MOLECULAR MECCANO, PART 23 - SELF-ASSEMBLING CYCLOPHANES AND CATENANES POSSESSING ELEMENTS OF PLANAR CHIRALITY

Cyclobis(paraquat-l,5-naphthalene) has been prepared by a template-dir ected 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) fo rm and two chiral forms, (RR) and (SS). Although the racemic modificat ion could not be separated from the meso isomer, dynamic NMR spectrosc opy together with molecular modeling indicate a diastereoisomeric rati o of 3:1 in favor of the (RR/SS) form. In addition, a couple of [2]cat enanes in which the pi-electron-deficient cyclobis(paraquat-1,5-naphth alene) component is interlocked with one of the pi-electron-rich macro cyclic polyethers bis-p-phenylene[34] crown-10 (BPP34C10) or bis-1/5-d inaphtho[38]-crown-10 (1/5DN38C10) self-assemble with diastereoselecti vities in the range 50-60%. Relative stereochemical assignments for th e different stereoisomeric forms have been achieved by dynamic NMR spe ctroscopy. In both [2]catenanes, the diastereoisomers in which the tet racationic cyclophanes have local D-2 symmetry predominate. When the d ynamic processes occurring within the [2]catenane containing the 1/5DN 38C10 macrocycle are slow on the NMR timescale, a third element of pla nar chirality can be observed. This element is associated with the ins ide 1,5-dioxynaphthalene residue belonging to the crown ether and is o rthogonal 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 15-disubstituted naphthalene residues i n both the cyclobis(paraquat-1,5-naphthalene) and the 1/5DN38C10 compo nents were found to have (SS) chirality; that is, the [2]catenane unde rgoes spontaneous resolution on crystallization.