Resolving the Mechanistic Complexity in Triarylborane-Induced Conjugate Additions

Mayer, Robert J.; Hampel, Nathalie; Ofial, Armin R. und Mayr, Herbert (2022): Resolving the Mechanistic Complexity in Triarylborane-Induced Conjugate Additions. In: ACS Catalysis, Bd. 12, Nr. 24: S. 15298-15309

Abstract

Previous studies have shown that the catalytically active species in the Lewis acid-catalyzed addition reactions of allyl silanes or silyl enol ethers to carbonyl compounds or Michael acceptors are often silylium-carbonyl adducts rather than the adducts of the carbonyl group with the Lewis acid used for the induction of the reaction. Indirect evidence for such catalyst variations has so far been derived from double-label crossover experiments and comparisons of absolute reaction rates. We have now performed a detailed investigation on the kinetics and mechanism of the triarylborane-initiated conjugate addition reactions of allylsilanes and silyl enol ethers to alpha,beta-unsaturated carbonyl compounds. NMR spectroscopic monitoring of such reactions gave rise to sigmoidal kinetic profiles, allowing us to directly follow the change of the catalytically active Lewis acid from triarylboranes in the induction period to silylium ions during the main part of the reaction. Crossover experiments and the isolation of four-and five-membered cyclic intramolecular trapping products provided further insight into the mechanism. DFT calculations of various mechanistic variants and kinetic modeling elucidated the operation of a complex reaction network, which rationalizes the experimental observations.

• Dokumententyp: Zeitschriftenartikel
• Fakultät: Chemie und Pharmazie > Department Chemie
• Themengebiete: 500 Naturwissenschaften und Mathematik > 540 Chemie
• ISSN: 2155-5435
• Sprache: Englisch