Molecular Recognition and Membrane Transport with Mixed-Ligand Borates

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Recently, there has been a surge of interest in the use of boronic acid compounds as artificial receptors for a variety of guest structures.1-6 In particular, boronic acids show a remarkable ability to form reversible, covalent complexes with diol-containing compounds in aqueous solution. This has resulted in the production of a range of molecular devices such as carriers for membrane transport,1 solid supports for chromatography,2 enzyme inhibitors,3 and chemosensors.4,5 While progress in this research area has been rapid, much of the future work is likely to be slowed due to the difficulty in synthesizing the next generation of structurally more-complicated receptors, particularly those with the capability of enantioselective recognition.5 These thoughts have motivated us to consider a new strategy for molecular recognition using boron acids. The strategy is predicated on the ability of boric acid to form reversible, covalent 1:2 mixedligand complexes.7 As shown in Figure 1, one of the borate ligands is considered to be the host, and the other ligand to be the guest. The mixed-ligand borate, 1, is thus the host/guest complex. Compared to the boronic acid approach, this mixedligand borate strategy introduces certain advantages, as well as disadvantages. A significant advantage is the host is no longer a boronic acid, but a bidentate chelating compound which in this study is a diol (Figure 1). This is an attractive feature, from the point of view of synthesis, as the methodology for diol preparation is highly advanced (particularly enantioselective synthesis).8 In addition, diols are generally easier to purify and are more stable than boronic acids. Another advantage is that borate complexation is generally well understood. A number of research groups have conducted careful, long-term studies on borate complexation reactions with a range of chelating compounds.7,9 The most apparent disadvantage is the likelihood of forming non-functional 1:2 borate complexes with both ligands being host or guest compounds. However, as proved by this report, there are a number of applications where this complication should not be disabling. Here, we demonstrate the utility of this new recognition strategy by inducing selective glycoside transport through a liquid organic membrane.10