Crystal structure analyses facilitate understanding of synthesis protocols in the preparation of 6,6′-dibromo-substituted BINOL compounds

A combination of crystallographic and spectroscopic techniques has been used in order to address thorough purification protocols for a series of atropisomeric 1,1'-binaphthalene-2,2'-diol (BINOL) derivatives to be used as building blocks for chiral nanoscale constructs. The compounds were obtained following an initial electrophilic aromatic bromination reaction at the 6,6' position of BINOL, which was used in either its racemic or its enantiopure form. The presence of regioisomeric compounds, in which bromine atoms are located at different positions of the naphthalene skeletons, could be hinted at by classical spectroscopy (H-1 NMR and IR) and fully confirmed by single-crystal X-ray analysis of the solids formed during crystal growth of the crude products, where solid solutions are identified in part due to the presence of bromine that is a relatively heavy atom. If purification by recrystallization in suitable solvents is not performed, further chemical elaboration by means of alkylation on the 2,2'-phenolic positions produces compounds in which the regioisomeric impurities are still present. The set of five X-ray structures highlights interesting packing features extremely useful for the crystal engineering of nanostructures integrating these axially chiral building blocks.