a,b
a Reaction conditions: Carbon rod anode (Ø = 6 mm), Carbon rod cathode (Ø = 6 mm), constant current = 12 mA, undivided cell, 1 (0.5 mmol), 2 (0.8 mmol), Te (0.6 mmol), Cs2CO3 (0.8 mmol), TBAI (0.5 mmol), MeCN (4 mL), H2O (2 mL), Room temperature, N2, 12 h. b Isolated yield.
To gain some insight into the mechanism of the electrocatalytic multicomponent reaction, some controlled experiments were conducted with elemental selenium as an example, as depicted in Scheme 7. First, without indole 1a , the reaction of Na-salt 2a with elemental selenium under the standard conditions did not produce Na-salt2a-I , suggesting that 2a-I might not be involved in the process (Scheme 7A). Next, to validate the Ts•and I• radical intermediates, a reaction was carried out with standard conditions using ethene-1,1-diyldibenzene as the trapping reagent. The compounds1aa-I and 1aa-II were detected by high-resolution mass spectrometry (Scheme 7B). Furthermore, when sodiump -toluenesulfinate 2a and elemental selenium reacted with styrene under the standard conditions (Scheme 7C), (E )-Se-styryl 4-methylbenzenesulfonoselenoate (3ar ) was obtained in 67% yield; further, a diselenide could be detected by gas chromatography (GC), indicating that radical TsSe• may be the intermediate in this transformation (Scheme 7D).
Scheme 7 Investigation of the mechanism
In addition, we also conducted a series of cyclic voltammetry (CV) experiments (Figure 1). In the case of TBAI, an oxidation peak occurred at 0.46 V vs. Ag/AgCl (SCE), corresponding to the electro-oxidation of I ions. Sodium p -tolylsulfinate 2aeasily lost a single electron and the oxidation potential was observed at 0.74 V vs. Ag/AgCl (SCE). No apparent oxidation peak corresponding to Se was visible in the region of 0.0–2.0 V, so Se oxidization on the anode can be ruled out (Figure 1, curve c). Curve d shows an oxidation peak for indole 1a at 1.32 V. However, in the absence of TBAI, no desired product 3aa could be detected (Table 1, entry 14), which might exclude 1a’ as the intermediate of this electrocatalytic transformation (Scheme 7E).