Title: Kinetic and Mechanistic Examination of Acid–Base Bifunctional Aminosilica Catalysts in Aldol and Nitroaldol Condensations

Abstract

The 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 played an important, yet subtle, role in the nitroaldol condensation, in which a p-methoxy electron-donating group was 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 reactionsmore » 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.« less

Authors:

Collier, Virginia E. ^[1]; Ellebracht, Nathan C. ^[1]; Lindy, George I. ^[1]; Moschetta, Eric G. ^[1]; Jones, Christopher W. ^[1]

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+ Show Author Affiliations

1. School of Chemical &, Biomolecular Engineering Georgia Institute of Technology 311 Ferst Drive NW, Atlanta, Georgia 30332, United States

Publication Date:

Fri Dec 04 00:00:00 EST 2015

Research Org.:

Georgia Institute of Technology, Atlanta, GA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES); USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division

OSTI Identifier:

1234153

Alternate Identifier(s):

OSTI ID: 1439317; OSTI ID: 1657457

Grant/Contract Number:  

FG02-03ER15459

Resource Type:

Published Article

Journal Name:

ACS Catalysis

Additional Journal Information:

Journal Name: ACS Catalysis Journal Volume: 6 Journal Issue: 1; Journal ID: ISSN 2155-5435

Publisher:

American Chemical Society (ACS)

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Arrhenius analysis; bifunctional acid-base catalysts; carbon-carbon coupling; condensation reactions; cooperative catalysis; Hammett analysis