A Metal-Free Approach for the C–H Activation and Transfer Borylation of Electron-Rich Alkenes
Vincent Desrosiers, Samantha M. Knight, Frédéric‐Georges Fontaine
Abstract
Alkenyl boronates are ubiquitous reagents in organic synthesis, as they allow the formation of crucial bonds through cross-coupling reactions. Transition-metal catalysis is the most common approach to this transformation; however, it has limitations in terms of selectivity and boron reagents. On the other hand, metal-free borylation reactions often require the use of harsh reagents, causing compatibility issues. Herein, we apply a metal-free isodesmic borylation strategy, a functional-group-tolerant approach, to the C–H borylation of electron-rich olefins. We show that 2-mercaptoimidazole compounds can efficiently catalyze the borylation of enol ethers, silyl enol ethers, and enamines. Furthermore, borylated compounds can be functionalized in one-pot transformations, making this a useful synthetic tool. The various deactivation pathways are explained and the mechanism is analyzed computationally, revealing that, unlike those of most transition metals, the mechanism occurs through C–H activation rather than a sequence of insertion and elimination. This work highlights transfer C–H borylation as a versatile and tolerant tool for the borylation and functionalization of alkenes.