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Special Catalysts and Processes: Sections 3.2.6–3.2.14
IntroductionElectronic Properties of TitanocenesLigands in the Coordination Sphere of TitanocenesReactions in the Coordination Sphere of TitanocenesAnalytical DataSelected Applications of Titanocene ComplexesCp-EquivalentsAcknowledgments
The process generated by the crossings of a fixed level, u, by the process Pn
(t) is considered, where
and the Xi
(t) are identical, independent, separable, stationary, zero mean, Gaussian processes. A simple formula is obtained for the expected number of upcrossings in a given time interval, sufficient conditions are given for the upcrossings process to tend to a Poisson process as u→∞, and it is shown that under suitable scaling the distribution of the length of an excursion of Pn
(t) above u tends to a Rayleigh distribution as u→ ∞.
This book describes the experience over 25 years of the senior author with the chemistry of organic free radicals. It begins with a mechanistic study of industrial importance on the pyrolysis of chlorinated alkanes. It continues with a theory on the biosynthesis of phenolate derived alkaloids involving phenolate radical coupling. There follows 20 years of practical work to prove the theory correct, especially in the case of morphine alkaloids. The book then describes the work on nitrile photolysis (Barton reaction) which involved the invention of new radical chemistry leading to a simple synthesis of the important hormone, aldosterone. There follows a description of the invention of an important new method for the deoxygenation of biologically important molecules, especially sugars and nucleosides, using radical chemistry applied to thiocarbonyl derivatives. Some years later, in a logical extension to carboxylic acids, another new reaction was invented which provides carbon, nitrogen, oxygen and other radicals under mild conditions. A final chapter summarizes recent applications of thiocarbonyl group derived radical reactions by other authors.
The homologation of methanol to ethanol was carried out with the aid of the mixed transition metal catalysts containing cobalt in conjunction with iodide promoters under the compressed synthesis gas (H2-CO). The Co-Ru system was found to exhibit a remarkable catalytic activity for homologation. The influences of reaction variables such as Ru/Co ratio, pressure, temperature, added iodide and solvents on the methanol conversion, the ethanol selectivity and the formation of by-products were examined in detail. Both the yield and selectivity of ethanol were significantly dependent on the Ru/Co ratio and the composition of synthesis gas. The yield increased with the charge ratio of RuCl3.n H2O to Co2(CO)3(Ru/Co), reached a maximum and then decreased with larger ratio. The optimum ratio of Ru/Co varied slightly from ca. 0.2 to 0.4 when the temperature and the partial pressure of H2 were decreased under the otherwise same conditions. It was also found that ethers such as 1,4-dioxane, THF, Methyl Cellosolve and Diglyme, served as a good solvent to give the higher yield (ca. 60%) and selectivity (ca. 80%) of ethanol.
Derivative cyclic voltammetry (DCV) can offer significant advantages in the investigation of electrode reactions when compared with conventional cyclic voltammetry (CV). Redox potentials can be accurately determined for incorporation in thermochemical cycles that allow for the determination of thermodynamic quantities that are of interest to organometallic chemists. Applications towards the determination of solution bond dissociation energies and Brønsted acidities for metal hydrides and metal hydride cation radicals are presented. However, perhaps the greatest advantages of DCV arise from the absence of double-layer charging currents in the derivative voltammograms. This effect makes the technique especially suitable for kinetic and mechanistic studies of the reactions of electrode-generated reactive intermediates. The use of DCV for this purpose is highlighted with case studies from our laboratories. This includes the substitution of one- and two-electron donors in 17-electron cation radicals by two-electron donor nucleophiles. Kinetic parameters show that both types of processes appear to involve 19-electron intermediates or transition states. Also included are some rather unusual reactions that proceed via initial reversible cation radical dimerization.
IntroductionS-Bound Thiophene Complexesη5-Thiophene Complexes and Related Derivativesη2- and η4-Thiophene ComplexesComplexes of Thiophene DerivativesTransition Metal Promoted Thiophene SynthesesCleavage and Desulfurization of Thiophene Rings by Transition Metal Complexes
IntroductionStereoselectivity in Reactions of Stable YlidsStereoselectivity in Reactions of Reactive YlidsApplication of Stereoselective Carbonyl Olefination ProceduresDiscussion
Reactions which result in the addition of the carbon–metal bond of an organometallic (1) across a carbon–carbon multiple bond (2) leading to a new organometallic (3) are called carbometallation reactions (equation 1).1–3 Only those reactions in which the newly formed carbon–metal bond of (3) can be used for further synthetic transformations will be considered here. Since the first carbometallation discovered by Ziegler and Bahr4 in 1927 (equation 2), an ever-increasing number of additions of organometallics to carbon–carbon multiple bonds have been reported.3 To be synthetically useful, a carbometallation reaction must show good chemo-, regio- and stereo-selectivity.
Different transition metal compounds in conjunction with suitable promoters have been reported in the literature to catalyze the alcohols homologation (1–9)(Equation 1).
The chapter focuses on the mechanistic pathways in the catalytic carbonylation of methanol by rodium and iridium complexes. Kinetic studies of the rhodium-catalyzed methanol carbonylation reaction show a remarkably simple behavior. The iodide promoter can be charged to the reaction in several different forms without marked differences in the reaction rate being noted. Many different types of rhodium compounds can be charged to the reaction, and at typical reaction temperatures of 1500–2000C, they function as effective catalysts. The generation of the initial metal–carbon bond in the catalytic cycle by reaction of methyl iodide with a metal carbonyl, containing species has been proposed as a key step in both the cobalt and rhodium catalyzed systems. Iridium is also an excellent homogeneous catalyst for the carbonylation of methanol under relatively mild reaction conditions. There are apparently complex interactions among solvent, water, iodide form, and carbon monoxide pressure, and this complicates interpretation. Some general kinetic observation can be studied as (1) the reaction rate is strongly dependent on water concentration, with a decrease in rate being observed at higher water levels; (2) in reaction media containing appreciable concentrations of iodide ion, the reaction rate increases with increasing carbon monoxide pressure; (3) the reaction rate is not first order with respect to methyl iodide concentration as in the rhodium system, but shows an optimum level; and (4) at low iodide levels using methyl acetate as the substrate with low levels of water present, the reaction rate is inversely dependent on carbon monoxide pressure.
This chapter discusses the homogeneous catalytic isomerization of olefins by transition metal complexes. Transition metal hydrides play a key role in the catalytic homogeneous isomerization of olefins. The pure hydrides, such as HCo(CO)4 can function as the catalyst or transition metals complexed to stabilizing ligands can function as catalysts. The catalysis almost certainly proceeds through hydride intermediates in many cases. The most attractive mechanism for olefin isomerization consists of initial complexing between olefin and metal, addition of H–M across the double bond to generate a σ carbon-metal bond, and then elimination of H–M in the opposite direction with eventual release of the isomerized olefin. The evidence for such a mechanism is compelling, especially with the heavier transition metals. However, when some σ-alkyl metals of cobalt and iron have been treated with olefins under conditions appropriate for isomerization, no transfer of the metal to the olefin occurred.
This chapter discusses the roles played by certain nucleophile adducts of metal–coordinated carbon monoxide in the stoichiometric and catalytic chemistry of metal–carbonyl complexes and relatively recent developments for systems where oxygen or nitrogen bases have been used as the nucleophiles. The syntheses, spectral characterizations, and chemical reaction studies of various nucleophile–carbonyl complexes are described with the goal of drawing attention to how such adduct formation not only activates the carbonyl directly involved for further reaction but also influences the reactivities of the balance of the complex. Preparation by direct addition of a nucleophile anion Nu– to a coordinated CO has been a widely applicable approach. Nuclear magnetic resonance (especially 13C NMR) and infrared spectroscopy have proved the most used and successful diagnostic tools in characterizing the formation and transformations of various LnM–(CO)Nu complexes and related species. Reactions of uncharged nucleophiles with coordinated CO provide a different situation. Kinetics studies of systems for which characterizable nucleophilecarbonyl adducts have been isolated are relatively few.
This chapter discusses homogeneous nickel-catalyzed olefin hydrocyanation. It describes the chemistry behind the du Pont adiponitrile process from a mechanistic viewpoint. The hydrocyanation process can be broken down into two major steps. In the first step, hydrogen cyanide (HCN) is added to butadiene in the presence of NiL4 catalyst to give 3-pentenenitrile (3PN) and 2-methyl-3-butenenitrile (2M3BN). In the second step, a Lewis acid promoter is added to the NiL4 (L = a phosphorus ligand) catalyst to effect the double bond isomerization of 3PN to 4-pentenenitrile (4PN) concurrently with the selective addition of HCN to 4PN. By-products in the second step include 2-methylglutaronitrile (MGN), ethylsuccinonitrile (ESN), and 2-pentenenitrile (2PN) arising, respectively, from Markovnikov addition to 4PN, direct addition of HCN to 3PN, and isomerization of 3PN to its conjugated isomer that is not hydrocyanated. The three different types of reactor system include (1) semibatch, (2) pulse, and (3) continuous. The semibatch reactor is the simplest. All reagents except the HCN are placed in a thermostated vessel (usually glass).
Electrochemistry of large numbers of organic species is well established, and the occurrence of multiple oxidation states is commonplace for transition-metal compounds. Many of the electrochemical studies of metal–carbonyl derivatives, and indeed of other species, have sought to correlate redox potentials with other physical or spectroscopic properties. Very few σ alkyl or aryl complexes, apart from those also containing carbony1 or cyclopentadienyl ligands, have been studied by electrochemical methods. A wealth of electrochemical data exists, particularly for dihalides, but their interpretation is controversial, and the isolation and full characterization of some of the postulated intermediates are necessary. Carbonyl substitution leads to large shifts in E 1/2 to more negative potentials, facilitating the isolation of paramagnetic monocations. A sandwich compound is regarded as one in which a metal is π bonded to two planar, delocalized rings. Such compounds almost invariably undergo electron-transfer reactions. The irreversible one-electron reduction of the cycloheptatrienyl complexes leads to reductive ring coupling, presumably via the neutral radical.
A wide variety of isomerizations can be achieved using transition metal species; the more important and most extensively studied of these involve alkenes. Some are stoichiometric reactions in which it may be necessary to isolate an intermediate organometallic compound, while others employ but small amounts of transition metal in catalytic reactions. Among the more well-understood catalytic reactions are the transition-metal-induced isomerizations of strained hydrocarbons to cyclic systems or alkenes, reactions that involve carbon—carbon bond fission. However, in terms of practical synthesis the most useful transition-metal-induced isomerizations are “double-bond migration processes” involving only carbon—hydrogen bond fission. These reactions often provide short routes to high yields of a desired isomer that cannot be obtained by other means.
Derivatisation of racemic ethylene-bridged bis(4,5,6,7tetrahydro-1-indenyl)-titanium and -zirconium dichlorides with O-acetyl-r-mandelic acid affords diastereomers which can be separately crystallised. Crystal and molecular structures of these diastereomers reveal different chelate ring conformations in the crystalline state. The separated diastereomers have been converted, via their dimethyl derivatives, into the corresponding titanocene and zirconocene dichloride enantiomers, the optical purity of these enantiomers has been demonstrated by their reconversion into the acetyl-r-mandelate derivatives.
Irradiation of dicarbonyl(η5-cyclopentadienyl)iron dimer 1 or decacarbonyldimanganese (2) in the presence of alkyl halides leads to C-centered radicals which can be trappedby alkenes and yields saturated and/or unsaturated addition products. Carbon radicals are generated via halogen abstraction by the initially formed metal-centered radicals resulting from homolysis of the metal-metal bond of dimeric mediators 1 and 2. No reaction occurs using octacarbonyldicobalt (3).
Water-soluble manganese(III) as well as iron(III) porphyrinates are introduced as light-sensitive precursor compounds for the photocatalytic activation of dioxygen in aqueous solutions. It is shown that in the presence of α-pinene (4) and the further cycloalkenes 11–13 photocatalytic oxygenation reactions occur. The dependence of the selectivity of the oxygen transfer to the olefin on both the presence of water and the variation of the substrate-to-catalyst ratio is discussed. The catalyst may be conveniently separated from the substrates/products by using aqueous solvent systems.
Die Fischer-Tropsch-Synthese gehört zu den vielseitig anwendbaren, eine breite Produktpalette ergebenden großtechnischen Verfahren und bietet seit Jahrzehnten die attraktivste Möglichkeit zur Nutzung der Kohlevorkommen für flüssige Heiz- und Kraftstoffe. Das denkbar einfache Reaktionsprinzip - katalytische Hydrierung von Kohlenmonoxid - führt generell auch zu einfachen, seit der Ölkrise der siebziger Jahre als Chemierohstoffe besonders erwünschten Kohlenwasserstoffen (z. B. kurzkettigen Olefinen), doch scheitert deren großtechnische, volkswirtschaftlich dringend erforderliche Gewinnung bisher noch an der äußerst geringen Selektivität des herkömmlichen Fischer-Tropsch-Prozesses. Zur Lösung dieses Problems ist die aktuelle Forschung nicht nur auf die Optimierung alter sowie die Entwicklung neuer Katalysator-Systeme ausgerichtet, sondern befaßt sich zunehmend auch mit reaktionsmechanistischen Fragen. Im vorliegenden Aufsatz werden an typischen Modellreaktionen die Grundzüge der Synthesegas-Chemie aus der Sicht des Organometall-Chemikers erörtert und mit den bisher üblichen Anschauungen über den Ablauf der Fischer-Tropsch-Synthese verglichen. Ungewöhnliche und grundlegende, innerhalb der Fischer-Tropsch-Chemie erforschte Verbindungsklassen werden auf ihre Bedeutung für den Aufbau von Kohlenwasserstoffen aus Kohlenmonoxid und Wasserstoff geprüft. Hierbei zeigt sich, daß der bereits von Franz Fischer und Hans Tropsch in Betracht gezogene Carbid/Methylen-Mechanismus die Primärschritte der reduktiven Oligomerisierung von Kohlenmonoxid am besten beschreibt.
The principal tendencies in the development of homogeneous
photocatalysis by transition metal complexes are examined. Attention is
concentrated on the photocatalytic reactions of organic substrates. The
bibliography includes 164 references.
Stabilized by “carbene” donor ligands, the Pd complex 1 catalyzes the Heck olefination of aryl halides unexpectedly efficiently and yet has long-term stability at elevated temperatures. The active Pd0 species can be generated during the Heck reaction or deliberately prepared by reduction of 1 with, for instance, hydrazine or sodium formate. Another similar catalyst can be synthesized in situ from Pd0 complexes and 1,3-dimethyldihydroimidazoline-2-ylidene.
The homogeneous selective hydrogenation of benzothiophene to 1,2-dihydrobenzothiophene and of quinoline to 1,2,3,4-tetrahydroquinoline is efficiently achieved by the use of RuCl2(PPh3)3, [RuHCl(CO)(PPh3)3], [OsHCl(CO)(PPh3)3], RhCl(PPh3)3, [Rh(cod)(PPh3)2]PF6, and [Ir(cod)(PPh3)2]PF6 (cod = cyclooctadiene) at 170°C and 115 bar and 150°C and 30 bar H2, respectively.
Selective dehydrodimerization of the title compounds can be carried out on a preparative scale at 1 atm. and at 25°–110°C in a simple apparatus by Hg-photosensitized reaction under H2.
The photooxygenation of vinyl silanes 1 in the presence of Ti(OiPr)4 afforded regio- and diastereo-selectively the epoxy alcohols 3.Reaction of the vinyl silanes 1 with 1O2 and Ti(OiPr)4 afforded the hydroxy epoxy silanes 3 regio- and diastereoselectively.
The reaction of Grignard reagents with ketones or aldehydes in the presence of a catalytic amount of Cp2TiCl2 leads to the corresponding reduction products in high yields. Cp2TiH intermediate was proposed to account for this observation.
Stereospecific annulation of cyclic alkenes for the preparation of cis-fused seven-membered ring systems was accomplished by sequential [2 + 2] cycloaddition, exo-allylation, hydrohalogenation, and free radical ring expansion. The new strategy extends our recently developed cyclobutanone-based ring expansion reactions.
Copper(II) chloride induced the chemo and regioselective chlorohydroperoxidation, as well as the site-selective chlorination of olefins, under photooxygenation conditions.
Cyclisations of various methylenecyclopropyl substituted malonate radicals have been studied. Radicals derived from malonates 4 and 9 underwent exclusive 5-exo cyclisation onto the methylenecyclopropane alkene, followed by opening of the resulting cyclopropylmethyl radical, to give methylenecyclohexane products. Radicals derived from malonates 5 and 10 largely gave bicyclo[1.5.0]octane products, in good yields, as a result of 7-endo cyclisation. Cyclisation of the radical derived from malonate 11 gave a bicyclo[1.5.0]nonane derivative in 31% yield, as a result of an 8-endo cyclisation.
Photochemically induced free-radical cyclisation of iodo acetylenic esters provides low yields of mixtures from which both (E)- and (Z)- iodoalkylidene lactones may be isolated. However, photoisomerisation of (E)-iodolkylidene lactones, which have previously been obtained in good yields by dibenzoyl peroxide induced cyclisation of iodo acetylenic esters, provides (E)/(Z)-mixtures from which the previously inaccessible (Z)-isomers may be isolated by chromatography. Also reported is a collection of IR, UV and NMR spectral data, all of which provide useful information relating to the stereochemistry of the iodoalkylidene lactones. A comparison of the crystal structures of the (E)- and (Z)-isomers of a trimethylsilyl iodomethylene lactone is also included.
Derivatisation of racemic ethylene-bridged bis(4,5,6,7tetrahydro-1-indenyl)-titanium and -zirconium dichlorides with O-acetyl-r-mandelic acid affords diastereomers which can be separately crystallised. Crystal and molecular structures of these diastereomers reveal different chelate ring conformations in the crystalline state. The separated diastereomers have been converted, via their dimethyl derivatives, into the corresponding titanocene and zirconocene dichloride enantiomers, the optical purity of these enantiomers has been demonstrated by their reconversion into the acetyl-r-mandelate derivatives.
The thermolysis and photolysis of (η4-cyclopentadiene)iron tricarbonyl to yield (principally) cyclopentadienyliron dicarbonyl dimer has been studied using the 5-exo deuterated compound (produced by cyanoborodeuteride reduction of cyclopentadienyliron tricarbonyl cation) as the principal mechanistic probe. The results of this study are interpreted in terms of a stereospecific free-radical chain abstraction of the deuterium by an Fe species as the major pathway for the thermal process. The photochemical process, on the other hand, seems to involve the intramolecular hydrogen transfer pathway previously proposed for the thermal reaction.
Polystyrene-bound bis(cyclopentadienyl)titanium dichloride has been shown to react with Grignard reagents to form a reactive alkene isomerization catalyst, which converts 1-alkenes primarily into E-2-alkenes at room temperature. The catalyst specificity seen for this polymer-bound catalyst is altered from that seen in an analogous homogeneous system. Hydrogenation of alkenes has also been accomplished when 1-alkenes are allowed to react with these same catalysts under hydrogen. Internal alkenes are unreactive toward these catalysts. The efficacy of organomagnesium versus organolithium reagents for preparation of these catalysts and the application of 13C NMR to the study of polymer-supported catalytic systems is described.
With the catalyst IrCl(CO)L2 acrylic and crotonic acid ethyl esters were homogeneously hydrogenated thermally and photochemistry without solvent. Photoactivation strongly depends on the ligand L. Turnover numbers up to 14000 and mean catalyst activities ā of 25 were found.
Trivalent phosphorus compounds are promoters for methyl formate homologation to ethanol and ethyl formate catalyzed by ruthenium compounds in the presence of iodide at 220°C and 27 MPa of synthesis gas. Under these conditions the phosphines are quaternized, but decomposition of phosphonium salts occurs during the reaction. Promotion is also observed for methyltriphenyl-phosphonium bromide and triphenylphosphine sulfide, but benzyltrimethyl-ammonium bromide, triphenylarsine, and triphenylantimony are not effective. The major ruthenium species present is Ru(CO)3I3- but with triphenylantimony a trimethylantimony complex, Ru(CO)2(Sb(CH3)3)2I2, can be isolated in high yield.
Methanolic solutions of tracarbonylcobaltate(—I) in the presence of an acid are simple and useful reducing agents for the selective hydrogenation of conjugated carbon-carbon double bonds. The reaction has the following stoichiometry:
The reaction of acetic [13C]formic anhydride with sodiumpentacarbonylmanganate proceeds rapidly at 0°C to give 13CO substituted pentacarbonylmanganese hydride as the predominant product. The results are consistent with the formation of a short-lived neutral formyl complex, (CO)5Mn13CHO.
CpFe(CO)2(η1-ally) Ia and CpFe(CO)2(η1-methally) Ib undergo photochemically initiated carbonyl substitution by bicyclic phosphite III to give IIa and IIb respectively. These latter complexes are more readily protonated than CpFe(CO)2(η1-allyl) complexes and the resulting cationic olefin complexes V and VI are more stable to olefin ligand displacement than similar complexes of the parent system. Both IIa and IIb exhibit high thermal stability, in contrast to Ia or Ib. CpFe(CO)2(η1-2-butenyl) and CpFe(CO)2(η1-cyclopentyl) fail to undergo photochemically initiated CO substitution in the presence of phosphite III, but give instead the dinuclear complex Cp2Fe2(CO)3(L) (L = III). The complex CpFe(CO)2(η1-cyclopentenyl) (VIII) can, however, be prepared indirectly from IIb → VI → VII → VIII. Complex IIa is 900 times more reactive than Ia cycloaddition reaction with β,??-dicyanostyrene.
In dichloromethane solution, cis- and trans-2-butene are catalytically isomerized by RhCl(PPh3)3. The slow reaction reaches the thermodynamic equilibrium of the three possible n-butenes. The isomerization shows a marked induction period. When pure cis-2-butene is isomerized, 1-butene temporarily reaches twice the equilibrium concentration. The kinetic data suggest that RhCl(PPh3)3 is not the catalytically active species. Dissociation of the original rhodium complex appears to be a requirement for the isomerization of the olefins.
The stereodifferentiating activity of Ziegler—Natta type Ti(IV) catalysts carrying the chiral ligands menthyl or neomenthyl cyclopentadienyl has been investigated in the asymmetric homogeneous catalytic reduction of a prochiral olefin such as 2-phenyl-1-butene, which is hydrogenated with acceptable optical yields to (R)-(−) or (S)-(+)-2-phenyl-1-butane, respectivelyThe effect on the optical yields of the reaction temperature, of the nature of the chiral cocatalyst or of achiral cocatalysts of different bulk has also been studied.
The synthesis of unsymmetrical ketones by means of the palladium-catalyzed carbonylative cross-coupling reaction of 9-alkyl-9-BBN derivatives with iodoalkanes under a carbon monoxide atmosphere is described.
The complex [TiCl3(THF)3] reacts with catalytically-prepared solid magnesium hydride (MgH2★) or dissolved magnesium hydride (MgH2′) with evolution of hydrogen to give a highly reactive titanium hydride, [HTiCl(THF)∼0.5]x (1). The well-known low valent titanium species, obtained by reduction of TiCl3 with LiAlH4, utilized in the McMurry reaction, has been shown to be 1. An X-ray absorption spectroscopy study (EXAFS) of 1 reveals that the Ti absorber is surrounded by O (from THF) and Cl atoms plus two types of Ti neighbors. Possible structural models for 1 compatible with the EXAFS results are proposed. Complex 1 is an active reagent for the coupling reaction of benzophenone to give tetraphenylethene. During the reaction hydrogen is liberated and the inorganic side product has been shown to be titanium(III) oxychloride; thus the McMurry reaction employing 1 as the reagent can be described byPh2CO+1THF or→toluen 0.5 PPh2CCPh2 + 0.5H2↑ + [TiOCl(THF)x].On the basis of these results, a new interpretation of the mechanism for the McMurry reaction is presented. Complex 1 also undergoes a number of carbenoid type reactions, which may proceed via a “titanium carbenoid” intermediate 9a–c.
The catalytic epoxidation of trans-3-hexene with electronically different bis(2-aryl-4,5,6,7-tetrahydroindenyl)titanium dichloride complexes in the presence of stoichiometric tert-butylhydroperoxide has been studied. The results show that both electron donating (4-methoxyphenyl and 4-methylphenyl) and withdrawing (4-bromophenyl) groups on the 2-position of 4,5,6,7-tetrahydroindene produce an enhancement in the number of turnovers of epoxide formed during 15 h at 80°C when compared to the 2-phenyl substituted complex. The facile preparation and purification of these previously unreported ligands and the corresponding titanocene complexes, and the results of the catalytic epoxidation of trans-3-hexene are described.
The enantioselective catalytic epoxidation of unfunctionalized alkyl and aryl olefins was conducted using titanocene and niobocene peroxide complexes incorporating a C2-symmetrical binaphthylcyclopentadienyl ligand (BpDMCp). Enantiomeric excesses up to 20% and activities up to 55 turnovers were obtained with the application of our chiral bis(binaphthylcyclopentadienyl)-derived metallocenes. A significant decrease in the enantioselectivity for the epoxidation of trans-3-hexene was observed when the metal was changed from titanium to niobium in these binaphthyl-derived C2-symmetrical complexes. The preparation of the bis(binaphthylcyclopentadienyl)-chloroniobium peroxide complexes and bis(indenyl)chloroniobium peroxide is described.