Carboxylic Acid-Promoted Single-Step Indole Construction from Simple Anilines and Ketones via Aerobic Cross-Dehydrogenative Coupling.

Abstract

The cross-dehydrogenative coupling (CDC) reaction is an efficient strategy for indole synthesis. However, most CDC methods require special substrates, and the presence of inherent groups limits the versatility for further transformation. A carboxylic acid-promoted aerobic catalytic system is developed herein for a single-step synthesis of indoles from simple anilines and ketones. This versatile system is featured by the broad substrate scope and the use of ambient oxygen as an oxidant and is convenient and economical for both laboratory and industry applications. The existence of the labile hydrogen at C-3 and the highly transformable carbonyl at C-2 makes the indoles versatile building blocks for organic synthesis in different contexts. Computational studies based on the density functional theory (DFT) suggest that the rate-determining step is carboxylic acid-assisted condensation of the substrates, rather than the functionalization of aryl C-H. Accordingly, a pathway via imine intermediates is deemed to be the preferred mechanism. In contrast to the general deduction, the in situ formed imine, instead of its enamine isomer, is believed to be involved in the first ligand exchange and later carbopalladation of the α-Me, which shed new light on this indolization mechanism.