Alkanediyl Bridged Calix[4]arenes: Synthesis, Conformational Analysis, and Rotational Barriers
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- 作者：Silvio E. Biali ; Volker Bö ; hmer ; Shmuel Cohen ; George Ferguson ; Cordula Grü ; ttner ; Flavio Grynszpan ; Erich F. Paulus ; Iris Thondorf ; and Walter Vogt
- 刊名：Journal of the American Chemical Society
- 出版年：1996
- 出版时间：December 25, 1996
- 年：1996
- 卷：118
- 期：51
- 页码：12938 - 12949
- 全文大小：420K
- 年卷期：v.118,no.51(December 25, 1996)
- ISSN：1520-5126
文摘
In calix[4]arenes when one methylene bridge carries analkyl or aryl substituent, two diastereomericconeconformations are possible in which this substituent assumes theequatorial or axial position. Two diastereomerswith cis or trans arrangement of the substituentsexist for the corresponding compounds with two substitutedbridges,and diastereomeric cone conformations have to be consideredadditionally in most cases. Molecular mechanicscalculations predict an energetical preference of the equatorialposition of the substituents in both systems. Thispreference is markedly more pronounced for alkyl groups than for arylgroups. To test these predictions a series ofcalix[4]arenes in which one (4) or two opposite(5) methylene bridges are substituted by alkyl or arylgroups wassynthesized by fragment condensation. For these calixarenes thesolution conformations, the equatorial/axialconformational equilibria, and the energy barriers for thecone to cone ring inversion were determined by1H NMRspectroscopy. The experimental energy differences between the twocone conformations correlate well with thecalculated ones. Free energies of activation
G
for the cone tocone ring inversion of the monoalkylsubstitutedcompounds 4 increase in the order methyl <tert-butyl < ethyl < isopropyl. For the bisalkylsubstituted compounds(5b-d) only the cis-isomer could be isolatedwhile cis- and trans-isomers were obtained for5a and for the bisarylcompounds 5e-g. Among thecis-isomers 5a-d existexclusively as the equatorial conformers, while theconformational equilibrium is strongly solvent dependent for5e-g. Single crystal X-raystructures were obtainedfor several calixarenes with one (4b) or two substitutedbridges (5e-g). Here thesubstituents are found exclusivelyin the equatorial position, and the molecular conformation is similarto the calculated one.
G for the cone tocone ring inversion of the monoalkylsubstitutedcompounds 4 increase in the order methyl <tert-butyl < ethyl < isopropyl. For the bisalkylsubstituted compounds(5b-d) only the cis-isomer could be isolatedwhile cis- and trans-isomers were obtained for5a and for the bisarylcompounds 5e-g. Among thecis-isomers 5a-d existexclusively as the equatorial conformers, while theconformational equilibrium is strongly solvent dependent for5e-g. Single crystal X-raystructures were obtainedfor several calixarenes with one (4b) or two substitutedbridges (5e-g). Here thesubstituents are found exclusivelyin the equatorial position, and the molecular conformation is similarto the calculated one.