Solid state investigation of BINOL and BINOL derivatives: A contribution to enantioselective symmetry breaking by crystallization

Abstract

Investigation of the solid state of 1,1′-binaphthalene-2,2′-diol (BINOL) and two BINOL derivatives, 2′-benzyloxy-1,1′-binaphthalene-2-ol (BINOL-OBn) and 1,1′-binaphthalene-2,2′-diyl bispivaloate (BINOL-OPiv), a class of compounds which find wide application in multiple fields of modern chemistry, particularly in asymmetric catalysis, is presented. Crystalline structures of the two enantiomers of these BINOL derivatives are solved for the first time, with the molecules presenting an almost perpendicular orientation of the mean planes of the naphthyl moieties. The thermal behavior of pure enantiomers is investigated as well as the solid-liquid phase diagrams of binary enantiomer mixtures, a crucial information to envisage resolution by crystallization. No phase transitions other than fusion were observed in heating runs performed on BINOL-OBn and BINOL-OPiv enantiomers and, in contrast to BINOL enantiomers, vitrification occurs on melt cooling which is consistent with their higher molecular complexity. Both BINOL derivatives, unlike BINOL, whose racemate is a racemic compound, give rise to the formation of conglomerates and are therefore amenable for resolution by preferential crystallization. This solid state study gives a first evidence that appropriate structural modulation of BINOL molecule may open a way to preparation of enantiomeric pure BINOL derivatives from racemates.