A Metal-Free Approach for the C-H Activation and Transfer Borylation of Electron-Rich Alkenes

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.