摘要/Abstract
本工作使用简单易得的三氟乙酸酐作为活化试剂，拓展了Stieglitz重排反应的底物适用范围，发展了一种通过C—C键断裂由苯乙基羟胺制备芳基伯胺的方法.该反应条件较为温和且对官能团具有较好的兼容性.机理研究表明，反应经历了活性三氟乙酸酯中间体的原位生成，并通过C—C和N—O键的断裂实现芳基迁移的过程.
关键词: C—C键断裂, Stieglitz重排反应, 羟胺, 苯胺, 芳基迁移
Hydroxylamines have a wide range of biological properties and have been used as a useful synthon in organic synthesis. In the past decades, many transformations of hydroxylamines have been developed and widely applied. In contrast, one of the interesting reactions of hydroxylamines through C—C bond cleavage, named Stieglitz rearrangement, was less developed. Due to the poor leaving ability of the hydroxyl groups, the reported Stieglitz rearrangement reactions suffered from the harsh conditions and the very limited substrate scope with triarylmethyl hydroxylamine substrates. Since an interesting C—C bond cleavage is involved which will extend the synthetic application of hydroxylamine, the practical method under mild conditions with broad substrate scope for Stieglitz rearrangement is very desired. However, there are three potential problems which need to be addressed. First, the activator must selectively react with the hydroxyl group but not the N-nucleophile of the hydroxylamine substrates. Secondary, a suitable leaving group must be generated to weaken the N—O bond. In addition, the employed activator must be inactive to the formed imine intermediates and the subsequent amine products. Herein, we developed an efficient Stieglitz rearrangement reaction of hydroxylamines under mild conditions for the preparation of corresponding primary aryl amines. This chemistry using simple trifluoroacetic anhydride (TFAA) as an activator resolves the issues mentioned above and therefore provides a practical protocol for the further transformation and application of hydroxylamines. Mechanistic studies demonstrate that the in situ generation of an active trifluoroacetate leaving group derived the aryl migration process via both of the C—C and N—O bond cleavage. A general procedure for the TFAA assisted stieglitz rearrangement is as follows: BF3·Et2O (28.4 mg, 0.2 mmol), TFAA (46.2 mg, 0.22 mmol) were added to the solution of hydroxylamine (0.2 mmol) in 2 mL hexafluoroisopropanol (HFIP). The reaction mixture was stirred at room temperature for 1 h. After that, the reaction was quenched by 4 mL 2 mol/L NaOH (aq.) and extracted by the mixture of petroleum ether and ethyl acetate (1∶1, V∶V). The combined organic phase was concentrated, and purified by flash chromatography on a short silica gel to afford the desired product (eluent: petroleum ether/ethyl acetate).
Key words: C—C bond cleavage, Stieglitz rearrangement, hydroxylamine, aniline, aryl migration
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