Oxazoline Complexes Research Articles

Applications of an Air Stable Pd Oxazoline Complex for Heck, Suzuki, Sonogashira and Related C—C Bond‐Forming Reactions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Electronic Effects Steer the Mechanism of Asymmetric Hydrogenations of Unfunctionalized Aryl-Substituted Alkenes

Density functional theory (PBE and B3LYP) was used to study asymmetric hydrogenations of alkenes catalyzed by an iridium imidazolylidine oxazoline complex. The calculation predicts that the alkene preferentially coordinates to the site trans to the carbene. The coordinated alkene then reacts first with the H2 ligand, then with the hydride to form alkane. Finally, the alkane is released by equilibrating with extrinsic H2 and alkene. Enantioface selectivities for hydrogenations of trisubstituted alkenes seem to be driven primarily by steric interactions with the adamantyl part of the ligand; only the smallest substituents can adopt a site close to it. Application of this theoretical model leads to correct predictions regarding the experimentally observed sense and magnitude of the enantioselectivities.

Application of an air stable Pd oxazoline complex for Heck, Suzuki, Sonogashira and related C–C bond-forming reactions

The novel complex trans-[PdCl 2(η 1- N-2-ethyl-2-oxazoline) 2] is shown to be an active and oxidatively robust catalyst for C–C bond-forming reactions (Heck, Sonogashira, Ullman, Suzuki), which can be carried out in air without rigorous solvent/substrate purification and in the absence of additional free ligand.

The Enantioselective Diethylzinc Addition to Imines Catalyzed by Chiral Cu(II)‐Oxazoline Complexes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

C 2-Symmetric Chiral Bis(thiazoline) and Bis(oxazoline) Ligands and their Application in the Catalytic Asymmetric Allylic Alkylation

A series of novel C 2 -symmetric bis(thiazoline) ligands with dibenzo[a,c]cycloheptadiene backbone as the substituent on the thiazoline ring were synthesized using the simple reagent phosphorus pentasulfide, and their application in catalytic allylic alkylation was investigated by comparison with the corresponding bis(oxazolines). Palladium-bis(oxazoline) complexes furnished good enantioselectivity (up to 87% ee), while bis(thiazolines) only gave up to moderate enantioselectivity (56% ee) with CH 2 Cl 2 as solvent. Preliminary explanation of the catalysis mechanism is given for the difference of two types of Pd(0)-ligand complexes.

Oxazoline chemistry — Part IV: Synthesis and characterization of oxazoline complexes of the zinc halides

The synthesis and characterization of 11 zinc halide derivatives that contain monodentate oxazoline ligands is described. The treatment of ether solutions of [ZnX2] (X = Cl, Br, I) with 2-aryl- or 2-methyl-2-oxazolines results in the formation of mildly hygroscopic complexes of the general formulae [ZnX2(ox)2] (ox = 2-methyl-2-oxazoline (1), 2,4,4-trimethyl-2-oxazoline (2), 2-phenyl-2-oxazoline (3), or 4,4-dimethyl-2-phenyl-2-oxazoline (4)), except in the case of ZnI2, which does not form an isolable complex — likely for steric reasons — with oxazoline 4. Treatment of [ZnBr2(4)2] with 1 reveals (1H NMR) that 1 only sluggishly displaces coordinated 4 at temperatures below 50 °C. The structural characterization, via single crystal X-ray diffraction, of six of the complexes, viz. [ZnI2(1)2], [ZnI2(2)2], [ZnX2(3)2] (X = Cl, Br, or I), and [ZnBr2(4)2], is also reported. All of these structurally characterized complexes are mononuclear zinc compounds with an overall distorted tetrahedral arrangement of the two halide and two oxazoline ligands around the zinc metal centre. The oxazoline series of complexes reported herein show little structural diversity, a facet which is in contrast to their substituted pyridine analogues.Key words: oxazoline, zinc, X-ray crystal structure, coordination complex, NMR spectroscopy, Zn(II).

The enantioselective diethylzinc addition to imines catalyzed by chiral Cu(II)–oxazoline complexes

A series of copper complexes of chiral bisoxazolines has been applied in the catalytic diethylzinc addition to N-sulfonyl imines. It has been found that the tridentate ligands 3– 5 provided higher enantioselectivity than bidentate ones. Addition of 4 Å molecular sieves to the reaction system benefits the enantioselectivity. The optimal procedure for diethylzinc addition to different imines resulted in moderate yields and enantioselectivities of up to 82% ee.

Iridium-mediated asymmetric hydrogenation of 2,3-diphenylbutadiene: a revealing kinetic study.

Hydrogenation of 2,3-diphenylbutadiene mediated by a chiral iridium carbene oxazoline complex was studied. The kinetics of the reaction showed that it occurred in two distinct stages. The first corresponded to slow consumption of the diene to form half-hydrogenated products, i.e., monoenes. After all the diene was consumed, however, the reaction was much faster and more stereoselective. These data were interpreted in terms of possible catalytic intermediates.

Enantioselective indole Friedel--Crafts alkylations catalyzed by bis(oxazolinyl)pyridine-scandium(III) triflate complexes.

A highly enantioselective Friedel-Crafts alkylation of electron-rich aromatic nucleophiles catalyzed by scandium(III) triflate-pyridyl(bis)oxazoline complexes has been accomplished. The reaction involves alpha,beta-unsaturated acyl phosphonates as electrophiles and primarily substituted indoles as nucleophiles. The reactive acyl phosphonate product is converted to the corresponding ester or amide in good overall yield by adding an alcohol or amine directly to the reaction mixture.

Diastereoselective lithiation of (eta6-arene)dicarbonyltriphenylphosphane chromium(0) oxazoline complexes--direct preparation of enantiopure complexes having planar chiral fragments of either configuration.

Incorporation of a triphenylphosphane ligand is the key to the direct lithiation of (η6-2-phenyloxazoline)chromium(0) complexes. As diastereoselection can be modulated by the presence or absence of N,N,N′,N′-tetramethylethylenediamine (TMEDA), both diastereomers of 1,2-disubstituted η6-arene chromium(0) oxazoline complexes, which possess planar chiral moieties of opposite absolute configuration, can be prepared in useful chemical yields and high diastereoselectivities (10–50:1).

Diastereoselective Lithiation of (η6-Arene)dicarbonyltriphenylphosphane Chromium(0) Oxazoline Complexes—Direct Preparation of Enantiopure Complexes Having Planar Chiral Fragments of Either Configuration

Komplementäre Stereoselektion: Der Austausch eines Carbonylliganden in Tricarbonylchromkomplexen von 2-Phenyloxazolinen gegen einen Phosphanliganden ermöglicht die direkte ortho-Lithiierung am Phenylring. Je nach An- oder Abwesenheit von N,N,N′,N′-Tetramethylethylendiamin (TMEDA) werden bei der anschließenden Umsetzung mit Elektrophilen Komplexe mit entgegengesetzter Konfiguration erhalten, wobei die Diastereoselektivitäten zwischen 10:1 und 50:1 liegen.

Chiral hexacatenar metallomesogens: supramolecular organization versus steric demand of chiral cores

Hexacatenar chiral oxazoline complexes were prepared in a stereospecific synthesis. Metals (Pd, Ni, Cu, Zn), lateral alkoxy chains (OC6H13, OC12H25), sterically demanding alkyl groups (CH3, C4H9) on the chiral rigid centre and the number of nuclei incorporated in the complex core (mononuclear, orthopalladated dinuclear) were varied in order to achieve supramolecular organization of the phasmid-like molecules in liquid crystal phases. Mesomorphic properties were not observed in neat materials, but phase diagrams of binary mixtures with TNF demonstrate that steric repulsion can be overcome by intercalation of the electron-acceptor TNF. In spite of the presence of six lateral chains, SmA phases were found exclusively. The structures of the SmA phases were found to have additional order with respect to SmA phases formed by calamitic molecules. Further examination of the complexes in host nematic materials demonstrated their use as chiral dopants to induce cholesteric LC phases.

Enantioselective hydrogenations of arylalkenes mediated by [Ir(cod) (JM-Phos) ]+ complexes.

Phosphine oxazoline ligands la-j were converted to the corresponding [Ir(cod)(phosphine oxazoline)]+ complexes 2a-j. X-ray diffraction analyses of complexes 2b, 2h, 2i, and 2j were performed. The tert-butyl-, 1,1-diphenylethyl-, and phenyl-oxazoline complexes (2b, 2h, and 2i, respectively) had typical square planar metal environments with chair-like metallocyclic rings. However, the 3,5-di-tert-butylphenyl oxazoline complex 2j was distorted toward a tetrahedral metal geometry. This library of complexes was tested in asymmetric hydrogenations of several arylalkenes. High enantioselectivities and conversions were observed for some substrates. A possible special role for the HPh2C-oxazoline substituent in asymmetric hydrogenations was identified and is discussed. In attempts to rationalize why high enantioselectivities were not observed for some alkenes, a series of deuterium labeling experiments were performed to probe for competing reactions that occurred prior to the hydrogenation step. Double bond migrations were inferred for several substrates, and this is a significant complication in asymmetric hydrogenations of arylalkenes that had not been discussed prior to this study. A mechanistic rationale is proposed involving competing double bond migration for some but not all substrates. Appreciation of this complication will be valuable in further studies aimed at optimization of enantioselection in asymmetric hydrogenations of unfunctionalized alkenes.

Enantioselective Michael addition catalyzed by chiral tripodal oxazoline– tBuOK complexes

Benzene-based tripodal oxazolines are found to be novel chiral ligands for the catalytic enantioselective Michael addition via potassium enolates. Thus, methyl phenylacetate undergoes 1,4-addition to methyl acrylate using a catalytic amount of a tBuOK–oxazoline complex in toluene at −78°C, and up to 82% ee is obtained.

Kinetics and mechanisms of catalytic oxygen atom transfer with oxorhenium(V) oxazoline complexes.

The rhenium(V) monooxo complexes (hoz)2Re(O)Cl (1) and [(hoz)2Re(O)(OH2)][OTf] (2) have been synthesized and fully characterized (hoz = 2-(2'-hydroxyphenyl)-2-oxazoline). A single-crystal X-ray structure of 2 has been solved: space group = P1, a = 13.61(2) A, b = 14.76(2) A, c = 11.871(14) A, alpha = 93.69(4) degrees, beta = 99.43(4) degrees, gamma = 108.44(4) degrees, Z = 4; the structure was refined to final residuals R = 0.0455 and Rw = 0.1055. 1 and 2 catalyze oxygen atom transfer from aryl sulfoxides to alkyl sulfides and oxygen-scrambling between sulfoxides to yield sulfone and sulfide. Superior catalytic activity has been observed for 2 due to the availability of a coordination site on the rhenium. The active form of the catalyst is a dioxo rhenium(VII) intermediate, [Re(O)2(hoz)2]+ (3). In the presence of sulfide, 3 is rapidly reduced to [Re(O)(hoz)2]+ with sulfoxide as the sole organic product. The transition state is very sensitive to electronic influences. A Hammett correlation plot with para-substituted thioanisole derivatives gave a reaction constant rho of -4.6 +/- 0.4, in agreement with an electrophilic oxygen transfer from rhenium. The catalytic reaction features inhibition by sulfides at high concentrations. The equilibrium constants for sulfide binding to complex 2 (cause of inhibition), K2 (L x mol(-1)), were determined for a few sulfides: Me2S (22 +/- 3), Et2S (14 +/- 2), and tBu2S (8 +/- 2). Thermodynamic data, obtained from equilibrium measurements in solution, show that the S=O bond in alkyl sulfoxides is stronger than in aryl sulfoxides. The Re=O bond strength in 3 was estimated to be about 20 kcal x mol(-1). The high activity and oxygen electrophilicity of complex 3 are discussed and related to analogous molybdenum systems.

Diastereoselective Transformation of Arenes into Highly Enantiomerically Enriched Substituted Cyclohexadienes

Sequential addition of a C-nucleophile and a C-electrophile to enantiomerically pure arene tricarbonyl chromium complexes 3a,b, 6, and 8b, containing aryl bound chiral oxazoline, SAMP-hydrazone and chiral imine auxiliaries affording substituted cyclohexadienes. C-Nucleophiles included alkyl-, vinyl-, and phenyl-lithium reagents. C-Electrophiles included methyl iodide, allyl bromide, benzyl bromide, and propargyl bromides. The 1,3-cylohexadienes were obtained with a 1,5,6-substitution pattern. The results are consistent with a diastereoselective exo-nucleophilic addition to an ortho position of the complexed arene, followed by addition of the electrophile to the metal center. With allyl, ben-zyl, and propargyl groups, direct reductive elimination then yielded trans-5,6-substituted products. With methyl iodide, reductive elimination was preceded by CO insertion and acetyl cyclohexadienes were formed exclusively whose in situ deprotonation/alkylation gave products in which three C-substituents had been added across an arene double bond with complete regio- and stereocontrol. The two path-ways reflect migratory aptitude to carbonylation. An X-ray structure determination of the phenyl oxazoline complex 3a allowed a rationalization of observed diastereoselectivity. Asymmetric induction was very high with the oxazoline and the SAMP-hydrazone complexes (>90% de) whereas the chiral benzaldehyde imine complex 8b afforded the substituted diene aldehydes in moderate enantiomeric purity (34-58% ee). Changing the reaction medium from THF to toluene in reactions with 8b resulted in products of the opposite chirality.

ChemInform Abstract: Enantioselective Diene Cyclization/Hydrosilylation Catalyzed by Optically Active Palladium Bisoxazoline and Pyridine-Oxazoline Complexes.

A 1:1 mixture of (N−N)Pd(Me)Cl [N−N = (S,S)-4,4‘-dibenzyl-4,5,4‘,5‘-tetrahydro-2,2‘-bisoxazoline] (S,S-4a) and NaBAr4 [Ar = 3,5-C6H3(CF3)2] (5 mol %) catalyzed the asymmetric cyclization/hydrosilylation of dimethyl diallylmalonate (2) and triethylsilane at −30 °C for 48 h to form an 8.1:1 mixture of the silylated carbocycle (S,S)-trans-1,1-dicarbomethoxy-4-methyl-3-[(triethylsilyl)methyl]cyclopentane (S,S-3) (95% de, 72% ee) and dimethyl 3,4-dimethylcyclopentane-1,1-dicarboxylate (S,S-6) in 64% combined yield. In comparison, a 1:1 mixture of the palladium pyridine−oxazoline complex (N−N)Pd(Me)Cl [N−N = (R)-(+)-4-isopropyl-2-(2-pyridinyl)-2-oxazoline] (R-5b) and NaBAr4 (5 mol %) catalyzed the asymmetric cyclization/hydrosilylation of 2 and triethylsilane at −32 °C for 24 h to form carbocycle S,S-3 in 82% yield (>95% de, 87% ee) as the exclusive product. Asymmetric diene cyclization catalyzed by complex R-5b was compatible with a range of functional groups and produced carbocycles with up to 91% ee. The pro...

FORMATION OF 2-PYRIDINYL-2-OXAZOLINES AND PYRIDINE-2-CARBOXAMIDINE IN THE COORDINATION SPHERE OF COPPER(II)

The reaction of pyridine-2-carbonitrile (2-CNpy) with 2-amino-2-hydroxymethyl-1,3-propanediol (L1) and 2-amino-2-methyl-1-propanol (L2) in methanolic solutions of anhydrous copper(II) chloride at room temperature led to the formation of solid complexes containing 2-(2-pyridinyl)-4,4-bis(hydroxynmethyl)-2-oxazoline (pyoxaL1) and 2-(2-pyridinyl)-4,4-dimethyl-2-oxazoline (pyoxaL2), respectively. With copper(II) bromide instead of copper(II) chloride, along with the oxazoline complexes, the complex dibromo-bis(pyridine-2-carbox-amidine)copper(II) was isolated. Under several hour reflux, the complexes dihalogenobis(pyridine-2-carboxamidine)copper(II) are the only isolable products both for chloride and bromide starting salts. The stereochemistry of the complexes and the mode of ligand coordination have been determined by spectroscopic and conductometric measurements. The crystal structure of bromobis[(2-(2-pyridinyl)-4,4-dimethyl)-2-oxazoline]copper(II) bromide hydrate was solved by X-ray diffraction techniques. The mechanism of 2-CNpy transformation to the final products is proposed.

Enantioselective diene Cyclization/Hydrosilylation catalyzed by optically active palladium bisoxazoline and pyridine-oxazoline complexes.

A 1:1 mixture of (N-N)Pd(Me)Cl ¿N-N = (S,S)-4,4'-dibenzyl-4,5,4', 5'-tetrahydro-2,2'-bisoxazoline (S,S-4a) and NaBAr(4) ¿Ar = 3, 5-C(6)H(3)(CF(3))(2) (5 mol %) catalyzed the asymmetric cyclization/hydrosilylation of dimethyl diallylmalonate (2) and triethylsilane at -30 degrees C for 48 h to form an 8.1:1 mixture of the silylated carbocycle (S,S)-trans-1, 1-dicarbomethoxy-4-methyl-3-¿(triethylsilyl)methylcyclop ent ane (S, S-3) (95% de, 72% ee) and dimethyl 3,4-dimethylcyclopentane-1, 1-dicarboxylate (S,S-6) in 64% combined yield. In comparison, a 1:1 mixture of the palladium pyridine-oxazoline complex (N-N)Pd(Me)Cl ¿N-N = (R)-(+)-4-isopropyl-2-(2-pyridinyl)-2-oxazoline (R-5b) and NaBAr(4) (5 mol %) catalyzed the asymmetric cyclization/hydrosilylation of 2 and triethylsilane at -32 degrees C for 24 h to form carbocycle S,S-3 in 82% yield (>95% de, 87% ee) as the exclusive product. Asymmetric diene cyclization catalyzed by complex R-5b was compatible with a range of functional groups and produced carbocycles with up to 91% ee. The procedure also tolerated substitution at a terminal olefinic position and at the allylic position of the diene.

The application of Pd-complexes of diphenylphosphinoferrocenyloxazoline ligands to catalytic enantioselective allylic amination

The preparation of cationic palladium (η 3-C 3H 5) complexes, possessing an enantiopure diphenylphosphinoferrocenyloxazoline unit, is described. These complexes were applied to the enantioselective amination of the test substrate, ethyl (2 E)-1,3-diphenylprop-2-enyl carbonate, with benzylamine in moderate to high conversions with enantioselectivities of up to 72% for the ( S)-valinol derived oxazoline complex. Enantioselectivities of up to 37% were obtained for the ( S)- tert-leucinol derived oxazoline complex.