Self-Assembly of Conformationally Rigid Dialcohols (Bis-Benzocyclobutenols): Supramolecular Cyclophanes and Arrays

Bis-benzocyclobutenols are synthetically important intermediates, and their structures have been established for the first time. These unique molecular systems allow investigation of the hydrogen bond-mediated aggregation of diols with rigid disposition of the hydroxyl groups on both sides of a flat arene core, namely bis-benzocyclobutene. The <i>syn</i>-diols <b>1a</b> and <b>2a</b> were found to assemble via O–H···O hydrogen bonds between the hydroxyl groups into dimers, which may be termed “supramolecular cyclophanes”. However, the analogous diols that are sterically encumbered by ethyl and phenyl groups were found to aggregate via adoption of tetrameric (<i>syn</i>-<b>2c</b>) or water-expanded dimeric (<i>syn</i>-<b>2b</b>) supramolecular synthons. The sterically unhindered <i>anti</i> diols were found to self-assemble via helical supramolecular synthon in contrast to the sterically encumbered ones; the latter were found to display preference to undergo cyclic tetrameric aggregation. The results thus point to intriguing possibilities in the adoption of supramolecular synthons when a given diol is constrained by the obligation to exploit two or more functional groups of the same kind