Showing papers in "Advanced Synthesis & Catalysis in 2007"
TL;DR: Different methods for the immobilization of enzymes are critically reviewed, with emphasis on relatively recent developments, such as the use of novel supports, e.g., mesoporous silicas, hydrogels, and smart polymers, novel entrapment methods and cross-linked enzyme aggregates (CLEAs).
Abstract: Immobilization is often the key to optimizing the operational performance of an enzyme in industrial processes, particularly for use in non-aqueous media. Different methods for the immobilization of enzymes are critically reviewed. The methods are divided into three main categories, viz. (i) binding to a prefabricated support (carrier), (ii) entrapment in organic or inorganic polymer matrices, and (iii) cross-linking of enzyme molecules. Emphasis is placed on relatively recent developments, such as the use of novel supports, e.g., mesoporous silicas, hydrogels, and smart polymers, novel entrapment methods and cross-linked enzyme aggregates (CLEAs).
1,857 citations
TL;DR: In this paper, the metal catalyst returned the hydrogen to the transformed carbonyl compound, leading to an overall process in which alcohols can be converted into amines, compounds containing CC bonds and β-functionalised alcohols.
Abstract: Alcohols can be temporarily converted into carbonyl compounds by the metal-catalysed removal of hydrogen. The carbonyl compounds are reactive in a wider range of transformations than the precursor alcohols and can react in situ to give imines, alkenes, and α-functionalised carbonyl compounds. The metal catalyst, which had borrowed the hydrogen, then returns it to the transformed carbonyl compound, leading to an overall process in which alcohols can be converted into amines, compounds containing CC bonds and β-functionalised alcohols.
929 citations
TL;DR: In this paper, a method for cellulose hydrolysis catalyzed by mineral acids in the ionic liquid 1-butyl-3-methylimidazolium chloride ([C(4)mim]Cl) has been developed that facilitates the hydro-sis of cellulose with dramatically accelerated reaction rates at 100 degrees C under atmospheric pressure and without pretreatment.
Abstract: A novel method for cellulose hydrolysis catalyzed by mineral acids in the ionic liquid 1-butyl-3-methylimidazolium chloride ([C(4)mim]Cl) has been developed that facilitates the hydrolysis of cellulose with dramatically accelerated reaction rates at 100 degrees C under atmospheric pressure and without pretreatment.
351 citations
TL;DR: Ball milling has been applied in numerous solvent-free carbon-carbon bond formations as mentioned in this paper, and in many cases, these transformations proved superior to the analogous reactions performed in solution, and the reaction scope involves purely organic addition and coupling reactions, transformations involving metals (in stoichiometric and catalytic amounts) as well as asymmetric organocatalyses.
Abstract: Ball milling has been applied in numerous solvent-free carbon-carbon bond formations. In many cases, these transformations proved superior to the analogous reactions performed in solution. The reaction scope involves purely organic addition and coupling reactions, transformations involving metals (in stoichiometric and catalytic amounts) as well as asymmetric organocatalyses.
317 citations
TL;DR: In this paper, a one-pot diaryliodonium triflates were synthesized from both electron-deficient and electron-rich arenes and aryl iodides with mCPBA and triflic acid.
Abstract: Symmetrical and unsymmetrical diaryliodonium triflates have been synthesized from both electron-deficient and electron-rich arenes and aryl iodides with mCPBA and triflic acid. A thorough investigation of the optimization, scope and limitations has resulted in an improved one-pot protocol that is fast, high-yielding, and operationally simple. The reaction has been extended to the direct synthesis of symmetrical iodonium salts from iodine and arenes, conveniently circumventing the need for aryl iodides.
244 citations
TL;DR: The last several years have produced some key advances in the area of alkene and alkyne metathesis by high oxidation state alkylidene and Alkylidyne complexes along with new applications in organic and polymer chemistry as mentioned in this paper.
Abstract: The last several years have produced some key advances in the area of alkene and alkyne metathesis by high oxidation state alkylidene and alkylidyne complexes along with new applications in organic and polymer chemistry. In this review we cover some of these developments and applications. The first part of this review concerns developments in catalyst synthesis and new catalysts. The second part concerns notable applications in organic and polymer chemistry. We discuss only high oxidation state alkylidene and alkylidyne chemistry of relevance to alkene or alkyne metathesis reactions and favor studies in the homogeneous phase.
241 citations
TL;DR: Alkyne metathesis has been studied extensively in the literature as mentioned in this paper, with an emphasis on comparing strengths and weaknesses of different types of catalysts, including two heterogeneous ones.
Abstract: There has been rapid progress and growing interest in alkyne metathesis within the past decade. The availability of highly active catalysts as well as their applications in both organic synthesis and polymer chemistry has served to motivate the advancement of this field. In this article, the development of several different metathesis catalysts, including two heterogeneous ones, are reviewed with an emphasis on comparing strengths and weaknesses. In Section 4, the applications of alkyne metathesis to synthesis of natural products, conjugated polymers as well as shape-persistent macrocycles are discussed. In the last section, a comparison of alkyne metathesis to the well established alkene metathesis is given. Developing an alkyne metathesis catalyst with both high reactivity and robustness to air and moisture remains an unsolved problem of this important and useful reaction.
237 citations
TL;DR: A small amount of iron(III) chloride (2 mol) catalyzes benzylic oxidations with tert-butyl hydroperoxide (TBHP) as oxidant in pyridine as discussed by the authors.
Abstract: A small amount of iron(III) chloride (2 mol %) catalyzes benzylic oxidations with tert-butyl hydroperoxide (TBHP) as oxidant in pyridine. The corresponding carbonyl compounds are obtained in high yields.
210 citations
TL;DR: This review is focused on advances over the last several years in the application of aldolases to organic synthesis, including directed evolution, discovery of new classes of a Aldolases in nature and the laboratory, and substrate and reaction engineering.
Abstract: This review is focused on advances over the last several years in the application of aldolases to organic synthesis. Several new technologies have been implemented to increase the scope and practicality of aldolases as tools for the synthetic chemist. These include directed evolution, discovery of new classes of aldolases in nature and the laboratory, and substrate and reaction engineering.
203 citations
TL;DR: Palladium(II) acetate-catalyzed Heck-type reactions have been performed from the arene 1 and alkenes (n-butyl acrylate, styrene) in acetic acid at room temperature, in the presence of a catalytic amount of benzoquinone or hydroquinone as discussed by the authors.
Abstract: Palladium(II) acetate-catalyzed Heck-type reactions have been performed from the arene 1 and alkenes (n-butyl acrylate, styrene) in acetic acid at room temperature, in the presence of a catalytic amount of benzoquinone or hydroquinone. The reactions have been made catalytic in benzoquinone [which is used to continuously oxidize the Pd(0) into to the active Pd(II) species able to activate the Ar-H bond] by the electrochemical oxidation of hydroquinone, formed in the reaction, back to benzoquinone.
197 citations
TL;DR: In this article, a review of the strategies that have been implemented to develop well-defined olefin metathesis catalysts immobilized on solid supports is presented, and the advantages, limitations and possible developments of these systems are discussed.
Abstract: This review is mainly devoted to a description of the strategies that have been implemented to develop well-defined olefin metathesis catalysts immobilized on solid supports. Two main approaches have been investigated involving supported homogenous catalysts or heterogeneous catalysts prepared by surface organometallic chemistry. Advantages, limitations and possible developments of these systems are discussed.
TL;DR: The introduction of siloxy groups at the hydroxy function of natural threonine resulted in efficient hydrophobic organocatalysts, which could efficiently catalyze the direct aldol reactions of both cyclic and acyclic ketones with aromatic aldehydes in water with excellent enantiomeric excess.
Abstract: The introduction of siloxy groups at the hydroxy function of natural threonine resulted in efficient hydrophobic organocatalysts, which could efficiently catalyze the direct aldol reactions of both cyclic and acyclic ketones with aromatic aldehydes in water with excellent enantiomeric excess.
TL;DR: In this article, a reaction pathway based on DFT calculations was proposed that accurately reflects the experimental results, such as the observed reactivity order of the substrates, for catalytic protodecarboxylation even of deactivated aromatic carboxylic acids.
Abstract: A catalyst generated from copper(I) oxide and 4,7-diphenyl-1,10-phenanthroline for the first time allows the catalytic protodecarboxylation even of deactivated aromatic carboxylic acids, giving rise to the corresponding arenes. Based on DFT calculations, a reaction pathway is proposed that accurately reflects the experimental results, such as the observed reactivity order of the substrates.
TL;DR: In this paper, a clean, facile, and ecologically friendly method for the production of biodiesel has been developed, where a solid acid, namely the heteropolyacid (HPA) Cs 2.5 H 0.5 PW 12 O 40, has been used as a heterogeneous catalyst, which shows almost the same activity under optimized reaction conditions as compared to a conventional homogeneous catalyst such as sodium hydroxide or sulfuric acid.
Abstract: A clean, facile, and ecologically friendly method for the production of biodiesel has been developed. A solid acid, namely the heteropolyacid (HPA) Cs 2.5 H 0.5 PW 12 O 40 , has been used as a heterogeneous catalyst for the production of biodiesel from Eruca sativa Gars. oils (ESG oil) with methanol at a certain temperature. A study for optimizing the reaction conditions such as the reaction time, temperature, the oil to methanol ratio, the amount of catalyst, and the usage times of the catalyst, has been performed. The Cs 2.5 H 0.5 PW 12 O 40 heterogeneous acid catalyst shows almost the same activity under the optimized reaction conditions as compared to a conventional homogeneous catalyst such as sodium hydroxide or sulfuric acid, and can easily be separated from the products and can be used for several more runs. The most important features of this catalyst are that the catalytic activity is not effected by the content of free fatty acids and content of water in the vegetable oil and that the esterification can occur at a lower temperature (room temperature) and be finished within a shorter time. The results illustrate that the Cs 2.5 H 0.5 PW 12 O 40 is an excellent, water-tolerant and environmentally benign solid acid catalyst for the production of biodiesel. The fuel properties of ESG biodiesel were found to be in agreement with the ASTM standard.
TL;DR: In this paper, the photocatalytic oxidation of 4-methoxybenzyl alcohol to p-anisaldehyde (PAA) was performed in water with organic-free suspensions of home-prepared and commercial titanium dioxide (TiO 2 ) catalysts.
Abstract: The photocatalytic oxidation of 4-methoxybenzyl alcohol to p-anisaldehyde (PAA) was performed in water with organic-free suspensions of home-prepared and commercial titanium dioxide (TiO 2 ) catalysts. The nanostructured TiO 2 samples were synthesised by boiling aqueous solutions of titanium tetrachloride (TiCl 4 ), under mild conditions, for different times. The crystallinity increased with the boiling time. The 4-methoxybenzyl alcohol oxidation rate followed the same pattern but the highest yield (41.5 % mol) to PAA was found for the least crystalline sample, that showed a quantum efficiency of 0.116%. A comparison with two commercial TiO 2 samples showed that all the home-prepared catalysts exhibited a PAA yield higher than that of commercial ones. The only by-products present were traces of 4-methoxybenzoic acid and aliphatic products, carbon dioxide being the other main oxidation product.
TL;DR: A highly chemo- and enantioselective organocatalytic cyclopropanation of aldehydes with bromomalonate and 2-bromoacetoacetate esters is presented and gives access to 2-formylcyclopropanes in high yields and up to 99?% ee.
Abstract: A highly chemo- and enantioselective organocatalytic cyclopropanation of ?,?-unsaturated aldehydes with bromomalonate and 2-bromoacetoacetate esters is presented. The reaction is catalyzed by chiral amines and gives access to 2-formylcyclopropanes in high yields and up to 99?% ee.
TL;DR: The Nozaki-Hiyama-Kishi (NHK) reaction was further developed with the discovery of the catalytic variant in the 1990s and has been applied in a variety of processes, including allylation, crotylation, methallylation, allenylation and propargylation of aldehydes as mentioned in this paper.
Abstract: This review collates the literature to date on the development of the Nozaki–Hiyama–Kishi (NHK) reaction into an important chromium-mediated, carbon-carbon bond forming process. The initial research employed stoichiometric quantities of chromium and this was exploited in the key steps of a range of total syntheses. Thereafter, the NHK reaction was further developed with the discovery of the catalytic variant. The focus of recent investigations is on the application of this reaction in asymmetric synthesis. The asymmetric NHK typically employed a range of salen- and oxazoline-derived chiral ligands and tethered bis(8-quinolinato)-chromium complexes. To date, good to high enantioselectivities have been obtained in a variety of NHK-type processes, including allylation, crotylation, methallylation, allenylation, propargylation and vinylation of a range of aldehydes, with limited examples employing ketones as substrates. Selected examples of the asymmetric NHK in total synthesis will also be described.
TL;DR: In this article, the authors used ultra small palladium(0) particles on magnetic nanoparticle supports, in which immobilized auxiliaries are used as stabilizing ligands.
Abstract: Application of transition metal nanoparticles as catalysts for organic transformations has been attracting wide interest as nanoparticle-based catalytic systems might exhibit superior catalytic activities than the corresponding bulk materials. However, the main difficulty is that such small particles are almost impossible to separate by conventional means, which can lead to the blocking of filters and valves by the nanoparticle catalyst. We have conveniently prepared ultra small palladium(0) particles (less than 1 nm) on magnetic nanoparticle supports, in which immobilized auxiliaries are used as stabilizing ligands. These catalysts were active for Suzuki cross-coupling and Heck reactions giving isolated yields of 83 % and 56 %, respectively. In addition, the catalyst can be easily separated using a magnet and reused several times with sustained activity.
TL;DR: Versatile, simple and inexpensive ligand-free, copper-catalyzed N-arylations of sulfoximines and nitrogen-containing heterocycles have been developed affording N-arylated products in high yields.
Abstract: Versatile, simple and inexpensive ligand-free, copper-catalyzed N-arylations of sulfoximines and nitrogen-containing heterocycles have been developed affording N-arylated products in high yields.
TL;DR: A review of successful metathesis reactions performed with amine-containing compounds can be found in this paper, where a special emphasis is placed on the different parameters that may influence the outcome of the reaction such as steric effects, amine basicity, and the nature of the catalyst.
Abstract: Olefin metathesis is one of the most powerful synthetic tool to access amine-containing heterocycles and alkaloids. A major drawback associated with the use of amines concerns their ability to coordinate to metal-alkylidene complexes and to interfere unproductively with catalytic activity. Based on literature precedents, it has been established as a "dogma" that efficient metathesis reactions are suppressed in the presence of basic amines and that such substrates must invariably be deactivated by conversion of the amines to the corresponding carbamates or ammonium salts. However, an increasing number of examples of amine-containing compounds that are good substrates for metathesis is being reported in the literature. How can this "non-classical" reactivity be rationalized and exploited? The purpose of this review is to provide an overview of successful metathesis reactions performed with amine-containing compounds in order to allow some guidelines to be formulated. A special emphasis is placed on the different parameters that may influence the outcome of the reaction such as steric effects, amine basicity, and the nature of the catalyst.
TL;DR: In this article, a method for supporting platinum nanoparticles on magnetite nanoparticles is described, which requires modification of the surface of the magnetic nanoparticles with ionic liquid groups.
Abstract: A method for supporting platinum nanoparticles on magnetite nanoparticles is described. The method requires modification of the surface of the magnetic nanoparticles with ionic liquid groups. Before modification, the magnetic nanoparticles are not stable and easily aggregate and, after modification, the magnetite nanoparticles become highly stable and soluble in polar or non-polar organic solvents depending on the alkyl group of the linked ionic liquids. The supporting of platinum nanoparticles on the modified magnetic nanoparticles was achieved by adsorbing platinum salts (K2PtCl4) on the surface of the magnetite nanoparticles via ion exchange with the linked ionic liquid groups and then reducing them by hydrazine. The supported platinum nanoparticles were applied in the catalytic hydrogenation of alkynes in which cis-alkenes were selectively produced, and in the hydrogenation of α,β-unsaturated aldehydes where the allyl alcohols were obtained as the exclusive products. The new catalyst can be easily separated from the reaction mixtures by applying an external magnetic field and recycled.
TL;DR: In this article, solid-state structure determination of the DABCO-copper complex has been carried out and it has been shown that the copper is in the +II oxidation state with trigonal bipyramidal geometry and exists in a linear polymeric structure due to strong hydrogen bonding.
Abstract: A DABCO-copper(I) chloride complex (5 mol %) together with TEMPO (5 mol %) in nitromethane as solvent has been used as an efficient catalytic system for the selective oxidation of benzylic and allylic alcohols into the corresponding carbonyl compounds at room temperature where molecular oxygen acts as an ultimate, stoichiometric oxidant and water is the only by-product. The solid-state structure determination of the DABCO-copper complex shows that the copper is in the +II oxidation state with trigonal bipyramidal geometry and exists in a linear polymeric structure due to strong hydrogen bonding.
TL;DR: A new protocol for the coupling of aryl iodides with thiophenols and alkanethiols catalyzed by copper nanoparticles under ligand-free condition has been developed and a plausible radical mechanism has been suggested.
Abstract: A new protocol for the coupling of aryl iodides with thiophenols and alkanethiols catalyzed by copper nanoparticles under ligand-free condition has been developed A variety of functionalized aryl sulfides are prepared in excellent yields under microwave irradiation for 5-7 min A plausible radical mechanism has been suggested
TL;DR: In this article, a simple catalytic system was proposed for the oxidation of primary and secondary benzylic alcohols to their corresponding carbonyl compounds in situ generated copper(II)-diimine complexes combined with TEMPO.
Abstract: In situ generated copper(II)-diimine complexes combined with TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl radical) were studied in the oxidation of benzylic alcohols, the focus being on enviromentally benign reaction conditions. In this respect, reactions were studied in aqueous alkaline solutions and dioxygen was used as an end oxidant. This simple catalytic system turned out to be highly efficient and selective in the oxidation of primary and secondary benzylic alcohols to their corresponding carbonyl compounds. Under optimised reaction conditions [5 mol % of TEMPO, 3 mol % of copper(II) diimine, pH 12.6–13.5, 80 °C, 10 bar O2] benzyl alcohol was quantitatively and selectively oxidised to benzaldehyde. According to ESI-MS studies, coordination of TEMPO, as well as deprotonated benzyl alcohol to the parent copper-diimine complex in aqueous solutions is feasible. Supported by these observations a plausible reaction mechanism is proposed for the oxidation reaction.
TL;DR: The wide specificity of enoate reductases for a range of α,β-unsaturated carbonyl compounds as well as coupling to glucose dehydrogenase for recycling of NAD(P)(H); however, the stability limitations the authors found need to be overcome to envision large-scale use of ERs in synthesis.
Abstract: Enoate reductases (ERs) selectively reduce carbon-carbon double bonds in α,β-unsaturated carbonyl compounds and thus can be employed to prepare enantiomerically pure aldehydes, ketones, and esters. Most known ERs, most notably Old Yellow Enzyme (OYE), are biochemically very well characterized. Some ERs have only been used in whole-cell systems, with endogenous ketoreductases often interfering with the ER activity. Not many ERs are biocatalytically characterized as to specificity and stability. Here, we cloned the genes and expressed three non-related ERs, two of them novel, in E. coli: XenA from Pseudomonas putida, KYE1 from Kluyveromyces lactis, and Yers-ER from Yersinia bercovieri. All three proteins showed broad ER specificity and broad temperature and pH optima but different specificity patterns. All three proteins prefer NADPH as cofactor over NADH and are stable up to 40 °C. By coupling Yers-ER with glucose dehydrogenase (GDH) to recycle NADP(H), conversion of >99 % within one hour was obtained for the reduction of 2-cyclohexenone. Upon lowering the loadings of Yers-ER and GDH, we discovered rapid deactivation of either enzyme, especially of the thermostable GDH. We found that the presence of enone substrate, rather than oxygen or elevated temperature, is responsible for deactivation. In summary, we successfully demonstrate the wide specificity of enoate reductases for a range of α,β-unsaturated carbonyl compounds as well as coupling to glucose dehydrogenase for recycling of NAD(P)(H); however, the stability limitations we found need to be overcome to envision large-scale use of ERs in synthesis.
TL;DR: In this article, the molybdenum and ruthenium car-benium complexes 1a and 1b were discovered by Schrock and Grubbs in 1990 and 1992, synthetic or- ganic chemistry has progressed with the use of these complexes as the catalyst for olefin metathesis.
Abstract: Since the molybdenum and ruthenium car- bene complexes 1a and 1b were discovered by Schrock and Grubbs in 1990 and 1992, synthetic or- ganic chemistry has progressed with the use of these complexes as the catalyst for olefin metathesis. It was found that the ruthenium carbene complex was also effective for enyne metathesis, which occurs between an alkene and an alkyne. Intramolecular enyne meta- thesis gives various cyclic compounds having a 1,3- diene moiety and cross-metathesis of an alkene with an alkyne is a useful method for the synthesis of a compound having a diene moiety. Furthermore, dien- yne metathesis, ROM of cycloalkenynes or tandem reactions of enyne metathesis have been developed. Using these various enyne metatheses, complicated natural products have been synthesized via novel routes. The syntheses of natural products and related compounds using enyne metathesis are described.
TL;DR: In this article, a 1,2-aza-Friedel-Crafts reaction of N -tert -butyldimethylsilylindole with N-tert-butoxycarbonyl aromatic imines is demonstrated using a BINOL-derived monophosphoric acid catalyst.
Abstract: A highly enantioselective 1,2-aza-Friedel-Crafts reaction of N -tert -butyldimethylsilylindole with N-tert-butoxycarbonyl aromatic imines is demonstrated using a BINOL-derived monophosphoric acid catalyst. The present approach provides efficient access to 3-indolylmethaneamines with aryl substituents in excellent enantioselectivities (up to 98% ee). An inversion in the sense of enantioselection was found between monophosphoric acid catalysts bearing different substituents introduced at the 3,3'-position of binaphthyl backbone. We also calculated the three-dimensional structure of the monophosphoric acid catalysts to speculate on the inversion of the stereochemical outcome.
TL;DR: Racemic Baylis-Hillman carbonates can be converted in the presence of a cata- lytic amount of a modified Cinchona alkaloid.
Abstract: Racemic Baylis-Hillman carbonates can be convertedin d ensely functionalizedprod ucts by reaction with cyano esters in the presence of a cata- lytic amount of a modified Cinchona alkaloidin high enantioselectivities andfair d iastereoselectivi- ties A rational for the observedstereoselectivity is presented
TL;DR: It is demonstrated that chiral diols, such as TADDOL, are effective enantioselective catalysts for the hydrogen-bond activation of aldimines.
Abstract: An efficient, metal-free Bronsted acid-catalyzed, enantioselective hydrocyanation of ketoimines has been developed. This BINOL phosphate-catalyzed Strecker reaction provides the corresponding amino nitriles, precursors of quaternary amino acids, in good isolated yields and enantioselectivities. Additionally, we demonstrate that chiral diols, such as TADDOL, are effective enantioselective catalysts for the hydrogen-bond activation of aldimines.
TL;DR: The benzylation of 4-hydroxycoumarin gives the pharmaceutically interesting 4-Hydroxy-3-(1-phenylethyl)-2H-chromen-2-ones, which is prepared in one step from commercially available substrates in 94 % yield.
Abstract: Various CH-acidic 1,3-dicarbonyl compounds and methyl 3-acetamidobut-2-enoate react with benzylic alcohols to give the corresponding 2-benzylated products in good to excellent yield. Typically, reactions proceed under mild conditions (50–80 °C; air) in the presence of catalytic amounts of inexpensive iron chloride hexahydrate. The benzylation of 4-hydroxycoumarin gives the pharmaceutically interesting 4-hydroxy-3-(1-phenylethyl)-2H-chromen-2-ones. As an example the anticoagulant Phenprocoumon is prepared in one step from commercially available substrates in 94 % yield.