Premium
Evidence That Protons Can Be the Active Catalysts in Lewis Acid Mediated Hetero‐Michael Addition Reactions
Author(s) -
Wabnitz Tobias C.,
Yu JinQuan,
Spencer Jonathan B.
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200305407
Subject(s) - lewis acids and bases , chemistry , nucleophile , catalysis , michael reaction , medicinal chemistry , lewis acid catalysis , conjugate acid , organic chemistry , photochemistry , ion
The mechanism of Lewis acid catalysed hetero‐Michael addition reactions of weakly basic nucleophiles to α , β‐unsaturated ketones was investigated. Protons, rather than metal ions, were identified as the active catalysts. Other mechanisms have been ruled out by analyses of side products and of stoichiometric enone–catalyst mixtures and by the use of radical inhibitors. No evidence for the involvement of π‐olefin–metal complexes or for carbonyl–metal‐ion interactions was obtained. The reactions did not proceed in the presence of the non‐coordinating base 2,6‐di‐ tert ‐butylpyridine. An excellent correlation of catalytic activities with cation hydrolysis constants was obtained. Different reactivities of mono‐ and dicarbonyl substrates have been rationalised. A 1 H NMR probe for the assessment of proton generation was established and Lewis acids have been classified according to their propensity to hydrolyse in organic solvents. Brønsted acid‐catalysed conjugate addition reactions of nitrogen, oxygen, sulfur and carbon nucleophiles are developed and implications for asymmetric Lewis acid catalysis are discussed.