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Chiral enol oxazolines and thiazolines as auxiliary ligands for the asymmetric synthesis of ruthenium-polypyridyl complexes



Chiral enol oxazolines and thiazolines as auxiliary ligands for the asymmetric synthesis of ruthenium-polypyridyl complexes



Chemistry, An Asian Journal 8(9): 2274-2280



Various ligands, such as (Z)-1-phenyl-2-[(4S)-4-phenyl-4,5-dihydro-1,3-oxazol-2-yl]ethen-1-ol ((S)-1a) and (Z)-1-phenyl-2-[(4S)-4-phenyl-4,5-dihydro-1,3-thiazol-2-yl]ethen-1-ol ((S)-1c), were investigated as auxiliaries for the asymmetric synthesis of chiral ruthenium(II) complexes. The reaction of these chiral auxiliary ligands with [RuCl2(dmso)4], 2,2′-bipyridine (bpy, 2.2 equiv), and triethylamine (10 equiv) in DMF/PhCl (1:8) at 140 °C for several hours diastereoselectively provided the complexes Λ-[Ru(bpy)2{(S)-1a-H}] (Λ-(S)-2a, 52 % yield, 56:1 d.r.) and Λ-[Ru(bpy)2{(S)-1c-H}] (Λ-(S)-2c, 48 % yield, >100:1 d.r.) in a single step after purification. Both Λ-(S)-2a and Λ-(S)-2c could be converted into Λ-[Ru(bpy)3](PF6)2 by replacing the bidentate enolato ligands with bpy, under retention of configuration, induced by either NH4PF6 as a weak acid (from Λ-(S)-2a: 73 % yield, 22:1 e.r.; from Λ-(S)-2c: 77 % yield, 22:1 e.r.), TFA as a strong acid (from Λ-(S)-2a: 72 % yield, 52:1 e.r.; from Λ-(S)-2c: 85 % yield, 25:1 e.r.), methylation with Meerwein′s salt (from Λ-(S)-2a: 59 % yield, 46:1 e.r.; from Λ-(S)-2c: 86 % yield, 37:1 e.r.), ozonolysis (from Λ-(S)-2a: 56 % yield, 22:1 e.r.; from Λ-(S)-2c: 43 % yield, 6.3:1 e.r.), or oxidation with a peroxy acid (from Λ-(S)-2a: 72 % yield, 45:1 e.r.; from Λ-(S)-2c: 79 % yield, 8.5:1 e.r.). This study shows that, except for the reaction with NH4PF6, oxazoline-enolato complex Λ-(S)-2a provides Λ-[Ru(bpy)3](PF6)2 with higher enantioselectivities than analogous thiazoline-enolato complex Λ-(S)-2c, which might be due to the higher coordinative stability of the thiazoline-enolato complex, thus requiring more prolonged reaction times. Thus, this study provides attractive new avenues for the asymmetric synthesis of non-racemic ruthenium(II)-polypyridyl complexes without the need for using a strong acid or a strong methylating reagent, as has been the case in all previously reported auxiliary methods from our group.

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Accession: 052063643

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PMID: 23818279

DOI: 10.1002/asia.201300598


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