Tuning of the cavity of water-soluble thiacalix[4]arene for the control of inclusion ability toward water-miscible organic molecules

Inclusion abilities of water-soluble thiacalix[4]arenetetrasulfonate (3) and mono-O-carboxy-methylated derivatives 5 and 6 toward water-miscible organic molecules such as alcohols, ketones, and nitriles in water were investigated by H-1 NMR. The limit chemical shift change of the guest upon inclusion in the host suggested that the hosts regioselectively encapsulate the guests from the side of aliphatic moiety. Large guests such as pentan-1-ol showed folding of the alkyl chain to be included inside the cavity. On the other hand, mono-O-substituted 6 included a guest molecule less deep in the cavity than 3 did. Binding constants ( K) of 3 toward guest molecules increased with hydrophobicity of the guest, suggesting that hydrophobic effect plays a main role for the complexation. On the contrary, mono-O-carboxymethyl derivatives 5 and 6 showed the opposite dependency of stability on hydrophobicity; higher affinities toward less hydrophobic guests. Among the guests examined, the smallest entities such as CH3OH and CH3CN were best included in 5 with the highest binding constant ever reported (K > 10(2) M-1). It was concluded that introduction of the carboxymethyl group into one of the phenolic OH of thiacalix[4] arene led to tuning of the cavity shape for stereospecific inclusion of small guests. The tuning of cavity by the O-alkylation was actually confirmed by X-ray crystal analysis of a complex of 5 with diethyl ketone.