process. This new protocol enables the direct functionalization of indoles with readily available and stable carboxylic acids using a non-precious metal based catalyst and hydrogen as reductant.

Bismuth nitrate-promoted disproportionative condensation of indoles with cyclohexanone: a new-type azafulvenium reactivity of indole

The bismuth nitrate-promoted disproportionative condensation of indoles with cyclohexanone in one pot, to yield C3-cyclohexyl substituted indoles and 1,3-di(1H-indol-3-yl)benzene derivatives is reported for the first time.

Synthesis of tryptamine derivatives via a direct, one-pot reductive alkylation of indoles

An efficient, one-pot reductive alkylation of indoles with N-protected aminoethyl acetals in the presence of TES/TFA is reported. It represents the first general method for the direct synthesis of tryptamine derivatives from indoles and nitrogen-functionalized acetals. This convergent and versatile approach employs safe and inexpensive reagents, proceeds under mild conditions, and tolerates several functional groups. The new procedure was efficiently applied to a gram-scale synthesis of both luzindole, a reference MT2-selective melatonin receptor antagonist, and melatonin.