UPPER RIM CALIXCROWNS - ELUCIDATION OF THE MECHANISM OF CONFORMATIONAL INTERCONVERSION OF CALIX[4]ARENES BY QUANTITATIVE 2-D EXSY NMR-SPECTROSCOPY

Tetramethoxycalix[4]arenes (4-8) bridged by a polyether chain at the upper rim (upper rim calixcrowns) were synthesized by reaction of 5,17-bis(chloromethyl)-25,26,27,28-tetramethoxycalix[4]arene (3) with poly(oxyethylene) glycols of different length (mono to penta). In CDCl3 solution at -20-degrees-C, these compounds exist as a mixture of two conformations, viz. the partial cone (P) and the cone (C). The P/C ratio varies between 4.0 and 1.7 depending on the length of the bridge. The upper rim calixcrowns are flexible molecules according to temperature-dependent H-1 NMR spectroscopy. For the first time the mechanism of conformational interconversion of calix[4]arenes (4 and 5) was elucidated by quantitative 2-D EXSY NMR spectroscopy. It proves that there are two different pathways (a slow and a fast process) by which the two P conformational topomers can interconvert. One interconversion proceeds via C (slow process), and the other process proceeds most likely via a short-lived 1,3-alternate (A) intermediate (fast process). The rate constants for both pathways were determined; calix[4]crown-2 4, k(PA) = 0.58, k(PC) = 0.054, and k(CP) = 0.27 s-1 at 298 K, and calix[4]crown-3 5, k(PA) = 5.4, k(PC) = 0.07, and k(CP) = 0.18 s-1 at 253 K. The overall rate diminishes dramatically going from a nine-atom bridge in 5 to a six-atom bridge in 4. Furthermore, shortening of the bridge has a much larger decelerating effect on the P --> A than on the P --> C process.