On the Syntheses and Reactions of Boroles and Boraindenes
Date
2014-10-08
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Highly Lewis-acidic boranes are an important class of compounds in both fundamental and applied chemistry. Recently, a sub-class of boranes known as boroles – five-membered unsaturated boracycles – has begun to receive attention due to the interesting properties afforded by the anti-aromatic nature of these compounds.
We synthesized and characterized a new class of boroles wherein the borole ring is fused to a benzene ring. These “boraindenes” were found to be highly Lewis acidic by the Gutmann-Beckett method, even more so than the ubiquitous tris-pentafluorophenylborane. Structural and computational evidence supports the notion that these compounds are anti-aromatic, which likely contributes to their Lewis acidity. A series of six derivatives was synthesized, and it was found that they had only marginally different absorption spectra.
Previous work in our group showed that perproteo and perfluoro-pentaphenylboroles are capable of irreversibly activating dihydrogen. The mechanism of this reaction was investigated both experimentally and computationally, and it was found that the reaction was bimolecular in nature. Furthermore, the reaction involves the formation of a transient dihydrogen-borole adduct which quickly adds dihydrogen across the boron-carbon bond. A 1,2-hydride shift can lead to a cisoid product, and cleavage of the boron-carbon bond results in a ring-opened intermediate that ultimately rearranges to give the transoid product; independent synthesis of this ring-opened intermediate confirms that it does indeed give the transoid product.
The most Lewis acidic of the boraindenes, perfluoro-1,2,3-triphenyl-1-boraindene, appears to react reversibly with dihydrogen gas. This compound has proven capable of mediating the hydrogenation of cyclohexene, though its precise role remains obscured in part due to the fact that it reacts with cyclohexene itself.
Perfluoro-1,2,3-triphenyl-1-boraindene was found to reversibly form an adduct with triethylsilane in solution, an adduct that was also isolated in the solid state. The solution-phase equilibrium was characterized by a titration followed by nuclear magnetic resonance spectroscopy, which revealed that the equilibrium is nearly thermoneutral at room temperature but favours adduct formation at lower temperatures. Infrared spectroscopy and X-ray diffraction analysis on solid samples confirm that the adduct is formed via a three-centre, two-electron silicon-hydrogen-boron bonding interaction. Perfluoro-1,2,3-triphenyl-1-boraindene was also found to be an active catalyst for the hydrosilation of olefins.
Description
Keywords
Chemistry--Inorganic
Citation
Houghton, A. Y. (2014). On the Syntheses and Reactions of Boroles and Boraindenes (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/27197