title: Stereolabile and configurationally stable atropisomers of hindered aryl carbinols.
abstract: Carbinols of the Ar-C(OH)R(2) type, Ar being o-isopropylphenyl, 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 (DeltaG++ = 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 DeltaG++ values being 9.4 and 8.8 kcal mol(-1) for the sc- and ap-atropisomer, respectively.
authors.affiliation: Department of Chemistry, University of Basilicata, Potenza, Italy.
authors.forename: Daniele
authors.initials: D
authors.lastname: Casarini
authors.forename: Carmine
authors.initials: C
authors.lastname: Coluccini
authors.forename: Lodovico
authors.initials: L
authors.lastname: Lunazzi
authors.forename: Andrea
authors.initials: A
authors.lastname: Mazzanti