Halogen and chalcogen cations (X(+) = Br(+), I(+), ArS(+), and ArSe(+)) were generated by low-temperature electrochemical oxidation in the presence of dimethyl sulfoxide (DMSO) and were accumulated in the solution. DFT calculations indicated that DMSO stabilizes these cations by coordination. The complexes of I(+) with one and two DMSO molecules were observed by cold-spray-ionization MS analyses. The stability of the resulting cation pools of X(+) increased in the order of Br(+) < I(+) < ArS(+) < ArSe(+), which could be explained in terms of the electronegativity of X. The cation pools served as versatile reagents for organic synthesis; the reactions with alkenes gave β-X-substituted alkoxysulfonium ions, which were converted to the corresponding carbonyl compounds by the treatment with triethylamine, whereas the treatment with methanol gave the corresponding alcohols. The reactions with aminoalkenes and 1,6-dienes gave the cyclized products.
Electrochemical oxidation of toluene derivatives in the presence of N-tosyldiphenylsulfilimine gave the corresponding benzylaminosulfonium ions, which were treated with tetrabutylammonium iodide under non-electrolytic conditions to give N-tosylbenzylamines. The transformation serves as a metal- and chemical-oxidant-free method for benzylic C-H amination. Because of high oxidation potential of N-tosyldiphenylsulfilimine the present method can be applied to synthesis of various benzylamines from functionalized toluene derivatives.
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.
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.
Reaction integration which combines multiple reactions in a single operation in one pot or in a flow system without isolating intermediates enhances the power and speed of organic synthesis to meet future demands for synthesis of various organic molecules. This article provides a brief outline of a new field of organic synthesis based on time integration and space integration of reactions using unstable reactive cationic intermediates generated by electrochemical oxidation.
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