Chiral zinc catalysts for asymmetric synthesis

Trickier than It Looks: Isomerization between Five- and Six-Coordinated Zinc in Heterometallic Li2Zn2 Molecule

This report describes the synthesis and characterization of two heterobimetallic Li-Zn coordination isomers [Li2Zn2(tbaoac)6] (tbaoac = tert-butyl acetoacetato) that have been isolated separately by the same stoichiometric reaction run in different organic solvents. The 6-coordinated zinc isomer (6-Zn) was synthesized in acetone with high yield, while the 5-coordinated one (5-Zn) was readily obtained from ethanol. The 5-Zn isomer has a low solubility in organic solvents such as alkanes and haloalkanes, while its 6-Zn counterpart exhibits a good solubility in almost all common solvents. Two isomeric molecules feature similar centrosymmetric tetranuclear cyclic assemblies, which are different in their arrangement of tbaoac ligands. While all ligands act as μ2-type in the structure of 5-Zn, the two tbaoac groups chelating Li appear as μ3-type in 6-Zn, thus providing an additional coordination for Zn ions. However, the real structural transformation between these isomers was shown to be more complex than simply making or breaking a couple of Zn-O bonds. X-ray single-crystal structure analysis, powder X-ray diffraction, multinuclear NMR, DART mass spectrometry, ICP-OES analysis, and TGA have been employed for the characterization of the isomers. The combination of powder X-ray diffraction and 1H NMR investigation revealed that 6-Zn isomer can be quantitatively transformed to 5-Zn in ethanol, while the reverse conversion instantly takes place in acetone.

Synthesis of chiral Cu(II) complexes from pro-chiral Schiff base ligand and investigation of their catalytic activity in the asymmetric synthesis of 1,2,3-triazoles

A pro-chiral Schiff base ligand (HL) was synthesized by the reaction of 2-amino-2-ethyl-1,3-propanediol and pyridine-2-carbaldehyde in methanol. The reaction of HL with CuCl2·2H2O and CuBr2 in methanol gave neutral mononuclear Cu(II) complexes with general formula of [Cu(HL)Cl2] (1) and [Cu(HL)Br2] (2), respectively. By slow evaporation of the methanolic solutions of 1 and 2, their enantiomers were isolated in crystalline format. The formation of pure chiral crystals in the racemic mixture was amply authenticated by single crystal X-ray analysis, which indicated that S-[Cu(HL)Cl2], R-[Cu(HL)Cl2], and S-[Cu(HL)Br2] are crystallized in chiral P212121 space group of orthorhombic system. Preferential crystallization was used to isolate the R and S enantiomers as single crystals and the isolated compounds were also studied by CD analysis. Structural studies indicated that the origin of the chirality in these compounds is related to the coordination mode of the employed pro-chiral ligand (HL) because one of its carbon atoms has been converted to a chiral center in the synthesized complexes. Subsequently, these complexes were used in click synthesis of a β-hydroxy-1,2,3-triazole and the results of catalytic studies indicated that 1 and 2 can act as enantioselective catalysts for the asymmetric synthesis of β-hydroxy-1,2,3-triazole product under mild condition. This study illustrates the significant capacity of the use of pro-chiral ligands in preparing chiral catalysts based on complexes which can also be considered as an effective approach to cheap chiral catalysts from achiral reagents.

An efficient catalytic method for hydrophosphination of heterocumulenes with diethylzinc as precatalyst without a solvent

Commercially available compound ZnEt₂ acts as an efficient precatalyst for the solvent-free hydrophosphinations of heterocumulenes using Ph₂PH as reagent. As far as we knew, this has been not reported in group 12 metal catalyzing reactions. A suggested mechanism of this reaction is explored, and the intermediate [{Ph₂PC(NⁱPr)₂}ZnEt]₂ is obtained and characterized by a single-crystal X-ray structural analysis.

Towards deeper understanding of multifaceted chemistry of magnesium alkylperoxides

Despite considerable progress in the multifaceted chemistry of non-redox-metal alkylperoxides, the knowledge about magnesium alkylperoxides is in its infancy and only started to gain momentum. Harnessing the well-defined dimeric magnesium tert-butylperoxide [(^f5BDI)Mg(μ-η²:η¹-OOtBu)]₂ incorporating a fluorinated β-diketiminate ligand, herein, we demonstrate its transformation at ambient temperature to a spiro-type, tetranuclear magnesium alkylperoxide [(^f5BDI)₂Mg₄(μ-OOtBu)₆]. The latter compound was characterized by single-crystal X-ray diffraction and its molecular structure can formally be considered as a homoleptic magnesium tert-butylperoxide [Mg(µ-OOtBu)₂]₂ terminated by two monomeric magnesium tert-butylperoxides. The formation of the tetranuclear magnesium alkylperoxide not only contradicts the notion of the high instability of magnesium alkylperoxides, but also highlights that there is much to be clarified with respect to the solution behaviour of these species. Finally, we probed the reactivity of the dimeric alkylperoxide in model oxygen transfer reactions like the commonly invoked metathesis reaction with the parent alkylmagnesium and the catalytic epoxidation of trans-chalcone with tert-butylhydroperoxide as an oxidant. The results showed that the investigated system is among the most active known catalysts for the epoxidation of enones.

Synergistic zinc catalyst mediated oxa-Michael kinetic resolution reaction

An oxa-Michael kinetic resolution reaction is developed to efficiently construct complexed polycyclic motifs by developing novel bifunctional zinc catalysts.

Preparative-Scale Synthesis of Vedejs Chiral DMAP Catalysts

A scalable synthesis of chiral Vedejs-type DMAP catalysts is reported. The key step of the synthesis is amination of the enantiomerically pure 4-chloropyridine derivative using well-defined ZnCl₂(amine)₂ complexes. A series of Zn(II)-amine complexes have been synthesized to explore the scope of the ZnCl₂-mediated amination of 4-halopyridines. Mechanistic studies support a Zn(II)-facilitated nucleophilic aromatic substitution as a plausible mechanism for the chlorine-to-amine exchange.

Di-Zinc-Aryl Complexes: CO2 Insertions and Applications in Polymerisation Catalysis

Two new di-zinc-aryl complexes, [LZn2 Ph2 ] and [LZn2 (C6 F5 )2 ], coordinated by a diphenol tetraamine macrocyclic ligand have been prepared and fully characterised, including by single-crystal X-ray diffraction experiments. The complexes' reactivities with monomers including carbon dioxide, cyclohexene oxide, phthalic anhydride, isopropanol and phenol were investigated using both experimental studies and density functional theory calculations. In particular, [LZn2 Ph2 ] readily inserts carbon dioxide to form a carboxylate, at 1 bar pressure, whereas [LZn2 (C6 F5 )2 ] does not react. Under these conditions [LZn2 Ph2 ] shows moderate activity in the ring-opening copolymerisation of cyclohexene oxide/carbon dioxide (TOF=20 h-1 ), cyclohexene oxide/phthalic anhydride (TOF=33 h-1 ) and the ring-opening polymerisations of rac-lactide (TOF=99 h-1 ) and ϵ-caprolactone (TOF=5280 h-1 ).

Dinuclear zinc complex catalyzed asymmetric methylation and alkynylation of aromatic aldehydes

A general AzePhenol dinuclear zinc catalytic system has been successfully developed and introduced into the asymmetric addition of dimethylzinc and alkynylzinc to aromatic aldehydes.

Development of a Simple Adjustable Zinc Acid/Base Hybrid Catalyst for C-C and C-O Bond-Forming and C-C Bond-Cleavage Reactions

A newly designed zinc Lewis acid/base hybrid catalyst was developed. By adjusting the Lewis acidity of the zinc center, aldol-type additions of 2-picolylamine Schiff base to aldehydes proceeded smoothly to afford syn-aldol adduct equivalents, trans-N,O-acetal adducts, in high yields with high selectivities. NMR experiments, including microchanneled cell for synthesis monitoring (MICCS) NMR analysis, revealed that anti-aldol adducts were formed at the initial stage of the reactions under kinetic control, but the final products were the trans-(syn)-N,O-acetal adducts that were produced through a retro-aldol process under thermodynamic control. In the whole reaction process, the zinc catalyst played three important roles: i) promotion of the aldol process (C-C bond formation), ii) cyclization process to the N,O-acetal product (C-O bond formation), and iii) retro-aldol process from the anti-aldol adduct to the syn-aldol adduct (C-C bond cleavage and C-C bond formation).

An Ir/Zn Dual Catalysis for Enantio- and Diastereodivergent α-Allylation of α-Hydroxyketones

An Ir/Zn dual catalysis has been developed for the enantio- and diastereodivergent α-allylation of unprotected α-hydroxyketones under mild conditions, in the absence of any additional base. The cooperative action of a chiral iridium complex derived from phosphoramidites and a chiral Zn-ProPhenol complex is most likely responsible for its high reactivity, excellent enantioselectivity (up to >99% ee), and good diastereoselectivity (up to >20:1 dr). All four product stereoisomers could be prepared from the same set of starting materials and under identical conditions by simple selection of appropriate catalyst combinations.

Renovation of Optically Active Phenanthrolines as Powerful Chiral Ligands for Versatile Asymmetric Metal Catalysis

In the field of asymmetric synthesis, the development of new chiral ligands has been regarded as an attractive challenge for decades. Novel chiral ligands can often have a great impact on synthetic protocols. In this context, we are currently interested in the application of 1,10-phenanthroline (phen) as an entirely new class of chiral ligand. To handle this issue, we designed a chiral phen ligand that provides the N,N,O-tridentate coordination of the phen moiety and an additional phenolic hydroxyl group. As phen possesses greater coordination ability with various ions, our chiral phen ligand would be valuable as one of the "privileged" chiral ligands applied to a broad range of metal catalysts and new reactions. This account summarizes the results of the application of the chiral phen ligand to various kinds of metal catalysis.

Asymmetric total synthesis of (+)-O-methylasparvenone, a rare nitrogen-free serotonin 2C receptor antagonist

The first enantioselective synthesis of the fungal metabolite (+)-O-methylasparvenone is reported.

Fine tuning of molecular and supramolecular properties of simple trianglimines – the role of the functional group

Readily available chiral, triangular poly-azamacrocycles can act as receptors, low molecular weight supergelators and ligands in asymmetric synthesis.

Mechanistic implications of the enantioselective addition of alkylzinc reagents to aldehydes catalyzed by nickel complexes with α-amino amide ligands

The enantioselective alkylation of aldehydes catalysed by nickel(ii)-complexes derived from α-amino amides was studied by means of density functional theory (DFT) and ONIOM (B3LYP:UFF) calculations.

Enantioselective Hydrosilylation of Imines Catalyzed by Chiral Zinc Acetate Complexes

A series of zinc acetate complexes with optically pure diphenylethanediamine (DPEDA)-derived ligands have been employed as enantioselective catalyst for the hydrosilylation of various imines. High control of stereoselectivity (up to 97% ee) and excellent yields (up to 96%) were gained for a broad range of N-phosphinoylimines by using (R,R)-N,N'-dibenzyl-1,2-diphenylethane-1,2-diamine. This is the first successful application of an air-stable and environmentally friendly chiral Zn(OAc)2 complex instead of the previously used harmful diethylzinc in the asymmetric reduction of the C═N double bond.

An overview of Zn-catalyzed enantioselective aldol type C–C bond formation

Enantioselective zinc-catalyzed aldol reactions provide an efficient route for the construction of a wide range of carbon–carbon bond-formation, which are described here.