Self-assembling cyclophanes and catenanes possessing elements of planar chirality

Abstract

Cyclobis(paraquat-1,5-naphthalene) has been prepared by a template-directed synthesis. As a result of the presence of the two chiral planes generated by its 1,5-disubstituted naphthalene residues, this compound can exist in three different stereoisomeric forms, an achiral (RS) form and two chiral forms, (RR) and (SS). Although the racemic modification could not be separated from the meso isomer, dynamic NMR spectroscopy together with molecular modeling indicate a diastereoisomeric ratio of 3:1 in favor of the (RR/SS) form. In addition, a couple of [2]catenanes in which the π-electron-deficient cyclobis(paraquat-1,5-naphthalene) component is interlocked with one of the π-electron-rich macrocyclic polyethers bis-p-phenylene[34]crown-10 (BPP34C10) or bis-1/5-dinaphtho[38]crown-10 (1/5DN38C10) self-assemble with diastereoselectivities in the range 50-60%. Relative stereochemical assignments for the different stereoisomeric forms have been achieved by dynamic NMR spectroscopy. In both [2]catenanes, the diastereoisomers in which the tetracationic cyclophanes have local D2 symmetry predominate. When the dynamic processes occurring within the [2]catenane containing the 1/5DN38C10 macrocycle are slow on the NMR timescale, a third element of planar chirality can be observed. This element is associated with the inside 1,5-dioxynaphthalene residue belonging to the crown ether and is orthogonal to the two chiral planes already present within the skeleton of the tetracationic cyclophane component. After repeated fractional crystallizations, single crystals were obtained, and one was subjected to X-ray crystallography. The 1,5-disubstituted naphthalene residues in both the cyclobis(paraquat-1,5-naphthalene) and the 1/5DN38C10 components were found to have (SS) chirality; that is, the [2]catenane undergoes spontaneous resolution on crystallization.