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Issue 8, 2009
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On the interactions of alkyl 2-hydroxycarboxylic acids with alkoxysilanes 2. Complexation and esterification of di- and tricarboxylic acids

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Abstract

The selective esterifications of L-malic, L-tartaric and citric acids with tetramethoxysilane Si(OMe)4, in methanol (MeOH) have been demonstrated for the first time. The interactions between these acids and Si(OMe)4, and also between oxalic acid and Si(OMe)4, were investigated using 1H, 13C and 29Si solution phase NMR, and electrospray mass spectrometry (ES-MS). Si(OMe)4 acts as a catalyst/reagent in the selective esterification of simple 2-hydroxycarboxylic acids (2HOAs), but with the di- and tri-carboxylic acids more complex selectivities are observed: the esterification of oxalic acid proceeds slower than in MeOH alone, L-malic acid is selectively esterified ∼1000 times faster at the 2-hydroxy acid, L-tartaric acid is esterified ∼1000 times faster to the mono- and diester, while citric acid is selectively methylated at the terminal (3-hydroxycarboxyl) groups ∼1000 times faster than in the absence of Si(OMe)4. Esterification is associated with the condensation of silane units to form oligomers. A mechanism is proposed in which 2HOAs attach to silicon via the alkoxy group and carboxyl groups to form various reactive cyclic intermediates. These intermediates may lead to accelerated esterification via nucleophilic attack of MeOH at the ligated carboxyl group, while a separate reaction pathway leads to condensation of silicon centres leading to oligosilanes. The mechanism has implications for the use of 2HOAs as templates in sol–gel silica preparation.

Graphical abstract: On the interactions of alkyl 2-hydroxycarboxylic acids with alkoxysilanes 2. Complexation and esterification of di- and tricarboxylic acids

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Publication details

The article was received on 20 Aug 2008, accepted on 14 Nov 2008 and first published on 16 Jan 2009


Article type: Paper
DOI: 10.1039/B814488E
Citation: Dalton Trans., 2009,0, 1460-1470
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    On the interactions of alkyl 2-hydroxycarboxylic acids with alkoxysilanes 2. Complexation and esterification of di- and tricarboxylic acids

    R. J. Ansell, J. E. Meegan, S. A. Barrett and S. L. Warrinner, Dalton Trans., 2009, 0, 1460
    DOI: 10.1039/B814488E

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