Alkane oxidation catalysed by a self-folded multi-iron complex

A preorganised ligand scaffold is capable of coordinating multiple Fe(II) centres to form an electrophilic CH oxidation catalyst. This catalyst oxidises unactivated hydrocarbons including simple, linear alkanes under mild conditions in good yields with selectivity for the oxidation of secondary CH bonds. Control complexes containing a single metal centre are incapable of oxidising unstrained linear hydrocarbons, indicating that participation of multiple centres aids the CH oxidation of challenging substrates.     

Kinetics and mechanism of alkane hydroperoxidation with tert-butyl hydroperoxide catalysed by a vanadate anion

tert-Butyl hydroperoxide oxidizes alkanes in acetonitrile at 60 degrees C if the soluble vanadium(v) salt, n-Bu4NVO3, is used as a catalyst. Alkyl hydroperoxides are formed as main products which decompose during the course of the reaction to produce the more stable corresponding alcohols and ketones. Turnover numbers (ie. numbers of moles of products per one mole of a catalyst) attained 250. The kinetics and selectivity of the reaction have been studied. The mechanism proposed involves the formation of a complex between the V(V) species and t-BuOOH (K5 was estimated to be 5 dm3 mol(-1)) followed by decomposition of this complex (k6 = 0.2 s(-1)). The generated V(IV) species reacts with another t-BuOOH molecule to produce an active t-BuO* radical which attacks the hydrocarbon.     

Oxidative polymerization of 2,6-dimethylphenol catalysed by insoluble polymer copper complexes

Catalysis by insoluble polymerCu complexes was studied for the oxidative polymerization of 2,6-dimethylphenol. The polymerization was heterogeneously catalysed by the insoluble complex. The catalyst was readily recovered by filtration and could be re-used. Compared with the homogeneous system, the catalytic activity and the molecular weight of the produced polymer were lower for the insoluble polymerCu complex because of steric hindrance, low flexibility of the ligand part and low concentration of the effective copper ion.     

Mechanistic aspects of vanadium catalysed oxidations with peroxides

The enhancement of the reactivity of peroxides, particularly hydrogen peroxide and alkylhydroperoxides, in the presence of vanadium catalysis is a very well known process. The catalytic effect is determined by the formation of an intermediate whose nature depends on the peroxides used and on its interaction with the metal precursor, high-valent peroxo vanadium species being usually the reactive oxidants. During the last decades the mechanistic details for several types of oxidation reactions have been elucidated. Interestingly, in a number of cases theoretical calculations offered support to the proposed reaction pathways.In general, V(V) peroxo species behave as electrophilic oxygen transfer reagents thus reacting preferentially with the more nucleophilic functional group present in the molecule. In several instances the chemoselectivity observed in such processes is very high when not absolute. As far as vanadium peroxides are concerned, a radical oxidative reactivity toward alkanes and aromatics has been also observed; also for this latter chemistry, diverse research groups studied in detail the mechanism. On the other hand, no clear-cut evidence of nucleophilic reactivity of vanadium peroxo complexes has been obtained.Here we collect a selection of recent achievements concerning the reaction mechanisms in the vanadium catalysed oxidation and bromination reactions with peroxides.     

Alkane activation by homogeneous palladium complexes

The conversion of adamantane exclusively to 1-adamantyl trifluoroacetate in a refluxing trifluoroacetic acid solution of palladium acetate at 1 atm of air or nitrogen was confirmed. The conversion can be made quantitative. The 1-adamantyl trifluoroacetate was hydrolysed to yield 1-adamantanol in 90% isolated yield. The adamantane functionalization is accompanied by concomitant formation of a palladium mirror.     

Homogeneous oxidation reactions catalysed by in situ-generated triazolylidene copper(I) complexes

Transition Metal Chemistry - Four new Cu complexes bearing triazolylidene ligands 1-(R)-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium-5-yl: R?=?phenyl (2a), mesitylenyl (2b), propyl (2c),...     

Polymerization of phenylacetylene catalysed by cyclopentadienylnickel complexes

A wide range of cyclopentadienylnickel compounds catalyse the reaction of phenylacetylene under solvent‐free conditions, giving a mixture of cyclotrimers, linear oligomers and poly(phenylacetylene). No reaction is observed in the case of internal acetylenes. Cyclotrimer formation is favoured by the presence of cyclopentadienylnickel catalysts bearing a chloro substituent at nickel. A reduction in reaction temperature results in lower conversion but favours linear oligomer and polymer formation. The main effect of the presence of solvent, regardless of whether it is potentially coordinating (toluene) or not (n‐octane), is to suppress almost completely reactions catalysed by nickelocene.     

Alkene epoxidation catalysed by binuclear manganese complexes

Binuclear manganese(II) complexes with macrocyclic ligands have been synthesized by template Schiff base condensation of diethylenetriamine and pentane-2,4-dione or 1,3-diphenyl-propane-1,3-dione. Catalytic epoxidation of simple olefins with hydrogen peroxide and t-BHP were studied using the above manganese complexes in the presence of a base. The influence of reaction temperature, the additive methanol and the cocatalyst had been investigated. The major products of the oxidations were the epoxides. The new manganese complexes showed significant catalytic activities for the epoxidation of alkenes using hydrogen peroxide as oxidant and ammonium acetate as cocatalyst.     

Alkene isomerization catalysed with platinum hydride complexes

The mechanism of but-1-ene, pent-1-ene and 3-methylbut-1-ene isomerization catalysed with trans-[PtH(SnX₃)L₂] (I, L = PPh₃, PMePh₂, PEt₃, PPr₃; X = Cl, Br) have been studied. Stoichiometric reactions of I with the alkenes proceed even at ?90°C giving cis-[Pt(alkyI-1) (SnX₃) L₂] (II). The equilibrium amounts of II are dependent on the nature of the phosphines, halogens and alkenes. The isomerization rates, determined at +20°C, change in parallel with the relative stabilities of II as a function of phosphine (PMePh₂ > PPh₃ > PAlk₃) and halogen (Br > Cl), and decrease with methyl substitution at γ- and δ- carbons of the alkenes. 2-Substituted alk-1-enes undergo no isomerization in the reactions under investigation. When L is PPh₃ or PMePh₂, the main platinum-containing species in the course of the isomerization are trans-[Pt(alkyl-1) (SnX₃)L₂], appearing as a result of cis-trans isomerization of II. The conversion of I, L = PAlk₃ into related trans-alkyl complexes, and oxidation of I, proceed more slowly than the isomerization of alkenes. The ratio of cis- to trans-alk-2-enes is dependent on the size of L and is a maximum for L = PPh₃.     

The coupling of butylvinyltellurides with organometallic reagents catalysed by nickel complexes

Vinylic tellurides couple efficiently with sp, sp² and sp³ hybridised organometallic compounds (Li, MgX and Zn species) in the presence of dichloro-bis(triphenylphosphine)nickel(II) as catalyst.     

Mechanistic studies on ethylene silylation with chlorosilanes catalysed by ruthenium complexes

Silylation of ethylene by chlorosilanes is catalysed by ruthenium complexes. Mechanistic investigations reveal the presence of a complicated network of reactions leading to new sigma-silane, ethylene and silyl complexes.     

Homo-metathesis of vinylsilanes catalysed by ruthenium carbene complexes

Effective homo-metathesis of a series of dichloro-substituted vinylsilanes H₂C = C(H)SiCl₂R (where R = Me, OSiMe₃, C₆H₅, C₆H₄–Me-4, C₆H₄–CF₃-4) in the presence of second generation Grubbs catalyst [Cl₂(PCy₃)(IMesH₂)Ru(=CHPh)] (I) and Hoveyda–Grubbs catalyst (II) leads to selective formation of E-1,2-bis(silyl)ethenes and ethene. On the basis of the results of experiments with deuterium-labelled reagents, a metallacarbene mechanism has been suggested for these reactions.     

Homogeneous hydrogenation of cyclic olefins catalysed by nitrosylcatecholatoiridium complexes

The catalytic activity of the nitrosylcatecholato complexes Ir(NO)(1,2-O₂C₆Br₄)(PPh₃) and Ir(NO)(1,2-O₂C₆H₄)(PPh₃) in homogeneous hydrogenation of cyclic olefins, dienes and trienes has been studied. The activities of the catalysts are related to the electron density on the central metal.     

Efficient alkyne homocoupling catalysed by copper immobilized on functionalized silica

Copper immobilized on a functionalized silica support is a good catalyst for the homocoupling of terminal alkynes. The so‐called Glaser–Hay coupling reaction can be run in air with catalytic amounts of base. The copper catalyst is active for multiple substituted alkynes, in both polar and non‐polar solvents, with good to excellent yields (75–95%). Depending on the alkyne, full conversion can be achieved within 3–24 h. The catalyst was characterized by TGA, inductively coupled plasma and X‐ray photoelectron spectroscopy. Leaching tests confirm that the catalyst is and remains heterogeneous. Importantly, the overall reaction requires only alkyne and oxygen (in this case, air) as reagents, making this a clean catalytic oxidative coupling reaction. Copyright © 2012 John Wiley & Sons, Ltd.     

Asymmetric ring opening of epoxides with cyanides catalysed by chiral binuclear titanium complexes

A series of Schiff bases obtained from salicylaldehydes and 3,3′-diformyl-BINOL were synthesized. The complexes of these Schiff bases with Ti(IV) were active for the asymmetric ring opening of epoxides with TMSCN. A mixture of unpurified ligands was found to be as effective as the best one. The influence of temperature, solvent polarity and structural modification of the pre-catalysts on the enantioselectivity of the process has also been investigated.     

Direct C-H bond arylation of arenes with aryltin reagents catalysed by palladium complexes

Direct C-H bond arylation of simple arenes with aryltin reagents has been successfully catalysed by PdCl2 in the presence of CuCl2. CuCl2 proved to be an activator for a palladium intermediate as well as an oxidant.     

Hydrocarbonylation of 2,2-dimethoxypropane catalysed by rhodium and cobalt complexes

The hydrocarbonylation of 2,2-dimethoxypropane (DMP) with synthesis gas catalysed by rhodium and cobalt complexes has been studied. The major product is either diethyl ether or acetaldehyde dimethylacetal (ADMA), arising from the homologation of the methoxy groups of the ketal. The operating conditions and ligand environment of the metal have been optimized to reduce the secondary aldolization reactions.     

Further studies on reactions of organic halides with disilanes catalysed by transition metal complexes

The interaction of a range of organic halides with (Cl₃Si)₂ or (Me₃Si)₂ in the presence of a variety of transition metal catalysts (very predominantly Pd⁰ or Pd^II complexes) have been examined. PhSiMe₃ was formed from PhCl[m.y., 15%] (m.y. - maximum yield), PhBr (m.y., 92%, with [PdL₂Br₂] as catalyst (L - PPh₃)), and (contrary to earlier reports) PhI (m.y. 51%, with [PdL₂I₂]). MeSiCl₃ was formed from MeBr (m.y., 78% with [PdL₄]) and MeI (m.y., 91% with [PdL₄]), and EtSiCl₃ from EtBr (m.y., 49%, with [PdL₂“Br₂]; L” - P(C₆H₄OMe-p)₃) and EtI (m.y. 45%, with [PdL₄]). Me₄Si was satisfactorily formed from MeBr (m.y. 42%, with [PdL₄]). Evidence was obtained for the formation of Me₃SiCF₃ from CF₃I. Very poor yields of XC₆H₄CH₂SiMe₃ were obtained from XC₆H₄CH2Br (X - H orp-Me) (with X - H some PhSiMe₃ was formed), butp-O₂NC₆H₄CH₂SiMe₃ was formed in 48% yield fromp-O₂NC₆H₄CH₂Cl with [PdL“₄] as catalyst. PhCOSiMe₃ was formed from PhCOCl (m.y. 52% with [PdL₂I₂]. The nickel complex [NiL₄] was moderately effective as a catalyst for reactions between (Cl₃Si)₂ and MeBr, EtBr, or PhCH₂Br. The new complex [PdL₂(SiCl₃)₂] was prepared by treatment of [PdL₄] with (Cl₃Si)₂ or Cl₃SiH, and shown to catalyse the reaction between MeBr and (Cl₃Si)₂.