An Electrochemical Route for Special Oxidative Ring-Opening of Indoles

Electrosynthesis of Spiro-indolenines via Dearomative Arylation of Indoles in Batch and Continuous Flow

An electrosynthesis of spiro-indolenines in batch and continuous flow was achieved through dearomative arylation of indoles with good functional group compatibility. User-friendly undivided cells were used under catalyst- and oxidant-free conditions. Moreover, the use of a flow electrolysis cell gave high daily productivity and excellent scale-up potential under less supporting electrolyte and higher substrate concentration conditions.

Electrooxidative Hofmann Rearrangement of Phthalimides to Access Anthranilate Derivatives

A simple and efficient electrooxidative Hofmann rearrangement reaction of phthalimides was developed. Anthranilate derivatives were synthesized in moderate to good yields under green and mild conditions using phthalimides as a rearrangement precursor. This approach not only provides a strategy for synthesizing anthranilates and deuterated anthranilate derivatives with high deuteration efficiency but also realizes efficient conversion at the gram scale. A possible reaction mechanism is proposed.

Electrochemical oxidation of 3-substituted indoles

2-Oxindoles are an abundant heteroaromatic motif in natural products and pharmaceuticals. An appealing method for accessing 2-oxindoles is by oxidation of the corresponding indole, a transformation currently executed using stoichiometric quantities of unsafe chemical oxidants that can also form unwanted side-products. Herein, we report that 3-substituted indoles undergo a logistically straightforward, electrochemical oxidation to the corresponding 2-oxindole in the presence of potassium bromide (>20 examples), with only traces of the oxidative dimer detected. Cyclic voltammetry and control studies infer that the reaction proceeds by electrochemical generation of elemental bromine (Br₂) that upon reaction with indole, followed by hydrolysis, delivers the 2-oxindole. This procedure is an appealing alternative to existing methods used to access 2-oxindoles by oxidation of the parent indole.

Copper-Catalyzed Aerobic Selective Oxidation of Tetrahydrocarbolines

We report a safe and convenient open-flask copper-catalyzed selective oxidation/functionalization methodology for tetrahydrocarbolines and tetrahydro-β-carbolines that employs atmospheric O₂ as the terminal oxidant. The system is applicable to oxidative rearrangement of tetrahydro-β-carbolines, tetrahydrocarboline oxidation to α-alkoxy carbazoles and spirooxindoles, and Witkop oxidation. Mechanistic experiments indicated that a single-electron oxidation process is responsible for the tunable selectivity control. This copper-catalysis protocol represents a significant advance in the field of indole oxidation.

Electrochemical oxidative rearrangement of tetrahydro-β-carbolines in a zero-gap flow cell

Oxidative rearrangement of tetrahydro-β-carbolines (THβCs) is one of the most efficient methods for the synthesis of biologically active spirooxindoles, including natural products and drug molecules. Here, we report the first electrochemical approach to achieve this important organic transformation in a flow cell. The key to the high efficiency was the use of a multifunctional LiBr electrolyte, where the bromide (Br⁻) ion acts as a mediator and catalyst and lithium ion (Li⁺) acts as a likely hydrophilic spectator, which might considerably reduce diffusion of THβCs into the double layer and thus prevent possible nonselective electrode oxidation of indoles. Additionally, we build a zero-gap flow cell to speed up mass transport and minimize concentration polarization, simultaneously achieving a high faradaic efficiency (FE) of 96% and an outstanding productivity of 0.144 mmol (h⁻¹ cm⁻²). This electrochemical method is demonstrated with twenty substrates, offering a general, green path towards bioactive spirooxindoles without using hazardous oxidants.

A zero-gap flow cell was designed for the first electro-oxidative rearrangement of tetrahydro-β-carbolines to spirooxindoles with high yield, faradaic efficiency and productivity when LiBr was discovered as a bi-functional mediator and catalyst.