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

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 H-1, C-13 and Si-29 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 similar to 1000 times faster at the 2-hydroxy acid, L-tartaric acid is esterified similar to 1000 times faster to the mono- and diester, while citric acid is selectively methylated at the terminal (3-hydroxycarboxyl) groups similar to 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.