The conformational distributions and interconversions of partially methylated calix[4]arenes


Hoorn, Willem P. van and Morshuis, Martijn G.H. and Veggel, Frank C.J.M. van and Reinhoudt, David N. (1998) The conformational distributions and interconversions of partially methylated calix[4]arenes. Journal of physical chemistry A: Molecules, spectroscopy, kinetics, environment, & general theory, 102 (7). pp. 1130-1138. ISSN 1089-5639

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Abstract:The conformational distributions and Cone-Inverted Cone (Cone') interconversions of monomethoxy-, 1,2-dimethoxy-, 1,3-dimethoxy-, and trimethoxycalix[4]arenes 3a-6a were investigated by molecular modeling. The calculated conformational distributions are generally in accordance with NMR data obtained for the p-tert-butyl derivatives 3b-6b. Careful examination of all possible inversion pathways showed that the high inversion barriers of Cone-Cone' interconversion are caused by the cooperative barriers of hydrogen bonds rupture and the steric barrier of rotating a methoxy group through the annulus. This cooperative effect is so strong that the Cone Cone' conversion is blocked at room temperature. The order of the calculated barriers for the Cone-Inverted Cone interconversions is monomethoxy- (35.1 kcal mol-1) > 1,2-dimethoxy- (32.2 kcal mol-1)=1,3-dimethoxy- (30.3 kcal mol-1) > trimethoxycalix[4]arene (27.0 kcal mol-1), values that are in good agreement with the available qualitative experimental results. The rate-limiting step involved in all cases is the rotation of a methoxy-bearing ring.
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Copyright:© 1998 American Chemical Society
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Metis ID: 105927