Thin Layer Chromatography with an Immobilized Azacyclophane Derivative as Analytical Agent and Probe for Host‐Guest Interactions

Abstract

131 Fluorescence spectroscopic measurements at higher concentrations, and methods which are based on changes in solubility or partition coefficients by the addition of complexing host compounds, do not enable any differentiation between surface association and inclusion [2dJ and, in certain cases give errors of more than a 100% in the complexation energies (H. [']. We have found that easily handleable chromatographic techniques are accessible by immobilization of the azacyclophane 1 on silica gel layers. As a rule, the azacyclophane derivative does not hinder detection by fluorescence under the UV lamp or by customary developing reagents [']. On application of the dissolved substances the starting spots are usually less spread out than on untreated layers. We hoped, by means of this technique, also to obtain information about the nature of host-guest interactions.-Experiments with aromatic guest molecules, inter alia with azo dyes, show that only compounds with acidic groups, particularly groups such as R=S03H, are selectively bound. Hereby, we define the selectivity S by the percent decrease in Rf obtained with 1. Besides the acidity of the substituents, the geometry of the hydrocarbon framework also has a decisive influence on the selectivity (cf. 5, 6 ) ; this is consistent with the order of lipophilicity and complexation observed, inter alia, in solution by NMR spectro~copy~~~. For the selective binding of different substrates, a most effective inclusion as possible in the cavity of immobilized 1 is essential. This follows from the generally far higher selectivities of naphthyl derivatives compared to the corresponding phenyl derivatives as well as from the considerably lower selectivities which are found with the immobilized open-chain analogue 2 : Here the S values obtained with sulfonic acids are only 20-40% instead of an average of 88%.Carboxylic acids are still complexed selectively if the lipophilic residue fits in the cavity of 1 (Scheme 2, compare 10 with 11 and 13) or if the electrostatic attraction can be realized by several substituents (cf. 12, 15). Phenolic groups are not acidic enough to lead to selectivity in methanol. On the other hand, the addition of water in this case leads, as also in solution"], to a strengthening of the binding (cf. 10, Scheme 2). A simple strategy for increasing the selectivity in the case of substrates with weakly dissociating groups consists in the use of an alkaline solvent, which enables novel separations to be realized, also in the case of natural products (Scheme 3). In the case of amino acids and peptides, protecting groups on the nitrogen which reduce the betaine participation and enhance the lipophilicity usually effect an increase in the selectivity.