Department of Chemistry, New York University, 100 Washington Square East, New York, NY 10003, USA.
Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, 2300 RA, Leiden, The Netherlands.
Angew Chem Int Ed Engl. 2022 Oct 17;61(42):e202209401. doi: 10.1002/anie.202209401. Epub 2022 Sep 16.
Acetal substitution reactions of α-halogenated five- and six-membered rings can be highly stereoselective. Erosion of stereoselectivity occurs as nucleophilicity increases, which is consistent with additions to a halogen-stabilized oxocarbenium ion, not a three-membered-ring halonium ion. Computational investigations confirmed that the open-form oxocarbenium ions are the reactive intermediates involved. Kinetic studies suggest that hyperconjugative effects and through-space electrostatic interactions can both contribute to the stabilization of halogen-substituted oxocarbenium ions.
α-卤代五元和六元环的缩醛取代反应具有高度的立体选择性。随着亲核性的增加，立体选择性会逐渐减弱，这与卤代稳定的氧杂碳正离子的加成反应一致，而不是与三员环正卤离子的加成反应。计算研究证实，开环氧杂碳正离子是涉及的反应中间体。动力学研究表明，超共轭效应和空间静电相互作用都可以有助于稳定卤代氧杂碳正离子。
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