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Chiral-at-metal complexes are used in many areas of modern chemistry. Besides applications in chemical biology, particularly the usage as catalysts has gained much attention lately. Single enantiomers with high enantiopurity are needed for molecular recognition and asymmetric catalysis. In order to increase the accessibility of enantiopure octahedral cyclometalated chiral-at-metal iridium(III) and rhodium(III) complexes, the presented thesis focuses on the development of new auxiliary-mediated methods for the enantiopure synthesis. A chiral auxiliary is used to transform a racemic precursor complex into a diastereomeric mixture, which is subsequently separated by achiral standard preparative chemistry techniques. By substitution of the auxiliary against an achiral ligand under retention of configuration, an enantiopure chiral-only-at-metal complex is obtained.
The first part of the thesis deals with iridium(III) complexes. Apart from an optimization of the diastereomeric separation by column chromatography using (4S)-4-tert-butyl-2-(2-hydroxyphenyl)-4,5-oxazoline as auxiliary, methods for the application of l-proline and l-α-methylproline are presented. The separation of the diastereomers can be achieved by washing, precipitation or crystallization. 2,2’-Phenylpyridine, 2-phenylbenzoxazole und 2-phenylbenzothiazole are used as cyclometalating ligands. The yields for the diastereomeric separations are in the range of 24–47% with a diastereomeric excess of up to >99% for the Lambda-(S)-diastereomers. Furthermore the thermal stability of the diastereomeric complexes is analyzed. Heating the diastereomeric mixtures results in a decomposition of the Delta-(S)-diastereomers leading to an accumulation of the Lambda-(S)-diastereomers. The substitution of the auxiliaries is achieved by Brønsted acid induced labilization of the auxiliary-metal bond under retention of configuration. The target complexes are thus accessible with enantiomeric excesses of up to >99% ee.
In the second part of the thesis the usage of (tolylsulfonyl)benzamides for the synthesis of rhodium(III) complexes is studied. The resulting complexes show a low stability against silica gel. The stability can be improved by introduction of electron withdrawing substituents at the auxiliary, albeit decomposition cannot be suppressed completely. Identification of the minor diastereomer signals within the 1H NMR spectra was not possible, therefore the isomeric excess can only be determined to 54–88% ee upon substitution of the auxiliary.