Synthesis and Metal Complexes of Chiral C₂‐Symmetric Diamino–Bisoxazoline Ligands

Abstract

A synthetic route to tetradentate chiral N(4) ligands has been developed with the aim to study the potential of corresponding iron and manganese complexes as catalysts for enantioselective epoxidation. These ligands, which contain two oxazoline rings and two trialkylamino groups as coordinating units, are readily prepared in enantiomerically pure form by the reaction of chiral 2-chloromethyloxazolines with achiral N,N'-dimethylethane-1,2-diamine or chiral (R,R)-N,N'-dimethylcyclohexane-1,2-diamine. The ligands derived from N,N'-dimethylethane-1,2-diamine reacted with anhydrous metal halides MnCl(2) and FeCl(2) in a stereoselective manner to give octahedral mononuclear complexes that have the general formula Delta-[(L)MCl(2)]. In contrast, the ligands derived from N,N'-dimethylcyclohexane-1,2-diamine formed complexes with different coordination modes depending on the diastereomer employed: in one case the metal ion was found to be pentacoordinate, in the other case a hexacoordinated complex was observed. The structure of a series of Fe and Mn complexes was determined by X-ray analysis. The coordination chemistry of these ligands was further studied by X-ray and NMR analyses of the diamagnetic isostructural complexes [(L)ZnCl(2)]. Analogous ionic complexes, which were prepared by removing chloride with silver trifluoromethanesulfonate or hexafluoroantimonate, were tested as catalysts for the epoxidation of olefins.

A new C2-symmetric azolium compound for Cu-catalyzed asymmetric conjugate addition of R2Zn to cyclic enone

“…An excellent ee value of 96% was obtained by allowing 13 to react with Bu 2 Zn in DMA to produce (S)-3-butylcycloheptanone (15) (Run 6). 4,4-Dimethyl-2-cyclohexen-1-one (16) also reacted smoothly with Et 2 Zn to form (R)-3-ethyl-4, 4-dimethylcyclohexanone (17) in good yield (Runs 7e8). A better result was obtained by using DMA as a solvent than that obtained using THF.…”

“…Reaction of (S)-valinol with bromoacetyl bromide or chloroacetyl chloride produced the corresponding abromo-or a-chloroacetamide derivative, 2a or 4a, in almost quantitative yield, respectively [17]. The hydroxyamide group was introduced into the benzimidazole ring by allowing 2a to react with benzimidazole (1) in the presence of KOH in CH 3 CN at room temperature.…”

Tridentate, anionic tethered N-heterocyclic carbene of Pd(II) complexes

“…Therefore, we proposed another synthetic route to the NHC proligand (Scheme 3). Reaction of chloroacetyl chloride with b-amino alcohol afforded a-chloroacetamide in almost quantitative yield [11], which subsequently coupled with N-alkylated azole to yield the corresponding azolium chloride. Since some N-alkylated imidazoles are commercially available, a variety of the desired azolium compounds can be synthesized using this route.…”

“…for C 18 H 20 ClN 3 O 2 ÁH 2 O: C, 59.42; H,6.09; N, 11.55. Found: C, 59.39; H, 5.88; N,11.57%. ½a 25 D ¼ þ105:…”

Non-heme manganese complexes of C1-symmetric N4 ligands: Synthesis, characterization and asymmetric epoxidations of α,β-enones

“…However, the observed enantiomeric excesses (ee) were low. Besides, a series of chiral N 4 ligands containing two oxazoline rings instead of pyridine rings were prepared by Pfaltz et al, and their manganese complexes also only exhibited low enantioselectivities in olefin epoxidation [25]. Costas et al demonstrated that manganese complexes with chiral tetradentate N 4 ligands (S,S,R-MCPP) derived from (R,R-mcp) by fusing pinene rings can promote the epoxidation reaction with modest enantiomeric excess [26].…”