Bis Oxazoline Complexes Research Articles

Lanthanide-catalyzed regio- and enantioselective hydrosilylation of aryl-substituted terminal and internal alkenes

Highly regio- and enantioselective hydrosilylation of aryl-substituted terminal/internal alkenes with primary silanes was achieved using a chiral La(iii)-bisoxazoline complex.

Conjugate Addition of α‐Substituted Acyl Imidazoles to Nitroalkenes Catalyzed by Nickel Bisoxazoline and B(C6F5)3

AbstractA diastereoselective and enantioselective conjugate addition of α‐substituted acyl imidazole to nitroalkene catalyzed by nickel bisoxazoline complex and B(C6F5)3 has been developed. This reaction tolerated a wide range of acyl imidazoles and nitroalkenes, affording a series of conjugate adducts in yields ranging from 38% to 99% with 4:1 to 99:1 dr and 13% to 98% ee. The use of strong Lewis acid B(C6F5)3 as co‐catalyst proved vital to the success of this process. The potential scalability and utility of the current protocol are demonstrated by a reaction on a 3 mmol scale and further transformations of the product.magnified image

Nickel-Catalyzed Regiodivergent Asymmetric Cycloadditions of α,β-Unsaturated Carbonyl Compounds

Nickel-Catalyzed Regiodivergent Asymmetric Cycloadditions of α,β-Unsaturated Carbonyl Compounds

Switching of Enantioselectivity in the Cu-Catalyzed Asymmetric Decarboxylative Aldol Reaction of Tryptanthrin with β-Keto Acids: An Unexpected Counteranion Effect.

Cu-bisoxazoline-catalyzed enantioselective decarboxylative aldol reaction of tryptanthrin with aryl-substituted β-keto acids is developed, providing a straightforward approach to deliver a series of phaitanthrin A analogues. Both enantiomers of the products can be obtained with good to high enantioselectivity in the presence of a single chiral ligand by simply changing the copper salts. Based on the X-ray crystallographic analysis of chiral Cu(II)-bisoxazoline complexes, the tentative stereochemical models are presented to account for the observed counteranion-induced switching in enantioselectivity.

Enantioselective synthesis of 4,5-disubstituted pyrrolidin-2-one derivatives with two stereocenters on the basis of Michael adducts

The addition of malonates to trans-β-alkyl-β-nitrostyrenes in the presence of chiral Mg2+ bisoxazoline complex and Ni2+ and Co2+ bis((S,S)- or (R,R)-N,N′-dibenzylcyclohexane-1,2-diamine) complexes resulted in the formation of Michael adducts with two stereo-centers which were used as key precursors for the preparation of potentially biologically active enantiopure 4,5-disubstituted pyrrolidin-2-one derivatives.

A ruthenium bisoxazoline complex as a photoredox catalyst for nitro compound reduction under visible light.

An unreported ruthenium(ii) complex containing bisoxazoline ligands has been synthesized and characterized. To test the catalytic ability of the ruthenium complex, the synthesis of anilines from nitro compounds in the presence of a mild reducing agent sodium borohydride and visible light has been developed. Mechanistic studies involving the experiment and DFT calculations suggest that the reaction could involve a radical pathway with the assistance of a photoredox catalyst.

Copper(II)-Catalyzed Asymmetric Photoredox Reactions: Enantioselective Alkylation of Imines Driven by Visible Light

Asymmetric photoredox catalysis offers exciting opportunities to develop new synthetic approaches to chiral molecules through novel reaction pathways. Employing the first-row transition metal complexes as the chiral photoredox catalysts remains, however, a formidable challenge, although these complexes are economic, environmentally friendly, and often exhibit special reactivities. We report in this Article the development of one class of highly efficient asymmetric/photoredox bifunctional catalysts based on the copper(II) bisoxazoline complexes (CuII-BOX) for the light-induced enantioselective alkylation of imines. The reactions proceed under very mild conditions and without a need for any other photosensitizer. The simple catalytic system and readily tunable chiral ligands enable a significantly high level of enantioselectivity for the formation of chiral amine products bearing a tetrasubstituted carbon stereocenter (36 examples, up to 98% ee). Overall, the CuII-BOX catalysts initiate the radical generation, and also govern the subsequent stereoselective transformations. This strategy utilizing chiral complexes comprised of a first-row transition metal and a flexible chiral ligand as the asymmetric photoredox catalysts provides an effective platform for the development of green asymmetric synthetic methods.

Copper‐Catalyzed Enantioselective Arylative Desymmetrization of Prochiral Cyclopentenes with Diaryliodonium Salts

AbstractA copper‐catalyzed enantioselective arylative desymmetrization of prochiral cyclopentenes with diaryliodonium salts was developed. In the presence of a catalytic amount of a chiral copper–bisoxazoline complex, which was generated in situ, the reaction of 4‐substituted or 4,4‐disubstituted cyclopent‐1‐enes with diaryliodonium hexafluoroarsenates afforded the chiral arylated products in good yields with excellent enantioselectivity. A cyclohexyl‐containing Box ligand was essential for the high enantioselectivity. Transformation of the enantiomerically enriched adducts into other chiral building blocks is also documented.

Enantioselective Conjugate Addition of 2-Acetyl Azaarenes to β,β-Disubstituted Nitroalkene for the Construction of All-Carbon Quaternary Stereocenters.

The first highly enantioselective conjugate addition of 2-acetyl azaarenes to α-substituted-β-nitroacrylates was successfully realized under mild conditions by a Ni(II)-bisoxazoline complex, providing the desired adducts bearing an all-carbon quaternary stereocenter in high yield with excellent enantioselectivity. The products obtained in this system could be readily converted into optically active β2,2-amino esters, succinates, lactones, and lactams.

Catalytic Asymmetric [3+1]-Cycloaddition Reaction of Ylides with Electrophilic Metallo-enolcarbene Intermediates.

The first asymmetric [3+1]-cycloaddition was successfully achieved by copper(I) triflate/double-sidearmed bisoxazoline complex catalyzed reactions of β-triisopropylsilyl-substituted enoldiazo compounds with sulfur ylides. This methodology delivered a series of chiral cyclobutenes in good yields with high enantio- and diastereoselectivities (up to 99 % ee, and >20:1 d.r.). Additionally, the [3+1]-cycloaddition of catalytically generated metallo-enolcarbenes was successfully extended to reaction with a stable benzylidene dichlororuthenium complex.

Catalytic Asymmetric [3+1]‐Cycloaddition Reaction of Ylides with Electrophilic Metallo‐enolcarbene Intermediates

AbstractThe first asymmetric [3+1]‐cycloaddition was successfully achieved by copper(I) triflate/double‐sidearmed bisoxazoline complex catalyzed reactions of β‐triisopropylsilyl‐substituted enoldiazo compounds with sulfur ylides. This methodology delivered a series of chiral cyclobutenes in good yields with high enantio‐ and diastereoselectivities (up to 99 % ee, and >20:1 d.r.). Additionally, the [3+1]‐cycloaddition of catalytically generated metallo‐enolcarbenes was successfully extended to reaction with a stable benzylidene dichlororuthenium complex.

Enantioselective [3+2] cycloaddition of azomethine ylides and aldehydes via Ni/bis(oxazoline)-catalyzed ring opening of N-tosylaziridines through a chirality transfer approach

A diastereo- and enantioselective formal [3+2] cycloaddition of N-tosylaziridines and aldehydes catalyzed by a Ni(ii)-bisoxazoline complex has been accomplished. The 1,3-oxazolidine products are obtained with high diastereoselectivity, good yields (up to 99%) and high ee values (up to 96% ee). The challenging long distant stereocenter control is achieved by a chirality transfer approach.

ChemInform Abstract: An Efficient Approach for the Construction of Trifluoromethylated All‐Carbon Quaternary Stereocenters: Enantioselective Ni(II)‐Catalyzed Michael Addition of 2‐Acetyl Azaarene to β,β‐Disubstituted Nitroalkenes.

The first example of a highly enantioselective Michael addition of 2-acetyl azaarenes with β,β-disubstituted nitroalkenes was achieved using a Ni(acac)2–bisoxazoline complex as a catalyst, which afforded chiral compounds with an all-carbon quaternary stereocenter bearing a CF3 group in good yields with excellent enantioselectivities (up to >99% ee). This reaction, featuring mild conditions, excellent enantioselectivity and broad generality, provides a new efficient strategy for the construction of trifluoromethylated all-carbon quaternary stereocenters.

ChemInform Abstract: Nickel‐Catalyzed Enantioselective Friedel—Crafts Alkylation of Indoles with β,β‐Disubstituted Nitroalkenes.

An enantioselective Friedel–Crafts alkylation of indoles with β,β-disubstituted nitroalkenes was developed by using a nickel(II) perchlorate–bisoxazoline complex as a catalyst. A range of nitroalkenes and indoles participated in this reaction, affording chiral indole compounds bearing all-carbon quaternary stereocenters in excellent yields and with moderate to good enantioselectivities (up to 80% ee).

Nickel-Catalyzed Enantioselective Friedel–Crafts Alkylation of Indoles with β,β-Disubstituted Nitroalkenes

An enantioselective Friedel–Crafts alkylation of indoles with β,β-disubstituted nitroalkenes was developed by using a nickel(II) perchlorate–bisoxazoline complex as a catalyst. A range of nitroalkenes and indoles participated in this reaction, affording chiral indole compounds bearing all-carbon quaternary stereocenters in excellent yields and with moderate to good enantioselectivities (up to 80% ee).

Enantioselective and Regiodivergent Copper-Catalyzed Electrophilic Arylation of Allylic Amides with Diaryliodonium Salts.

A catalytic enantioselective and regiodivergent arylation of alkenes is described. Chiral copper(II)bisoxazoline complexes catalyze the addition of diaryliodonium salts to allylic amides in excellent ee. Moreover, the arylation can be controlled by the electronic nature of the diaryliodonium salt enabling the preparation of nonracemic diaryloxazines or β,β′-diaryl enamides.

ChemInform Abstract: Cobalt—Bisoxazoline‐Catalyzed Asymmetric Kumada Cross‐Coupling of Racemic α‐Bromo Esters with Aryl Grignard Reagents.

The first cobalt-catalyzed asymmetric Kumada cross-coupling with high enantioselectivity has been developed. The reaction affords a unique strategy for the enantioselective arylation of α-bromo esters catalyzed by a cobalt–bisoxazoline complex. A variety of chiral α-arylalkanoic esters were prepared in excellent enantioselectivity and yield (up to 97% ee and 96% yield). The arylated products were transformed into α-arylcarboxylic acids and primary alcohols without erosion of ee. The new enantioenriched α-arylpropionic esters synthesized herein are potentially useful in the development of nonsteroidal anti-inflammatory drugs. This method was conducted on gram-scale and applied to the synthesis of highly enantioenriched (S)-fenoprofen and (S)-ar-turmerone.

Epoxidation of alkenes and oxidation of sulfides catalyzed by a new binuclear vanadium bis-oxazoline complex

Bis(oxazoline) ligand, [2,2′-(1,3-phenylene)bis(oxazole-4,2-diyl)]-dimethanol, derived from dicyanobenzene was applied as ligand for complexation with vanadium. The catalyst was characterized by FT-IR, UV–Vis, 1H NMR spectroscopic methods, CHNS, ICP and thermal analyses, and magnetic susceptibility. The catalytic activity of this complex was then studied in the epoxidation of alkenes with TBHP (tert-butyl hydroperoxide), in acetonitrile. The effect of reaction parameters such as kind of solvent and oxygen donors was studied in the epoxidation of cyclooctene. The catalytic activity of this catalyst was also investigated in the oxidation of sulfides with H2O2 in ethanol and the corresponding sulfoxides and sulfones were produced.

ChemInform Abstract: Asymmetric Friedel—Crafts Alkylation of α‐Substituted β‐Nitroacrylates: Access to β2,2‐Amino Acids Bearing Indolic All‐Carbon Quaternary Stereocenters.

AbstractA highly enantioselective Friedel—Crafts alkylation reaction of indoles with acyclic α‐substituted β‐nitroacrylates is developed under the catalysis of a Ni(ClO4)2—bisoxazoline complex, affording chiral indolic β‐nitroesters bearing an all‐carbon quaternary stereocenter.

Oxidation of alkenes and sulfides catalyzed by a new binuclear molybdenum bis-oxazoline complex

A novel bis(oxazoline) ligand derived from 1,3-dicyanobenzene was prepared and applied as a ligand for the preparation of a new binuclear molybdenyl complex. This ligand was characterized by UV–Vis, mass, 1H NMR, and FT-IR spectroscopic methods, thermal and elemental analysis and X-ray diffraction. The molybdenum complex was prepared by the reaction of this ligand with MoO2(acac)2. The catalyst was also characterized by FT-IR, UV–Vis, and ICP spectroscopy, elemental and thermal analysis. This catalytic system was efficiently used for the oxidation of alkenes and sulfides in the presence of TBHP. The effect of different solvents and kind of oxygen donor was also studied in the oxidation reactions.