Employing a directed ortho-metalation route, chiral ferrocene-based hydroxyloxazolines have been synthesized, and their capability to serve as bidentate ligand precursors in the diethylzinc addition to benzaldehyde has been investigated. On the basis of the X-ray crystal structures of two ferrocenes, (S,R p )-6 and (S,S p )-10, it became possible to determine the general importance of the element of planar chirality for diastereomeric ferrocene complexes in this process. Comparison of the assumed transition states revealed a preferential catalyst conformation which led to the design of an adequate arene compound [(S)-20] that displayed catalytic activity similar to that of (S,R p )-6.

COMMUNICATIONSUsing the plates thus impregnated, we have succeeded in performing thin-layer chromatographic enantiomeric resolution via ligand exchange: 2 pL of each of the racemic a-amino acids in Table 1 was applied as a 1% solution to the TLC plates and eluted with the given eluent (30-45 min). After drying, the spots were visualized using 0.1% ninhydrin reagent. The resolution of the enantiomers was so good that the respective antipodes could be determined at trace levels; the lowest level of detection was found to be 2 1%, although this value can probably be improved. Received: September 14, 1983; witheld for publication at the authors' request [Z 549 IE] German version: Angew. Chem. 96 (1984) 514 Thin-Layer Chromatographic Enantiomeric Resolution via Ligand ExchangeBy Kurt Giinther, Jurgen Martens*, and Maren Schickedanz Today, gas"] and liquid chromatography ['] are the methods of choice for the resolution of enantiomeric organic compounds in analysis. However, these methods necessitate the use of expensive apparatus and, in part, derivatization of the samples. Chiral amino acids are increasingly gaining importance as building blocks for the preparation of biologically active peptides in asymmetric syntheses[31 and as p h a r m a~a~~] .A simple and rapid method of monitoring optical purity is therefore desirable.A glass plate covered with hydrophobiated silicic acidacetate solution (methanol :water= 1 : 9 v/v) and subsequently dried. The plate is then immersed in a 0.8% methanolic solution of the chiral selector (1 min). A suitable selector is (2S,4R,2'RS)-4-hydroxy-l-(2'-hydroxydodecyl)-proline l [']. After being allowed to dry in the air, the plate is ready for chromatography. ClOHZl Topological Charge Stabilization and the Equilibrium between PAS3 and AS,&**By Benjamin Gimarc* and P. J. Joseph Recently, Blachnik and Wickel"] reported the results of their experimental studies on the equilibrium system P4S3/ As4S3. When the reactants are mixed in the stoichiometric ratio (1 :3), the equilibrium lies predominantly to the side of PS3As3 [eq. (a)].P4S3 and As4& are cage or end-capped triangular prism structures with a unique apical atom, three equivalent bridging atoms, and three atoms in the equilateral triangular In these molecules the sulfur atoms occupy the bridging positions, while atoms of the less electronegative group 5B elements are located at apical and basal sites.Presumably the particularly stable equilibrium product PS3As3 has a similar structure, with a P atom at the apex, bridging S atoms, and basal As atoms. Blachnik and Wickel attribute the stability of this product to the greater strength of PS compared to ASS bonds and to the increase in the number of PS bonds from 6 in the educt to 12 in the prodWe report here an alternative explanation. The stability of PS3As3 is a beautiful example of the operation of the rule of "topological charge stabilization"[41. The P:" ion is isoelectronic and isostructural ['] with P4S3, AS&, and uct P S~A S~.[

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