Structure, Conformation, and Stereodynamics of the Atropisomers of Highly Hindered Benzyl Ethers

Low-temperature and NOE NMR spectra of four of the title compounds indicate that they adopt a synclinal (sc) conformation, in agreement with the prediction of ab initio computations. In the case of the most-hindered derivative (compound 4), the conformation is syn-periplanar (sp), as is also shown by X-ray diffraction. Such stereolabile sp- or sc-atropisomers exist as two conformational enantiomers:  the corresponding enantiomerization barriers, covering the range 6.6 to 9.7 kcal mol-1, could be measured for all the examined compounds. In two cases (compounds 3 and 5), the minor antiperiplanar (ap) atropisomer has been also observed, and the sc to ap interconversion barrier measured (11.7 and 11.9 kcal mol-1, respectively). In addition, restricted rotation of the isopropyl and tert-butyl substituents has been detected, and the corresponding barriers have been determined.