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Kinetic and Mechanistic Examination of Acid–Base Bifunctional Aminosilica Catalysts in Aldol and Nitroaldol Condensations
journal contribution
posted on 2015-12-18, 15:41 authored by Virginia
E. Collier, Nathan C. Ellebracht, George I. Lindy, Eric G. Moschetta, Christopher W. JonesThe kinetic and mechanistic understanding
of cooperatively catalyzed
aldol and nitroaldol condensations is probed using a series of mesoporous
silicas functionalized with aminosilanes to provide bifunctional acid–base
character. Mechanistically, a Hammett analysis is performed to determine
the effects of electron-donating and electron-withdrawing groups of
para-substituted benzaldehyde derivatives on the catalytic activity
of each condensation reaction. This information is also used to discuss
the validity of previously proposed catalytic mechanisms and to propose
a revised mechanism with plausible reaction intermediates. For both
reactions, electron-withdrawing groups increase the observed rates
of reaction, though resonance effects play an important, yet subtle,
role in the nitroaldol condensation, in which a p-methoxy electron-donating group is also able to stabilize the proposed
carbocation intermediate. Additionally, activation energies and pre-exponential
factors are calculated via the Arrhenius analysis of two catalysts
with similar amine loadings: one catalyst had silanols available for
cooperative interactions (acid–base catalysis), while the other
was treated with a silanol-capping reagent to prevent such cooperativity
(base-only catalysis). The values obtained for activation energies
and pre-exponential factors in each reaction are discussed in the
context of the proposed mechanisms and the importance of cooperative
interactions in each reaction. The catalytic activity decreases for
all reactions when the silanols are capped with trimethylsilyl groups,
and higher temperatures are required to make accurate rate measurements,
emphasizing the vital role the weakly acidic silanols play in the
catalytic cycles. The results indicate that loss of acid sites is
more detrimental to the catalytic activity of the aldol condensation
than the nitroaldol condensation, as evidenced by the significant
decrease in the pre-exponential factor for the aldol condensation
when silanols are unavailable for cooperative interactions. Cooperative
catalysis is evidenced by significant changes in the pre-exponential
factor, rather than the activation energy for the aldol condensation.