Rösel Sören, Becker Jonathan, Allen Wesley D, Schreiner Peter R
Institute of Organic Chemistry , Justus Liebig University , Heinrich-Buff-Ring 17 , 35392 Giessen , Germany.
Institute of Inorganic and Analytical Chemistry , Justus Liebig University , Heinrich-Buff-Ring 17 , 35392 Giessen , Germany.
J Am Chem Soc. 2018 Oct 31;140(43):14421-14432. doi: 10.1021/jacs.8b09145. Epub 2018 Oct 17.
The long-known, ubiquitously present, and always attractive London dispersion (LD) interaction was probed with hexaphenylethane (HPE) derivatives. A series of all- meta hydrocarbyl [Me, Pr, Bu, Cy, Ph, 1-adamantyl (Ad)]-substituted triphenylmethyl (TPM) derivatives [TPM-H, TPM-OH, (TPM-O), TPM] was synthesized en route, and several derivatives were characterized by single-crystal X-ray diffraction (SC-XRD). Multiple dimeric head-to-head SC-XRD structures feature an excellent geometric fit between the meta-substituents; this is particularly true for the sterically most demanding Bu and Ad substituents. NMR spectra of the Pr-, Bu-, and Cy-derived trityl radicals were obtained and reveal, together with EPR and UV-Vis spectroscopic data, that the effects of all- meta alkyl substitution on the electronic properties of the trityl scaffold are marginal. Therefore, we concluded that the most important factor for HPE stability arises from LD interactions. Beyond all- meta Bu-HPE we also identified the hitherto unreported all- meta Ad-HPE. An intricate mathematical analysis of the temperature-dependent dissociation constants allowed us to extract Δ G(exptl) = 0.3(5) kcal mol from NMR experiments for all- meta Bu-HPE, in good agreement with previous experimental values and B3LYP-D3(BJ)/def2-TZVPP(C-PCM) computations. These computations show a stabilizing trend with substituent size in line with all- meta Ad-HPE (Δ G(exptl) = 2.1(6) kcal mol) being more stable than its Bu congener. That is, large, rigid, and symmetric hydrocarbon moieties act as excellent dispersion energy donors. Provided a good geometric fit, they are able to stabilize labile molecules such as HPE via strong intramolecular LD interactions, even in solution.
利用六苯基乙烷（HPE）衍生物对长期以来广为人知、普遍存在且一直具有吸引力的伦敦色散（LD）相互作用进行了探究。在此过程中合成了一系列全间位烃基[甲基、丙基、丁基、环己基、苯基、1-金刚烷基（Ad）]取代的三苯甲基（TPM）衍生物[TPM-H、TPM-OH、(TPM-O)、TPM]，并通过单晶X射线衍射（SC-XRD）对几种衍生物进行了表征。多个二聚体头对头的SC-XRD结构显示间位取代基之间具有出色的几何匹配；对于空间位阻最大的丁基和金刚烷基取代基而言尤其如此。获得了丙基、丁基和环己基衍生的三苯甲基自由基的核磁共振谱，这些谱图与电子顺磁共振（EPR）和紫外可见光谱数据一起表明，全间位烷基取代对三苯甲基骨架电子性质的影响很小。因此，我们得出结论，HPE稳定性的最重要因素来自LD相互作用。除了全间位丁基-HPE，我们还鉴定出了迄今未报道的全间位金刚烷基-HPE。对温度依赖性解离常数进行的复杂数学分析使我们能够从核磁共振实验中提取全间位丁基-HPE的ΔG（实验值）= 0.3(5) kcal/mol，这与先前的实验值以及B3LYP-D3(BJ)/def2-TZVPP(C-PCM)计算结果吻合良好。这些计算表明，随着取代基尺寸增大呈现出稳定趋势，全间位金刚烷基-HPE（ΔG（实验值）= 2.1(6) kcal/mol）比其丁基同系物更稳定。也就是说，大的、刚性的和对称的烃基部分是出色的色散能供体。只要几何匹配良好，它们就能通过强分子内LD相互作用稳定诸如HPE之类的不稳定分子，即使在溶液中也是如此。
