Axially chiral C2-symmetric N-heterocyclic carbene (NHC) palladium complex-catalyzed asymmetric α-hydroxylation of β-keto esters

Axially chiral C₂-symmetric N-heterocyclic carbene (NHC) palladium diaquo complexes 1a and 1b, derived from (R)-BINAM, are effective catalysts for the enantioselective α-hydroxylation of β-keto esters using oxaziridine 2a as the oxidant to give the corresponding products in high yields along with moderate enantioselectivities.     

Chiral Pt(II)/Pd(II) pincer complexes that show C-H?Cl hydrogen bonding: Synthesis and applications to catalytic asymmetric aldol and silylcyanation reactions

A chiral C₂-symmetric NCN ligand, (5R,7R)-1,3-bis(6,6-dimethyl-5,6,7,8-tetrahydro-5,7-methanoquinolin-2-yl)benzene has been synthesized. A direct cyclometalation of this ligand with K₂MCl₄ (M = Pt, Pd) in dry acetic acid offered the corresponding pincer complexes, [(5R,7R)-1,3-bis(6,6-dimethyl-5,6,7,8-tetrahydro-5,7-methanoquinolin-2-yl)phenyl]platinum(II) chloride 5a and its palladium(II) analogue 5b. The Pt(II) and Pd(II) complexes 5 were characterized by NMR spectroscopy, and X-ray crystal structure analysis was done for the Pt(II) complex. The NMR data for both the complexes and X-ray crystal structural data for the chloro-Pt(II) complex indicate the existence of intramolecular C-H?Cl hydrogen bonding both in solution and in solid states. Chloride abstraction from 5a by treatment with silver triflate resulted in the corresponding triflate complex 6a, which generates the corresponding cationic aqua complex 7a in the presence of water molecules. The Pt(II) complex 6a/7a was used as asymmetric catalyst in the aldol reaction between methyl isocyanoacetate and aldehydes and also in the silylcyanation of aldehydes.     

Enantioselective alkynylation to aldimines catalyzed by chiral 2,2′-di(2-aminoaryloxy)-1,1′-binaphthyl-copper(I) complexes

The enantioselective alkynylation of aldimines with terminal acetylenes catalyzed by chiral Cu(I) complexes with (R)-2,2′-di(2-aminoaryloxy)-1,1′-binaphthyl ligands (7) was examined. Chiral C₂-symmetric N,N-ligands 7, which have primary aniline moieties, were readily prepared from inexpensive (R)-1,1′-binaphthol (BINOL) as a chiral source. In particular, the reaction of N-benzylidenebenzeneamine 1a with phenylacetylene 2a proceeded smoothly in the presence of 5 mol % of (CuOTf)₂·C₆H₅CH₃ and 10 mol % of (R)-7d at room temperature for 24 h, and the corresponding propargylamine 3a was obtained with up to 82% ee.     

Doubly [SiMe2]-bridged Cs- and C2v-symmetric zirconocene catalysts for propylene polymerization. Synthesis and polymerization characteristics

The C_s- and C_2v-symmetric doubly dimethylsilylene-bridged zirconocene complexes (Me₂Si)₂{η⁵-C₅H₂-4-CHMe₂}{η⁵-C₅H-3,5-(SiMe₃)₂}ZrCl₂, 6a, and (Me₂Si)₂{η⁵-C₅H-3,5-(CHMe₂)₂}₂ZrCl₂, 6b, have been synthesized by reaction of corresponding dilithio ligand salts with ZrCl₄ in CH₂Cl₂. When activated with methyl aluminoxane, C_s-symmetric zirconocene 6a catalyzes the polymerization of propylene (liquid propylene, 0°C) to yield a syndiotactic polymer of Mw=1,000,000 and rrrr%=75.4. By contrast, C_2v-symmetric zirconocene 6b displays a low activity and affords an atactic polymer.     

Chiroptical and conformational properties of (R)-1-phenylethylamine derivatives of persubstituted benzene

Chiral derivatives of 2,4,5,6-tetrachloro-1,3-dicyanobenzene 1 with one, two and three (R)-1-phenylethylamino ((R)-PEA) units 2–4 are prepared and their chiroptical and conformational properties discussed on the bases of the UV/CD, NMR and MM2 data. High polarity of the persubstituted benzene ring leads to peculiar UV and IR spectra of achiral model compounds 5–7, whereas relatively rigid conformations of the chiral analogues 2–4 are reflected in the CD spectra. Strong exciton coupling (EC) appears in the CD spectrum of pseudo-C₃-symmetric 4; this type of interaction seems not to be present in the C₁-symmetric 2 and C₂-symmetric 3. The absence of a molecular cleft in the chiral structures 2–4 could explain their inability to recognise the enantiomers of some racemates in the NMR experiment.     

Synthesis of C2-symmetric chiral crown ethers by lipase-catalyzed reactions

Kinetic resolution of a racemic mixture of C₂-symmetric 18-crown-6 diols (rac-1a) and 15-crown-5 diol (rac-1c) was achieved by lipase-catalyzed acetylation. The enantiomeric excess of the chiral crown diols (95% ee and 82% ee) was determined by ¹H NMR spectroscopy, using (R)-(+)-1-(1-naphthyl)ethylammonium hydrochloride as a shift reagent. The C₂-symmetric chiral 15-crown-5 diol (>95% ee) was also obtained by kinetic resolution of the racemic diacetate (rac-2c) using lipase-catalyzed solvolysis.     

Desymmetrization of 2,2′,6,6′-tetramethoxybiphenyl by regioselective sulfenylation reaction

A practical regioselective sulfenylation reaction of the achiral title compound 1 provides desymmetrization and access to C₂-symmetric sulphur derivatives. Resolution of 2,2′,6,6′-tetramethoxy-3,3′-dimercapto-1,1′-biphenyl 7 was achieved by conversion to the corresponding dithiocarbonate diastereomers. The absolute configuration of (aR)-(−)-7 and (aS)-(+)-7 was assigned unambiguously.     

Asymmetric addition of diethylzinc to benzaldehyde catalyzed by novel chiral C2-symmetric tetrakis(sulfonamide)–titanium(IV) complexes

A series of chiral C₂-symmetric tetrakis(sulfonamides) 3a–f, 4a–f were prepared and employed as ligands for titanium(IV) complexes in the asymmetric addition of diethylzinc to benzaldehyde. The chiral secondary alcohols were obtained in high yields and in moderate enantioselectivities.     

Synthesis and characterization of cationic and zwitterionic allyl zirconium complexes derived from trimethylenemethane (TMM) cyclopentadienylzirconium acetamidinates

Protonation of the trimethylenemethane derivatives, Cp*Zr(σ²,π-C₄H₆)[N(R¹)C(Me)N(R²)] (1a: R¹=R²=i-Pr and 1b: R¹=Et, R²=t-Bu) (Cp*=η⁵-C₅Me₅), by [PhNMe₂H][B(C₆F₅)₄] in chlorobenzene at −10 °C provides the cationic methallyl complexes, Cp*Zr(η³-C₄H₇)[N(R¹)C(Me)N(R²)] (2a: R¹=R²=i-Pr and 2b: R¹=Et, R²=t-Bu), which are thermally robust in solution at elevated temperatures as determined by ¹H NMR spectroscopy. Addition of B(C₆F₅)₃ to 1a and 1b provides the zwitterionic allyl complexes, Cp*Zr{η³-CH₂C[CH₂B(C₆F₅)₃]CH₂}[N(R¹)C(Me)N(R²)] (3a: R¹=R²=i-Pr and 3b: R¹=Et, R²=t-Bu). The crystal structures of 2b and 3a have been determined. Neither the cationic complexes 2 or the zwitterionic complexes 3 are active initiators for the Ziegler-Natta polymerization of ethylene and α-olefins.     

Chiral thiophosphoramide and selenophosphoramide ligands in the Cu(I)-promoted catalytic enantioselective 1,3-dipolar cycloaddition reactions of azomethine ylides and pyrrole-2,5-dione derivatives

Chiral C₂-symmetric diphenylthiophosphoramide ligand L1 prepared from C₂-symmetric (1S,2S)-(−)-1,2-diphenylethylenediamine was found to be a fairly effective chiral ligand for Cu(I)-promoted 1,3-dipolar cycloaddition of imines and pyrrole-2,5-dione derivatives to give the corresponding adducts in moderate enantioselectivities and good yields.     

Synthesis and characterization of novel five-membered palladacycles

3-(2-Chloroquinolin-3-yl)-1,5-bis(3,4,5-trimethoxy-phenyl)-pentane-2,4-dione derivatives 3a–b were conveniently synthesized in excellent yields (82% each) by tandem Knoevenagel condensation reactions of 2-chloro-3-carbaldehyde-quinoline 1a–b with 3,4,5-trimethoxy acetophenone, followed by a base catalyzed Michael addition, such as DBU (1,8-diazabicyclo[5,4,0]undec-7-ene) with or without solvent. The reactions of 3a–b with Pd(dba)₂ in the presence of PPh₃ (1:2) in degassed acetone provided the dinuclear palladium complexes {Pd(C,N-2-C₉H₄N–CH–[–CH₂CO(3,4,5-(OMe-)₃–C₆H₂-]₂–3-R-6)Cl(PPh₃)}₂ [(R = H (4a), R = OMe (4b)] in moderate yields (38% and 43%), which in turn reacted with an excess of isonitrile XyNC (Xy = 2,6-Me₂C₆H₃) to give the corresponding palladacycles 5a–b in moderate yields (45% and 43%). The palladacycles 5a–b were also obtained in similar yields (32% and 33%) via a one-pot oxidative addition reaction of 3a-b with isonitrile XyNC:Pd(dba)₂ (4:1). The products were characterized by satisfactory elemental analysis and spectral studies (IR, ¹H, and ³¹P NMR). The crystal structure of 5a was determined by X-ray crystallography diffraction studies.     

Carbon-nitrogen bond formation in the reaction of the reaction of diphenyl diazomethane Ph2 CN2 with diiron-carbene complexes (Fe2(CO)7x03BD;-C(R)C(NEt2)x03BD;-C(R)C(NEtx03BC;-C(R)C(NEt2)N(NCPh2)x03BC;-C(R)C(NEtx03BC;-C(R)C(NEt2)NN(CPh2)x03BC;-C(R)C(NEt)]

The diiron ynamine complex [Fe₂(CO)₇{μ-CR)C(NEt₂)}] (1:R=Me,2:R = C₃H₅.3:R=SiMe₃.4:R = Ph) reacts at room temperature with diphenyldiazomethane Ph₂CN₂, in hexane to yield complexes [Fe₂(CO)₆{C(R)C(NEt₂)N (NCPh₂)] (5a:R=Me,6a:R=C₃H₅.7a R=SiMe₃.8a:R=Ph) resulting from the insertion of the terminal nitrogen atom into the Fe=C carbene bond. Insertion the second nitrogen atom and formation of compounds [Fe₂(CO)₆zμ-C(R)C(NEt₂)NN(CPh₂)}] (5b:R=Me,6b:R=C₃H₅,7b:R=SiMe₃,8b:R=Ph) is observed when compounds5a-5a are treated in refluxing hexane. Transformation of compoundsa tob is also obtained at room temperature within a few days. All compounds were identified by their¹H NMR spectra. Compounds6a, 7a, 8a, and8b were characterized by single crystal X-ray diffraction analyses. Crystal data: for6a: space group = P2₁/n,a=12.853(1) A,b=24.800(7) A,c=8.947(6) A,β=99.29(3)°,Z=4, 2227 rellectionsR=0,038; for7a: space group=Pl,a=ll.483(4) A,b=14.975(4) A,c = 17.890(8) A,α = 82.80(3)°,β=94.29(7)°,γ=85.42(2),Z = 4, 5888 reflectionR = 0.035: for8a: space group = Pcab.a = 31.023(8) A.b=20.137(1) A.c=9.686(2) A.Z=8. 1651 reflections,R=0.071; for8b: space group=P2₁/n,a=21.459(4),b=10,100(3) A,c=28,439(8) A,ß=103.86(4)°,Z=8. 2431 reflections.R=0.057.     

Chelating diamide complexes of titanium: new catalyst precursors for the highly active and living polymerization of α-olefins

The reaction of RHN(CH₂)₃NHR (1a,b) (a, R=2,6-ⁱPr₂C₆H₃; b, R=2,6-Me₂C₆H₃) with 2 equiv of BuLi followed by 2 equiv of ClSiMe₃ yields the silylated diamines R(Me₃Si)N(CH₂)₃N(SiMe₃)R (3a,b). The reaction of 3a,b with TiCl₄ yields the dichloride complexes [RN(CH₂)₃NR]TiCl₂ (4a,b) and two equiv of ClSiMe₃. An X-ray study of 4a (P2₁/n, a=9.771(1) Å, b=14.189(1) Å, c=21.081(2) Å, β=96.27(1)°, V=2905.2(5) ų, Z=4, T=25°C, R=0.0701, R_w=0.1495) revealed a distorted tetrahedral geometry about titanium with the aryl groups lying perpendicular to the TiN₂-plane. Compounds 4a,b react with 2 equiv of MeMgBr to give the dimethyl derivatives [RN(CH₂)₃NR]TiMe₂ (5a,b). An X-ray study of 5b (P2₁2₁2₁, a=8.0955(10) Å, b=15.288(4) Å, c=16.909(3) Å, V=2092.8(7) ų, Z=4, T=23°C, R=0.0759, R_w=0.1458) again revealed a distorted tetrahedral geometry about titanium with titanium–methyl bond lengths of 2.100(9) Å and 2.077(9) Å. These titanium dimethyl complexes are active catalysts for the polymerization of 1-hexene, when activated with methylaluminoxane (MAO). Activities up to 350,000 g of poly(1-hexene)/mmol catalyst·h were obtained in neat 1-hexene. These systems actively engage in chain transfer to aluminum. Equimolar amounts of 5a or 5b and B(C₆F₅)₃ catalyze the living aspecific polymerization 1-hexene. Polydispersities (M_w/M_n) as low as 1.05 were measured. Highly active living systems are obtained when 5a is activated with {Ph₃C}⁺[B(C₆F₅)₄]⁻. A primary insertion mode (1,2 insertion) has been assigned based on both the initiation of the polymer chain and its purposeful termination with iodine.     

The absolute configuration of the palmarumycins C9, C10, and C12 by quantum-mechanical calculations of CD spectra

The absolute configurations of the palmarumycins C₉ 1a, C₁₀ 2, and C₁₂ 3 were assigned by comparison of the quantum-mechanically calculated with the experimental CD spectra as (2R,3S,4aS,8aR), (2R,3R,4S,4aS,8aR), and (2R,3R,4R), respectively.     

C2-Symmetric diphenylphosphoramide and diphenylthiophosphoramide derived from (1R,2R)-1,2-diaminocyclohexane as ligands for the titanium(IV) alkoxide-promoted addition of diethylzinc to aldehydes

Chiral C₂-symmetric diphenylphosphoramide 4 and diphenylthiophosphoramide 5 were prepared from the reaction of diphenylphosphinic chloride and diphenylthiophosphinic chloride with (1R,2R)-(−)-1,2-diaminocyclohexane in the presence of diisopropylethylamine in high yields. They were used as chiral ligands in the catalytic asymmetric addition reaction of diethylzinc to aldehydes in the presence of titanium(IV) isopropoxide to give the corresponding sec-alcohols in 70–83% ee with an (R)-configuration and in 40–50% ee with an (S)-configuration, respectively.     

Asymmetric Diels–Alder and ene reactions promoted by a Ti(IV) complex bearing a C2-symmetric tridentate ligand

A new Ti(IV) complex obtained from the C₂-symmetric amino diol (1R,5R)-3-aza-3-benzyl-1,5-diphenyl pentan-1,5-diol, (1R,5R)-1, is used effectively as a Lewis acid promoter in asymmetric Diels–Alder reactions. Using various Evans' oxazolidinones as dienophiles and cyclopentadiene as the diene high yields of the adducts with moderate enantioselectivity, under different reaction conditions are achieved. The effects of solvent, temperature and ligand on the enantioselectivity of the Diels–Alder products are reported. Molecular modelling studies provide an understanding of the diastereofacial selectivity of the Diels–Alder reactions. Asymmetric carbonyl-ene reactions between various glyoxylate esters and α-methyl styrene are also described.     

General theoretical analysis of three-connected polyhedral molecules and their capped derivatives

The synthesis of the half-sandwich compound Na[(C₅H₅)Ni{P(S)(CH₃)₂}₂] is described. The anions [(C₅H₅)Ni{P(S)R₂}₂]^?, 1a (R = OCH₃) and 1b (R = CH₃) react as bidentate sulfur ligands with [Ni₂(C₅H₅)₃]⁺, giving nickelocene and weakly paramagnetic dinuclear complexes of the type [(C₅H₅)Ni{P(S)R₂}₂Ni(C₅H₅)] (2a,b). In these compounds, the P(S)R₂ units form NiPSNi bridges in such a fashion as to generate a (C₅H₅)NiP₂ and a (C₅H₅)NiS₂ unit. A temperature-dependent singlettriplet spin equilibrium is observed, which is essentially localized on the (C₅H₅)NiS₂ side. Accordingly, the position of the cyclopentadienyl peak of the (C₅H₅)Ni unit bound to the two sulfur donor centers displays a very large temperature dependence in the ¹H NMR spectra. MO model calculations (EHT) for P(S)H₂^?, [(C₅H₅)Ni{P(S)H₂}₂]^? (1c), [(C₅H₅)Ni{P(S)H₂}₂Ni(C₅H₅)] (2c) and its isomer 3c allow the observed spin crossover to be explained as a consequence of the pronounced π-donor properties of the sulfur centers and allow predictions for related complexes.The green complexes 2a,b isomerize completely and irreversibly in a first-order reaction to yield the diamagnetic red compounds [{(C₅H₅)NiP(S)R₂}₂] (3a,b), in which each (C₅H₅)Ni unit is coordinated to one P and one S donor atom. The rate constant of isomerization of 2a, k (7.6 ± 0.3) × 10^?4s^?1 at 306 K, and the energy of activation, E_a 76 kJ mol^?1, have been determined. The rate of isomerization is independent of the solvent, and crossover experiments verify that the isomerization is an intramolecular process without involvement of the monomeric units [(C₅H₅)NiP(S)R₂].     

Novel enantiopure non-C2-symmetric NCN-pincer palladium complexes with l-proline chiral auxiliaries: mer η3-N,C,N versus square planar η4-N,C,N,O coordination

New chiral NCN-pincer palladium complexes containing proline ester moieties as chiral auxiliaries have been synthesized. The parent ligands 2,6-bis{[(S)-2-(methoxycarbonyl)-1-pyrrolidinyl]methyl}-1-bromobenzene L^Me and 2,6-bis{[(S)-2-(benzoxycarbonyl)-1-pyrrolidinyl]methyl}-1-bromobenzene L^Bn were prepared in a single synthetic step and were obtained enantiomerically pure. Neutral arylpalladium bromide complexes 1a and 1b, formed upon treatment of the respective ligands L^Me and L^Bn with [Pd₂(dba)₃]·CHCl₃, were isolated as mixtures of three stereoisomers (S_NS_NS_CS_C, R_NS_NS_CS_C and R_NR_NS_CS_C). The ratio of stereoisomers is approximately 1:1:0.6 in the case of methyl ester derivative 1a, whereas the bulkier benzyl ester derivative 1b predominantly forms the (S_NS_NS_CS_C)-stereoisomer. Upon abstraction of the bromide ion from unresolved mixtures of 1a and 1b, cationic complexes 2 and 3, respectively, form as single diastereoisomers in which one of the ester prolinate carbonyl groups is coordinated to palladium according to X-ray crystal structure determination. This coordination of a carbonyl group to the metal has a substantial influence on the stereochemistry and results in the formation of a single diastereoisomer, having the (R_NR_NS_CS_C)-configuration, regardless of the stereochemistry or ratio of stereoisomers of the starting bromide compound. The structures of compounds 2 and 3 were somewhat unexpected since formation of the corresponding cationic [Pd(NCN)(OH₂)]⁺ complexes was anticipated. In preliminary tests of these cationic complexes as catalysts in the enantioselective aldol condensation of benzaldehyde with methyl isocyanoacetate, modest selectivities were observed.     

Synthesis and structure of {{amino(triorganosilyl)carbene}pentacarbonyltungsten(0)} complexes and attempted synthesis of {[2-bis(trimethylsilyl)methyl-3-triorganosilyl-2H-azaphosphirene-κP]pentacarbonyltungsten(0)}

First examples of tungsten aminocarbene complexes [(OC₅)W{C(SiR¹_nR²_3-n)NH₂}] 2a-d (R¹ = Ph, R² = Me) were synthesized via ammonolysis of the corresponding methoxycarbene complexes 1a-d. They were characterized by NMR spectroscopy, MS, IR, UV/Vis and elemental analysis, and in the case of the C-triphenylsilyl derivative 2a by single-crystal X-ray structure analysis. The reaction of P-chloro alkylidenephosphane 3 with complexes 2a-d, meant to give 2H-azaphosphirene complexes, was monitored by ³¹P NMR spectroscopy to reveal the formation of the products 4-7, which were presumably formed via decomposition of the transient complexes 10a-d.     

Synthesis, characterization and photoluminescent properties of platinum complexes with novel bis(imidazoline) pincer ligands

Chiral C₂-symmetric bis(imidazoline) pincer ligands 2a-d have been synthesized for the first time. Direct cycloplatination of these ligands with K₂PtCl₄ in dry acetic acid afforded the corresponding cycloplatinated pincer complexes 3a-d. The X-ray single-crystal structure of platinum complex 3d and the preliminary studies on the photoluminescent properties of 3 are reported.