Figure 1 CV of 0.1mol/L of LiClO4 in 4 mL of MeCN and 2 mL of H2O solution containing different compounds: (a) blank experiment; (b) Sodium p -tolylsulfinate2a (0.8 mmol); (c) Se (0.6 mmol); (d) Indole 1a (0.5 mmol); (e) TBAI (0.5 mmol); (f) Sodium p -tolylsulfinate2a (0.8 mmol), Se (0.6 mmol), Indole 1a (0.5 mmol), TBAI (0.5 mmol); with a GC disk working electrode, Pt counter electrode, and Ag/AgCl reference electrode at 0.01 V/s scan rate.
Based on these findings and those of previous studies,[13] we propose the reaction mechanism depicted in Scheme 8. Initially, single-electron-transfer (SET) oxidation of 2a at the anode generates a sulfonyl radicalA that reacts with elemental selenium to generate the free radical intermediate TsSe• (B ). Subsequently, the radical addition of 1a with intermediate B gives the radical species C . Then 3-iodoindoline E is obtained through radical cross-coupling of C and the iodine radical from the anodic oxidation of iodine ions. Alternatively, SET oxidation ofC at the anode produces the cation intermediate D that is quickly attacked by I to give intermediateE . Subsequent elimination of HI could afford the desired product 3aa .
Scheme 8 Plausible reaction mechanism
Owing to the unique scaffolding of chalcogenosulfonates, we envision that the obtained compounds will serve as a promising platform for the design and discovery of fungicides. A total of 28 chalcogenosulfonates were tested for in vitro antifungal activity against two phytopathogenic fungi at 100 μ g/L, using commercialized fungicide azoxystrobin as positive control agent. As shown in the Supplementary Information (Table S5), compounds 3ag (79.5% inhibition),3ai (81.2% inhibition), 3al (82.2% inhibition),3bf (83.4% inhibition), 4ae (79.1% inhibition), and5ag (79.7% inhibition) showed better antifungal activities than azoxystrobin (78.5% inhibition) against Botrytis. cinerea . Notably, compounds 3ag (86.4% inhibition), 3ai(89.4% inhibition), 3al (92.6% inhibition), 3bf(90.2% inhibition), 4ae (87.9% inhibition), and 5ag(85.5% inhibition) showed potent antifungal activities that were higher than the activities of azoxystrobin (85.3% inhibition) againstRhizoctonia. solani .
To better understand the antifungal activities of these compounds, the half maximal effective concentration (EC50) values of compounds with more than 75% inhibition rate at 100 μg/L were considered (Figure 2). Compounds 3ai and 4ae showed the best antifungal activities among 28 chalcogenosulfonates, and their EC50 values were much lower than those of commercial azoxystrobin pesticides. These findings enable the adoption of a new course in studying the biological activity of chalcogen-containing organic compounds.