Conformational Studies by Dynamic NMR. 86. Structure, Stereodynamics, and Cryogenic Enantioseparation of the Stereolabile Isomers of o-Dinaphthylphenyl Derivatives

Static and dynamic stereochemistry of the hydrocarbon comprising a phenyl ring bearing two α-naphthyl substituents in the ortho positions, i.e., 1,2-di-(4-methyl-naphth-1-yl)-benzene 1, has been studied by a combination of variable temperature NMR, cryogenic HPLC, and MM calculations. Whereas in solution both syn (meso) and anti (chiral) forms were observed and the corresponding interconversion barrier was determined (ΔG^⧧ = 19.5 kcal mol^-1), only the diastereoisomer anti was found to be present in the crystalline state (X-ray diffraction). When the molecule is rendered asymmetric by introduction of a nitro group in the phenyl ring as in 1,2-di-(4-methyl-naphth-1-yl)-4-nitrobenzene 2, the chiral syn and anti diastereoisomers are simultaneously present both in solution and in the solid state, albeit in different proportions. Cryogenic chromatography on a HPLC chiral stationary phase at −20 °C allowed the stereolabile diastereoisomers and the corresponding enantiomers to be separated.