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Carbonyl Activation by Borane Lewis Acid Complexation: Transition States of H-2 Splitting at the Activated Carbonyl Carbon Atom in a Lewis Basic Solvent and the Proton-Transfer Dynamics of the Boroalkoxide Intermediate
Stockholm University, Faculty of Science, Department of Organic Chemistry.
Stockholm University, Faculty of Science, Department of Organic Chemistry.
Number of Authors: 2
2017 (English)In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 23, no 38, 9098-9113 p.Article in journal (Refereed) Published
Abstract [en]

By using transition-state (TS) calculations, we examined how Lewis acid (LA) complexation activates carbonyl compounds in the context of hydrogenation of carbonyl compounds by H-2 in Lewis basic (ethereal) solvents containing borane LAs of the type (C6F5)(3)B. According to our calculations, LA complexation does not activate a ketone sufficiently enough for the direct addition of H-2 to the O = C unsaturated bond; but, calculations indicate a possibly facile heterolytic cleavage of H-2 at the activated and thus sufficiently Lewis acidic carbonyl carbon atom with the assistance of the Lewis basic solvent (i.e., 1,4-dioxane or THF). For the solvent-assisted H-2 splitting at the carbonyl carbon atom of (C6F5)(3)B adducts with different ketones, a number of TSs are computed and the obtained results are related to insights from experiment. By using the Born-Oppenheimer molecular dynamics with the DFT for electronic structure calculations, the evolution of the (C6F5)(3)B-alkoxide ionic intermediate and the proton transfer to the alkoxide oxygen atom were investigated. The results indicate a plausible hydrogenation mechanism with a LA, that is, (C6F5)(3)B, as a catalyst, namely, 1) the step of H-2 cleavage that involves a Lewis basic solvent molecule plus the carbonyl carbon atom of thermodynamically stable and experimentally identifiable (C6F5)(3)B-ketone adducts in which (C6F5)(3)B is the Lewis acid promoter, 2) the transfer of the solvent-bound proton to the oxygen atom of the (C6F5)(3)B-alkoxide intermediate giving the (C6F5)(3)B-alcohol adduct, and 3) the S(N)2-style displacement of the alcohol by a ketone or a Lewis basic solvent molecule.

Place, publisher, year, edition, pages
2017. Vol. 23, no 38, 9098-9113 p.
Keyword [en]
boranes, hydrogenation, ketones, Lewis acids, Lewis bases
National Category
Organic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-145312DOI: 10.1002/chem.201700437ISI: 000404854700016OAI: oai:DiVA.org:su-145312DiVA: diva2:1128419
Available from: 2017-07-25 Created: 2017-07-25 Last updated: 2017-07-25Bibliographically approved

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