Synthesis of 1, 4-Bis (n-propoxy) pillar [7] arene and its host-guest chemistry

Abstract

Pillar[n]arenes, as a new type of macrocyclic hosts, have attracted more and more attention in recent years. Their repeating units are connected by methylene bridges at the para-positions so they have a pillar architecture, which is different from the basket-shaped structure of meta-bridged calixarenes. Herein, 1, 4-Bis (n-propoxy) pillar [5] arene (P5), 1, 4-bis (n-propoxy) pillar [6] arene (P6) and 1, 4-bis (n-propoxy) pillar [7] arene (P7) were synthesized from the condensation of 1, 4-di (n-propoxy)-2, 5-bis (methoxymethyl) benzene using p-toluenesulfonic acid as the catalyst in dichloromethane. They were characterized by 1H NMR, 13C NMR and mass spectrometry. They had different 1H NMR spectra but similar 13C NMR spectra. The diameter of the internal cavity of P5 is ca. 4.6 Å, similar to those of cucurbit [6] uril (ca. 5.8 Å) and α-cyclodextrin (ca. 4.7 Å). The diameter of the internal cavity of P6 is ca. 6.7 Å, similar to those of cucurbit [7] uril (ca. 7.3 Å) and β-cyclodextrin (ca. 6.0 Å). The diameter of the internal cavity of P7 is ca. 8.7 Å, similar to those of cucurbit [8] uril (ca. 8.8 Å) and γ-cyclodextrin (ca. 7.5 Å). The host-guest binding properties of them were investigated with n-octyltriethyl ammonium hexafluorophosphate G as a model guest. P6 showed complexation with G while weak complexation was observed between G and P5, but no complexation occurred between P7 and G. The 1H NMR spectrum of an equimolar solution of P6 and G in chloroform-d showed only one set of peaks, indicating fast-exchange complexation between P6 and G on the 1H NMR time scale at 22°C. A mole ratio plot indicated that the complexation stoichiometry was 1:1 between P6 and G. This was further confirmed by a low resolution electrospray ionization mass spectroscopy peak at m/z 1451.7 corresponding to [P6⊃G-PF6]+. The association constant of the 1:1 complex P6⊃G in chloroform-d was determined to be (740±86) L•mol-1 by a proton NMR titration. These experiments demonstrate that G fits the cavity of P6 while it is too big for the cavity of P5 and too small for the cavity of P7. © 2012 Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences.