Institute for Integrated Catalysis, Pacific Northwest National Laboratory , P.O. Box 999, Richland, Washington 99352, United States.
Department of Chemistry and Catalysis Research Center, TU München , Lichtenbergstrasse 4, 85748 Garching, Germany.
J Am Chem Soc. 2017 Jul 12;139(27):9178-9185. doi: 10.1021/jacs.7b02153. Epub 2017 Jun 27.
The reaction mechanism of solid-acid-catalyzed phenol alkylation with cyclohexanol and cyclohexene in the apolar solvent decalin has been studied using in situ C MAS NMR spectroscopy. Phenol alkylation with cyclohexanol sets in only after a majority of cyclohexanol is dehydrated to cyclohexene. As phenol and cyclohexanol show similar adsorption strength, this strict reaction sequence is not caused by the limited access of phenol to cyclohexanol, but is due to the absence of a reactive electrophile as long as a significant fraction of cyclohexanol is present. C isotope labeling demonstrates that the reactive electrophile, the cyclohexyl carbenium ion, is directly formed in a protonation step when cyclohexene is the coreactant. In the presence of cyclohexanol, its protonated dimers at Brønsted acid sites hinder the adsorption of cyclohexene and the formation of a carbenium ion. Thus, it is demonstrated that protonated cyclohexanol dimers dehydrate without the formation of a carbenium ion, which would otherwise have contributed to the alkylation in the kinetically relevant step. Isotope scrambling shows that intramolecular rearrangement of cyclohexyl phenyl ether does not significantly contribute to alkylation at the aromatic ring.
使用原位 C MAS NMR 光谱研究了在非极性溶剂十氢萘中固体酸催化的苯酚与环己醇和环己烯的烷基化反应机理。只有在大多数环己醇脱水为环己烯后，苯酚与环己醇的烷基化反应才会开始。由于苯酚和环己醇的吸附强度相似，这种严格的反应顺序不是由于苯酚对环己醇的有限接触引起的，而是由于只要存在相当一部分环己醇，就不存在反应性亲电试剂。C 同位素标记表明，当环己烯为共反应物时，反应性亲电试剂环己基碳正离子直接在质子化步骤中形成。在环己醇存在下，其在 Brønsted 酸位上质子化的二聚体阻碍了环己烯的吸附和碳正离子的形成。因此，证明了质子化的环己醇二聚体在没有形成碳正离子的情况下脱水，否则这将有助于在动力学相关步骤中的烷基化。同位素交换表明，环己基苯基醚的分子内重排对芳环上的烷基化没有显著贡献。
