Chiral Salen Ligands Research Articles

Switching of Circularly Polarized Luminescence via Dynamic Axial Chirality Control of Chiral Bis(Boron Difluoride) Complexes with Salen Ligands

AbstractThe intensity and handedness of circularly polarized luminescence (CPL) were successfully controlled by dynamic molecular motion in solutions. Bis(boron difluoride) complexes with chiral salen ligands were synthesized and their photophysical properties were investigated. Although these complexes showed rapid molecular rotation about the C−N bond axis in solution at room temperature, two conformers assigned as atropisomers were observed in the NMR spectra at low temperature. Furthermore, the equilibrium of these atropisomers was found to change depending on the external environment, such as the solvent and temperature, allowing precise control of the intensity and handedness of CPL without luminescence color shifts. Theoretical calculations based on density functional theory (DFT) revealed that intramolecular chiral exciton coupling is the key to changes in CPL properties.

Synthesis of chiral salen-MnIII complexes tagged with ionic liquid as recoverable homogeneous catalysts for the oxidative kinetic resolution of (±)-secondary alcohols

New chiral salen ligands tagged with imidazolium based ionic liquids (ILs) were synthesized from the 5-bromopentyl benzoate functionality at C-5 position of salicylaldehyde moiety, N-methylimidazole, and (1R,2R)-1,2-diphenylethane-1,2-diamine/(1R,2R)-cyclohexane-1,2-diamine in 81–92% yields. The chiral salen ligands were further reacted with Mn(OAc)2.4H2O in dry methanol under N2 atmosphere, followed by air oxidation with LiCl to produce the IL tagged chiral salen-MnIII complexes 1(a–c) and 2(a–c) in 79-86% yields. The catalytic activities of these complexes were evaluated in the oxidative kinetic resolution of (±)-1-phenylethanol (8a) with N-bromosuccinimide (NBS) as an oxidizer in biphasic solvent system [H2O - organic solvent (2: 1, v/v)] at 0 °C. The enantioselective oxidation of various (±)-secondary alcohols catalyzed by complex 1c (1 mol%) afforded the chiral secondary alcohols with up to 99% enantiomeric excess and 29.6 krel (selectivity factor) value. The IL tagged chiral salen-MnIII complex 1c was easily recovered with hexane and subsequently reused for five recycles with no significant loss in conversion of (±)-1-phenylethanol (8a) and ee of the resolved product.

Asymmetric Epoxidation of Unfunctionalized Olefins Catalyzed by Chiral (Pyrrolidine Salen) Mn (III) Complexes with Proline Sidearms

Four (R, R)-3, 4-diaminopyrrolidine-based chiral Salen ligands with L-Boc-N-proline, D-Boc-N-proline, L-proline, D-proline as respective sidearms, as well as their manganese complexes were synthesized and characterized. These Salen-Mn(III) complexes were applied to the asymmetric epoxidations of unfunctionalized olefins with NaClO, and m-CPBA as respective oxidants. The catalysis of the Salen-Mn (III) complexes varied with the change of sidearms. The one with D-Boc-N-proline as a sidearm showed the best asymmetric catalysis among the four chiral manganese complexes, and its catalysis was also better than that of the one with a benzyl sidearm, and comparable with that of Jacobsen's catalyst. Moderate to excellent enantioselectivities and yields were achieved in the asymmetric of various olefins over this complex. The big enhancement of the D-Boc-N-proline sidearm on the enantioselectivity of the asymmetric epoxidation might be ascribed to its large steric hindrance and appropriate configuration, allowing the Boc moiety to be away from the side-on approach pathway of olefins to the active center but near the opposite side one in some degree, thus facilitating the approach of olefins to the active center from one direction.

Synthesis of new chiral Mn(iii)–salen complexes as recoverable and reusable homogeneous catalysts for the asymmetric epoxidation of styrenes and chromenes

New chiral Mn(iii)–salen complexes 1a–e and 2a–e were synthesized from the reaction of C2-symmetric chiral salen ligands and Mn(CH3COO)2·4H2O under an inert atmosphere followed by aerobic oxidation.

Solid Phase Nitrosylation of Enantiomeric Cobalt(II) Complexes

Accompanied by a change in color from red to black, the enantiomorphic phases of the cobalt complexes of a chiral salen ligand (L2−, Co(L)·CS2, and Co(L) (L = LS,S or LR,R)) chemisorb NO (g) at atmospheric pressure and rt over hours for the CS2 solvated phase, and within seconds for the desolvated phase. NO is installed as an axial nitrosyl ligand. Aligned but unconnected voids in the CS2 desorbed Co(LR,R)·CS2 structure indicate conduits for the directional desorption of CS2 and reversible sorption of NO, which occur without loss of crystallinity. Vibrational circular dichroism (VCD) spectra have been recorded for both hands of LH2, Zn(L), Co(L)·CS2, Co(L), Co(NO)(L), and Co(NO)(L)·CS2, revealing significant differences between the solution-state and solid-state spectra. Chiral induction enables the detection of the νNO band in both condensed states, and surprisingly also the achiral lattice solvent (CS2 (νCS at 1514 cm−1)) in the solid-state VCD. Solution-state spectra of the paramagnetic Co(II) complex shows a nearly 10-fold enhancement and more extensive inversion of polarity of the vibrations of dominant VCD bands compared to the spectra of the diamagnetic compounds. This enhancement is less pronounced when there are fewer polarity inversions in the solid state VCD spectra.

Polymerization of Ethylene in the Presence of Titanium(IV), Zirconium(IV), and Vanadium(V) Coordination Compounds with Salen Ligands

Coordination compounds of titanium(IV), zirconium(IV), and vanadium(V) with chiral salen ligands have been obtained and characterized, and their catalytic activity in the polymerization reaction of ethylene has been studied. It has been shown that in the presence of polymethylaluminoxane as a cocatalyst, the activity of the titanium-containing catalyst system reaches 944 kgPE mol(Ti)−1 h−1 and leads to the preparation of ultrahigh-molecular-weight polyethylene.

The chiral boronate-catalyzed asymmetric transfer hydrogenation of various aromatic ketones to high-value alcohols: Preparation and spectroscopic studies

This work deals with the synthesis, spectroscopic studies and catalytic evaluation of the novel chiral salen (L1H2) and (L2H2) ligands and their chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes. Initially, the reaction of 5-azidomethyl salicylaldehyde and (R)-(−)-2-amino-1-butanol in absolute ethanol afforded a new chiral salen ligand (L1H2). Then, a novel chiral salen ligand (L2H2) have been prepared from chiral salen ligand (L1H2) for the synthesis of boronate [L2(B1-4)] complexes through click reaction approach under ambient conditions. The reaction of chiral salen (L1H2) and (L2H2) ligands with various boronic acids afforded a new tetra-coordinated mononuclear chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes. All the compounds are remarkably stable crystalline solids and were obtained in good yields. For the full characterization of newly synthesized chiral salen ligands and their boronate complexes, the FT-IR, UV–Vis, NMR (1H, 13C, and 11B), LC-MS, and elemental analysis techniques have been used. The well-shaped chiral boronate compounds were investigated as catalyst for the asymmetric transfer hydrogenation (ATH) of aromatic ketones under appropriate settings. Particularly, it was proved that the ferrocene-based boronate compounds can afford an efficient catalytic conversion compared to the other boronate complexes in the asymmetric transfer hydrogenation catalytic studies.

Lanthanide complexes combined with chiral salen ligands: application in the enantioselective epoxidation reaction of α,β-unsaturated ketones.

Readily available lanthanide amides Ln[N(SiMe3)2]3 (Ln = Nd (1), Sm (2), Eu (3), Yb (4), La (5)), combined with chiral salen ligands H2La ((S,S)-N,N′-di-(3,5-disubstituted-salicylidene)-1,2-cyclohexanediamine) and H2Lb ((S,S)-N,N′-di-(3,5-disubstituted-salicylidene)-1,2-diphenyl-1,2-ethanediamine) were employed in the enantioselective epoxidation of α,β-unsaturated ketones. It was found that the salen–La complex shows the highest efficiency and enantioselectivity. A relatively broad scope of α,β-unsaturated ketones was investigated, and excellent yields (up to 99%) and moderate to good enantioselectivities (37–87%) of the target molecules were achieved.

Rare-earth/zinc heterometallic complexes containing both alkoxy-amino-bis(phenolato) and chiral salen ligands: synthesis and catalytic application for copolymerization of CO2 with cyclohexene oxide.

Ten rare-earth/zinc heterometallic complexes containing both alkoxy-amino-bis(phenolato) and chiral salen ligands were synthesized from the reactions of alkoxy-amino-bis(phenolato) rare-earth complexes with chiral salen zinc complexes via ligand redistribution and THF disassociation. Six of them were characterized by single-crystal X-ray diffraction, which showed 6-coordinate rare-earth and 5-coordinate zinc centres in the complexes. A primary catalytic application of these heterometallic complexes was studied, and it was found that samarium/zinc and dysprosium/zinc heterometallic complexes are highly efficient catalysts for copolymerization of CO2 with cyclohexene oxide while the alkoxy-amino-bis(phenolato) samarium complex and chiral salen zinc complex show very low catalytic activity or no catalytic activity for this copolymerization.

Stereochemical Properties of Multidentate Nitrogen Donor Ligands and Their Copper Complexes by Electronic CD and DFT.

UV-Vis and electronic circular dichroism (ECD) spectroscopy, complemented with Density Functional Theory (DFT) calculations, were used to elucidate the structural diversities of three multidentate nitrogen donor ligands and two associated copper complexes in solution directly. The three chiral salen ligands all consist of trans-cyclohexane-1,2-diamine as a chiral scaffold and also of pyridine rings as chromophores, differing only in the linking groups between the two functional groups mentioned above. Very different ECD intensities and somewhat different ECD patterns were observed for these ligands and satisfactorily interpreted theoretically. For the geometry optimization and spectral simulation of the open-shell metal complexes, the LANL2DZ basis set with effective core potential for the Cu and Cl atoms and pure cc-pVTZ for the rest of the atoms was utilized. The performance of the same calculations with the polarization functions (f,g) from the cc-pVTZ basis added to the LANL2DZ basis was compared. While the three ligands exhibit different conformational flexibility, the associated copper complexes show great rigidity imposed by the metal-ligand coordination, taking on a single structure in each case. In addition, dispersion interactions were shown to change the conformational stability ordering of the ligands noticeably and to exert considerable influence on the simulated UV-Vis and ECD spectra. Chirality 28:545-555, 2016. © 2016 Wiley Periodicals, Inc.

The Origin of Relative Stability of Di-µ-oxo M–M Chiral Salen Complexes [M–M = Ti(IV)–Ti(IV), V(IV)–V(IV), Cr(IV)–Cr(IV), and Mn(IV)–Mn(IV)]: A Quantum-Chemical Analysis

Abstract Di-µ-oxo dimetal cores can form dinuclear complexes with definite stereochemistries. Recent studies describing the synthesis and evaluation of titanium(IV) and manganese(IV) di-µ-oxo dimetal chiral salen complexes prompted us to explore the corresponding vanadium(IV) and chromium(IV) complexes. We therefore modeled a series of di-µ-oxo dimetal complexes of titanium(IV), vanadium(IV), chromium(IV), and manganese(IV) with tetradentate salen ligands bearing different degrees of steric bulk. Quantum mechanical calculations were conducted to compare the stabilities and molecular structures of these complexes. The results of natural population analysis gave an insight into the charge distribution characteristics of the di-µ-oxo dimetal cores of these complexes. Our calculations showed that the di-µ-oxo dimetal complexes bearing the least sterically encumbered model ligand 1 preferred the M-helical form over the P-helical one. In the titanium(IV) and chromium(IV) complexes containing a practical chiral salen ligand 2, the P-helical form was found to be more stable than the corresponding M-form. In contrast, the corresponding vanadium(IV) and manganese(IV) complexes preferred to exist in the M-helical form. The degree of distortion of the di-µ-oxo dimetal core and the inter ligand interactions were key factors for determining the stability of these complexes.

Conformational Study of an Axially Chiral Salen Ligand in Solution using Two-Photon Circular Dichroism and the Fragment-Recombination Approach

Herein we report on the conformational study of a chiral Salen ligand, AFX-155, in THF solution using two-photon absorption (TPA) and, even more importantly, two-photon circular dichroism (TPCD). The fragment-recombination approach (FRA), employed to perform computations on half-AXF-155 (AXF-169') and the center (C-AXF-155), allowed us to overcome the current computational limitations found in calculations of the TPCD spectra of large molecules. The comparative experimental-theoretical analysis of AXF-155 showed that its TPA is mainly determined by AXF-169' and validated, through TPCD, the presence of the two dominant Trans_R-Intra(NHB)//Trans_R-Extra(HB) and Trans_R-Intra(NHB)//Trans_R-Extra(NHB) structure in THF with a ΔΔ propeller chirality on the diphenylamine moieties at the end of each AXF-169'. The application of FRA for the analysis of the TPCD spectra of large chiral molecules has been proven to be effective.

ChemInform Abstract: The Synthesis of Tetradentate Salens Derived from (3R,4R)‐N‐Substituted‐3,4‐diaminopyrrolidines and Their Application in the Enantioselective Trimethylsilylcyanation of Aromatic Aldehydes.

AbstractIn situ formed Ti(IV) complexes of pyrrolidine‐based chiral salen ligands (I) derived from L‐tartaric acid are evaluated in the enantioselective trimethylsilylcyanation of aromatic aldehydes.

The synthesis of tetradentate salens derived from (3R,4R)-N-substituted-3,4-diaminopyrrolidines and their application in the enantioselective trimethylsilylcyanation of aromatic aldehydes

The in situ formed Ti(IV) complexes of pyrrolidine-based chiral salen ligands derived from natural l-tartaric acid were evaluated as catalysts in the enantioselective trimethylsilylcyanation of aromatic aldehydes. The different activity and selectivity of the catalysts in the formation of the products were found to be dependent on the N-substituent of the pyrrolidine.

Stable chiral salen Mn(III) complexes with built-in phase-transfer capability for the asymmetric epoxidation of unfunctionalized olefins using NaOCl as an oxidant

A series of novel chiral salen Mn(III) complexes with inherent phase-transfer capability were prepared by covalent linkage of the limidazolium IL moieties containing various PEG chains with chiral salen ligand at two sides of 5,5′-position. Technologies of characterization well suggested the presence of polyether chain, the IL linker, and the intact active sites in the complexes. The amphipathic nature of the PEG chain allowed the PEG-based catalysts to undergo built-in phase transfer, which in turn increased the reaction rates in water/organic biphasic systems. Enantioselective epoxidation of styrene, α-methylstyrene, indene, 1,2-dihydronaphthalene, 6-cyano-2,2-dimethylchromene, and 6-nitro-2,2-dimethylchromene catalyzed by the complexes with NaOCl gave >99% conversions within 60min. The enantiomeric excess (ee) for the epoxides was in the range of 68–93%, except for styrene (ee, 35%) and α-methylstyrene (ee, 42%). Furthermore, the PEG-based complexes were stable and could be separated from the reaction mixture by control of the solvent.

Chiral salen ligands designed to form polymetallic complexes

Chiral salen ligands capable of forming polymetallic complexes have been designed. The ligands possess substituents in the 4,4′-positions, but have no substituent in the 3,3′-positions to allow a second metal ion access to the salen oxygen atoms. Ligands in which a polyether chain links the 4,4′-positions were prepared and complexed to copper. In addition, acyclic ligands with potential metal coordinating substituents in the 4,4′-positions were prepared and complexed to copper and cobalt. The crystal structure of one of the cobalt complexes shows it to be a trimetallic complex in which a Co(II)(OAc)2 group coordinates to the salen oxygen atoms of two Co(III)(salen)(OAc) units. In contrast, the crystal structure of a Co(salen) complex with tert-butyl groups attached to the 3,3′-positions is found to be mononuclear. All of the complexes were tested as asymmetric phase transfer catalysts for the asymmetric alkylation of an alanine methyl ester, forming (R)-α-methyl phenylalanine methyl ester with up to 85% ee.

Stereoselective synthesis of a chiral ferrosalen ligand using an aromatization strategy

A new chiral salen ligand based on two ferrocenyl groups is designed. Unlike known salen ligands, of which chirality originates from central and axial chiral centers, the chirality of this ligand comes from the planar chiral ferrocenyl groups. The ligand is synthesized stereoselectively using a novel aromatization strategy starting from a ferrocene derivative, which was readily prepared using a known chiral auxiliary approach.

ChemInform Abstract: Designing Hydroxy‐Functionalized Chiral Salen Ligand (III) and Its Use in the Synthesis of Dioxadiazasilamacroheterocycles

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Do Trinuclear Triplesalen Complexes Exhibit Cooperative Effects? Synthesis, Characterization, and Enantioselective Catalytic Sulfoxidation by Chiral Trinuclear FeIII Triplesalen Complexes

The chiral triplesalen ligand H(6)chand provides three chiral salen ligand compartments in a meta-phenylene arrangement by a phloroglucinol backbone. The two diastereomeric versions H(6)chand(RR) and H(6)chand(rac) have been used to synthesize the enantiomerically pure chiral complex [(FeCl)(3)(chand(RR))] (3(RR)) and the racemic complex [(FeCl)(3)(chand(rac))] (3(rac)). The molecular structure of the free ligand H(6)chand(rac) exhibits at the terminal donor sides the O-protonated phenol-imine tautomer and at the central donor sides the N-protonated keto-enamine tautomer. The trinuclear complexes are comprised of five-coordinate square-pyramidal Fe(III) ions with a chloride at the axial positions. The crystal structure of 3(rac) exhibits collinear chiral channels of approximately 11 A in diameter making up 33.6 % of the volume of the crystals, whereas the crystal structure of 3(RR) exhibits voids of 560 A(3). Mössbauer spectroscopy demonstrates the presence of Fe(III) high-spin ions. UV/Vis spectroscopy is in accordance with a large delocalized system in the central backbone evidenced by strong low-energy shifts of the imine pi-pi* transitions relative to that of the terminal units. Magnetic measurements reveal weak intramolecular exchange interactions but strong magnetic anisotropies of the Fe(III) ions. Complexes 3(rac) and 3(RR) are good catalysts for the sulfoxidation of sulfides providing very good yields and high selectivities with 3(RR) being enantioselective. A comparison of 3(RR) and [FeCl(salen')] provides higher yields and selectivities but lower enantiomeric excess values (ee values) for 3(RR) relative to [FeCl(salen')]. The low ee values of 3(RR) appeared to be connected to a strong ligand folding in 3(RR), opening access to the catalytically active high-valent Fe-O species. The higher selectivity is assigned to a cooperative stabilization of the catalytically active high-valent Fe-O species through the phloroglucinol backbone in the trinuclear complexes.

Cyclic Carbonate Synthesis Catalysed by Bimetallic Aluminium–Salen Complexes

The development of bimetallic aluminium-salen complexes [{Al(salen)}(2)O] as catalysts for the synthesis of cyclic carbonates (including the commercially important ethylene and propylene carbonates) from a wide range of terminal epoxides in the presence of tetrabutylammonium bromide as a cocatalyst is reported. The bimetallic structure of one complex was confirmed by X-ray crystallography. The bimetallic complexes displayed exceptionally high catalytic activity and in the presence of tetrabutylammonium bromide could catalyse cyclic carbonate synthesis at atmospheric pressure and room temperature. Catalyst-reuse experiments demonstrated that one bimetallic complex was stable for over 60 reactions, though the tetrabutylammonium bromide decomposed in situ by a retro-Menschutkin reaction to form tributylamine and had to be regularly replaced. The mild reaction conditions allowed a full analysis of the reaction kinetics to be carried out and this showed that the reaction was first order in aluminium complex concentration, first order in epoxide concentration, first order in carbon dioxide concentration (except when used in excess) and unexpectedly second order in tetrabutylammonium bromide concentration. Further kinetic experiments demonstrated that the tributylamine formed in situ was involved in the catalysis and that addition of butyl bromide to reconvert the tributylamine into tetrabutylammonium bromide resulted in inhibition of the reaction. The reaction kinetics also indicated that no kinetic resolution of racemic epoxides was possible with this class of catalysts, even when the catalyst was derived from a chiral salen ligand. However, it was shown that if enantiomerically pure styrene oxide was used as substrate, then enantiomerically pure styrene carbonate was formed. On the basis of the kinetic and other experimental data, a catalytic cycle that explains why the bimetallic complexes display such high catalytic activity has been developed.