Synthesis and characterization of cyclometallated palladium(II) and platinum(II) complexes with amide-thiolate ligands

The redox reaction of bis(2-benzamidophenyl) disulfide (H₂L-LH₂) with [Pd(PPh₃)₄] in a 1:1 ratio gave mononuclear and dinuclear palladium(II) complexes with 2-benzamidobenzenethiolate (H₂L⁻), [Pd(H₂L-S)₂(PPh₃)₂] (1) and [Pd₂(H₂L-S)₂ (μ-H₂L-S)₂(PPh₃)₂] (2). A similar reaction with [Pt(PPh₃)₄] produced only the corresponding mononuclear platinum(II) complex, [Pt(H₂L-S)₂(PPh₃)₂] (3). Treatment of these complexes with KOH led to the formation of cyclometallated palladium(II) and platinum(II) complexes, [Pd(L-C,N,S)(PPh₃)]⁻ ([4]⁻) and [Pt(L-C,N,S) (PPh₃)]⁻ ([5]⁻). The molecular structures of 2, 3 and [4]⁻ were determined by X-ray crystallography.     

Trifluoromethyl and mixed hydrido trifluoromethyl complexes of iridium(III) as potential precursors of an iridium(I) trifluoromethyl complex

Abstraction of iodide from Ir(CF₃)ClI(CO)(PPh₃)₂ (1) by AgSbF₆ in the presence of acetonitrile yields the cationic complex [Ir(CF₃)Cl(MeCN)(CO)(PPh₃)₂]⁺ [SbF₆]⁻ (2). The acetonitrile group of 2 is readily displaced, and 2 reacts with para-tolyl isocyanide to yield [Ir(CF₃)Cl(CN-p-tolyl)(CO)(PPh₃)₂]⁺ [SbF₆]⁻ (3). The addition of NaOMe to 3 results in the methoxyester complex Ir(CF₃)(COOMe)Cl(CN-p-tolyl) (PPh₃)₂ (4). The acetonitrile ligand of 2 is also displaced by anions, including H⁻. Thus, 2 reacts with LiEt₃BH to give Ir(CF₃)HCl(CO)(PPh₃)₂ (5), in which the hydrido and trifluoromethyl ligands are mutually trans. In contrast, the addition of excess NaBH₄ to 2 affords the novel dihydrido complex trans-Ir(CF3)H₂(CO)(PPh₃)₂ (6). Investigations into the potential use of 5 and 6 as precursors of an iridium(I) complex such as Ir(CF₃)(CO)(PPh₃)₂ are also described.     

Synthesis and characterization of binuclear μ-oxalato nickel(II), copper(II) and zinc(II) complexes with 3,3′-diamino-N-methyl-dipropylamine or trans-1,2-diaminocyclohexane

New binuclear complexes of the type [(Ni(Medpt)NO₃)₂ox] (1) (Medpt=3,3′-diamino-N-methyl-dipropylamine, H₂ox=oxalic acid), [(Ni(dach)₂)₂ox]NO₃·2H₂O (2) (dach=trans-1,2-diaminocyclohexane), [(Cu(Medpt))₂ox]X₂·yH₂O (X=NO₃, y=2 2/3 (3); X=ClO₄, y=0 (4)) and [(Zn(dach)₂)₂ox](ClO₄)₂·2H₂O (5) have been prepared and characterized by IR and UV–Vis spectroscopies. Spectroscopic data are consistent with oxalate-bridged structures between six-coordinated (N₃O₃ or N₄O₂) Ni(II) (compounds 1 or 2), five-coordinated (N₃O₂) Cu(II) (compounds 3 and 4) or six-coordinated (N₄O₂) Zn(II) (compound 5). The crystal structure of [(Cu(Medpt))₂ox](NO₃)₂·2 2/3 H₂O (3) has been determined by single-crystal X-ray analysis. The structure of (3) consists of centrosymmetric binuclear cations [(Medpt)Cu(ox)Cu(Medpt)]²⁺, nitrate anions and water molecules of crystallization. The copper atom is five-coordinated by two oxalate–oxygen and three Medpt–nitrogen atoms, in a hybrid arrangement between trigonal–bipyramidal and square–pyramidal. The temperature dependence of magnetic susceptibility (1.8–300 K) was measured for compounds 1–4. Magnetochemical measurements show that Ni(II) complexes are antiferromagnetically coupled, J=−29.4 (1) and −32.7 cm⁻¹ (2) (H=−JS₁S₂) while the Cu(II) complexes present a very weak coupling, J=−2.6 (3) and +1.9 cm⁻¹ (4), being antiferro- and ferromagnetic in nature.     

Catalysis of hydrosilylation: XI. Rhodium(I)-siloxyalkylphosphine complexes; synthesis,characteristics and catalytic activity

Rhodium(I) complexes with 1,5-cyclooctadiene (COD) and disiloxydiphosphines {O[Si(CH₃)₂(CH₂)_nPPh₂]₂ where n = 1–3; (B-1, B-2, B-3, respectively)}; and/or with trisiloxytriphosphine Ph₂P(CH₂)₃(CH₃)Si[OSi(CH₃)₂(CH₂)₂PPh₃]₂ (C-2) were synthesized. Their composition and structure were determined using elemental analysis, molecular weight measurements and spectroscopic (IR, ¹H NMR and vis) methods, and were then compared with the corresponding data for RuCl(COD)PPh₃ (A) and RhCl(COD)₂ (D). The analytical and physico-chemical data all confirm the square planar geometry of the rhodium siloxyphosphine (the same as for rhodium triphenylphosphine) complexes with the general formula [(COD)RhCl(PPh₂)(CH₂)_n]_mZ where m = 2 and Z = (CH₃)₂SiOSi(CH₃)₂ or m = 3 and Z = (CH₃)₂SiOSi(CH₃)OSi(CH₃)₂. The structure is independent of the type of phosphine ligand, and the molar ratio of Rh:P is always 1:1. Catalytic activity of the complexes prepared was tested in the hydrosilylation of 1-hexene by triethoxysilane which showed a slight decrease in turnover number (A–C) compared with Wilkinson's catalyst (E) but the activation energies for the rhodium-siloxyphosphine complexes (B and C) are higher than those for the rhodium phosphine complexes (A and E).     

Formation and isomerization of cis,cis-Ir(H)2(--Ph)(CO)(PPh3)2 and cis,cis-Ir(H)(--Ph)2(CO)(PPh3)2 in oxidative addition of hydrogen and phenylacetylene to trans-Ir(CO)(--Ph)(PPh3)2

Oxidative addition of H–R (H--Ph and H₂) to trans-Ir(--Ph)(CO)(PPh₃)₂ (2) gives the initial products, cis, cis-Ir(H)(--Ph)₂(CO)(PPh₃)₂ (3a) and cis, cis-Ir(H)₂(--Ph)(CO)(PPh₃)₂ (3b), respectively. Both cis-bis(PPh₃) complexes, 3a and 3b undergo isomerization to give the trans-bis(PPh₃) complexes, trans, trans-Ir(H)(--Ph)₂(CO)(PPh₃)₂ (4a) and cis, trans-Ir(H)₂(--Ph)(CO)(PPh₃)₂ (4b). The isomerization, 3b→4b is first order with respect to 3b with k₁=6.37×10⁻⁴ s⁻¹ at 25°C under N₂ in CDCl₃. The reaction rate (k₁) seems independent of the concentration of H₂. A large negative entropy of activation (ΔS^≠=−24.9±5.7 cal deg⁻¹ mol⁻¹) and a relatively small enthalpy of activation (ΔH^≠=14.5±3.3 kcal mol⁻¹) were obtained in the temperature range 15∼35°C for the isomerization, 3b→4b under 1 atm of H₂.     

One-pot synthesis of quinazolin-4(3H)-ones and fused quinazolinones by a palladium-catalyzed domino process

An efficient one-pot synthesis of quinazolin-4(3H)-ones, benzoimidazo[2,1-b]quinazolin-12(6H)-ones and imidazo[2,1-b]quinazolin-5(1H)-ones via a palladium-catalyzed domino process has been developed. The Pd-catalyzed reactions of 2-azidobenzamides 1 with isocyanides 2 produced quinazolin-4(3H)-ones 4 at room temperature by a domino Pd-catalyzed cross-coupling/carbodiimide-mediated cyclization. However, as 2-azido-N-(2-bromophenyl)benzamides 1 were used under heating condition in the presence of Cs₂CO₃, the benzoimidazo[2,1-b]quinazolin-12(6H)-ones 5 were directly obtained by twice Pd-catalyzed domino cyclization. A domino reogioselective 5-exo-dig intramolecular cyclization reaction of alkynyl-containing azides 6 with isocyanides 2 generated imidazo[2,1-b]quinazolin-5(1H)-ones 9 in 74–93% yields in the presence of catalyst Pd(PPh₃)₄ and K₂CO₃.     

Styryl coupling vs. styryl acetate formation in reactions of styryl tellurides with palladium(II) salts

In sharp contrast to the expected formation of a telluride-palladium complex, the treatment of either (E,E)- and (Z,Z)-distyryl tellurides (1 and 2) or (E)- and (Z)-styryl phenyl tellurides (5 and 6) with Li₂PdCl₄ in acetonitrile at 25°C results in the formation of stereoisomeric 1,4-diphenylbuta-1,3-dienes, whereas the treatment of 1 and 2 with Pd(OAc)₂ produces styryl acetates solely or mainly.     

Synthesis and X-ray structure of two isomeric aminocarbene complexes of tungsten containing a free carbon-carbon double bond

The reaction of allylamine with (CO)₅WC(OCH₂CH₃)CH₃ gives two isomeric aminocarbene complexes (CO)₅WC(NHCH₂CHCH₂)CH₃ 2E and 2Z. Refluxing of a solution of this mixture in benzene gives the complexes (CO)₄WC(η²NHCH₂CHCH₂)CH₂ (3) and 2E, which have been separated. 2E was fully characterized by X-ray diffraction. Crystals of 2E are monoclinic, space group P2₁/n with Z = 4, a 7.188(3), b 14.312(2), c 12.530(2) Å and β 91.06(3)°.The same mixture when treated with lithium diisopropylamide (LDA) followed by allyl bromide gives a mixture of (CO)₅WC(N(CH₂CHCH₂)₂)CH₃ (4) and 2Z. These complexes were separated, and 2Z fully characterized by X-ray diffraction. Crystals of 2Z are monoclinic, space group P2₁/c, with Z = 4, a 6.593(5), b 14.584(3), c 13.323(1) Å and β 95.13(4)°.     

P-stereogenic wide bite angle diphosphine ligands

Two modular synthetic approaches for the preparation of novel wide bite angle diphosphine ligands containing stereogenic P-atoms have been developed, leading to compounds (S,S)-2,2′-bis(methylphenylphosphino)diphenyl ether (L1) and (S,S)-2,2′-bis(ferrocenylphenylphosphino)diphenyl ether (L2) in very good diastereomeric ratios. Both protocols involve diphenyl ether as backbone and (2R_P,4S_C,5R_C)-(+)-3,4-dimethyl-2,5-diphenyl-1,3,2-oxazaphospholidine borane (R_P)-5 as initial auxiliary to induce chirality at phosphorus. The absolute configuration of intermediates (S,S)-9-(BH₃)₂ and (R,R)-10-(BH₃)₂ as well as the ligands (S,S)-L1-BH3 and (S,S)-L2 was determined by X-ray crystallographic analysis.     

Enantioselective synthesis of β-amino acids. Part 10: Preparation of novel α,α- and β,β-disubstituted β-amino acids from (S)-asparagine

Perhydropyrimidinone (S)-1 is alkylated with very high diastereoselectivity to give trans products (2S,5R)-3, (2S,5R)–4 and (2S,5R)-5. Dialkylation of (S)-1 also proceeds with complete stereoselectivity to afford adducts (2S,5R)-6, (2S,5S)-6, (2S,5R)-7 and (2S,5S)-7. Hydrolysis (6N HCl, 100°C) of monoalkylated derivative (2S,5R)-3 gives enantiopure α-substituted β-amino acid (R)-8. Hydrolysis of dialkylated adducts 6 and 7 affords enantiopure α,α-disubstituted β-amino acids (R)- or (S)-9 and (R)- or (S)-10. Related iminoester (2S,6S)-2 is alkylated with complete diastereoselectivity to give products (2S,6S)-11–13 whose hydrolysis under relatively mild conditions (2N CF₃CO₂H, CH₃OH, 100°C) affords enantiopure N-benzoylated β,β-disubstituted β-amino acid esters (S)-14–16, with intact double bonds in the olefinic substituents.     

Synthesis,crystal structures and spectroscopic studies on trans and cis isomers of Co(II) complexes with 1-benzyl-2-hydroxymethylimidazole

By carrying out the synthesis in a special way, two novel cobalt(II) isomers of trans(O)-[Co(1-Bz-2-CH₂OHIm)₄](NO₃)₂ (1) and cis(O)-[Co(1-Bz-2-CH₂OIm)₄](NO₃)₂·1.5H₂O (2) have been separated. The crystal structures of the Co(II) isomers show the triclinic space group P1̄ (1) and the monoclinic space group C2/c (2). The coordination geometry around the Co atom is approximately octahedral (1) or very distorted octahedral (2) and the Co(II) ions are surrounded by four nitrogen atoms of the four imidazole rings and two oxygen atoms of the hydroxymethyl group. Two of the ligands act as a monodentate and two as a bidentate, forming the five–membered chelate ring with the central ion. The structural data obtained for the Co(II) isomers were confirmed by IR and UV–Vis spectroscopic methods.     

Structural and electrochemical studies on trithia macrocyclic complexes of palladium

The single crystal X-ray structure of [Pd(1)₂](PF₆)₂ (1 = 1,4,7-trithiacyclononane) shows a crystallographically centrosymmetric cation with a distorted octahedral stereochemistry about the Pd^II centre with PdS_eq 2.332(3) and 2.311(3) Å for the equatorial thia donors, and PdS_ax 2.952(4) Å for the two apically coordinated donors. The crystals have space group C2/C, with a 17.879(8), b 15.627(13), c 11.476(8) Å, β 125.92(4)° and Z = 4. Least squares refinement gave R = 0.0565 for 1153 unique observed reflections measured by counter diffracometry using Mo-K_α radiation. This green complex undergoes a chemically reversible, one-electron oxidation in CH₃CN, E_pa = +0.65V, E_pc = +0.56 V vs. Fc/Fc⁺, ΔE_p = 84 mV. Oxidation of [Pd(1)₂](PF₆)₂ by controlled potential electrolysis at +0.7 V affords an orange, ESR active product which may be tentatively assigned to the corresponding palladium(III) species. These results are contrasted with data for the related homoleptic thia complexes [Pd(L)]²⁺ (L = 1,4,8,11-tetrathiacyclotetradecane (2), 1,4,7,10,13,16-hexathiacyclooctadecane (3)). The syntheses of the complexes cis-[Pd(1)Cl₂], cis-[Pt(1)Cl₂], cis-[Pd(1)(PPh₃)₂](PF₆)₂ and cis-[Pt(1)(PPh₃)₂](PF₆)₂ are also described.     

Synthesis and characterization of binuclear tantalum hydride complexes

Reduction of the quadruply-bridged (2Cl, 2H) tantalum(IV) dimer, Ta₂Cl₆ (PMe₃)₄H₂ (2) with sodium amalgam in glyme or THF at 0°C provides deep green Ta₂Cl₄(PMe₃)₄H₂ (3) in 70% yield. Dimer 3 has a D_2d Ta₂Cl₄(PMe₃)₄ substructure which closely resembles that of the quadruply metal-metal-bonded dimer W₂Cl₄(PMe₃)₄. The hydride ligands of 3 are located on a diagonal plane, bridging the two tantalum atoms and the Ta-Ta separation is 2.545(1) Å. 3 reacts cleanly with Cl₂, HCl and H₂ in diethyl ether to provide the quadruply-bridged dimers 2, Ta₂Cl₅(PMe₃)₄H₃ (4), and Ta₂Cl₄(PMe₃)₄H₄ (5), respectively, in high yield. Dimer 5 can also be prepared in high yield via thermolysis of the tantalum(IV) hydride TaCl₂H₂(PMe₃)₄ (6) in refluxing methylcyclohexane. The X-ray structure of 5 shows that the (μ-H)₄ group is staggered by 45° with respect to the eclipsed pyramidal TaCl₂(PMe₃)₂ end groups. The molecular symmetry of 5 is D_2d and the Ta-Ta separation is 2.511(2)Å. Multiple-scattering Xα calculations on the model compounds Ta₂Cl₄(PH₃)₄H₂ and Ta₂Cl₄(PH₃)₄ are used to elucidate the ground-state electronic structures of 3 and 5, and to probe the question of (μ-H)_x rotation about the metal-metal bonds in these complexes. Crystal data (at 160°C) are as follows: for 3, monoclinic space group C2/c, a = 18.371(5) Å, b = 9.520(3) Å, c = 18.942(6) Å, β = 125.36(2)°, V = 2701.8 ų, Z = 4,d_calc. = 1.991 g cm⁻³; for 5, tetragonal space group P4/nbm, a = b = 12.579(2) Å, c = 10.205(2) Å, V = 1614.7 ų, Z = 2, d_calc. = 1.670 g cm⁻³.     

Reactions of trans-[Pt(H2){P(C6H11)3}2] with heterocumulenes. The crystal and molecular structure of trans-[Pt{P(C6H11)3}2(H){OCHC(C6H5)2}]

Trans-PtH₂(PCy₃)₂ (1) reacts with phenylisocyanate (2) and with diphenylketene (3) to yield the formamido complex (4) and the vinyloxo complex (5), respectively. The structure of 5 has been determined by X-ray diffraction.     

Synthesis, structure, and catalytic activity of titanium complexes with new chiral 11,12-diamino-9,10-dihydro-9,10-ethanoanthracene-based ligands

A new series of organo-titanium complexes have been prepared from the reaction between Ti(NMe₂)₄ and C₂-symmetric ligands, (R,R)-11,12-bis(pyrrol-2-ylmethyleneamino)-9,10-dihydro-9,10-ethanoanthracene (1H₂), and (R,R)-bis(diphenylthiophosphoramino)-9,10-dihydro-9,10-ethanoanthracene (2H₂), (R,R)-11,12-bis(mesitylenesulphonylamino)-9,10-dihydro-9,10-ethanoanthracene (3H₂) and (R,R)-bis(diphenylthiophosphoramino)-1,2-cyclohexane (4H₂). Treatment of Ti(NMe₂)₄ with 1 equiv of 1H₂ gives, after recrystallization from a benzene solution, the binuclear double helicate titanium amide (1)₂[Ti(NMe₂)₂]₂⋅(5) in 71% yield. While under similar reaction conditions, reaction of Ti(NMe₂)₄ with 1 equiv of 2H₂, 3H₂ or 4H₂ gives, after recrystallization from a toluene or benzene solution, the mononuclear single helicate titanium amides (2)Ti(NMe₂)₂ (6), (3)Ti(NMe₂)₂ (7) and (4)Ti(NMe₂)₂ (8), respectively, in good yields. All new compounds have been characterized by various spectroscopic techniques, and elemental analyses. The solid-state structures of complexes 5-8 have further been confirmed by X-ray diffraction analyses. The titanium amides are active catalysts for the polymerization of rac-lactide, leading to the isotactic-rich polylactides.     

Structural control in palladium(II)-catalyzed enantioselective allylic alkylation by new chiral phosphine-phosphite and pyridine-phosphite ligands

The ligands 6-[(diphenylphosphanyl)methoxy]-4,8-di-tert-butyl-2,10-dimethoxy-5,7-dioxa-6-phosphadibenzo[a,c]cycloheptene, 1, (S)-4-[(diphenylphosphanyl)methoxy]-3,5-dioxa-4-phosphacyclohepta[2,1-a;3,4a′]dinaphthalene, (S)-2, and (S)-4-[(diphenylphosphanyl)methoxy]-2,6-bis-trimethylsilanyl-3,5-dioxa-4-phosphacyclohepta[2,1-a;3,4-a′]dinaphthalene, (S)-3, (S)-2-(3,5-dioxa-4-phosphacyclohepta[2,1-a;3,4-a′]dinaphthalen-4-yloxymethyl)pyridine, (S)-4, and (S)-2-(3,5-dioxa-4-phosphacyclohepta[2,1-a;3,4-a′]dinaphthalen-4-yloxy)pyridine, (S)-5, have been easily prepared.The cationic complexes [Pd(η³-C₃H₅)(L-L′)]CF₃SO₃ (L–L′=1–(S)-5) and [Pd(η³-PhCHCHCHPh)(L–L′)]CF₃SO₃ (L–L′=(S)-2–(S)-4) were synthesized by conventional methods starting from the complexes [Pd(η³-C₃H₅)Cl]₂ and [Pd(η³-PhCHCHCHPh)Cl]₂, respectively. The behavior in solution of all the π-allyl- and π-phenylallyl-(L–L′)palladium derivatives 6–14 was studied by ¹H, ³¹P{¹H}, ¹³C{¹H} NMR and 2D-NOESY spectroscopy. As concerns the ligands (S)-4 and (S)-5, a satisfactory analysis of the structures in solution was possible only for palladium–allyl complexes [Pd(η³-C₃H₅)((S)-4)]CF₃SO₃, 11, and [Pd(η³-C₃H₅)((S)-5)]CF₃SO₃, 12, since the corresponding species [Pd(η³-PhCHCHCHPh)((S)-4)]CF₃SO₃, 13, and [Pd(η³-PhCHCHCHPh)((S)-5)]CF₃SO₃, 14, revealed low stability in solution for a long time. The new ligands (S)-2–(S)-5 were tested in the palladium-catalyzed enantioselective substitution of (1,3-diphenyl-1,2-propenyl)acetate by dimethylmalonate. The precatalyst [Pd(η³-C₃H₅)((S)-2)]CF₃SO₃ afforded the allyl substituted product in good yield (95%) and acceptable enantioselectivities (71% e.e. in the S form). A similar result was achieved with the precatalyst [Pd(η³-C₃H₅)((S)-3)]CF₃SO₃. The nucleophilic attack of the malonate occurred preferentially at allylic carbon far from the binaphthalene moiety, namely trans to the phosphite group. When the complexes containing ligands (S)-4 and (S)-5 were used as precatalysts, the product was obtained as a racemic mixture in high yield. The number of the configurational isomers of the Pd-allyl intermediates present in solution in the allylic alkylation and the relative concentrations are considered a determining factor for the enantioselectivity of the process.     

Palladium-catalyzed asymmetric allylic substitution using novel phosphino-ester (PHEST) ligands with 1,1′-binaphthyl skeleton

The asymmetric allylic alkylation of rac-1,3-diphenyl-2-propenyl acetate 1 with dimethyl malonate 2a proceeded smoothly in the presence of lithium acetate, BSA (N,O-bis(trimethylsilyl)acetamide), [Pd(η³-C₃H₅)Cl]₂, and the chiral ligand (R)-i-Pr₂N-PHEST (R)-5a to give the allylic alkylation product (R)-3a in 89% yield with 99% ee. Furthermore, the asymmetric allylic amination of 1 with potassium phthalimide 2c has been carried out using the same ligand to give the allylic amination product (S)-3c in 10% yield with 66% ee.     

New octahedral bis(diphenylphosphino)methanide derivatives and heterometallic species from mixed ligand isocyanide-dppm iron(II) complexes

The cationic [FeL(dppm)(CNPh)₃]ⁿ⁺ (1a: L = I, n = 1; 1b: L = CNPh, n = 2) are readily deprotonated by KOH to give [FeL(dppm-H)(CNPh)₃]ⁿ⁻¹ (2a and 2b). 2a reacts with [thtAuPPh₃]PF₆ to give mer-[FeI((PPh₂)₂C(H)(AuPPh₃))-(CNPh)₃]PF₆ (3). The new heterotrimetallic species [FeL((PPh₂)₂C(AuPPh₃)₂)-(CNPh)₃]ⁿ⁺ (4a and 4b) have been obtained from 1a and 1b by treatment with ClAuPPh₃ in the presence of KOH.     

Ligand design for dual enantioselective control in Cu-catalyzed asymmetric conjugate addition of R2Zn to cyclic enone

A new chiral N-heterocyclic carbene (NHC) ligand derived from a natural α-aminoester has been designed and synthesized. The coupling of N-methylbenzimidazole with an α-chloroacetamide derivative, which was prepared from chloroacetyl chloride and (S)-serine methyl ester, gave the corresponding ester/amide-functionalized azolium compound 20. The reaction of 2-cyclohexen-1-one (17) with Et₂Zn in the presence of catalytic amounts of Cu(OTf)₂ and 20 produced (R)-3-ethylcyclohexanone (18) as a major product. In contrast, the enantioselective conjugate addition (ECA) reaction catalyzed by Cu(OTf)₂ under the influence of a hydroxy-amide-functionalized azolium compound 15, which was derived from (S)-tert-leucinol, produced (S)-18 in preference to (R)-18. A series of azolium salts were synthesized from (S)-serine esters, and the reaction conditions for the ECA reaction were optimized to produce (R)-18 with 69% ee. The best results were obtained in the case of the reaction of 4,4-dimethyl-2-cyclohexen-1-one (34) with Et₂Zn catalyzed by Cu(OTf)₂ in combination with azolium compounds. When the reaction of 34 with Et₂Zn was carried out in the presence of catalytic amounts of Cu(OTf)₂ and 20, (S)-3-ethyl-4,4-dimethylcyclohexanone (35) was obtained with 97% ee, whereas the ECA reaction under the influence of hydroxy-amide-functionalized azolium 15 afforded (R)-35 with >99% ee. In this manner, the reversal of enantioselectivity was achieved by controlling the structure of chiral ligands.     

Ruthenium(II) and iron(II) complexes of N-pyridyl substituted imidazolin-2-ylidenes

N-mesityl-N′-pyridyl-imidazolium chloride 1a and the corresponding bromide salt 1b have been deprotonated with NaH in THF giving the free N-heterocyclic carbene N-mesityl-N′-pyridyl-imidazolin-2-ylidene 2 in 80% yield (starting from 1a). Imidazolium salt 1a reacts with RuCl₃ · xH₂O to give a racemic mixture of dinuclear di-μ-chloro bridged ruthenium complexes [(κ²-2)₂Ru(μ-Cl)₂Ru(κ²-2)₂]²⁺ [3a]²⁺. The carbene carbon atoms as well as the halides are arranged in cis-positions to each other whereas the nitrogen atoms adopt a trans-configuration. The di-μ-bromo bridged derivative [(κ²-2)₂Ru(μ-Br)₂Ru(κ²-2)₂]²⁺ [3b]²⁺ was obtained from RuCl₃ · xH₂O and 1b. The bridging halide ligands can be removed by the reaction with silver or sodium salts of bidentate Lewis acids. Complex [3a]²⁺ reacts with silver pyridylcarboxylate to give a racemic mixture of the mononuclear complex [4]⁺. Reaction of [3a]²⁺ with the sodium salt of l-proline resulted in a diastereomeric mixture of complexes [5]⁺. The free N-heterocyclic carbene 2 reacts with [FeCl₂(PPh₃)₂] to give after anion exchange with NaBPh₄ cis/cis/trans coordinated [Fe(κ²-2)₂(MeCN)₂](BPh₄)₂ [6](BPh₄)₂. The molecular structures of [3b](PF₆)₂, [4]PF₆ and [6](BPh₄)₂ · H₂O are reported.