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.