A reaction between an N-phosphine oxide-substituted imidazolylidene (PoxIm) and MOTf (M = Li or Na) in THF afforded the complexes [(PoxIm) x (MOTf) y (THF) z ] through the coordination of the N-phosphoryl group to Na + or Li + , which was unambiguously confirmed by 13 C and 31 P NMR spectroscopy and single-crystal X-ray diffraction analyses. These measurements confirmed that the carbene moieties in these complexes remain intact. In solution, PoxIm and MOTf might form the complexes in a 3:2 ratio (i.e., x = 3, y = 2), while ratios of 1:1 (M = Li; x = 1, y = 1) and 2:1 (M = Na; x = 2, y = 1) were observed in the solid state.
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.
A strategy for modulating the Lewis acidity of triarylboranes is proposed based on the concept of ‘remote’ back strain. Steric repulsion and non-covalent interactions, both generated between the aryl meta-substituents of triarylboranes, are found to be critical for determining the strength of the remote back strain. Applying this concept, we synthesized B(2,6-F2-3,5-TMS2-C6H)3 and the liquid B(2,6-F2-3,5-allyl2-C6H)3 and demonstrated their superior catalytic activity for the hydrogenation of quinoline relative to B(C6F5)3 and B(2,6-F2-C6H3)3. Moreover, we established the first example of the catalytic hydrogenation of quinoline using B(2,6-F2-3,5-allyl2-C6H)3 in the presence of a gaseous mixture of H2/CO/CO2 (1/1/1 molar ratio).
Combined experimental and theoretical studies allowed clarifying the reaction
mechanism for the revival of frustrated carbene−borane pairs from external-stimuli-responsive
classical Lewis adducts comprised of N-phosphine oxide-substituted imidazolylidenes and
triarylboranes. A borane-transfer process from the carbene carbon atom to the N-phosphinoyl
oxygen atom was identified as the rate-determining event for the regeneration of the FLP
species, eventually enabling the heterolytic cleavage of H2.<br>
Combined experimental and theoretical studies allowed clarifying the reaction
mechanism for the revival of frustrated carbene−borane pairs from external-stimuli-responsive
classical Lewis adducts comprised of N-phosphine oxide-substituted imidazolylidenes and
triarylboranes. A borane-transfer process from the carbene carbon atom to the N-phosphinoyl
oxygen atom was identified as the rate-determining event for the regeneration of the FLP
species, eventually enabling the heterolytic cleavage of H2.<br>
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