Preparation of optically active bridged biphenyls via a stereoselective synthesis of their mono (tricarbonylchromium) complexes. Chiroptical properties and absolute chiralities of the complexes and ligands12 |
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Authors: | M Eyer K Schlögl R Schölm |
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Institution: | Institut für Organische Chemie der Universität Wien, A-1090 Wien, Austria |
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Abstract: | Reaction of the mono(tricarbonylchromium) complex 3 of diphenic acid anhydride with chiral amines gave the optically active imides 9-11. 10 and 11, from (S)-(?) and (R)(+)-phenylalaninol, respectively were separated by chromatography into exo- and endo-diastereomers (a and b) with opposite metallocene chiralities but with the same axial chirality. The (S)-(?) amino-alcohol induces (S)a-chirality and vice versa. The absolute configurations of the metallocene and axial-chiral parts of the biphenyl system were deduced from the circular dichroism spectra, the former (R)m or (S)m] by comparison with the CD of benchrotrenes of known absolute configuration, the latter (R)a or (S)a] by applying the exciton model of coupled oscillators to the optically stable biphenyl ligands i.e. the imides (?)- and (+)-8 easily accessible from 10 and 11 by photochemical decomplexation]. (R)m-(S)a for the complex (+)-10a (exo) and (S)m-(S)a for the endo-isomer (?)-10b, and vice versa for the complex 11. The parent ligands have the configurations (S)a(?)-8 and (R)a(+)-8, respectively.Photochemical cleavage of the optically active bis(tricarbonylchromium) complex (?)-13 of the biphenyl lactone 12 at ?60°C afforded optically labile 12 with a half-life time of racemization of ca 10 min at ?20°. On the basis of its CD the configuration (R)a is tentatively proposed for 12.The work described represents the first example of the preparation of optically-active biphenyls via benchrotrene precursors. |
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