Conformational studies by dynamic NMR spectroscopy. Part 96: Stereomutations of highly hindered naphthylphenyl atropisomers in solution and in the solids

When a benzene ring bears two 2-methyl-1-naphthyl moieties in the para, meta or ortho positions as in 1,4-bis(2-methyl-1-naphthyl)benzene, , 1,3-bis(2-methyl-1-naphthyl)benzene, and 1,2-bis(2-methyl-1-naphthyl)benzene , two rotational isomers (atropisomers) are generated, with the two naphthyl substituents in a syn or anti relationship. In the case of the para and meta derivatives ( and , respectively) these atropisomers could not be separated but were detected by NMR spectroscopy, that also allowed the determination of their syn–anti interconversion barriers in solution (19.5 and 20.4 kcal mol−1, respectively) and, in the case of , also in the solid state (26.7 kcal mol−1). In the more hindered ortho derivative , the syn (meso) and anti (racemic) atropisomers interconvert in solution with a barrier (31.2 kcal mol−1) sufficiently high to allow their physical separation. The racemic form could also be separated (by enantioselective HPLC) into the PP and MM enantiomers. Analysis of the corresponding CD spectra allowed the assignment of the absolute configuration. When three such naphthyl substituents are bonded to the phenyl in a meta relationship, two atropisomers in statistical proportions were observed: the anti (Cs symmetry) and the syn (C3v symmetry) display a 3:1 ratio at the equilibrium in solution. This ratio is different in the solid state, as is the interconversion barrier (22.1 and 32.1 kcal mol−1 in solution and in the solid, respectively).