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.