Rongalite-mediated hydride-free chemoselective reduction of the CC bond of isatin-derived Michael acceptors

Abstract

A chemoselective and sustainable reduction of the CC bond in isatin-derived Michael acceptors has been achieved using rongalite as an inexpensive hydride-free reductant. The transformation proceeds efficiently in ethanol at room temperature with p-TSA as an additive, without the need for transition metals, external hydrides, visible light, or additional catalysts. Under these mild conditions, a wide range of substrates were converted to the corresponding reduced products in excellent yields, highlighting the broad applicability of the protocol. Mechanistic studies support a rongalite-mediated 1,4-conjugate addition pathway, which accounts for the observed high selectivity. The practicality and sustainability of the method are further validated by gram-scale synthesis. Furthermore, the protocol is compatible with bioactive isatin derivatives and the corresponding products serve as valuable intermediates for downstream transformations, including spirocyclization and dimerization reactions.