Resolution and Racemization of a Planar-Chiral A1/A2-Disubstituted Pillar[5]arene

Abstract

Butoxycarbonyl (Boc)-protected pillar[4]arene[1]-diaminobenzene (BP) was synthesized by introducing the Boc protection onto the A1/A2 positions of BP. The oxygen-through-annulus rotation was partially inhibited because of the presence of the middle-sized Boc substituents. We succeeded in isolating the enantiopure RP (RP, RP, RP, RP, and RP)- and SP (SP, SP, SP, SP, and SP)-BP, and studied their circular dichroism (CD) spectral properties. As the Boc substituent is not large enough to completely prevent the flip of the benzene units, enantiopure BP-f1 underwent racemization in solution. It is found that the racemization kinetics is a function of the solvent and temperature employed. The chirality of the BP-f1 could be maintained in n-hexane and CH2Cl2 for a long period at room temperature, whereas increasing the temperature or using solvents that cannot enter into the cavity of BP-f1 accelerated the racemization of BP-f1. The racemization kinetics and the thermodynamic parameters of racemization were studied in several different organic solvents.