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首页> 外文期刊>Chemistry: A European journal >Electronic and Steric Substituent Influences on the Conformational Equilibria of Cyclohexyl Esters: The Anomeric Effect Is Not Anomalous!
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Electronic and Steric Substituent Influences on the Conformational Equilibria of Cyclohexyl Esters: The Anomeric Effect Is Not Anomalous!

机译：电子和立体取代基对环己基酯构象平衡的影响：端基效应并非异常！

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摘要

The cyclohexyl esters of a series of carboxylic acids, RCO_2H, spanning a range of electronegativities and quotients of steric hindrance for the R substituent (R=Me, Et, iPr, tBu, CF_3, CH_2Cl, CHCl_2, CCl_3, CH_2Br, CHBr_2, and CBr_3) were prepared. Their conformational equilivbria in CD_2Cl_2 were examined by low-temperature ~1H NMR spectroscopy to study the axial or equatorial orientation of the ester functionality with respect to the adopted chair conformation of the cyclohexane ring. The ab initio and DFT geometry-optimized structures and relative free energies of the axial and equatorial conformers were also calculated at the HF/6-311G~**, MP2/6-311G~**, and B3LYP/6-31G~** levels of theory, both in the gas phase and in solution. In th4e latter case, a self-consdistent isodensity polarized continuum model was employed. Only by including electron correlation in the modeling calculations for the solvated molecules was it possible to obtain a reasonable correlation between DELTAG~deg_calcd and DELTAG~deg_exp. Both the structures and the free energy differences of the axial and equatorial conformers were evaluated with respect to the factors ormally influencing conformational preference, namely, 1,3-diaxial steric interactions in the axial conformer and hyperconjugation. It was assessed that hypercon-jugative interactions, sigma_C-C/sigma_C-H and sigma_(C-O)~*, together with a steric effect-the destabilization of the equatorial conformer with inceasing bulkj of the R group-were the determinant factors for the position of the conformational equilibria. Thus, because hypercionjugation is held responsible as the mitigating factor for the anomeric effect in 2-substituted, six-membered saturated heterocyclic rings, and since it is also similarly responsible, at least partly, in these moosubstituted cyclohexanes for a preferential shift towards the axial conformer, the question is therefore raised: can the anomeric effect really be construed as anomalous?
机译：一系列羧酸的环己基酯RCO_2H跨越一定范围的电负性和R取代基的空间位阻商（R = Me，Et，iPr，tBu，CF_3，CH_2Cl，CHCl_2，CCl_3，CH_2Br，CHBr_2和制备CBr_3）。通过低温〜1H NMR光谱检查了它们在CD_2Cl_2中的构象平衡，以研究酯官能度相对于采用的环己烷环的椅子构象的轴向或赤道取向。还在HF / 6-311G〜**，MP2 / 6-311G〜**和B3LYP / 6-31G〜*处计算了从头算和DFT几何优化的结构以及轴向和赤道构象的相对自由能。 *气相和溶液中的理论水平。在后一种情况下，采用自等价极化连续体模型。仅通过在溶剂化分子的建模计算中包括电子相关性，才可能在DELTAG_deg_calcd和DELTAG_deg_exp之间获得合理的相关性。轴向和赤道构象异构体的结构和自由能差异都针对影响构象偏好的因素进行了评估，即轴向构象异构体中的1,3-双轴空间相互作用和超共轭。据评估，超共轭相互作用，sigma_C-C / sigma_C-H和sigma_（CO）〜*，以及空间效应-赤道构象体的不稳定和R基团的增加j-是决定性的决定因素。构象平衡的位置。因此，因为超阳离子接合被认为是2-取代的六元饱和杂环中的异头作用的缓解因子，并且由于它同样至少部分地在这些moo取代的环己烷中负责优先向轴向转移。顺应者，因此提出了一个问题：异头作用真的可以解释为异常吗？

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