A Computational Study of Vicinal Fluorination in 2,3-Difluorobutane: Implications for Conformational Control in Alkane Chains
A Computational Study of Vicinal Fluorination in 2,3-Difluorobutane: Implications for Conformational Control in Alkane Chains
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DOI：
10.1002/chem.201405317
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发表时间：
2015-01-19
影响因子：
4.3
通讯作者：
Linclau, Bruno
中科院分区：
文献类型：
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作者：
Fox, Stephen J.;Gourdain, Stephanie;Linclau, Bruno
基于真空和溶液中的密度泛函理论计算以及溶液中的核磁共振实验，对 2,3-二氟丁烷非对映异构体进行了全面的构象分析。虽然对于 1,2-二氟乙烷来说，氟错觉效应显然是决定其构象的主要效应，但人们发现，对于 2,3-二氟丁烷来说，存在多种效应的复杂相互作用，这些效应的大小相似，但通常符号相反。结果，观察到二面角、相对构象能和布居数的意外偏差，而这些偏差不能仅通过化学直觉来合理化。此外，人们发现，考虑各种构象异构体的自由能很重要，因为与仅基于电子能的计算相比，这些构象异构体的自由能在真空和溶剂中会导致性质不同的结果。与预期相反，发现丁烷中邻位顺式二氟化物的引入以及由此延伸的更长的烃链预计不会导致线性构象的有效稳定。我们的研究结果对于使用连位二氟化物基序进行构象控制具有重要意义。
A comprehensive conformational analysis of both 2,3-difluorobutane diastereomers is presented based on density functional theory calculations in vacuum and in solution, as well as NMR experiments in solution. While for 1,2-difluoroethane the fluorine gauche effect is clearly the dominant effect determining its conformation, it was found that for 2,3-difluorobutane there is a complex interplay of several effects, which are of similar magnitude but often of opposite sign. As a result, unexpected deviations in dihedral angles, relative conformational energies and populations are observed which cannot be rationalised only by chemical intuition. Furthermore, it was found that it is important to consider the free energies of the various conformers, as these lead to qualitatively different results both in vacuum and in solvent, when compared to calculations based only on the electronic energies. In contrast to expectations, it was found that vicinal syn-difluoride introduction in the butane and by extension, longer hydrocarbon chains, is not expected to lead to an effective stabilisation of the linear conformation. Our findings have implications for the use of the vicinal difluoride motif for conformational control.