Organic Synthesis by Electrolysis. VI. Anodic Oxidation of Arylcyclopropanes

Electrochemical Formation of Oxazolines by 1,3-Oxyfluorination of Non-activated Cyclopropanes

The C-C bond in non-activated cyclopropanes can be intramolecularly cleaved with an electrochemically generated amidyl radical forming oxazolines. In the presence of TBABF4, this provides 1,3-oxyfluorination products. C-C bond cleavage of cyclopropane proceeds with inversion of the configuration, suggesting an intramolecular homolytic substitution (SHi) mechanism. The performance of TBABF4 as an efficient fluoride source was explained by accumulation of the BF4- anion at the anode surface, at which a carbocation is formed by the oxidation of the C-centered radical.

Electrochemical Synthesis and Reactivity of Three-Membered Strained Carbo- and Heterocycles

Three-membered carbocyclic and heterocyclic ring structures are versatile synthetic building blocks in organic synthesis with biological importance. Moreover, the inherent strain of these three-membered rings leads to their ring-opening functionalization through C->C, C->N, and C-O bond cleavage. Traditional synthesis and ring-opening methods for these molecules require the use of acid catalysts or transition metals. Recently, electro-organic synthesis has emerged as a powerful tool for initiating new chemical transformations. In this review, the synthetic and mechanistic aspects of electro-mediated synthesis and ring-opening functionalization of three-membered carbo- and heterocycles are highlighted.

Electrochemical 1,3-Alkyloxylimidation of Arylcyclopropane Radical Cations: Four-Component Access to Imide Derivatives

Herein, a general electrochemical radical-cation-mediated four-component ring-opening 1,3-alkyloxylimidation of arylcyclopropanes, acetonitrile, carboxylic acids, and alcohols is described, providing a facile and sustainable approach to quickly construct structurally diverse imide derivatives from easily available raw materials in an operationally simple undivided cell. This metal-catalyst- and oxidant-free single-electron oxidation strategy offers a green alternative for the formation of highly reactive cyclopropane-derived radical cations, and this protocol features a broad functional group tolerance.

Electrochemical Oxidation Enables Regioselective 1,3-Hydroxyfunctionalization of Cyclopropanes

The direct construction of 1,3-hydroxyfunctionalized molecules is still a significant challenge, as they can currently be obtained through multiple synthetic steps. Herein, we report a general and efficient 1,3-hydroxyfunctionalization of arylcyclopropanes by electrochemical oxidation with a strategic choice of nucleophiles and H2O. 1,3-Amino alcohols, 1,3-alkynyl alcohols, 1,3-hydroxyesters, and 1,3-halo alcohols are achieved with high levels of chemo- and regio-selectivity, opening a new dimension for 1,3-difunctionalization reaction.

Deconstructive Functionalization of Unstrained Cycloalkanols via Electrochemically Generated Aromatic Radical Cations

Herein we report an electrochemical approach for the deconstructive functionalization of cycloalkanols, where various alcohols, carboxylic acids, and N-heterocycles are employed as nucleophiles. The method has been demonstrated across a broad range of cycloalkanol substrates, including various ring sizes and substituents, to access useful remotely functionalized ketone products (36 examples). The method was demonstrated on a gram scale via single-pass continuous flow, which exhibited increased productivity in relation to the batch process.

Critical role of oxygen for rapid degradation of organic contaminants in permanganate/bisulfite process

Organic contaminants can be oxidized rapidly in permanganate/bisulfite (PM/BS) process due to generation of Mn(III)_aq. However, the influence of O₂ and its role in contaminants degradation in PM/BS process are poorly understood. Thus, the influence of O₂ concentration on phenol removal was quantified and isotopic oxygen tracers (¹⁸O-labeled water and ¹⁸O₂) were employed to further unravel the mechanisms of PM/BS process. The introduction of O₂ to PM/BS process appreciably improved phenol oxidation. Under oxic conditions the oxidation products of bisulfite included SO₄^2- and HSO₅^- while SO₄^2- was the single stable oxidation product under anaerobic conditions. The oxygen isotope labeling experiments confirmed that HSO₅^- originated from the oxidation of SO₃^- by O₂ to SO₅^- and the following reduction of SO₅^- by HSO₃^-. Incorporation of oxygen isotope from H₂¹⁸O to MnO₂ due to the disproportionation of Mn(III)_aq was observed, which verified the generation of Mn(III)_aq in PM/BS process. The critical influence of O₂ on PM/BS process was mainly associated with rapid oxidation of SO₃^- by O₂ to SO₅^-, which otherwise competed with target contaminants for Mn(III)_aq. Oxygen from H₂O and/or O₂ can also be incorporated in the degradation products of organics in PM/BS process.

Nanosecond Redox Equilibrium Method for Determining Oxidation Potentials in Organic Media

A general, nanosecond equilibrium method is described for determining thermodynamically meaningful oxidation potentials in organic media for compounds that form highly reactive cation radicals upon one-electron oxidation. The method provides oxidation potentials with unusually high precision and accuracy. Redox ladders have been constructed of appropriate reference compounds in dichloromethane and in acetonitrile that can be used to set up electron-transfer equilibria with compounds with unknown oxidation potentials. The method has been successfully applied to determining equilibrium oxidation potentials for a series of aryl-alkylcyclopropanes, whose oxidation potentials were imprecisely known previously. Structure-property trends for oxidation potentials of the cyclopropanes are discussed.

Electron transfer in P450 mechanisms. Microsomal metabolism of cyclopropylbenzene and p-cyclopropylanisole

1. The metabolism of cyclopropylbenzene (1a) and 4-cyclopropylanisole (1b) was studied using liver microsomal preparations from control, phenobarbital- and beta-naphthoflavone treated rats. 2. With all three types of microsomes 1a was metabolized by benzylic hydroxylation to give 1-phenylcyclopropanol and by aromatic hydroxylation at C-4; the former predominated by a factor of 2-4. BNF-induced microsomes also formed 2-cyclopropylphenol. No cyclopropyl ring-opened metabolites of 1a, including benzoic acid, were detected in any of the incubations. 3. With PB-induced microsomes 1b underwent O-demethylation (90%) and benzylic hydroxylation; no other metabolites were detected. 4. Progress curves for metabolism of 1a are markedly nonlinear after only limited conversion of substrate, suggesting the possibility that 1a, like other cyclopropyl compounds, could be a suicide substrate for one or more isozymes of P450. 5. For both 1a and b, metabolite formation and enzyme inactivation can be explained by conventional P450 reaction mechanisms not involving electron abstraction.