Pyridine-oxazoline Ligand Research Articles

Synthesis, Characterization, and Catalytic Behaviors in Isoprene Polymerization of Pyridine-Oxazoline-Ligated Cobalt Complexes.

A family of pyridine-oxazoline-ligated cobalt complexes L2CoCl23a-h were synthesized and characterized. Determined via single-crystal X-ray diffraction, complexes 3a and 3d, ligated by two ligands, displayed a distorted tetrahedral coordination of a cobalt center. The X-ray structure indicated the pyridine-oxazoline ligands acted as unusual mono-dentate ligands by coordinating only to Noxazoline. Upon activation with AlEt2Cl (diethylaluminum chloride), these cobalt complexes all exhibited high catalytic activity (up to 2.5 × 106 g·molCo-1·h-1), affording cis-1,4-co-3,4-polyisoprene with molecular weights of 4.4-176 kg mol-1 and a narrow Ð of 1.79-3.42, suggesting a single-site nature of the active sites. The structure of cobalt catalysts and reaction parameters, especially co-catalysts and the reaction temperature, all have significant influence on the polymerization activity but not on the microstructure of polyisoprene.

Copper-Catalyzed Asymmetric Synthesis of Silicon-Stereogenic Benzoxasiloles.

A Cu-catalyzed asymmetric synthesis of silicon-stereogenic benzoxasiloles has been realized via intramolecular Si-O coupling of [2-(hydroxymethyl)phenyl]silanes. Cu(I)/difluorphos is found to be an efficient catalytic system for enantioselective Si-C bond cleavage and Si-O bond formation. In addition, kinetic resolution of racemic substituted [2-(hydroxymethyl)phenyl]silanes using Cu(I)/ PyrOx (pyridine-oxazoline ligands) as the catalytic system is developed to afford carbon- and silicon-stereogenic benzoxasiloles. Ring-opening reactions of chiral benzoxasiloles with organolithiums and Grignard reagents yield various enantioenriched functionalized tetraorganosilanes.

Immobilization of Trifluoromethyl-Substituted Pyridine-Oxazoline Ligand and Its Application in Asymmetric Continuous Flow Synthesis of Benzosultams.

This study presents an improved synthetic route to ligand (S)-4-(tert-butyl)-2-(5-(trifluoromethyl)pyridin-2-yl)-4,5-dihydrooxazole and its application as a highly active and enantioselective catalyst in the addition of arylboronic acids to cyclic N-sulfonylketimines. Immobilization of such a ligand was achieved using a commercially available starting material and a PS-PEG TentaGel S NH2 support, resulting in a stable heterogeneous catalyst. Although the anchored catalyst exhibited a slight reduction in enantioselectivity and a 4-fold decrease in reaction rate, it displayed remarkable stability, enabling 10 consecutive reaction cycles. Furthermore, the successful transition to a continuous flow system demonstrated even higher turnover numbers compared to batch arrangements. These findings provide valuable insights into the development of efficient flow reactors for continuous synthesis of benzosultams, further advancing the field of asymmetric catalysis.

Nickel/Titanocene-Catalyzed Electrophilic Acylation Coupling of Styrene Oxides.

The cross-coupling of epoxides with acyl chlorides or anhydrides by a nickel/titanocene dual catalytic system is established. A variety of synthetically useful β-hydroxy ketones were obtained in good to high yields by using modified pyridine-oxazoline ligand. The reaction proceeds via the cooperation of titanocene-catalyzed ring-opening of epoxides and nickel-catalyzed acylation of the benzylic radical intermediate.

Non-nucleophilic Grignard Synthesis of Bridged Pyridine–Oxazoline Ligands and Evaluation in Palladium-Catalysed Allylic Alkylation

AbstractThe synthesis of eight pyridine-oxazoline ligands, five of which have never been reported previously, is described. The ligands were prepared in two efficient steps, initially preparing 2-pyridyl alkylnitriles, followed by their conversion into oxazolines ligands using chiral amino alcohols and zinc chloride. The 2-pyridyl nitriles are prepared via a novel SNAr alkylation reaction of 2-bromopyridine with alkyl nitriles using methylmagnesium chloride as a non-nucleophilic base in conjunction with an amine mediator. This methodology allows preparation of the existing gem-dimethyl motif and its elaboration beyond previously prepared ligands in fewer steps with simplicity and scalability. The toleration of variation in the nitrile, halopyridine, and amino alcohol starting materials allows for other novel bridging substitution which gives new ligands with enhanced reactivity. The library of bridging ligands was applied to the Pd-catalysed allylic alkylation of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate and afforded conversions of up to 100% and enantioselectivities of up to 68%.

Palladium-Catalyzed Alkynylation of Enones with Alkynylsilanes via C-C Bond Activation.

We report herein the synthesis of 1,3-enynes via palladium-catalyzed cross-coupling between enone derivatives and alkynylsilanes. The employment of an appropriate pyridine-oxazoline ligand is the key to the C-C cleavage and the high E/Z stereoselectivity. This protocol features broad substrate scope and wide functional-group tolerance, affording the desired products in moderate-to-good yields. Late-stage diversification of natural product β-ionone further demonstrated the synthetic utility of this protocol.

Functionalized styrene synthesis via palladium-catalyzed C[sbnd]C cleavage of aryl ketones

We report herein the synthesis of functionalized styrenes via palladium-catalyzed Suzuki–Miyaura cross-coupling reaction between aryl ketone derivatives and potassium vinyltrifluoroborate. The employment of pyridine-oxazoline ligand was the key to the cleavage of unstrained CC bond. A variety of functional groups and biologically important moleculars were well tolerated. The orthogonal Suzuki–Miyaura coupling demonstrated the synthetic practicability.

Catalytic remote hydrohalogenation of internal alkenes.

Primary alkyl halides have broad utility as fine chemicals in organic synthesis. The direct halogenation of alkenes is one of the most efficient approaches for the synthesis of these halides. Internal alkenes, in particular mixtures of isomers from refineries, constitute readily available and inexpensive feedstock and are the most attractive starting materials for this synthesis. However, the hydrohalogenation of alkenes generally affords branched alkyl halides; there are no catalytic methods to prepare linear alkyl halides directly from internal alkenes, let alone from a mixture of alkene isomers. Here we report the remote oxidative halogenation of alkenes under palladium catalysis via which both terminal and internal alkenes yield primary alkyl halides efficiently. Engineering pyridine-oxazoline ligands by introducing a hydroxyl group is essential for achieving excellent chemo- and regioselectivity. The catalytic system is also good for the mixture of alkene isomers generated from dehydrogenation of alkanes, providing a window to investigate the high-value utilization of inexpensive alkanes.

Arylketones as Aryl Donors in Palladium-Catalyzed Suzuki-Miyaura Couplings.

Herein, we report the arylation, alkylation, and alkenylation of aryl ketones via a palladium-catalyzed Suzuki-Miyaura cross-coupling reaction. The use of the pyridine-oxazoline ligand is the key to the cleavage of the unstrained C-C bond. The late-stage arylation of aryl ketones derived from drugs and natural products demonstrated the synthetic utility of this protocol.

Redox Activity of Pyridine-Oxazoline Ligands in the Stabilization of Low-Valent Organonickel Radical Complexes.

Low-valent organonickel radical complexes are common intermediates in cross-coupling reactions and metalloenzyme-mediated processes. The electronic structures of N-ligand supported nickel complexes appear to vary depending on the actor ligands and the coordination number. The reduction products of a series of divalent (pyrox)Ni complexes establish the redox activity of pyrox in stabilizing electron-rich Ni(II)-alkyl and -aryl complexes by adopting a ligand-centered radical configuration. The reduced pyrox imparts an enhanced trans-influence. In contrast, such redox activity was not observed in a (pyrox)Ni(I)-bromide species. The excellent capability of pyrox in stabilizing electron-rich Ni species resonates with its proclivity in promoting the reductive activation of C(sp3) electrophiles in cross-coupling reactions.

Iron-Catalyzed Cyanoalkylation of Glycine Derivatives Promoted by Pyridine-Oxazoline Ligands

An iron-catalyzed C(sp3)–H cyanoalkylation of glycine derivatives with cyclobutanone oxime esters was established for the incorporation of a cyano group into amino acids and peptides. In this reaction, Fe(NTf2)2 and pyridine-oxazoline ligands form highly active catalysts that could simultaneously and selectively activate both substrates. Preliminary mechanistic studies revealed the excellent chemo-selectivity may stem from an in situ formed imine intermediate and a consecutive radical addition. The evidence suggested the interaction between glycinate substrate and the Fe-catalyst has a prominent impact on the catalytic activity, and the catalyst may also activate the imine intermediate as a Lewis acid.

Transformations of Aryl Ketones via Ligand-Promoted C-C Bond Activation.

The coupling of aromatic electrophiles (aryl halides, aryl ethers, aryl acids, aryl nitriles etc.) with nucleophiles is a core methodology for the synthesis of aryl compounds. Transformations of aryl ketones in an analogous manner via carbon-carbon bond activation could greatly expand the toolbox for the synthesis of aryl compounds due to the abundance of aryl ketones. An exploratory study of this approach is typically based on carbon-carbon cleavage triggered by ring-strain release and chelation assistance, and the products are also limited to a specific structural motif. Here we report a ligand-promoted β-carbon elimination strategy to activate the carbon-carbon bonds, which results in a range of transformations of aryl ketones, leading to useful aryl borates, and also to biaryls, aryl nitriles, and aryl alkenes. The use of a pyridine-oxazoline ligand is crucial for this catalytic transformation. A gram-scale borylation reaction of an aryl ketone via a simple one-pot operation is reported. The potential utility of this strategy is also demonstrated by the late-stage diversification of drug molecules probenecid, adapalene, and desoxyestrone, the fragrance tonalid as well as the natural product apocynin.

Mechanism of Au(III)-Mediated Alkoxycyclization of a 1,6-Enyne.

Gold-mediated homogeneous catalysis is a powerful tool for construction of valuable molecules and has lately received growing attention. Whereas Au(I)-catalyzed processes have become well established, those mediated by Au(III) have so far barely been explored, and their mechanistic understanding remains basic. Herein, we disclose the combined NMR spectroscopic, single-crystal X-ray crystallographic, and computational (DFT) investigation of the Au(III)-mediated alkoxycyclization of a 1,6-enyne in the presence of a bidentate pyridine-oxazoline ligand. The roles of the counterion, the solvent, and the type of Au(III) complex have been assessed. Au(III) is demonstrated to be the active catalyst in alkoxycyclization. Alkyne coordination to Au(III) involves decoordination of the pyridine nitrogen and is the rate-limiting step.

Asymmetric Palladium-Catalyzed C-H Functionalization Cascade for Synthesis of Chiral 3,4-Dihydroisoquinolones.

A palladium-catalyzed C-H functionalization/intramolecular asymmetric allylation cascade of N-sulfonyl benzamides with 1,3-dienes has been developed. In the presence of a chiral pyridine-oxazoline ligand, this protocol enables the synthesis of chiral 3,4-dihydroisoquinolones in yields of up to 83% with enantioselectivities of up to 96%, using environmentally friendly air as the terminal oxidant.

Cobalt-Catalyzed Asymmetric Hydrogenation of Vinylsilanes with a Phosphine–Pyridine–Oxazoline Ligand: Synthesis of Optically Active Organosilanes and Silacycles

The asymmetric hydrogenation of vinylsilanes catalyzed by a new C1-symmetric phosphine–pyridine–oxazoline cobalt complex is described. The method provides an efficient approach to chiral tertiary s...

An autonomous self-optimizing flow machine for the synthesis of pyridine–oxazoline (PyOX) ligands

A continuous flow synthesis of PyOX ligands has been developed using an autonomous self-optimizing flow reactor.

Enantioselective C2-Alkylation of Indoles through a Redox-Relay Heck Reaction of 2-Indole Triflates.

A palladium-catalyzed enantioselective redox-relay Heck reaction of 2-indole triflates and disubstituted alkenes is reported. This process combines readily available indole triflates with a variety of alkenes to afford a range of indole derivatives bearing a stereocenter adjacent to C2. Enantioselectivity is achieved through use of a simple pyridine-oxazoline ligand. Tuning the electronics of the indole, through judicious choice of N-protecting group, is required to ensure selective β-hydride elimination away from the indole core. Utility of this method is highlighted in a modular formal synthesis of an S1P1 agonist precursor developed by Merck.

Pd(II)-Catalyzed Asymmetric Oxidative 1,2-Diamination of Conjugated Dienes with Ureas.

A palladium(II)-catalyzed asymmetric 1,2-diamination of 1,3-dienes with readily available dialkylureas was established by using a chiral pyridine-oxazoline ligand. The diamination reaction exclusively occurs at the terminal C-C double bond of the 1,3-dienes to give 4-vinylimidazolidin-2-ones in high yields and with excellent levels of enantioselectivity (up to 99% yield, 97% ee). The reaction could feasibly be applied for gram-scale synthesis with a 1:1 ratio of the diene and the urea.

Pd(ii)-Catalyzed asymmetric 1,6-conjugate addition of arylboronic acids to Meldrum's acid-derived dienes.

A Pd(ii)-catalyzed asymmetric 1,6-addition of arylboronic acids to Meldrum's acid-derived dienes was developed. A new substituted In-Pyrox ligand was designed to enable this reaction with high enantioselectivity via a remote steric effect. A series of Meldrum's acid-derived dienes and arylboronic acids were tolerable to the reaction conditions, giving the desired adducts in moderate to high yields with good enantioselectivities (up to 96% ee). This reaction provides the first method of Pd(ii)-catalyzed asymmetric 1,6-conjugate addition of arylboron.

Developing Comprehensive Computational Parameter Sets To Describe the Performance of Pyridine-Oxazoline and Related Ligands

The applicability of computational descriptors extracted from metal pyridine-oxazoline complexes to relate both site and enantioselectivity to structural diversity was investigated. A group of computationally derived features (e.g., metal NBO charges, steric descriptors, torsion angles) were acquired for a library of pyridine-oxazoline ligands. Correlation studies were employed to examine steric/electronic features described by each descriptor, followed by application of the said descriptors in modeling the results of two reaction types, the site-selective redox-relay Heck reaction and the enantioselective Carroll rearrangement, affording simple, well-validated models. Through experimental validation and extrapolation, parameters derived from ground state metal complexes were found to be advantageous over those from the free ligand.