Mechanisms of Amine-Catalyzed Organosilicate Hydrolysis at Circum-Neutral pH
Citations Over TimeTop 10% of 2006 papers
Abstract
Mono- and polyamines can catalyze the hydrolysis and condensation of organosilicate starting materials in biomimetic silica synthesis pathways at circum-neutral pHs and room temperature. Our study is focused on understanding the mechanistic role of amines in catalyzing the hydrolysis process that precedes condensation. We have conducted (29)Si NMR experimental studies over a range of temperature and pHs for the hydrolysis rates of trimethylethoxysilane (TMES), a model compound with only one hydrolyzable bond, combined with quantum mechanical hybrid density functional theory calculations of putative intermediate and transition-state structures for TMES and tetramethyl orthosilicate (TMOS). Comparison of calculated energies with experimentally determined activation energies indicates that amine catalysis of TMES is primarily a consequence of the amine's acidity at neutral pH. The proton released by the amine is transferred to the organosilicate, producing a protonated ethoxy leaving group that can be displaced by water in an S(N)2 reaction. For TMOS, the activation energy of proton-transfer coupled with S(N)2 substitution is comparable to that for Corriu's nucleophile-activated nucleophilic displacement, such that the mechanism of amine-catalyzed hydrolysis is dependent mostly on the ambient pH conditions as well as the type of amine. The relevance of our results to biological silica precipitation is discussed.
Related Papers
- → Nucleophilic substitution at phosphorus: stereochemistry and mechanisms(2017)76 cited
- → Electrostatic Switching between SN1 and SN2 Pathways(2018)50 cited
- → Do the solvolysis reactions of secondary substrates occur by the SN1 or SN2 mechanism: or something else?(2011)4 cited
- → Nucleophilic Substitution and Elimination(2015)
- → Gas phase nucleophilic substitution reactions; Transaminations of silanamines(1978)