The metal-free or Lewis acid-catalyzed C5–H methylthiomethylation of oxindoles with DMSO

The metal-free C5-H methylthiomethylation of indolin-2-ones and the Lewis acid-catalyzed C5-H methylthiomethylation of isatins with DMSO featuring a reusable system are described herein. Two vital ionic liquids (CH₂S⁺-Me and Me₃S⁺) are formed <i>in situ</i>, leading to the formation of a biphasic system from the initial homogeneous system. The ionic liquids accumulate in the lower layer, serving as key reaction intermediates, dissolution agents, and catalysts, while the products remain in the upper layer. The lower layer can be easily isolated by decanting, and can be reused as the catalyst for several cycles without considerable loss of the initial activity. Furthermore, the lower layer can mediate an unprecedented C5-H methylsulfonylmethylation of indolin-2-ones under solvent-free conditions. This protocol offers several key advantages: oxidant/additive-free C(sp²)-H and C(sp³)-H activation, practicality, cost-effectiveness, and reusability. Moreover, DFT calculations reveal that the atomic charge distribution governs the site selectivity.