Metal-free or Lewis acid-catalyzed C5−H methylthiomethylation of oxindoles with DMSO

Abstract

Thanks to the homogeneous-to-heterogeneous phase transition, metal-free C5-H methylthiomethylation of indolin-2-ones and Lewis acid-catalyzed C5-H methylthiomethylation of isatins with DMSO featuring with a reusable system have been disclosed. In metal-free system, the reactive ionic liquid (CH 2 =S + -Me) is formed and accumulates in the lower layer that also serves as reactant. Contrastively, another non-reactive ionic liquid, trimethylsulfonium (Me 3 S + ), is formed as main component in the Lewis acid-catalytic system, which can catch the catalyst to dissolve in the lower layer. While the products remained in the upper layer, and the lower layer can be easily isolated by decantation and can be reused as catalyst for several cycles without considerable loss in its 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(sp 2 )-H and C(sp 3 )-H activation, practical, cost-effective, and reusable. Lots of key intermediates are characterized by ESI-MS, indicating an anion-cation coupling pathway. Moreover, DFT calculations reveal that atomic charge distribution governs the site selectivity.