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首页> 外文期刊>Chemistry: A European journal >On the interactions of alkyl 2-hydroxycarboxylic acids with alkoxysilanes: Selective esterification of simple 2-hydroxycarboxylic acids
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On the interactions of alkyl 2-hydroxycarboxylic acids with alkoxysilanes: Selective esterification of simple 2-hydroxycarboxylic acids

机译：关于烷基2-羟基羧酸与烷氧基硅烷的相互作用：简单的2-羟基羧酸的选择性酯化

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摘要

The interactions of a range of monocarboxylic acids with tetramethoxysilane Si(OMe)(4) (TMOS), in methanol (MeOH), have been investigated by using H-1, C-13 and Si-29 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 sot-gel silica preparation.
机译：使用H-1，C-13和Si-29溶液相NMR光谱和电喷雾研究了一系列单羧酸与四甲氧基硅烷Si（OMe）（4）（TMOS）在甲醇（MeOH）中的相互作用质谱（ESMS）。 Si（OMe）（4）在室温下于MeOH中的2-羟基羧酸（2HOA）选择性甲基化中充当催化剂/试剂：乙醇酸，乳酸和2-羟基丁酸在室温下被酯化的速度快100倍以上MeOH和Si（OMe）（4）比仅在MeOH中。对于缺少2-羟基的羧酸，没有观察到甲基化的加速。 2HOA的甲基化与各个硅氧烷单元的缩合形成低聚物有关。提出了一种机制，其中2HOA通过烷氧基连接到硅上，然后在分子内重排中通过羧基连接到硅上，形成不稳定的反应性环状中间体。该中间体可通过MeOH在羰基上的亲核攻击而导致羧酸的加速甲基化，而单独的反应路径导致硅烷醇和/或烷氧基硅烷的缩合，从而形成低聚硅氧烷。该机制对于在溶胶凝胶二氧化硅制备中使用2HOA作为模板具有重要意义。

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