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Aryl–aryl cross-coupling reactions are important reactions for the production of various biaryl compounds. This short review covers the various aryl–aryl cross-coupling reactions carried out in flow, focusing on the metal species of the aryl nucleophiles used in the cross-coupling reactions.1 Introduction2 Suzuki–Miyaura Coupling (B)3 Migita–Kosugi–Stille Coupling (Sn)4 Negishi Coupling (Zn)5 Kumada–Tamao–Corriu Coupling (Mg)6 Murahashi Coupling (Li)7 Conclusion

β-Haloalkoxysulfonium ions generated by the reaction of electrogenerated Br+ and I+ ions stabilized by dimethyl sulfoxide (DMSO) reacted with sodium hydroxide and sodium methoxide to give the corresponding halohydrins and epoxides, respectively, whereas the treatment with triethylamine gave α-halocarbonyl compounds.

Arylbis(arylthio)sulfonium ions (ArS(ArSSAr)+), which were generated and accumulated by the low-temperature electrolysis of diaryl disulfide (ArSSAr), were found to serve as ArS+ equivalent in electrophilic substitution reactions of aromatic compounds, enolizable ketones, enol acetates, ketene silyl acetals and allylsilanes to give the corresponding arylthiolated products.

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.

The reaction of electrochemically generated N-acyliminium ion pools with alkenes bearing a heteroatom nucleophilic functionality such as a hydroxy group, a carboxy group, and an oxime moiety led to...

Arylbis(arylthio)sulfonium ions (ArS(ArSSAr)+), which were generated and accumulated by the low-temperature electrolysis of diaryl disulfide (ArSSAr), were found to serve as ArS+ equivalent in electrophilic substitution reactions of aromatic compounds, enolizable ketones, enol acetates, ketene silyl acetals and allylsilanes to give the corresponding arylthiolated products.

Anodic oxidation of toluene derivatives in the presence of DMSO followed by hydrolysis of the alkoxysulfonium ion intermediates allows a new and selective access to benzylic alcohols.

Organosulfur compounds have received much attention because they serve as useful building blocks in organic synthesis and exhibit interesting physical properties in materials chemistry. To synthesize such compounds, the addition of ArSSAr to carbon-carbon multiple bonds is one of the most efficient methods. We have recently reported the reactions of alkenes with a stoichiometric amount of ArS(ArSSAr)+, which is generated by electrochemical oxidation and accumulated as a cation pool of ArSSAr.1) Thiiranium ions are generated as initial intermediates. Quenching with hard nucleophiles such as MeOH and H2O leads to the addition of an ArS group and RO group (R = Me or H) across the carbon-carbon double bond. In contrast, the use of soft nucleophiles such as ketene silyl acetals, allylsilanes, and Et3N leads to the addition of two ArS groups across the carbon-carbon double bond. Presumably, soft nucleophiles assist nucleophilic attack of ArSSAr on the thiiranium ion intermediate. We envisioned that the addition reaction of ArSSAr to alkenes may proceed with a catalytic amount of ArS(ArSSAr)+ if another molecule of ArSSAr plays a role of a soft nucleophile. This is indeed the case, if we use Bu4NB(C6F5)4 as a supporting electrolyte, and herein, we report this chemistry (Scheme 1).2) We focused on the reaction of ArSSAr (Ar = p-FC6H4) and (E)-β-methylstyrene with a catalytic amount of electrochemically generated ArS(ArSSAr)+ (Ar = p-FC6H4), as a model reaction. The reaction gave the addition product in 78% yield (Scheme 1). The addition took place anti-selectively, which was confirmed by the comparison of the NMR spectra of the product with those reported in the literature.1a) The present reaction seems to proceed by a cation chain mechanism. In the prsentation, the details of optimization conditions and the scope and limitations of the present reaction will be discussed. References 1) (a) Fujie, S.; Matsumoto, K.; Suga, S.; Nokami, T.; Yoshida, J. Tetrahedron 2010, 66, 2823-2829. See also, (b) Matsumoto, K.; Suga, S.; Yoshida, J. Org. Biomol. Chem. 2011, 9, 2586-2596. 2) Matsumoto, K.; Sanada, T.; Shimazaki, H.; Shimada, K.; Hagiwara, S.; Fujie, S.; Ashikari, Y.; Suga, S.; Kashimura, S.; Yoshida, J. Asian J. Org. Chem. 2013, 2, 325-329.

An integration of electrooxidative cyclization and chemical oxidation was achieved. Electrochemical oxidation of alkenes having a nucleophilic moiety in the presence of DMSO gave cyclized alkoxysulfonium ions, which were converted to the corresponding ketones by treatment with triethylamine in a one-pot sequential manner. The method is also an effective tool for cyclization of 1,6-dienes affording five-membered ring diketones in high stereoselectivity.