Conformational studies by dynamic nuclear magnetic resonance .59. Stereodynamics of conformational enantiomers in the atropisomers of hindered naphthylcarbinols

Naphthyldialkylmethanols ArRR'COH (Ar = 1-naphthyl or 1-naphthyl-2-methyl exist as a pair of atropisomers created by the restricted rotation about the Ar-COH bond. They can be detected by low-temperature NMR spectroscopy but can also be separated as stable compounds at room temperature if both the alkyl substituents are bulky tert-butyl groups (one such example is provided by compound 1, R = R' = But with Ar = 1-naphthyl). The free energies of activation (Delta G double dagger) for the interconversion of these atropisomers were found to vary between 7.6 kcal mol(-1) (as in 7, R = R' Me, Ar = 1-naphthyl-2-methyl) and 32.9 kcal mol(-1) (as in 1). The syn-periplanar (sp) or antiperiplanar tap) structures were assigned either by means of difference NOE experiments or by taking advantage of the H-8 chemical shifts which are vastly different in the two atropisomers. Depending on the substituents the more stable species at the equilibrium can be either the sp or the ap atropisomer. When R = R' = Pr-i and R = R' = Et (respectively 2 and 3 if Ar = 1-naphthyl), the sp atropisomers adopt an asymmetric conformation, thus creating a pair of conformational enantiomers which interchange by rotating the isopropyl or the ethyl groups about the appropriate sp(3)-sp(3) bonds, with Delta G double dagger values of 7.2 and 6.1 kcal mol(-1), respectively. On the contrary the corresponding 3-ap and 2-ap atropisomers adopt a symmetric (meso) conformation, as predicted by molecular mechanics calculations. In the case of R = Pr-i, R' = Et, and Ar = l-naphthyl (10-sp atropisomer), two asymmetric conformers were found to be appreciably populated (ratio 9:1 at -135 degrees C).