High Catalyst Turnover Numbers Research Articles

Catalytic Asymmetric Access to Structurally Diverse N-Alkoxy Amines via a Kinetic Resolution Strategy.

Chiral N-alkoxy amines are increasingly vital substrates in bioscience. However, asymmetric synthetic strategies for these compounds remain scarce. Catalytic kinetic resolution represents an attractive approach to prepare structurally diverse enantiopure N-alkoxy amines, which has remained elusive due to the notably reduced nucleophilicity of the nitrogen atom together with the low bond dissociation energies of labile NO-C and N-O bonds. We here report a general kinetic resolution of N-alkoxy amines through chemo- and enantioselective oxygenation. The mild and green titanium-catalyzed approach features broad substrate scope (55 examples), noteworthy functional group compatibility, high catalyst turnover number (up to 5200), excellent selectivity factor (s > 150), and scalability.

In-situ Kinetic Studies of Rh(II)-Catalyzed C-H Functionalization to Achieve High Catalyst Turnover Numbers.

Detailed kinetic studies on the functionalization of unactivated hydrocarbon sp3 C-H bonds by dirhodium-catalyzed reaction of aryldiazoacetates revealed that the C-H functionalization step is rate-determining. The efficiency of this step was increased by using the hydrocarbon as solvent and using donor/acceptor carbenes with an electron-withdrawing substituent on the aryl donor group. The optimum catalyst for these reactions is the tetraphenylphthalimido derivative Rh2(R-TPPTTL)4 and a further beneficial refinement was obtained by using N,N'-dicyclohexylcarbodiimide as an additive. Under the optimum conditions with a catalyst loading of 0.001 mol %, effective enantioselective C-H functionalization (66-97% yield, 83-97% ee) was achieved of cycloalkanes with a range of aryldiazoacetates as long as the aryldiazoacetate was not to sterically demanding. The reaction with cyclohexane using a catalyst loading of 0.0005 mol % could be recharged twice with additional aryldiazoacetate, resulting in an overall dirhodium catalyst turnover number of 580,000.

Polyaniline-supported tungsten-catalyzed oxidative deoximation reaction with high catalyst turnover number

Polyaniline-supported tungsten (W@PANI) was easily prepared by immersing polyaniline (PANI) in the aqueous solution of Na2WO4. It was found to be an efficient catalyst for oxidative deoximation reaction, the very important transformation for pharmaceutical industry. Besides the green features, the method employed very few of catalytic tungsten (0.048 mol% vs. oxime substrates), resulting in the high turnover numbers (TONs) of the catalyst (ca. 103 mol/mol) and the low metal residues in product (<0.1 ppm). The reaction is applicable for a variety of substrates, including those containing heterocycles, which are key intermediates in medicine synthesis. It has also been successfully magnified to kilogram scale production to afford the desired carbonyl products smoothly.

Selenium-doped carbon: An unexpected efficient solid acid catalyst for Beckmann rearrangement of ethyl 2-(2-aminothiazole-4-yl)-2-hydroxyiminoacetate

In this study, Se-doped carbon, which is different from reported Se-containing compounds or materials, did not act as an oxygen carrier catalyst in the reaction of ethyl 2-(2-aminothiazole-4-yl)-2-hydroxyiminoacetate with H2O2 but participated in the reaction as a solid acid catalyst, and thus, the Beckmann rearrangement occurred to produce the useful Cefixime derivative. Unique steric hindrance of carbon support restrained the side reactions through nucleophilic reaction paths. As the first example for using Se-containing materials as solid acid catalyst, this work may inspire new ideas for Se catalyst development. This novel material has advantages of easy fabrication method and high catalyst turnover number (TON = 1.1 × 104).

Design and fabrication of the Fe/Cl-doped Al foil-supported copper nano-material as the high turnover number catalyst for Suzuki coupling

Cu@Al nano-materials were easily fabricated by immersing Al foil in related metal salt solutions. Although simple Cu@Al was a poor catalyst, after Fe and Cl doping, it became highly active in Suzuki coupling reactions of aryl iodides with aryl boric acids and led to very high product yields (up to 99%) with extremely high catalyst turnover numbers (TON up to 9.1 × 103 mol/mol). Fe/Cl doping affected the morphologies of the materials, endowing more active sites as well as facilitating better contact with the reaction substrates. This is the first report on cheap metal “tea bag” style catalyst, which can be easily removed, overcoming the interruptions by the generated insoluble by-products or tars during the reaction processes.

Increasing Catalyst Efficiency in C-H Activation Catalysis.

C-H activation reactions with high catalyst turnover numbers are still very rare in the literature and 10 mol % is a common catalyst loading in this field. We offer a representative overview of efficient C-H activation catalysis to highlight this neglected aspect of catalysis development and inspire future effort towards more efficient C-H activation. Examples ranging from palladium catalysis, Cp*RhIII - and Cp*CoIII -catalysis, the C-H borylation and silylation to methane C-H activation are presented. In these reactions, up to tens of thousands of catalyst turnovers have been observed.

Highly Efficient Olefin Hydrosilylation Catalyzed by Iron Complexes with Iminobipyridine Ligand

Abstract Several iron complexes bearing an iminobipyridine ligand, (RBPIAr)FeBr2 (R = H, Me; Ar = Mes, Dipp), were synthesized. These iron complexes exhibited high catalytic activities with the help of NaBHEt3 for the hydrosilylation of 1-octene with primary, secondary, and tertiary silanes. The reaction between 1-octene and PhSiH3 produced three different species: mono- (Ph(octyl)SiH2), di- (Ph(octyl)2SiH), and trialkylated (Ph(octyl)3Si) silanes. The selective formation of Ph(octyl)SiH2 and Ph(octyl)2SiH could be achieved under different conditions. Single or double hydrosilylation reactions of secondary silanes (PhMeSiH2 and Ph2SiH2) afforded tertiary and quaternary silanes, respectively. The iron iminobipyridine complexes exhibited catalytic activity in organic solvents such as hexane, toluene, diethyl ether, and THF; an appropriate solvent choice could reverse the mono/dialkylated selectivity of the reacting silanes. In addition, a high catalyst turnover number of 12038 was achieved for the reaction between 1-octene and Ph2SiH2, and the catalyst remained active at 100 °C without any loss of activity.

Heteropoly Acid Catalysts for the Synthesis of Fragrance Compounds from Biorenewables: The Alkoxylation of Monoterpenes

AbstractThe cesium salt of tungstophosphoric heteropoly acid, Cs2.5H0.5PW12O40, is an active and environmentally friendly solid‐acid catalyst for the liquid‐phase alkoxylation of widespread monoterpenes, such as camphene, limonene, α‐pinene, and β‐pinene. These reactions provide isobornyl or α‐terpenyl ethers, useful as fragrances, in good to excellent yields. The reactions are equilibrium‐controlled and occur with high catalyst turnover numbers (TONs) up to 1500–4200 without catalyst leaching. Heteropoly acid H3PW12O40 also efficiently catalyzes the alkoxylation of these monoterpenes under homogeneous conditions with TONs up to 1500–11 300.

High-density monolayers of metal complexes: preparation and catalysis.

Catalysts are one of the key materials for realizing a sustainable society. However, we may encounter problematic cases where conventional catalyst systems cannot provide effective solutions. We thus believe that the establishment of novel methods of catalyst preparation is currently necessary. Utilization of high-density monolayers of molecular metal complexes is our strategy, and we expect that this methodology will enable facile and systematic screening of unique and efficient catalysts. This Personal Account describes our challenges to establish such an immature method in catalyst preparation as well as the related background and perspective. Preparation and catalysis by high-density monolayers of Rh complexes with N-heterocyclic carbene, structurally compact phosphine and diisocyanide ligands on gold surfaces are presented. The catalytic application of a high-density Pd-bisoxazoline complex prepared on a single-crystal silicon surface is also shown. Uniquely high catalyst turnover numbers and high chemoselectivities were observed with these catalyst systems.

Experimental strategies for increasing the catalyst turnover number in a continuous Heck coupling reaction

This work presents a continuous Heck coupling reaction combined with organic solvent nanofiltration (OSN) separation of the catalyst in situ, using polymeric membranes at high temperature (80°C) and high concentration of base (>0.9molL−1). Two reactor configurations are investigated: a continuous single stirred tank reactor/membrane separator (m-CSTR) and a plug flow reactor (PFR) followed by m-CSTR (PFR–m-CSTR). The combined PFR–m-CSTR configuration was found to be the most promising, achieving conversions above 98% and high catalyst turnover numbers (TONs) of ∼20,000. In addition, low contamination of the product stream (∼27mgPdperkg of product) makes this process configuration attractive for the pharmaceutical industry.

ChemInform Abstract: Solvent‐Free Catalytic Enantioselective C—C Bond Forming Reactions with Very High Catalyst Turnover Numbers.

AbstractA combination of donor—acceptor‐substituted rhodium carbenoid catalysts with solvent‐free reaction conditions is used to significantly enhance the turnover numbers (TON) in Rh‐catalyzed C—C bond formation reactions.

Solvent-free catalytic enantioselective C–C bond forming reactions with very high catalyst turnover numbers

Rhodium(II)-catalyzed reactions of donor/acceptor carbenoids can be achieved with catalyst loadings as low as 6 × 10−7 mol equivalents when the reactions are conducted in the absence of solvent. ReactIR studies demonstrate that donor/acceptor carbenoids are ideal for achieving high catalyst turnover numbers due to the rapid reactions of their diazo precursors and the enhanced selectivity of this class of carbenoids.

Synthesis of Bis(benzo[b]thiophenyl)methanes by Gold-Catalyzed Double Carbothiolation

Gold-catalyzed double cyclization of bis(2-alkynylphenylthio)acetals (1) produced bis(benzo[b]thiophen-3-yl)methanes (2) in good to excellent yields with high catalyst turnover number. For example, the reaction of (phenylmethylene)bis((2-(phenylethynyl)phenyl)sulfane) (1b) in the presence of 1 mol% of AuCl in toluene at 25 °C for 1 hour gave 3,3'-(phenylmethylene)bis(2-phenylbenzo[b]thiophene) (2b) in 97% isolated yield. The present reaction proceeded through two successive intramolecular carbon-sulfur bond addition reactions, or the so-called carbothiolation.

Mild oxidation of alkanes and toluene by tert-butylhydroperoxide catalyzed by an homogeneous and immobilized Mn(salen) complex

Mild oxidations with t-BuOOH of linear ( n-pentane, n-hexane, n-heptane and n-octane) and cyclic (1,2-dimethylcyclohexane) alkanes to the corresponding secondary alcohols and ketones, as well as of toluene to benzaldehyde, catalyzed by homogeneous and immobilized Mn(salen) (on a polydimethylsiloxane (PDMS) based membrane, Mn(salen)-PM) systems [Mn(salen) = {(R,R)-N,N-bis(3,5-di-tert-butylsalicylidenato)-1,2-cyclohexanediamine(2-)}chloro manganese(III)], have been studied. These oxidations are markedly promoted by the addition of various acids, namely nitric, pyrazine-2-carboxylic, acetic, benzoic and oxalic acids. Regio- and stereoselectivity parameters are dependent on the immobilization of the catalyst on the support and on the type of acid promoter, and correlate with the sorption values of alkane substrates on the PDMS membrane. The oxidation was shown to proceed with participation of t-BuO and alkylperoxyl (ROO ) radicals. The toluene oxidation systems feature a high selectivity towards the formation of benzaldehyde (mainly in the case of the supported catalyst) and remarkably high catalyst turnover numbers (up to 1.7 × 10 3).

Immobilization of manganese tetraphenylporphyrin on boehmite and its catalysis for aerobic oxidation of cyclohexane

The search for a reusable material as an efficient and economic catalyst for hydrocarbon oxidation is an important field of research. In this study, we investigated the highly effective combination of support and metalloporphyrin provided by manganese tetraphenylporphyrin adsorbed onto a precursor of freshly precipitated aluminium hydroxide, followed by conversion to a manganese porphyrin/boehmite compound. This was characterized using FT-IR, UV–vis spectroscopic techniques, N 2 BET analysis, X-ray powder diffraction, transmission electron micrograph technique and thermal analysis. Aerobic oxidation of cyclohexane has been investigated using the compound as a catalyst under mild conditions and in the absence of any solvent and reductant. The results indicate that it has higher catalytic activity than manganese tetraphenylporphyrin and the corresponding chitosan-supported catalyst. In particular, the catalyst exhibits good atom economy for cyclohexane oxidation, high catalyst turnover number, and good ketone and alcohol selectivity. In particular, it has excellent reusability, with no change in original catalytic activity; a quantity of catalyst containing only 1 mg of Mn TPP can be reused 8 times under the promotion of beomite (BM). In addition, we examined the reaction conditions for the use of such nanoparticle-supported manganese tetraphenylporphyrin as a mild and environmental friendly oxidation catalyst.

New Catalysts for Suzuki−Miyaura Coupling Reactions of Heteroatom-Substituted Heteroaryl Chlorides

The new air-stable PdCl2[PR2(Ph-R')]2 complexes, readily prepared from commercial reagents, exhibit unique efficiency as catalysts for the Suzuki-Miyaura coupling reactions of a variety of heteroatom-substituted heteroaryl chlorides with a diverse range of aryl/heteroaryl boronic acids. The coupling reactions catalyzed by the new complexes exhibit high product yields (88-99%) and high catalyst turnover numbers (up to 10,000 TON).

Metalloporphyrin Catalysts for Organic Synthesis

AbstractFor Abstract see ChemInform Abstract in Full Text.

Direct Catalytic Asymmetric Mannich‐Type Reaction of Hydroxyketone Using a Et2Zn/Linked‐BINOL Complex: Synthesis of Either anti‐ or syn‐β‐Amino Alcohols.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Metalloporphyrin catalysts for organic synthesis

Novel chiral systems for the catalytic asymmetric oxidation and cyclopropanation of olefins based on metalloporphyrins containing iron, ruthenium and manganese, have been recently introduced. High catalyst turnover numbers and sometimes high enantiomeric excess were observed. New catalytic reactions with metalloporphyrins have recently been reported; these are the olefination of aldehydes and cyclotrimerization of terminal alkynes. Dendrimers and polymers containing metalloporphyrins, have also been found to be efficient catalysts for oxidation and carbene transfer.

Direct catalytic asymmetric Mannich-type reaction of hydroxyketone using a Et2Zn/linked-BINOL complex: synthesis of either anti- or syn-beta-amino alcohols.

Full details of a direct catalytic asymmetric Mannich-type reaction of a hydroxyketone using a Et2Zn/(S,S)-linked-BINOL complex are described. By choosing the proper protective groups on imine nitrogen, either anti- or syn-beta-amino alcohol was obtained in good diastereomeric ratio, yield, and excellent enantiomeric excess using the same zinc catalysis. N-Diphenylphosphinoyl (Dpp) imine 3 gave anti-beta-amino alcohols in anti/syn = up to >98/2, up to >99% yield, and up to >99.5% ee, while Boc-imine 4 gave syn-beta-amino alcohols in anti/syn = up to 5/95, up to >99% yield, and up to >99.5% ee. The high catalyst turnover number (TON) is also noteworthy. Catalyst loading was successfully reduced to 0.02 mol % (TON = up to 4920) for the anti-selective reaction and 0.05 mol % (TON = up to 1760) for the syn-selective reaction. The Et2Zn/(S,S)-linked-BINOL complex exhibited far better TON than in previous reports of catalytic asymmetric Mannich-type reactions. Mechanistic studies to clarify the reason for the high catalyst efficiency as well as transformations of Mannich adducts are also described.