Conformation, Inversion Barrier, and Solvent-Induced Conformational Shift in <i>Exo</i>- and <i>Endo</i>/<i>Exo</i>-Calix[4]arenes
作者:Silvio E. Biali、Volker Böhmer、Jörg Brenn、Michael Frings、Iris Thondorf、Walter Vogt、Jens Wöhnert
DOI:10.1021/jo970967u
日期:1997.11.1
solution exo-calixarene 5a adopts a 1,2-alternate conformation with the methyl group at the bridge located in an axial position, while in DMSO-d(6) the conformation adopted is the partial cone. Similar solvent-induced conformational shifts were found for the exo-calixarenes 3b and 3d. MM3 calculations predict that the cone form is the lowest energy conformation of 4 and the exo-calixarenes 3 and 5. The calculations
通过逐步策略合成在末端和外位具有羟基的杯芳烃4a和4b以及乙二氧基桥接的外杯芳烃5a。获得了4a和外杯芳烃3d的单晶X射线结构,表明分子以1,2-交替构象存在,在溶液中4a,b也存在。4a和4b的反转势垒(10.3和10.8 kcal mol(-1))与内二羟基杯芳烃12的反转势垒相似,这表明exo OH基团之间的额外分子内氢键不会降低分子的柔韧性。在CDCl(3)溶液中,exo-calixarene 5a采用1,2-交替构型,其中甲基位于轴向位置的桥上,而在DMSO-d(6)中,采用的构型是部分圆锥。对于exo-calixarenes 3b和3d,发现了类似的溶剂诱导的构象变化。MM3计算预测圆锥形式是4的最低能量构象,外杯芳烃3和5。该计算表明桥基上的甲基对于轴向或赤道位置的构象偏好在很大程度上取决于与羟基的排斥性空间相互作用。通过计算令人满意地再现了4b的反转势垒,这表明外环的旋转比