Proline and α-Methylproline as Chiral Auxiliaries for the Synthesis of Enantiopure Bis-Cyclometalated Iridium(III) Complexes

A convenient proline- and α-methylproline-mediated method for the synthesis of enantiomerically pure bis-cyclometalated iridium(III) complexes is reported. The reactions of l-proline or l-α-methylproline with [Ir(µ-Cl)(C^N)₂]₂ (C^N = cyclometalating 2-phenylpyridine, 2-phenylbenzoxazole, or 2-phenylbenzothiazole ligand) afforded diastereomeric mixtures of intermediate prolinatoiridium(III) complexes from which the Λ-(S) diastereomers were isolated with excellent diastereomeric purity by washing, precipitation, or crystallization. A subsequent trifluoroacetic acid (TFA) induced substitution of the prolinate ligands with 2,2′-bipyridine with the retention of configuration provided the chiral-only-at-metal complexes with >99 % ee.

Proline and α-methylproline are very suitable chiral auxiliaries for the synthesis of enantiopure bis-cyclometalated iridium(III) complexes in a straightforward and convenient fashion without the tedious chromatographic separation of the intermediate diastereomers.

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