Resolving the Mechanistic Complexity in Triarylborane-Induced Conjugate Additions

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 α,β-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.