A series of calix[4]arenes containing a single
p-nitrophenol unit (or two p-nitrophenol units) have
been synthesized by fragment condensation. Their first acid constant
(pKa1) has been determined in
2-methoxyethanol–water (9:1) by optical titration.
Relative to the corresponding linear trimers with a
p-nitrophenol in the middle, a decrease of
pKa1 by 2.1 units or more is observed, which can be
explained entirely by intramolecular hydrogen bonds stabilising the
monoanion. In this way electron-withdrawing p-substituents in
the opposite phenolic unit lead to a further decrease in
pKa1, while a distortion of the cone conformation by
m-methyl groups causes a slight increase. The structure of one
calix[4]arene was further confirmed by single crystal X-ray analysis
showing the molecule in the usual cone conformation.
通过片段缩合法合成了一系列含有一个对硝基苯酚单元(或两个对硝基苯酚单元)的钙[4]炔。通过光学滴定法测定了它们在 2-甲氧基乙醇-水(9:1)中的第一酸常数(pKa1)。 与中间含有一个对硝基苯酚的相应线性三聚体相比,pKa1 下降了 2.1 个单位或更多,这完全可以用稳定单阴离子的分子内氢键来解释。这样,相反的酚单元中的抽电子 p 取代基会导致 pKa1 进一步降低,而 m-甲基基团对锥形构象的扭曲则会导致 pKa1 略微升高。单晶 X 射线分析进一步证实了一种钙[4]炔的结构,显示该分子呈通常的锥形构象。
A stepwise synthesis of functionalized calix [4]arenes and a calix[6]arene with alternate electron-withdrawing substituents
作者:Javier de Mendoza、Pedro M. Nieto、Pilar Prados、Concha Sánchez
DOI:10.1016/s0040-4020(01)85448-x
日期:1990.1
The acidity of calix[5]arenes and their linear analogues
lower (ΔpKa > 2) than for the trimer, while no difference has been found in comparison to structurally analogous calix[4]arenes. Rigidification of the calix[5]arene skeleton by introducing a single alkanediyl bridge opposite to the p-nitrophenol unit has no effect on pKal. All values are in agreement with a stabilisation of the monoanion via two chains of consecutive intramolecular hydrogen bonds OH…OH…O(−)