Abstract
β-Hydroxyketones are versatile building blocks in organic synthesis, which can be conveniently synthesized from ketones and aldehydes by aldol reactions. Unfortunately, these reactions often suffer from dehydration of the initially formed β-hydroxyketones. Previously, tetrahedral 3,5-difluorophenylborate was shown to be an efficient and selective catalyst for this reaction. The present investigation concerns the catalytic performance of phenyl borates with different substitution patterns in the aldol reaction. It appears that the dehydration reaction can be suppressed by selecting substituents and substituent positions with reduced electron withdrawing effects on the borate function. Optimal suppression of the dehydration of β-hydroxyketones was obtained for compounds corresponding to phenylboronic acids with a pK
a > 7. The reactions between benzaldehyde and butanone or 3-pentanone did not show diastereoselectivity, which suggests that the catalysts merely act as bases rather than as templates for the transition state of the aldol reaction. Sterically more demanding ketones were not converted.
摘要
β-羟基酮是有机合成中多用途的构建模块,可以通过醛和酮的羟醛反应方便地合成。不幸的是,这些反应常常受到最初形成的β-羟基酮脱水的困扰。之前的研究表明,四面体3,5-二氟苯基硼酸酯是这种反应的高效且选择性催化剂。当前的调查研究涉及到不同取代模式的苯基硼酸酯在羟醛反应中的催化性能。看来,通过选择具有减少硼酸功能电子吸引效果的取代基和取代基位置,可以抑制脱水反应。β-羟基酮的脱水反应在对应于苯硼酸的化合物的pKa > 7时得到了最佳抑制。苯甲醛与丁酮或3-戊酮的反应没有显示出对映选择性,这表明催化剂仅仅是作为碱而非羟醛反应过渡态的模板。空间位阻更大的酮没有发生转化。