On the Interactions of Alkyl 2‐Hydroxycarboxylic Acids with Alkoxysilanes: Selective Esterification of Simple 2‐Hydroxycarboxylic Acids

Abstract

The interactions of a range of monocarboxylic acids with tetramethoxysilane Si(OMe)(4) (TMOS), in methanol (MeOH), have been investigated by using (1)H, (13)C and (29)Si solution-phase NMR spectroscopy and electrospray mass spectrometry (ESMS). Si(OMe)(4) acts as a catalyst/reagent in the selective methylation of 2-hydroxycarboxylic acids (2HOAs) in MeOH at room temperature: glycolic acid, lactic acid and 2-hydroxybutyric acid are esterified more than a hundred times faster in MeOH and Si(OMe)(4) than in MeOH alone. No acceleration of methylation is observed for carboxylic acids lacking the 2-hydroxy group. Methylation of the 2HOAs is associated with the condensation of individual siloxane units to form oligomers. A mechanism is proposed in which 2HOAs attach to silicon via the alkoxy group, then subsequently via the carboxyl group in an intramolecular rearrangement to form an unstable and reactive cyclic intermediate. This intermediate may lead to accelerated methylation of the carboxylic acid via nucleophilic attack of MeOH at the carbonyl group, while a separate reaction pathway leads to condensation of silanols and/or alkoxysilanes leading to oligosiloxanes. The mechanism has implications for the use of 2HOAs as templates in sol-gel silica preparation.

Hybrid Coatings Enriched with Tetraethoxysilane for Corrosion Mitigation of Hot-Dip Galvanized Steel in Chloride Contaminated Simulated Concrete Pore Solutions

“…The 13 C spectra of hybrid samples (Figure 4 and Table 2) reveal the signals of residual ethoxyde groups suggesting that the organosilane hydrolysis is not complete in spite of the hydrolysis ratio used in the hybrid synthesis (Table 1). The content of citric acid in the mixtures is about 10% with respect to the whole silane amount (Table 1) and the complexation of the Si units by citric acid cannot be excluded, taking into account the well-known reactivity of carboxylic acids towards silicon alkoxides [54]. …”

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

“…23 Further, there is ample evidence that silicon-diol complexes such as 6 are stable in solution, even under aqueous conditions. 21 In previous studies reacting glycolic, lactic, and 2-hydroxybutyric acids 13 with Si(OMe) 4 in methanol (MeOH) at room temperature, complexes of the HOAs with silicon were not detected by NMR but were found using ESI-MS. A mechanism for the observed esterification was proposed involving the formation of a 5-membered cyclic intermediate in which both the alkoxy and carboxy groups are bonded to silicon, which is reactive and labile so cannot be detected spectroscopically, yet can react with MeOH to yield the methyl 2HOA, and with ∫SiOH to yield oligomers.…”

“…Thus, it was of interest to determine whether OA, MA, CA and TA react with Si(OMe) 4 in MeOH to form complexes, and whether similar reactions occur as with the simpler 2HOAs. Since esterification of 2-hydroxybutyric acid is accelerated under these conditions but that of 3-hydroxybutyric acid is not, 13 regioselective esterification of MA and CA was expected. 24,25 Materials and methods NMR 1 H NMR were recorded on a Bruker Avance 500 spectrometer at 500 MHz.…”

“…24,25 Materials and methods NMR 1 H NMR were recorded on a Bruker Avance 500 spectrometer at 500 MHz. Proton-decoupled 13 C NMR were recorded on a Bruker DPX 300 spectrometer at 75.5 MHz. Acid (0.375 mmol) was dissolved completely in CD 3 OD (0.75 mL), and 1 H and 13 C spectra were recorded.…”

“…It was shown in a preceding paper 13 that the simple 2HOAs glycolic, lactic and 2-hydroxybutyric acid react with tetramethoxysilane, Si(OMe) 4 , in methanol (MeOH) at room temperature with rapid formation of methyl esters (by contrast, esterification of acetic and 3-hydroxybutyric acids is not accelerated under these conditions). This selectivity for 2HOAs suggests the process may be synthetically useful.…”