An efficient route to 2,3-disubstituted indoles via reductive alkylation using H2 as reductant

Pd/C-Catalyzed transfer hydrogenation of N-H indoles with trifluoroethanol and tetrahydroxydiboron as the hydrogen source

Under the guidance of the known mechanism of the hydrogenation of indoles and transfer hydrogenation with tetrahydroxydiboron (B₂(OH)₄), Pd/C catalyzed transfer hydrogenation of N-H indoles with trifluoroethanol and tetrahydroxydiborane as the hydrogen source has been developed. This provides an efficient strategy and catalytic system for the reduction of un-activated N-H indoles, and N-H indolines are obtained with good to excellent yields. In addition, a series of the isotopic labelling experiments were carried out to probe the mechanism.

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.

Radical-based regioselective cross-coupling of indoles and cycloalkanes

An investigation on regiochemistry of radical functionalization of indoles using cycloalkanes through di-tert-butyl peroxide (DTBP)-promoted C(sp³)–H activation was conducted. A wide range of indoles bearing substituents at different position was functionalized directly with simple cycloalkanes in moderate to high regioselctivity.

Hydrophobically directed, catalyst-free, multi-component synthesis of functionalized 3,4-dihydroquinazolin-2(1H)-ones

A one-pot three-component reaction of o-formyl carbamate, a primary amine, a nucleophile (1H-indole or 2-napthol) in water generated a variety of functionalised 3,4-dihydroquinazolin-2(1H)-one derivatives.

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.