Electrochemical synthesis of dendritic diarylcarbenium ion pools

Electrochemical Assembly for Synthesis of Middle-Sized Organic Molecules

Electrochemical methods offer a powerful, reliable, and environmentally benign approach for the synthesis of small organic molecules, and such methods are useful not only for the transformation of small molecules, but also for the preparation of oligomers and polymers. Electrochemical assembly is a concept that allows structurally well-defined middle-sized organic molecules to be synthesized by applying electrochemical methods. The preparation of dendrimers, dendronized polymers, and oligosaccharides are introduced as examples of such an approach. Automated electrochemical assembly of oligosaccharides is also demonstrated using the electrochemical synthesizer developed by our group.

Electrochemical strategies for C-H functionalization and C-N bond formation

Conventional methods for carrying out carbon-hydrogen functionalization and carbon-nitrogen bond formation are typically conducted at elevated temperatures, and rely on expensive catalysts as well as the use of stoichiometric, and perhaps toxic, oxidants. In this regard, electrochemical synthesis has recently been recognized as a sustainable and scalable strategy for the construction of challenging carbon-carbon and carbon-heteroatom bonds. Here, electrosynthesis has proven to be an environmentally benign, highly effective and versatile platform for achieving a wide range of nonclassical bond disconnections via generation of radical intermediates under mild reaction conditions. This review provides an overview on the use of anodic electrochemical methods for expediting the development of carbon-hydrogen functionalization and carbon-nitrogen bond formation strategies. Emphasis is placed on methodology development and mechanistic insight and aims to provide inspiration for future synthetic applications in the field of electrosynthesis.

Synthetic Organic Electrochemical Methods Since 2000: On the Verge of a Renaissance

Electrochemistry represents one of the most intimate ways of interacting with molecules. This review discusses advances in synthetic organic electrochemistry since 2000. Enabling methods and synthetic applications are analyzed alongside innate advantages as well as future challenges of electroorganic chemistry.

Cathodic Aromatic C,C Cross-Coupling Reaction via Single Electron Transfer Pathway

We have successfully developed a novel cathodic cross-coupling reaction of aryl halides with arenes. Utilization of the cathodic single electron transfer (SET) mechanism for activation of aryl halides enables the cross-coupling reaction to proceed without the need for any transition metal catalysts or single electron donors in a mild condition. The SET from a cathode to an aryl halide initiates a radical chain by giving an anion radical of the aryl halide. The following propagation cycle also consists entirely of anion radical intermediates.

Synthesis of bench-stable diarylmethylium tetrafluoroborates

A representative number of bench-stable nonsymmetric diarylcarbenium tetrafluoroborates have been isolated via the direct coupling of aryl (or heteroaryl) aldehydes and N-heteroarenes and fully characterized. They have proven to be highly stable in the presence of both EDG and EWG substituents. An (E)-iminium vinylogous substructure has been shown as the common cation scaffold by X-ray analysis and by NOE determination.

Redox active dendronized polystyrenes equipped with peripheral triarylamines

Dendronized polystyrene having peripheral bromo groups was prepared from the dendronization of unfunctionalized polystyrene with dendritic diarylcarbenium ions bearing peripheral bromo groups using the “cation pool” method. The palladium-catalyzed amination of the peripheral bromo groups with diarylamine gave dendronized polystyrene equipped with peripheral triarylamines, which exhibited two sets of reversible redox peaks in the cyclic voltammetry curves.

Palladium-catalyzed C(sp3)-H arylation of diarylmethanes at room temperature: synthesis of triarylmethanes via deprotonative-cross-coupling processes

Although metal-catalyzed direct arylation reactions of non- or weakly acidic C-H bonds have recently received much attention, chemists have relied heavily on substrates with appropriately placed directing groups to steer reactivity. To date, examples of intermolecular arylation of unactivated C(sp(3))-H bonds in the absence of a directing group remain scarce. We report herein the first general, high-yielding, and scalable method for palladium-catalyzed C(sp(3))-H arylation of simple diarylmethane derivatives with aryl bromides at room temperature. This method facilitates access to a variety of sterically and electronically diverse hetero- and nonheteroaryl-containing triarylmethanes, a class of compounds with various applications and interesting biological activity. Key to the success of this approach is an in situ metalation of the substrate via C-H deprotonation under catalytic cross-coupling conditions, which is referred to as a deprotonative-cross-coupling process (DCCP). Base and catalyst identification were performed by high-throughput experimentation (HTE) and led to a unique base/catalyst combination [KN(SiMe(3))(2)/Pd-NiXantphos] that proved to efficiently promote the room-temperature DCCP of diarylmethanes. Additionally, the DCCP exhibits remarkable chemoselectivity in the presence of substrates that are known to undergo O-, N-, enolate-, and C(sp(2))-H arylation.

Metal- and chemical-oxidant-free C-H/C-H cross-coupling of aromatic compounds: the use of radical-cation pools

Pool and couple: A method for oxidative C-H/C-H cross-coupling has been developed using "radical-cation pools". Aromatic compounds react with aryl radical cations, which are generated and accumulated by low-temperature electrolysis (see scheme). This method avoids both the nonselective oxidation of substrates and oxidation of products and effects the C-H/C-H cross-coupling of aromatic compounds without metal complexes and chemical oxidants.

Oxidative hydroxylation mediated by alkoxysulfonium ions

Oxidative hydroxylation of toluene derivatives via alkoxysulfonium ion intermediates was achieved by integration of anodic oxidation and hydrolysis to give benzyl alcohols which are also susceptible to oxidation. Alkenes were also oxidized to give 1,2-diols without overoxidation. The integration of electrochemical oxidative cyclization and hydrolysis was achieved using alkenes bearing a nitrogen atom in an appropriate position to give cyclic β-amino-substituted alcohols.

Integrated electrochemical-chemical oxidation mediated by alkoxysulfonium ions

Generation of carbocations by the "cation pool" method followed by reaction with dimethyl sulfoxide (DMSO) gave the corresponding alkoxysulfonium ions. Alkoxysulfonium ions could also be generated by in situ DMSO trapping of electrochemically generated carbocations. The resulting alkoxysulfonium ions were transformed into carbonyl compounds by treatment with triethylamine. The present integrated electrochemical-chemical oxidation can be applied to the oxidation of diarylmethanes to diaryl ketones, toluenes to benzaldehydes, and aryl-substituted alkenes to 1,2-diketones. Moreover, the oxidation of unsaturated compounds bearing a nucleophilic group in an appropriate position gives cyclized carbonyl compounds.