Compared with traditional, heat-promoted organic reactions, visible-light-induced organic reactions are milder, environmentally benign and operationally simpler. Moreover, visible light is a readily available source, so it has promising applications in science and industry [1-5]. In recent years, great progress has been achieved in visible-light-induced reactions. For example, various C-C and C-hetero bond-formation reactions induced by visible light have been successfully achieved [6-21] Among these, visible-light-induced sulfonylation reactions with sulfinic acids have become a hot topic. In 2015, a visible-light-induced reaction for the synthesis of sulfonated oxindoles [22] and coumarin derivatives [23] using arylsulfinic acids was developed by Wang's group. Jiang's group [24] developed a visible-light photocatalytic arylsulfonylation and bicyclization reaction of C(sp3)-tethered 1, 7-enynes with sulfinic acid which afforded sulfone-containing benzo[a]-fluorene-5-ones in generally good yields [24]. The reaction of terminal alkynes and sulfinic acids mediated by visible light for the synthesis of α-substituted vinyl sulfones with exclusive Markovnikov regioselectivity was reported by Lei and coworkers [25]. Mechanistic investigations showed that radical/radical cross-coupling might be the key step in this transformation. In the absence of additional photocatalyst, three-component reactions of isocyanides, alkynes, and sulfinic acids were described by Wang's group [26]. C6-polyfunctionalized phenanthridines with high selectivity were formed. It is interesting that the novel electron donor-acceptor (EDA) complex generated from the reaction of arylsulfinic acid and biaryl isocyanide drives the reaction. Although various sulfonylated compounds have been synthesized via visible-light-induced sulfonylation reactions using sulfinic acids, it is highly desirable to expand the substrate scope of the sulfonylation reaction.

2-Sulfonylquinoline derivatives are important nitrogen-containing heterocyclic compounds in agricultural and pharmaceutical chemistry [27-41]. Therefore, significant efforts have been focused on the development of efficient and economically feasible protocols for their preparation [42-49]. At present, sodium benzenesulfinate [50-53], sulfonyl chlorides [54-56], and sulfonyl hydrazides [57, 58], have been shown to be efficient sulfonylation reagents for the preparation of deoxygenative 2-sulfonylquinolines. However, to the best of our knowledge, there is no example of visible-light-induced C2-sulfonylation of quinoline N-oxides with sulfinic acids. Consistent with our interest in green chemistry [59-66], we describe visible-light-induced reactions for the synthesis of 2-sulfonylquinolines using sulfinic acids via a radical mechanism.

Initially, to optimize the reaction conditions, the reaction between quinoline 1-oxide and benzenesulfinic acid was examined. As can be seen in Table 1, solvents play an important role in the reaction. The desired product can be obtained with an 83% yield when acetone is used as the solvent (Table 1, entry 1). The use of DMSO and CH₃CN as solvents led to lower yields (Table 1, entries 3 and 4, respectively). To our disappointment, no product was obtained when DMF, H₂O, 1, 2-dimethoxyethane (DME), or EtOAc were used as solvents (Table 1, entries 2 and 5-7, respectively). Dichloroethane (DCE) was found to be an excellent solvent for this reaction, providing the highest yield (88%) of the desired products (Table 1, entry 8). Several different catalysts for this reaction were also investigated. Methylene blue was found to be effective in this reaction, yielding moderate product isolation (62%, Table 1, entry 10). Other catalysts, such as Acid Red 94, Acridine Red, Eosin Y, and Ru(bpy)₃Cl₂, were less effective for this reaction (Table 1, entries 9 and 11-13, respectively). An examination of the effects of the oxidant on the reaction was then performed. tert-Butyl peroxybenzoate (TBPB), dibutylperoxide (DTBP), and K₂S₂O₈ were less effective than tert-butyl hydroperoxide (TBHP) (Table 1, entries 14-16, respectively). Next, different visible-light sources such as green, UV, white, and blue were tested. Blue light showed the best result (Table 1, entries 8 and 17-19, respectively). When the reaction was conducted in the absence of light, catalyst, or oxidant, it was completely inhibited (Table 1, entries 20-22, respectively). Increasing the loading of the catalyst to 5 mol% also did not improve the efficiency of the reaction (Table 1, entry 23).

To survey the versatility of the deoxygenative sulfonylation reaction, a variety of quinoline N-oxides and arylsulfinic acids were tested, with the results summarized in Table 2. Quinoline N-oxides with electron-neutral, electron-poor, and electron-rich groups all reacted efficiently with benzenesulfic acid to generate the corresponding products in moderate to high yields (Table 2, 3a-3h). For substituted sulfinic acids, various groups such as F, Cl, Br, CH₃, and tBu were all suitable for this reaction (Table 2, 3i-3p). Steric hindrance did not have any obvious impact on this reaction, as compounds 3f, 3m, and 3n were obtained in 72%-82% yields. It is interesting to note that naphthalene-2-sulfinic acid also reacted well with quinoline N-oxides, affording the desired products in 77%-80% yields (Table 2, 3q and 3r). Br and CH₃OCO are active groups which render the corresponding products ready for further functionalization.

To investigate the mechanism of this transformation, radical-trapping reagents, 2, 2, 6, 6-tetramethyl-1-piperidinyloxy (TEMPO) and 2, 6-di-tert-butyl-4-methylphenol (BHT), were introduced to the reaction, with no desired product being formed (Scheme 1). This suggests that free-radical intermediates may be involved in the presented transformation. Visible light, photocatalyst, and oxidant also play important roles in the sulfonylation reaction, because the reaction will not proceed if any of these are absent (Table 1, entries 20-22, respectively).

Based on our initial mechanistic studies and previous work [23-25, 50-53, 59-66], a possible mechanism for visible-light-induced deoxygenative sulfonylation of quinoline N-oxides is diagrammed in Scheme 2. First, excited photocatalyst species Na₂-eosinY* is generated from the irradiation of Na₂-eosinY by visible light. Then, Na₂-eosinY* reacts with TBHP, leading to production of a tert-butoxyl radical. Hydrogen can be abstracted by the tert-butoxyl radical from sulfinic acid to afford a sulfonyl radical A. Then the sulfonyl radical A reacts with quinoline N-oxide to provide intermediate B. Intermediate B further reacts with the sulfonyl radical to afford intermediate C. Finally, desired product D is obtained from aromatization of C with concomitant release of sulfonic acid. Alternatively, intermediate B could also abstract a hydrogen atom from sulfinic acid. This would produce another equivalent of a sulfonyl radical and lead to the elimination of H₂O from hydrogenated E, affording the desired products. This mechanism may also be possible.

In conclusion, we have described a novel and efficient visible-light-induced reaction for preparation of 2-sulfonylquinoline. Various functional groups were well tolerated in this reaction and the corresponding products were obtained in moderate to high yields. Free-radical intermediates may be involved in the reaction process. Further investigations toward expanding substrate scope and mechanistic study are underway in our lab.