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Chiral Enol Oxazolines and Thiazolines as Auxiliary Ligands for the Asymmetric Synthesis of Ruthenium‐Polypyridyl Complexes
Author(s) -
Huang Biao,
Wang Lun,
Gong Lei,
Meggers Eric
Publication year - 2013
Publication title -
chemistry – an asian journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.18
H-Index - 106
eISSN - 1861-471X
pISSN - 1861-4728
DOI - 10.1002/asia.201300598
Subject(s) - yield (engineering) , chemistry , ruthenium , medicinal chemistry , denticity , triethylamine , stereochemistry , catalysis , organic chemistry , crystal structure , materials science , metallurgy
Various ligands, such as ( Z )‐1‐phenyl‐2‐[(4 S )‐4‐phenyl‐4,5‐dihydro‐1,3‐oxazol‐2‐yl]ethen‐1‐ol (( S )‐ 1a ) and ( Z )‐1‐phenyl‐2‐[(4 S )‐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 [RuCl 2 (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 ](PF 6 ) 2 by replacing the bidentate enolato ligands with bpy, under retention of configuration, induced by either NH 4 PF 6 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 NH 4 PF 6 , oxazoline‐enolato complex Λ‐( S )‐ 2a provides Λ‐[Ru(bpy) 3 ](PF 6 ) 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.