Constitutional isomers of brominated-functionalized copillar[5]arenes: synthesis, characterization, and crystal structures

Abstract

We herein report the preparation of constitutional isomers of brominated-functionalized pillar[5]arenes via co-condensation of 1,4-bis(2-bromoethoxy)benzene and 1,4-dimethoxybenzene. The structures of the obtained isomers were then established using single crystal X-ray diffraction. We also found that the isomeric yield distribution of the different constitutional isomers was independent of the monomer's mole feed ratio, as revealed by HPLC analysis of the crude mixture. Finally, further characterization of the separated constitutional isomers indicated that they possess different melting points, NMR spectra, crystal structures, binding constants and stacking patterns in the solid state.

Fig. 1. Chemical structures of all possible pillar[5]arenes from the co-cyclization reaction of 1,4-dimethoxybenzene with1,4-bis(2-bromoethoxy)benzene.

Fig. 2. ¹H NMR spectra (600 MHz, CDCl₃) of the constitutional isomers of Pillars-3a–3b and 4a–4b and insets showing the X-ray single crystal diffraction structures. (a) The 1,3-alternate isomer, Pillar-3a, (b) the 1,2-alternate isomer, Pillar-3b, (c) the 1,2-alternate isomer, Pillar-4a, and (d) the 1,3-alternate isomer, Pillar-4b.

Fig. 3. Crystal structures of the constitutional isomers of tetra-and hexabromo-functionalized pillar[5]arenes obtained for single crystal X-ray diffraction analysis.

Fig. 4. HPLC chromatogram for the crude reaction mixtures of the copillar[5]arenes synthesized from monomers 1,4-dimethoxybenzene and 1,4-bis(2-bromoethoxy)benzene and insets showing the chromatogram of separated constitutional isomers of Pillar-3 and Pillar-4 (Table 1, entry 6).

Fig. 5. ¹H NMR (600 MHz, CDCl₃ at 298 K) spectra of (a) 8.4 mM OMA, (b) 3.4 × 10⁻²M of Pillar-3a and 8.4 mM OMA, and (c) 8.4 mM of Pillar-3a.