Chiral Cobalt Research Articles

Liang-Qiu Lu.

"My first experiment was to prepare a new chiral bifunctional organocatalyst which was designed by myself. Chemistry is fun because whatever you find, you will benefit by gaining new knowledge …" Find out more about Liang-Qiu Lu in his Author Profile.

Exploration of a Chiral Cobalt Catalyst for Visible‐Light‐Induced Enantioselective Radical Conjugate Addition

AbstractChiral catalysts tolerating photochemical reactions are in great demand for the vast development of visible‐light‐induced asymmetric synthesis. Now, chiral octahedral complexes based on earth‐abundant metal and chiral N4 ligands are reported. One well‐defined chiral CoII‐complex is shown to be an efficient catalyst in the visible‐light‐induced conjugated addition of enones by alkyl and acyl radicals, providing synthetically valued chiral ketones and 1,4‐dicarbonyls in 47–>99 % yields with up to 97:3 e.r.

Exploration of a Chiral Cobalt Catalyst for Visible-Light-Induced Enantioselective Radical Conjugate Addition.

Chiral catalysts tolerating photochemical reactions are in great demand for the vast development of visible-light-induced asymmetric synthesis. Now, chiral octahedral complexes based on earth-abundant metal and chiral N4 ligands are reported. One well-defined chiral CoII -complex is shown to be an efficient catalyst in the visible-light-induced conjugated addition of enones by alkyl and acyl radicals, providing synthetically valued chiral ketones and 1,4-dicarbonyls in 47->99 % yields with up to 97:3 e.r.

Ab Initio Study of Circular Dichroism and Circularly Polarized Luminescence of Spin-Allowed and Spin-Forbidden Transitions: From Organic Ketones to Lanthanide Complexes.

Complete and restricted active space self-consistent field (CAS-/RAS-SCF) wave function methods are applied for the calculation of circular dichroism (CD) and circularly polarized luminescence (CPL) of a series of molecules comprising four organic ketones, the chiral cobalt(III) complex [Co(en)3]3+, and the europium(III) complex [Eu(DPA)3]3-. The ab initio results are in good agreement with the experimental data and previous results obtained with Kohn-Sham density functional theory in the case of the spin-allowed transitions. CD and CPL properties are calculated ab initio for the spin-forbidden transitions of both a transition metal and a lanthanide complex.

Lipophilic chiral cobalt (III) complexes of hexaamine ligands: Efficacies as enantioselective hydrogen bond donor catalysts

Four known chiral enantiopure octahedral cobalt(III) trichloride salts with aliphatic hexaamine ligands are converted to new CH2Cl2 soluble tris(tetraarylborate) or 3BArf− salts (BArf = B(3,5-C6H3(CF3)2)4 in biphasic reactions. These include sepulchrate and sarcophagine complexes in which the hexaamine ligands are bicyclic (Z(CH2NHCH2CH2NHCH2)3Z, Z=N, CX), as well as a truncated sarcophagine in which a missing carbon vertex renders the ligand acyclic and tripodal, with three terminal NH2 groups that define a trigonal C3 symmetric face (CH3C(CH2NHCH2CH2NH2)3). In the presence of tertiary amine bases, these are effective catalysts for two Michael type carbon-carbon bond forming reactions and a related carbon-nitrogen bond forming reaction. However, only the last complex affords significant enantioselectivities (30–57% ee). Rationales, and directions for future catalyst optimization, are proposed.

Brønsted Acids of Anionic Chiral Cobalt(III) Complexes as Catalysts for the Iodoglycosylation or Iodocarboxylation of Glycals

Brønsted acids of anionic chiral Co(III) complexes were found to act as efficient phase-transfer catalysts for the diastereoselective iodoglycosylation or iodocarboxylation of glycals with a variety of alcohols or carboxylic acids, respectively, with N-iodosuccinimide as the iodo cation source. The corresponding 2-deoxy-2-iodoglycosides, including monosaccharides and disaccharides, and 2-deoxy-2-iodoglycosyl carboxylates, which are of high synthetic and biological importance, were obtained in high yields (up to 88%) with good diastereoselectivities (up to 9:1 dr).

An indirect approach for encapsulation of chiral cobalt catalyst in microporous Zeolite-Y

Hierarchical structure in microporous zeolite-Y was introduced to overcome the diffusional restriction for direct encapsulation of chiral cobalt (II) Schiff base complexes. Mesopores with cylindrical channels of ∼5 nm dimensions were formed on post modification of zeolite-Y and was evident from the transmission electron microscopy and surface area analysis. Two different chiral cobalt (II) Schiff base complexes with minor variation in the ligand structure were constructed inside the mesoporous zeolite-Y cavity. The formation of the metal complexes was ascertained from the decrease in surface area and mesoporous pore volume. The two synthesized heterogeneous chiral cobalt Schiff base complexes were used as catalysts for asymmetric Henry reaction. At −35 °C, more than 80% ee with S-major nitro-aldol product was achieved within 3 h. Initial activation of the reaction mixture with microwave irradiation was found to have dramatic influence on the catalytic performance of the two catalysts.

Chiral Cobalt Complexes Synthesised From L-Valine: As Potential Catalysts for Asymmetric Henry Reaction

Chiral Cobalt Complexes Synthesised From L-Valine: As Potential Catalysts for Asymmetric Henry Reaction

Asymmetric Hydrolytic and Aminolytic Kinetic Resolution of Racemic Epoxides using Recyclable Macrocyclic Chiral Cobalt(III) Salen Complexes

AbstractNew chiral macrocyclic cobalt(III) salen complexes were synthesized and used as catalyst for the asymmetric kinetic resolution (AKR) of terminal epoxides and glycidyl ethers with aromatic/aliphatic amines and water as nucleophiles. This is the first occasion where a Co(III) salen complex demonstrated its ability to catalyze AKR as well as hydrolytic kinetic resolution (HKR) reactions. Excellent enantiomeric excesses of the epoxides, the corresponding amino alcohols and diols (upto 99%) with quantitative yields were achieved by using the chiral Co(III) salen complexes in dichloromethane at room temperature. This protocol was further extended for the synthesis of two important drug molecules, i.e., (S)‐propranolol and (R)‐naftopidil. The catalytic system was also explored for the synthesis of chirally pure diols and chiral cyclic carbonates using carbon dioxide as a greener renewable C1 source. The catalyst was recycled for upto 5 catalytic cycles with retention of enantioselectivity.magnified image

Superior Effect of Ultrasonic Homogenization to Mechanical Agitation on Accelerating Reaction Rates in Asymmetric Ring Opening of Epoxides

Double‐layered mesoporous silica was prepared for use as an efficient support for the encapsulation of chiral salen Co‐BF3 complexes in the pore channels. The heterogenized chiral salen complex was applied successfully to the asymmetric ring opening (ARO) of terminal epoxides using water, alcohols, and carboxylic acids as nucleophiles. Although the homogeneous salen catalyst exhibited higher catalytic activity than the heterogenized catalyst in ARO reactions as expected, the heterogeneous salen catalysts exhibited high catalytic activities that were sufficient for practical applications. This study evaluated the unique contribution of ultrasonic irradiation as a powerful mixing medium to improve the kinetics in comparison to conventional mechanical agitation during the ARO reactions. Both the homogeneous and heterogeneous chiral salen complexes catalyzed enantioselectively the racemic or chiral epoxides under ultrasonication. The adoption of ultrasonic irradiation in the catalysis resulted in a superior mixing effect to vigorous mechanical stirring. The reaction time was shortened to obtain the highest enantiomeric excess (%ee) when ultrasonic irradiation was applied to the reaction system. Through the ARO reaction of epoxide by phenol derivatives, the highly enantio‐enriched β‐blockers could be synthesized easily from racemic terminal epoxides or chiral ones using chiral cobalt–salen catalysts. The regioselective ARO reaction of (R)‐ECH with propionic or butyric acid proceeded smoothly at room temperature under ultrasonic exposure in the presence of the heterogenized catalyst. The corresponding glycidyl alkylates were produced with excellent optical purity of up to 98%ee. The chiral cobalt–salen complexes encapsulated in MCM‐41 retained its catalytic activity and enantioselectivity for three recycles. Consequently, the heterogeneous chiral catalysts anchored in MCM‐41 offered the practical advantages of facile reuse through simple recovery from the reactant and products.

Asymmetric Cyclopropanation of Olefins Catalyzed by a Chiral Cobalt(II) Porphyrin

The cobalt(II) complex of the Halterman porphyrin, 5,10,15,20-tetrakis[(1S, 4R, 5R, 8S)-1,2,3,4,5,6,7,8octahydro- 1,4: 5,8-dimethanoanthracene-9-yl] porphyrinato cobalt(II) [Co(por*)], was synthesized and its structure was identified by X-ray analysis. Up to 80: 20 trans: cis diastereomeric ratio and 82% ee were achieved in the cyclopropanation of styrene with ethyl diazoacetate by using this cobalt(II) porphyrin complex as catalyst.

A Homochiral {CoΙΙ16 CoΙΙΙ4 } Supertetrahedral T4 Cluster from a Racemic Ligand with Ferromagnetic Behavior and High Photocatalytic Activity.

A homochiral mixed-valence cobalt cluster [CoΙΙ16 CoΙΙΙ4 (μ6 -O)4 (μ3 -OH)12 (S-bme)12 (OAc)6 ]Cl6 ⋅5 CH3 OH⋅18 H2 O (1, Hbme=1H-(benzimidazol-2-yl)ethanol) was synthesized from a racemic ligand and three cobalt salts of CoCl2 ⋅6 H2 O, Co(Ac)2 ⋅4 H2 O and Co(NO3 )2 ⋅6 H2 O in a DMF/MeOH mixed solvent. The enantioselective coordination occurs when a large excess of cobalt ions added in the solution and only the S-configuration of the racemic ligand involved in crystallization. The CD spectra of three crystal samples show identical Cotton signals, indicating the repeatability and the enantiomeric purity of the single crystals. This compound presents a beautiful two-shell Matryoshka-type supertetrahedral T4 cluster constructed by an inner CoΙΙΙ4 O4 cubane and four exterior CoΙΙ4 O4 cubanes bridged by μ6 -O2- and μ3 -OH- ions. This highest nuclear chiral cobalt cluster is the first example of enantiopure cobalt cluster separated from a racemic ligand and is the largest supertetrahedral cobalt cluster up to now. The magnetic studies reveal it behaves as a ferromagnet. Photocatalytic properties of 1 show high catalytic activities for the degradation of the highly toxic triphenyl dye crystal violet (CV) in the presence of H2 O2 under visible light in aqueous solution. The degradation rate almost reach 100 % at 45 min and can maintain 98.54 % after 8 cycles.

Asymmetric Formal [3+2] Cycloaddition Catalyzed by a Chiral Cobalt(II) Complex

Asymmetric Formal [3+2] Cycloaddition Catalyzed by a Chiral Cobalt(II) Complex

Synthesis of chiral and achiral cobalt complexes for hydrogenation studies.

Catalytic hydrogenation reactions have diverse applications in the petrochemical, food, pharmaceutical, and agricultural industries for the efficient synthesis of useful molecules. Specifically, asymmetric hydrogenation of olefins is a key method in the synthesis of single-enantiomer products necessary for the pharmaceutical industry, in particular. However, precious metals such as ruthenium, rhodium, and iridium are the basis for many of the current catalysts. It would be advantageous to replace these more expensive and toxic metals with first-row transition metals that are abundant and environmentally compatible. In addition, most catalytic systems for asymmetric hydrogenation are supported by phosphine ligands. Thus, chiral nitrogen-based ligands provide an opportunity to investigate a relatively underexplored area of hydrogenation chemistry. To this end, we are pursuing cobalt complexes supported by chiral nitrogen-based ligands. The synthesis of a previously reported chiral nitrogen ligand, (R,R)-2,6-bis[1-(N-piperidinyl)ethyl]pyridine, is underway with characterization by NMR spectroscopy. Upon successful synthesis of the ligand, cobalt complexes will be targeted in order to investigate asymmetric hydrogenation reactivity. In addition, synthesis of cobalt complexes bearing the achiral congener ligand, pip2NNN [pip2NNN = 2,6-bis(piperidylmethyl)pyridine], is underway in order to compare reactivity of chiral versus achiral complexes.

Chiral cobalt(ii) complex catalyzed Friedel-Crafts aromatization for the synthesis of axially chiral biaryldiols.

An efficient atroposelective synthesis of axially chiral biaryldiols via asymmetric Friedel-Crafts aromatization between p-quinones and 2-naphthols was developed. A chiral cobalt(ii) complex of N,N'-dioxide enabled the process to generate axially chiral biaryldiols in up to 98% yield and 95% ee. A large range of substituents at different positions of p-quinones and 2-naphthols was tolerable. The configuration of the product and the chiral N,N'-dioxide-Co(ClO4)2 catalyst was identified by X-ray crystal diffraction analysis and a possible catalytic model was suggested.

A chiral cobalt(ii) complex catalyzed asymmetric formal [3+2] cycloaddition for the synthesis of 1,2,4-triazolines.

A highly efficient catalytic asymmetric formal [3+2] cycloaddition reaction of 5-alkoxyoxazoles with azodicarboxylate compounds has been realized by a chiral N,N'-dioxide/Co(BF4)2·6H2O complex. A series of poly-substituted 1,2,4-triazolines compounds were obtained in moderate to excellent yields (70-99%) with excellent enantioselectivities (82-98% ee).

ChemInform Abstract: Palladium(II)‐Catalyzed Enantioselective Reactions Using COP Catalysts

AbstractReview: 47 refs.

Palladium(II)-Catalyzed Enantioselective Reactions Using COP Catalysts.

Allylic amides, amines, and esters are key synthetic building blocks. Their enantioselective syntheses under mild conditions is a continuing pursuit of organic synthesis methods development. One opportunity for the synthesis of these building blocks is by functionalization of prochiral double bonds using palladium(II) catalysis. In these reactions, nucleopalladation mediated by a chiral palladium(II) catalyst generates a new heteroatom-substituted chiral center. However, reactions where nucleopalladation occurs with antarafacial stereoselectivity are difficult to render enantioselective because of the challenge of transferring chiral ligand information across the square-planar palladium complex to the incoming nucleophile. In this Account, we describe the development and use of enantiopure palladium(II) catalysts of the COP (chiral cobalt oxazoline palladacyclic) family for the synthesis of enantioenriched products from starting materials derived from prochiral allylic alcohols. We begin with initial studies aimed at rendering catalyzed [3,3]-sigmatropic rearrangements of allylic imidates enantioselective, which ultimately led to the identification of the significant utility of the COP family of Pd(II) catalysts. The first use of an enantioselective COP catalyst was reported by Richards' and our laboratories in 2003 for the enantioselective rearrangement of allylic N-arylimidates. Shortly thereafter, we discovered that the chloride-bridged COP dimer, [COP-Cl]2, was an excellent enantioselective catalyst for the rearrangement of (E)-allylic trichloroacetimidates to enantioenriched allylic trichloroacetamides, this conversion being the most widely used of the allylic imidate rearrangements. We then turn to discuss SN2' reactions catalyzed by the acetate-bridged COP dimer, [COP-OAc]2, which proceed by a unique mechanism to provide branched allylic esters and allylic phenyl ethers in high enantioselectivity. Furthermore, because of the unique nucleopalladation/deoxypalladation mechanism of these SN2' reactions, they provide exclusively the branched allylic product. Importantly, both enantiomers of the [COP-Cl]2 and [COP-OAc]2 catalysts are commercially available. We also briefly consider several other enantioselective reactions catalyzed by COP complexes. The mechanism of enantioselective COP-catalyzed allylic rearrangements and allylic substitutions is discussed in some detail. In both reactions, nucleopalladation is found to be the enantiodetermining step. The cyclobutadienyl "floor" of the COP catalyst is critical for transmitting chiral information across the palladium square plane in these reactions. This structural feature enables high enantioselection to be realized in spite of the nearly 180° angle between the catalyst, electrophile and nucleophile in the enantiodetermining step. Our discussion concludes by considering several uses of the COP family of catalysts by other researchers for the enantioselective synthesis of biologically active chiral molecules. We anticipate that additional uses for COP catalysts will emerge in the future. In addition, the structural features of these catalysts that we have identified as important for achieving high enantioselection should be useful in the future development of improved enantioselective Pd(II) catalysts.

Enantioselective Hydrogenation of Ketones Catalyzed by Chiral Cobalt Complexes Containing PNNP Ligand

AbstractNovel chiral cobalt complexes containing a PNNP‐type ligand were synthesized using a straightforward method. The structures of the cobalt complexes have been fully characterized by X‐ray crystallography, high resolution mass spectrometry (HRMS), and electron paramagnetic resonance (EPR). Using H2 as the hydrogen source, the cobalt‐catalyzed asymmetric hydrogenation of various ketones was investigated, and the corresponding chiral alcohols were afforded with up to 99 % yield and 95 % ee. To the best of our knowledge, this is the first example of a cobalt‐catalyzed enantioselective hydrogenation of ketones with molecular hydrogen.

ChemInform Abstract: Chiral Cobalt(III) Complexes as Bifunctional Broensted Acid‐Lewis Base Catalysts for the Preparation of Cyclic Organic Carbonates.

AbstractThe catalyst is recycled and reused in two additional catalytic cycles without loss of its activity.