A boron-transfer mechanism mediating the thermally induced revival of frustrated carbene-borane pairs from their shelf-stable adducts

Chemists have designed strategies that trigger the conformational isomerization of molecules in response to external stimuli, which can be further applied to regulate the complexation between Lewis acids and bases. We have recently developed a system in which frustrated carbene-borane pairs are revived from shelf-stable but external-stimuli-responsive carbene-borane adducts comprised of N-phosphine-oxide-substituted imidazolylidenes (PoxIms) and triarylboranes. Herein, we report the detailed mechanism on this revival process. A thermally induced borane-transfer process from the carbene carbon atom to the N-phosphinoyl oxygen atom initiates the transformation of the carbene-borane adduct. Subsequent conformational isomerization via the rotation of the N-phosphinoyl group in PoxIm moieties eventually leads to the revival of frustrated carbene-borane pairs that can cleave H-2. We believe that this work illustrates an essential role of dynamic conformational isomerization in the regulation of the reactivity of external-stimuli-responsive Lewis acid-base adducts that contain multifunctional substituents. Frustrated Lewis pairs can be thermally revived from classical Lewis adducts, but the mechanism by which this occurs is not well understood. Here a combined experimental and theoretical study supports a pathway mediated by boron-jumping and dynamic conformational isomerization.

Automated summary

This study reports a mechanism for the revival of frustrated carbene-borane pairs in an external-stimuli responsive system, involving thermal-induced borane transfer and conformational isomerization to cleave H-2. The pathway mediated by boron-jumping and dynamic conformational isomerization is supported by combined experimental and theoretical studies.