Chiral Enol Oxazolines and Thiazolines as Auxiliary Ligands for the Asymmetric Synthesis of Ruthenium‐Polypyridyl Complexes

Abstract

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.

Divalent cobalt and copper coordination complexes of κ2-N, O-derivatives of (Z)-1-R-2-(2′-oxazolin-2′-yl)-eth-1-en-1-ates: Structure and reactivity patterns

“…This was due to a combination of the observed inherent instability of the Co materials (oxidation; ligand exchange), the issues with the production and isolation of stable Cu derivatives in general (vide supra) and the generally more facile isolation of the ligands themselves. [28][29][30][31][32][33][34][35][36][37] Compounds 1aa -1ll were all found (Experimental section) to give acceptable yields (56 − 99%) of air-stable Co(II) complexes of general formula Co(1 − H)2 (i.e., complexes 2aa -2ll: Scheme 3). These complexes vary in colour from orange to pink to red depending on the nature of the various R groups.…”

“…[28][29][30][31] <Insert Figure 1> These organics have been investigated in some detail by Yasuo Tohda since 1984 and indeed random related examples of this general structural class have been studied since the 1970s. [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] The compounds themselves exist in the solid-state in the enamine form (Figure 1: centre) and in some cases equilibrate in solution to include the keto isomer (Figure 1: right). [28-31, 32, 37, 43] This aspect depends on both the solvent and the nature of the R / R' groups.…”

“…[28-31, 32, 37, 43] This aspect depends on both the solvent and the nature of the R / R' groups. [28][29][30][31][32][33][34][35][36][37][38][39][40] Protonation at the N-atom leads to complete conversion to the keto tautomer. [43] The presence of base typically leads to H + loss and the formation of a reactive enolate.…”

“…[43] The presence of base typically leads to H + loss and the formation of a reactive enolate. [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] Despite the fact that these materials have been studied for many years, there are only a few reports of metal chelation by these reagents and their analogues. [41][42][43] Sometime ago, we briefly reported on such a Cu(II) example of formula bis-([Z]-1-phenyl-2-{4',4'-dimethyl-2'oxazolin-2'yl}-eth-1-en-1-ate)copper(II), i.e., complex 3aa: Figure 2).…”