Cheng-Chu Zeng

Novel Triarylimidazole Redox Catalysts: Synthesis, Electrochemical Properties, and Applicability to Electrooxidative C–H Activation

A new class of metal-free, easy to synthesize redox catalysts based on a triarylimidazole framework is described. With those synthesized thus far, one can access a potential range of ca. 410 mV. They proved to be useful mediators for the activation of benzylic C-H bonds under mild conditions.

Electrochemically Initiated Oxidative Amination of Benzoxazoles Using Tetraalkylammonium Halides As Redox Catalysts

An electrochemically promoted coupling of benzoxazoles and amines has been developed, leading directly to the formation of 2-aminobenzoxazoles. The chemistry utilizes catalytic quantities of a tetraalkylammonium halide redox catalyst and is carried out under constant current conditions in a simple undivided cell. The use of excess chemical oxidant or large amounts of supporting electrolyte is avoided. This greatly simplifies the workup and isolation process and leads to a reduction in waste.

Electrocatalytic Aziridination of Alkenes Mediated by n-Bu4NI: A Radical Pathway

Efficient electrocatalytic aziridination of alkenes has been achieved for the first time. A structurally broad range of aziridines was easily accessed using an undivided cell operated at constant current and mediated by a catalytic quantity of n-Bu4NI. The electrocatalytic reaction also proceeded in the absence of additional conducting salt. The aziridination is proposed to follow a radical mechanism.

Use of Electrochemistry in the Synthesis of Heterocyclic Structures

The preparation and transformation of heterocyclic structures have always been of great interest in organic chemistry. Electrochemical technique provides a versatile and powerful approach to the assembly of various heterocyclic structures. In this review, we examine the advance in relation to the electrochemical construction of heterocyclic compounds published since 2000 via intra- and intermolecular cyclization reactions.

Polymeric Ionic Liquid and Carbon Black Composite as a Reusable Supporting Electrolyte: Modification of the Electrode Surface

One of the major impediments to using electroorganic synthesis is the need for large amounts of a supporting electrolyte to ensure the passage of charge. Frequently this causes separation and waste problems. To address these issues, a polymeric ionic liquid-Super P carbon black composite has been formulated. The system enables electrolyses to be performed without adding an additional supporting electrolyte, and its efficient recovery and reuse. In addition, the ability of the composite to modify the electrode surface in situ leads to improved kinetics. A practical consequence is that one can decrease catalyst loading without sacrificing efficiency.

Triarylimidazole redox catalysts: electrochemical analysis and empirical correlations.

A series of triarylimidazoles was synthesized and characterized electrochemically. The synthetic route is general, providing a pathway to 30 redox mediators that exhibit a > 700 mV range of accessible potentials. Most of the triarylimidazoles display three oxidation peaks where the first redox couple is quasi-reversible. The electronic character of the substituents affects the oxidation potential. This is exemplified by a linear correlation between the first oxidation potential and the sum of the Hammett σ(+) substituent constants, as well as with a series of calculated ionization potentials. We close by putting forward a rule of thumb stating that for a given mediator, the upper limit of accessible potentials can be extended by at least 500 mV beyond the largest recorded value. A rationale, the conditions under which the rule is likely to apply, and an example are provided.

Electrochemical C–H functionalization and subsequent C–S and C–N bond formation: paired electrosynthesis of 3-amino-2-thiocyanato-α,β-unsaturated carbonyl derivatives mediated by bromide ions

An efficient paired electrosynthesis involving C–H functionalization and subsequent C–S and C–N bond formation for the assembly of valuable 3-amino-2-thiocyanato-α,β-unsaturated carbonyl derivatives has been developed. In the paired electrolysis, the amino and thiocyanato moieties originate from a single reagent or a combination of ammonium acetate and potassium isocyanate. The chemistry proceeds in a simple undivided cell employing a sub-stoichiometric amount of NH4Br that serves both as an inner sphere type redox catalyst and a supporting electrolyte; in this manner an additional conducting salt is not required. The reaction also works using a catalytic amount of NH4Br. Cyclic voltammetry and the results of control experiments demonstrate that the reaction proceeds via an anodically initiated C–H functionalization of the 1,3-dicarbonyl substrates that occurs via the electrochemical oxidation of bromide and simultaneous cathodic reduction of ammonium ions.

Formation Mechanism of NDMA from Ranitidine, Trimethylamine, and Other Tertiary Amines during Chloramination: A Computational Study

Chloramination of drinking waters has been associated with N-nitrosodimethylamine (NDMA) formation as a disinfection byproduct. NDMA is classified as a probable carcinogen and thus its formation during chloramination has recently become the focus of considerable research interest. In this study, the formation mechanisms of NDMA from ranitidine and trimethylamine (TMA), as models of tertiary amines, during chloramination were investigated by using density functional theory (DFT). A new four-step formation pathway of NDMA was proposed involving nucleophilic substitution by chloramine, oxidation, and dehydration followed by nitrosation. The results suggested that nitrosation reaction is the rate-limiting step and determines the NDMA yield for tertiary amines. When 45 other tertiary amines were examined, the proposed mechanism was found to be more applicable to aromatic tertiary amines, and there may be still some additional factors or pathways that need to be considered for aliphatic tertiary amines. The heterolytic ONN(Me)2–R+ bond dissociation energy to release NDMA and carbocation R+ was found to be a criterion for evaluating the reactivity of aromatic tertiary amines. A structure–activity study indicates that tertiary amines with benzyl, aromatic heterocyclic ring, and diene-substituted methenyl adjacent to the DMA moiety are potentially significant NDMA precursors. The findings of this study are helpful for understanding NDMA formation mechanism and predicting NDMA yield of a precursor.

Electrochemical Oxidative Amination of Sodium Sulfinates: Synthesis of Sulfonamides Mediated by NH4I as a Redox Catalyst

An efficient protocol for the synthesis of sulfonamides via the electrochemical oxidative amination of sodium sulfinates has been developed. The chemistry proceeds in a simple undivided cell employing a substoichiometric amount of NH4I that serves both as a redox catalyst and a supporting electrolyte; in this manner additional conducting salt is not required. A wide range of substrates, including aliphatic or aromatic secondary and primary amines, as well as aqueous ammonia, proved to be compatible with the protocol. Scale-up was possible, thereby demonstrating the practicality of the approach. The electrolytic process avoids the utilization of external oxidants or corrosive molecular iodine and therefore represents an environmentally benign means by which to achieve the transformation.

Electrochemically catalyzed amino-oxygenation of styrenes: n-Bu4NI induced C–N followed by a C–O bond formation cascade for the synthesis of indolines

Efficient electrochemical amino-oxygenation of styrenes has been developed for the synthesis of 3-methoxyindolines and 3-ethoxyindoline, using a simple beaker-type undivided cell with n-Bu4NI serving as a redox catalyst under constant current electrolysis (CCE) conditions. The chemistry proceeds in a paired electrolysis mode, avoiding the utilization of external oxidants and bases and therefore represents an environmentally benign means. The process also works in the absence of an additional conducting salt. Gram-scale reaction further demonstrates the practicability of the protocol. Proton NMR spectroscopy was used to demonstrate that the amino-oxygenation of N-(2-vinylphenyl)sulfonamides likely involves the initial iodoamination of the alkene, followed by nucleophilic substitution of the methoxide/ethoxide generated at the cathode.