Abstract
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 H2. 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.
Highlights
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
In 2015, we demonstrated a strategy to generate frustrated Lewis pairs (FLPs) from shelf-stable classical Lewis adducts (CLAs) (PoxIm·B1 in Fig. 1) that are comprised of N-phosphine-oxide-substituted imidazolylidenes (PoxIms; 1) and B(C6F5)[3] (B1)
Full consumption of 1a was confirmed after 20 min, resulting in the formation of two CLAs, i.e., 2aB2, which contains a N-phosphinoyl oxygen–boron bond, and 3aB2, which contains a carbene–boron bond, in 61%
Summary
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. Chemists have developed strategies that trigger the conformational isomerization of molecules in response to external-stimuli[10,11,12,13] These strategies can be used to generate transient FLP species from classical Lewis adducts (CLAs) that act like their shelf-stable precursors[14,15,16,17,18,19,20,21,22,23,24,25,26]. Clarifying the relationship between external-stimuli-responsive conformational isomerizations and the interconversion that occurs between frustrated and quenched Lewis pairs is of great importance This would allow a significant expansion of different strategies to design and apply FLP species[19].
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