The Addition of ArSSAr to Alkenes: The Implications of a Cationic Chain Mechanism Initiated by Electrogenerated ArS(ArSSAr)+
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.
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.