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Reductive Labilization of a Cyclometalating Ligand Applied to Auxiliary-Mediated Asymmetric Coordination Chemistry

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posted on 23.09.2013, 00:00 by Marianne Kraack, Klaus Harms, Eric Meggers
(4S)-4-Isopropyl-2-(3-nitrophenyl)-4,5-dihydrooxazole ((S)-Phox) is introduced as a novel chiral auxiliary for the asymmetric synthesis of ruthenium polypyridyl complexes. A simply accessible (S)-Phox-bearing precursor serves as the starting point for diastereoselective coordination chemistry: The stereogenic carbon atom of the cyclometalating auxiliary controls the spatial arrangement of incoming 1,10-phenanthrolines during ligand substitution reactions (ratio Λ:Δ up to 14:1), and further precipitation affords diastereopure compounds. In the following key step, the labilization of the auxiliary ligand is achieved by reduction, thus permitting its replacement against a third polypyridyl ligand with complete retention of the configuration at the metal center (er > 99:1) under mildly acidic conditions, in contrast with previously developed systems that require strong acid. On the basis of results of NMR experiments and X-ray analysis obtained for intermediate compounds, mechanistic considerations for the formation of diastereomeric complexes were made, revealing a Δ → Λ isomerization as the reason for the observed limitations in selectivity optimization. This work expands the pool of methods available for the asymmetric synthesis of tris-heteroleptic ruthenium polypyridyl complexes and additionally may serve as an inspiration for the synthesis of other nonracemic octahedral chiral-at-metal compounds.