Electrochemical Synthesis of Dimeric 2-Oxindole Sharing Vicinal Quaternary Centers Employing Proton-Coupled Electron Transfer.

Dimerization of 3-substituted 2-oxindoles has been developed under a mild electrochemical condition, avoiding toxic chemical oxidants and metal by-products. This methodology forms a C(sp)-C(sp) bond at the pseudobenzylic position of two partners of 2-oxindoles with a broad substrate scope. These dimeric structural motifs are important building blocks for the total synthesis of pyrroloindoline alkaloids. Furthermore, this work demonstrates in-depth mechanistic insights employing electrochemistry, which suggests a stepwise one proton transfer (PT) and two electron transfer (ET) processes. Most significantly, reaction rate acceleration has been demonstrated by exploiting the base-assisted proton-coupled electron transfer (PCET) pathway. Hence, this work brings a new dimension in the field of electro-organic synthesis with the help of nature's favorite kinetic route, i.e., PCET, to lower the kinetic barrier.