CONFORMATIONAL CHARACTERISTICS OF ETHERS AND ESTERS OF P-TERT-BUTYLCALIX[5]ARENE

As part of a program involving the chemistry of the calix[5]arenes, a variety of ethers and esters of p-tert-butylcalix[5]arene (Ib) have been synthesized and their conformational behavior studied by means of variable temperature H-1 NMR spectroscopy. The following is shown: (a) The Delta G(double dagger) values for conformational interconversion diminish progressively as the OH groups of Ib are changed to OMe groups, ascribed to continuously diminishing intramolecular hydrogen bonding that is not offset by the corresponding increase in steric hindrance. (b) The Delta G(double dagger) values for conformational interconversion of the pentaethers of Ib increase as the OR groups get larger, ascribed to increasing steric hindrance. (c) The Delta G(double dagger) values for the conformational interconversion of the pentaesters of Ib also increase with the size of the ester group and are somewhat higher than those for the pentaethers of corresponding carbon content, ascribed to the additional steric hindrance provided by the carbonyl oxygen. (d) The Delta G(double dagger) values for the conformational interconversion of the monoethers are higher than those of the corresponding pentaethers, ascribed to intramolecular hydrogen bonding and steric hindrance factors acting cooperatively. (e) Sufficiently large ether or ester groups confer Limited conformational mobility, as in the case of the mono-n-propyl ether and the cyclopropylcarbanoate of Ib, or complete conformational immobility, as in the case of the monobenzyl ether, pentabenzyl ether, pentaisobutanoate, pentapivaloate, pentabenzoate, and pentatosylate of Ib. The conformational assignments are supported in three cases with X-ray crystallographic structures of the pentaisopropyl ether, pentaallyl ether, and mono-n-propyl ether.