Stereolabile and Configurationally Stable Atropisomers of Hindered Aryl Carbinols.

Carbinols of the Ar-C(OH)R2 type, Ar being ortho-isopropyl phenyl, exist as stereolabile syn-clinal (sc) and anti-periplanar (ap) atropisomers when R = Me, Et, i-Pr. In the latter compound the major atropisomer also comprises two enantiomeric forms that interchange with a barrier of 6.4 kcal mol-1. X-ray diffraction, NOE experiments and ab-initio calculations indicate that the sc-atropisomer is the more stable form when R = Me, i-Pr, t-Bu but is the less stable one when R = Et. NMR spectra at variable temperature allowed the determination of the barriers for the interconversion of the sc- into the ap-atropisomers (DG¹= 7.6, 8.8 and 13.5 kcal mol-1 for Me, Et, i-Pr, respectively). When R is a tert-butyl group the two atropisomers are configurationally stable: the ap-atropisomer is obtained as the kinetic controlled compound, which can be transformed into the thermodynamically more stable sc-atropisomer with a free energy of activation of 29.3 kcal mol-1. Both atropisomers exhibit restricted rotation of the tert-butyl moiety, the corresponding DG¹ values being 9.4 and 8.8 kcal mol-1 for the sc- and ap-atropisomer, respectively.