The Conformational Distributions and Interconversions of Partially Methylated Calix[4]arenes

摘要

The conformational distributions and Cone → Inverted Cone (Cone') interconversions of monomethoxy-, 1, 2-dimethoxy-1, 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.