A new procedure to measure the equilibrium constants for the dimerization (homochiral and heterochiral) reactions of enantiomers in solution was applied to two different compounds, namely omeprazole and Pirkle×s alcohol, both in CHCl 3 . This procedure is based on the measurement of the optical rotation of the solution as a function of its composition, which exhibits a nonlinear dependence on the enantiomer enrichment. Such nonlinearity depends on the extent of dimerization, and it is a strong effect in the case of omeprazole, whose equilibrium constants are 14.0 AE 3.5 l/mol and 25.2 AE 4.0 l/mol for the formation of homochiral and of heterochiral dimers, respectively. Pirkle×s alcohol exhibits a weaker effect that allows only to estimate the order of magnitude of these constants, i.e., ca. 0.10 and 0.34 l/mol, respectively. Although, in solution, enantiomers and racemates of chiral compounds usually exhibit identical NMR spectra, small differences in the chemical shifts are sometimes observed (see [18] for an overview, as well as the ref. cit. therein). It is also known that nonracemic mixtures of enantiomers of compounds that strongly self-associate, e.g., amino alcohols, amides, and carboxylic acids, can exhibit anisochrony, thus reflecting the enantiomer composition even in the absence of shift reagents [19]. Association is often limited to the formation of dimers that may take place through the formation of H-bonds between enantiomers. This is the case for the enantiomers of the chiral [1,1'-binaphthalene]-2,2'-diol ( binaphthol) and 1-(anthracen-9-yl)-2,2,2-trifluoroethanol ( Pirkle×s alcohol). In the case of the enantiomers of binaphthol in CHCl 3 , we have recently been able, using an achiral chromatography column, to collect two fractions constituted of the pure enantiomer present in excess (reaching 100% e.e.) and of the racemic mixture, respectively [20]. In the case of Pirkle×s alcohol, the enantiomer enrichment of a nonracemic solution in hexane/CH 2 Cl 2 was also achieved through achiral chromatography on a silica stationary phase [12].

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The synthesis of the superphanes 4-8 could be achieved from {(1,2,11,12-η 4 )-tricyclo-[10.8.0.0 2,11 ]eicosa-1,11-diene-6,17-diyne}(η 5 -cyclopentadienyl)cobalt ( 15) and its methoxycarbonyl congener 14 by heating with R-CpCo(CO) 2 (R ) H, CH 3 , (CH 3 ) 5 ). X-ray investigations on the superphanes 3, 6, and 8 reveal distances between the two cyclobutadiene units between 2.922 Å (3) and 2.941 Å (6). Investigations by means of cyclic voltammetry show a decrease of the first oxidation potentials with increasing number of CH 3 substituents at the cyclopentadienyl ligands. The comparison between the first oxidation potentials of 2-6, 4-7, and 5-8 gives evidence for a partial charge delocalization over both CpCoCb fragments of the superphanes.

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