Martins Francisco A, de Azevedo Santos Lucas, Rodrigues Silva Daniela, Fonseca Guerra Célia, Bickelhaupt F Matthias, Freitas Matheus P
Department of Chemistry, Institute of Natural Sciences, Federal University of Lavras, 37200-900 Lavras, Minas Gerais, Brazil.
Department of Theoretical Chemistry, Amsterdam Institute of Molecular and Life Sciences (AIMMS), Amsterdam Center for Multiscale Modeling (ACMM), Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands.
J Org Chem. 2022 Sep 2;87(17):11625-11633. doi: 10.1021/acs.joc.2c01258. Epub 2022 Aug 19.
The conformer in 1-X,2-Y-disubstituted ethanes, that is, the staggered orientation in which X and Y are in closer contact, is only favored for relatively small substituents that do not give rise to large X···Y steric repulsion. For more diffuse substituents, weakly attractive orbital interactions between antiperiplanar bonds (i.e., hyperconjugation) cannot overrule the repulsive forces between X and Y. Our quantum chemical analyses of the rotational isomerism of XCHCHY (X = F, OH; Y = I) at ZORA-BP86-D3(BJ)/QZ4P reveal that indeed the conformer is generally favored due to a less destabilizing I···F and I···O-H steric repulsion. The only case when the conformer is preferred is when the hydroxyl hydrogen is oriented toward the iodine atom in the 2-iodoethanol. This is because of the significantly stabilizing covalent component of the I···H-O intramolecular hydrogen bond. Therefore, we show that strong intramolecular interactions can overcome the steric repulsion between bulky substituents in 1,2-disubstituted ethanes and cause the effect. Our quantum chemical computations have guided nuclear magnetic resonance experiments that confirm the increase in the population as X goes from F to OH.
在1-X,2-Y-二取代乙烷中，即X和Y处于更紧密接触的交错构象，仅对于不会产生大的X···Y空间排斥的相对较小的取代基才是有利的。对于更弥散的取代基，反式共平面键之间的弱吸引轨道相互作用（即超共轭）无法克服X和Y之间的排斥力。我们在ZORA-BP86-D3(BJ)/QZ4P水平下对XCHCHY（X = F，OH；Y = I）的旋转异构进行的量子化学分析表明，由于I···F和I···O-H空间排斥的去稳定作用较小，构象通常是有利的。构象更受青睐的唯一情况是在2-碘乙醇中羟基氢朝向碘原子时。这是因为I···H-O分子内氢键具有显著的稳定共价成分。因此，我们表明强分子内相互作用可以克服1,2-二取代乙烷中庞大取代基之间的空间排斥并导致效应。我们的量子化学计算指导了核磁共振实验，证实了随着X从F变为OH，构象体数量增加。
