Asymmetric Diels–Alder reactions of N-allenoyloxazolidinones catalyzed by Cu(II)–bis(oxazoline) complexes

Catalytic asymmetric Diels–Alder reactions of N-allenoyloxazolidinones were investigated. Various chiral metal–bis(oxazoline) and metal–pyridinebis(oxazoline) complexes were screened. Cu(SbF₆)₂(H₂O)₂(t-BuBox) was found to be the most effective catalyst, giving the product in high yield, enantioselectivity, and endo:exo selectivity. The relative reactivity between N-allenoyloxazolidinones and N-alkenoyloxazolidinones was also investigated. A model for stereoinduction was proposed to account for the enantioselectivity and endo:exo selectivity.     

Reversal of enantioselection in an aldol reaction catalyzed by sterically congested bis(oxazoline)–Cu(II) complexes

The use of sterically congested C₂-symmetrical bis(oxazoline) ligands with methylene and ethylene spacers between the oxazoline rings results in the reversal of the enantioselection for aldol reactions catalyzed by bis(oxazoline)–Cu(II) complexes.     

Asymmetric glyoxylate-ene reaction catalyzed by C2-symmetric chiral bis(oxazoline)–lanthanide complexes

Efficient C₂-symmetric chiral bis(oxazoline)-lanthanide catalysts are developed for the glyoxylate-ene reaction to afford the α-hydroxy esters in 85% yield and up to 54% ee. An enhanced level of diastereoselectivity (81% de) was obtained in the reaction between menthyl glyoxylate and alkenes catalyzed by bis(oxazoline)–Ln(OTf)₃ complex.     

Facile and rapid immobilization of copper(II) bis(oxazoline) catalysts on silica: application to Diels-Alder reactions, recycling, and unexpected effects on enantioselectivity

Two chiral copper(II) bis(oxazoline) complexes have been immobilized on silica via electrostatic interactions using a remarkably straightforward procedure. The immobilized catalysts were tested in a standard Diels-Alder reaction and gave surprising results. Where the immobilized Cu((S,S)-phenyl-box)(OTf)₂ catalyst was used, the predominant enantiomer formed was the opposite of that produced using the same catalyst in a homogeneous reaction. This is a startling result given that the only difference is the electrostatic immobilization of the catalyst on amorphous silica. The activity of the catalyst in a hetero Diels-Alder reaction was also tested. This catalyst was also recycled, successfully maintaining a similar activity to the homogeneous analogue through a number of cycles.     

Heterogeneous asymmetric Diels–Alder reactions using a copper–chiral bis(oxazoline) complex immobilized on mesoporous silica

A chiral bis(oxazoline)–copper complex was immobilized onto mesoporous silica and the resulting heterogeneous catalyst was employed in asymmetric Diels–Alder reactions. Reactions using the catalyst exhibited good enantioselectivity of 78% enantiomeric excess and endo/exo-selectivity (17:1) better than the corresponding homogeneous reaction. The catalyst could be easily recovered and reused several times without significant loss of the remarkable reactivity, diastereoselectivity and enantioselectivity.     

Asymmetric cyclopropanation catalyzed by fluorous bis(oxazolines)–copper complexes

Fluorous bis(oxazoline)–copper(I) complexes generated in situ were tested as catalysts in the metal-catalyzed cyclopropanation of styrene with various α-diazoacetates. Under optimized conditions in CH₂Cl₂, quite good yields were obtained. Diastereoselectivities were found to be substrate and, to a lesser extent, ligand dependent, with trans/cis ratios ranging from 62/38 (cyclopropanation with α-ethyldiazoacetate 2a using ligand 1a) to 98/2 (cyclopropanation with methyl phenyldiazoacetate 2c using ligand 1b). Enantioselectivities up to 84% ee for the trans-isomer and 81% ee for the cis-isomer were observed using ligand 1b. Fluorous bis(oxazoline)–copper(I) complexes could be very easily separated from the products by simple precipitation using hexane, and recycled without loss of stereo- as well as enantioselectivities.     

Sulfur‐Containing Chiral Bis(oxazolines) Tested in Copper‐Catalyzed Asymmetric Cyclopropanation

The novel C₂‐symmetric sulfur‐containing chiral bis(oxazoline) compound has been synthesized and characterized by X‐ray crystallography. Highly enantioselective and diastereoselective cyclopropanation reactions have been performed using the copper‐bis(oxazoline) catalyst.     

Amino acid recognition of pyridine bis(oxazoline)-copper(II) complex in aqueous solvent

Enantioselective recognition of amino acids has been studied with C₂-symmetric chiral pyridine bis(oxazoline)-copper(II) complexes at physiological pH condition. UV-visible titration revealed strong binding of submillimolar dissociation constant between pyridine bis(oxazoline)-copper(II) complex and amino acids in aqueous solution. Moderate selectivity of up to 2:1 between d- and l-amino acids was achieved. The enantiomers were baseline resolved by capillary electrophoresis, using the bis(l-lysine)-copper(II) complex as a chiral selector.     

Highly enantioselective hetero Diels–Alder reactions of N-sulfinyl dienophiles promoted by copper(II)- and zinc(II)-chiral bis(oxazoline) complexes

Hetero Diels–Alder reactions of 1,3-cyclohexadiene with N-sulfinyl dienophiles 1a or 1b, promoted by Cu(II)- or Zn(II)-chiral bis(oxazoline) complexes, afforded endo adducts in high yields (up to 85%) and enantiomeric excess (e.e. of up to >98%) under stoichiometric conditions. With 10 mol% loading of the Zn(II) catalyst up to 75% e.e. was obtained for the endo adduct (endo:exo=10:1, total yield 68%).     

Novel chiral dibenzo[a,c]cycloheptadiene bis(oxazoline) and catalytic enantioselective cyclopropanation of styrene

A series of chiral C₂-symmetric bis(oxazoline) ligands containing dibenzo[a,c]cycloheptadiene units were synthesized. The copper complexes, prepared in situ from copper (I)-triflate and the new enantiopure oxazoline ligands, were assessed as chiral catalysts in the enantioselective cyclopropanation of styrene with diazoacetate. Enantioselectivities up to 82 and 62%, respectively, for trans- and cis-2-phenylcyclopropanecarboxylate were observed.     

Metallocene‐mediated cationic ring‐opening polymerization of 2‐methyl‐ and 2‐phenyl‐oxazoline

The cationic ring‐opening polymerization of 2‐methyl‐2‐oxazoline and 2‐phenyl‐2‐oxazoline was efficiently used using bis(η⁵‐cyclopentadienyl)dimethyl zirconium, Cp₂ZrMe₂, or bis(η⁵‐tert‐butyl‐cyclopentadienyl)dimethyl hafnium in combination with either tris(pentafluorophenyl)borate or tetrakis(pentafluorophenyl)borate dimethylanilinum salt as initiation systems. The evolution of polymer yield, molecular weight, and molecular weight distribution with time was examined. In addition, the influence of the initiation system and the monomer on the control of the polymerization was studied. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 000: 000–000, 2011     

Synthesis and polymerization reactions of cyclic imino ethers. 5: naphthalene unit‐containing poly(ether amide)s

Poly(ether amide)s containing naphthalene unit were prepared either by the polyaddition reaction of aromatic bis(2‐oxazoline)s with the different dihydroxynaphthalenes or by the homopolyaddition of a monomer containing an oxazoline, a hydroxy, and naphthalene moieties. First, polymerization method represents AA + BB mode where 1,4‐phenylene‐2,2′‐bis(2‐oxazoline) (A) and 1,3‐phenylene‐2,2′‐bis(2‐oxazoline) (B) were used as AA monomers and four different dihydroxynaphthalenes 1–4 were used as BB monomers. In the second case, 2‐(6‐hydroxynaphthalene‐2‐yl)‐2‐oxazoline (5) was used as AB‐type monomer in thermally induced polymerizations. The time dependences of polyadditions in bulk as well as in the solution were examined. The reduction of molar mass was observed after the initial fast increase of molar mass. This can be explained by the presence of side and degradation reactions. In both cases, polyadditions resulted in the linear poly(ether amide)s, which were characterized by ¹H and ¹³C NMR spectroscopy. Thermal properties of the prepared polymers were studied by differential scanning calorimetry (DSC) measurements. Comparison of the temperatures of glass transition for polymers prepared in AA + BB mode shows the strong dependence of thermal properties on the structure of the polymers. The values were in the range of 136–171°C. The glass‐transition temperature (T_g) of poly[2‐(6‐hydroxynaphthalene‐2‐yl)‐2‐oxazoline] prepared by AB‐type polyaddition is 183°C, which corresponds to the higher contents of hard aromatic segments in the latter type of polymers compared to the polymers prepared in the AA + BB‐type polyadditions. The described polymers represent novel naphthalene unit‐containing poly(ether amide)s for different applications in material science. Copyright © 2011 John Wiley & Sons, Ltd.     

负载型双噁唑啉催化剂上的不对称Diels-Alder反应

 分别以乙烷桥键磺酸官能化的有机-无机杂化介孔材料、十二钨磷酸铯、活化硅胶以及SBA-15为载体,通过非共价键作用制备了负载型双噁唑啉催化剂,并将该催化剂用于催化3-((E)-2-丁烯酰基)-1,3-噁唑啉-2-酮和环戊二烯的不对称Diels-Alder反应. 研究表明,催化剂的性能取决于载体本身以及载体表面阴离子的性质. 以SBA-15为载体时产物的对映体选择性较低,可归因于载体表面较低的羟基浓度.     

Asymmetric Diels-Alder reactions with hydrogen bonding heterogeneous catalysts and mechanistic studies on the reversal of enantioselectivity

Chiral bis(oxazoline) complexes of Cu(II), Zn(II) and Mg(II) have been immobilized on silica support via hydrogen-bonding interactions. Up to 93% ee is obtained in the Diels-Alder reaction between 3-((E)-2-butenoyl)-1,3-oxazolin-2-one and cyclopentadiene at room temperature with the heterogeneous bis(oxazoline) complexes, and the catalysts can be recycled without losing enantioselectivity. Experimental and theoretical studies show that the reversal of the absolute product configuration upon immobilization of the PhBOX-Cu(II) catalyst is triggered by the anion dissociation from Cu(II) onto the surface of the support.     

The first example of an enantiopure planar chiral hydroxyferrocene ligand

Lithiation of 2-ferrocenyl-4,4-dimethyloxazoline followed by addition of bis(trimethylsilyl)peroxide led to the isolation of air-stable 2-(2-hydroxyferrocenyl)-4,4-dimethyloxazoline. Repetition of this procedure on (S)-2-ferrocenyl-4-(1-methylethyl)oxazoline gave the lithium salt of (S)-2-((_pS)-2-hydroxyferrocenyl)-4-(1-methylethyl)oxazoline, a stable precursor to the first example of an enantiopure hydroxyferrocene ligand.     

Tautomerism in bis(oxazolines): an experimental and theoretical study of proton transfer in 1,1-bis(4,4-dimethyl-1,3-oxazolin-2-yl)-ethane

The serendipitous isolation of an unusual protonated bis(oxazoline) prompted us to discuss the role of H⁺ on the kinetics and thermodynamics of the equilibrium between its ketimine and enamine forms. X-ray diffraction analysis revealed that the protonated bis(oxazoline) is in the Z-enamine form, the unipositive charge is counterbalanced by [(FeCl₃)₂O]²⁻ and [FeCl₄]⁻ anions. DFT calculation at the BP86/TZVP level showed that relative stability of enamine tautomer versus ketimine tautomer increases with the protonation of the nitrogen atom of the oxazolidine ring. At the same time, the barrier energy of tautomerism decreases.     

Asymmetric Hydrogenation of Isoxazolium Triflates with a Chiral Iridium Catalyst

The iridium catalyst [IrCl(cod)]₂–phosphine–I₂ (cod=1,5‐cyclooctadiene) selectively reduced isoxazolium triflates to isoxazolines or isoxazolidines in the presence of H₂. The iridium‐catalyzed hydrogenation proceeded in high‐to‐good enantioselectivity when an optically active phosphine–oxazoline ligand was used. The 3‐substituted 5‐arylisoxazolium salts were transformed into 4‐isoxazolines with up to 95:5 enantiomeric ratio (e.r.). Chiral cis‐isoxazolidines were obtained in up to 89:11 e.r., with no formation of their trans isomers, when the substrates had a primary alkyl substituent at the 5‐position. The mechanistic studies indicate that the hydridoiridium(III) species prefers to deliver its hydride to the C5 atom of the isoxazole ring. The hydride attack leads to the formation of the chiral isoxazolidine via a 3‐isoxazoline intermediate. Meanwhile, in the selective formation of 4‐isoxazolines, hydride attack at the C5 atom may be obstructed by steric hindrance from the 5‐aryl substituent.     

Cobalt‐Catalyzed Alkyne Hydrosilylation and Sequential Vinylsilane Hydroboration with Markovnikov Selectivity

A pyridinebis(oxazoline) cobalt complex is a very efficient precatalyst for the hydrosilylation of terminal alkynes with Ph₂SiH₂, providing α‐vinylsilanes with high (Markovnikov) regioselectivity and broad functional‐group tolerance. The vinylsilane products can be further converted into geminal borosilanes through Markovnikov hydroboration with pinacolborane and a bis(imino)pyridine cobalt catalyst.     

Synthesis of oxazoline functionalized polypropene using metallocene catalysts

Using two different zirconocene/MAO catalyst systems, propene was copolymerized with the comonomers 2‐(9‐decene‐1‐yl)‐1,3‐oxazoline and 2‐(4‐(10‐undecene‐1‐oxo)phenyl)‐1,3‐oxazoline, respectively. The catalysts used were rac‐Et[Ind]₂ZrCl₂ and rac‐Me₂Si[2‐Me‐4, 5‐BenzInd]₂ZrCl₂. Up to 0.53 mol‐% oxazoline could be incorporated into polypropene. Oxazoline content, molecular weight, degree of isotacticity and melting behavior were dependent on the catalyst system, comonomer structure and comonomer concentration in the feed.     

Highly Enantioselective Carbonyl–Ene Reactions of 2,3‐Diketoesters: Efficient and Atom‐Economical Process to Functionalized Chiral α‐Hydroxy‐β‐Ketoesters

Carbonyl–ene reactions of 2,3‐diketoesters catalyzed by [Cu{(S,S)‐tBu‐box}](SbF₆)₂ [box=bis(oxazoline)] generate chiral α‐functionalized α‐hydroxy‐β‐ketoesters in up to 94 % yield and 97 % ee. The 2,3‐diketoesters are conveniently accessed from the corresponding α‐diazo‐β‐ketoester, and a catalyst loading as low as 1.0 mol % can be achieved.