Showing papers on "Lewis acids and bases published in 1999"

Diastereoselection in lewis-Acid-mediated aldol additions.

Abstract: The aldol addition is one of the most important methods for stereoselective construction of carboncarbon bonds. New and powerful variants of these classical reactions have been developed in recent years.1 Two classes were mainly used for asymmetric induction in these reactions: the use of asymmetric modified enolates or electrophiles2 and the use of chiral Lewis acids.3 The chiral enolate or electrophile approach is much more general and gives high stereoselectivities due to the highly ordered nature of transition structures (“closed” transition models). The chiral center has to be removed after the completed aldol addition. To avoid this additional reaction step, a stategy is employed whereby achiral enolates can be reacted with achiral carbonyl compounds in the presence of additional chiral auxiliaries. This method requires the careful use of a chiral auxiliary.4 Unfortunately, however, stoichiometric amounts of the chiral information are necessary. Up to now and apart from enzymatic transformations, the so-called Mukaiyama reaction has opened an enantioselective and catalytic approach using chiral Lewis acids. This review covers the evolution of stereoselective Lewis-acid-mediated aldol-type addition up to the recent development of chiral Lewis acids. Mukaiyama et al. found that silyl enol ether reacts with carbonyl compounds in the presence of Lewis acids to give aldol products (for initial studies, see ref 5). The main advantages in the Mukaiyama approach are the chemoselectivity of the reaction and the possibility of stereoselective execution. Since the mid-1970s, the Mukaiyama reaction has become a useful method for chemoand regioselective carboncarbon bond formation.6 About 10 years later, investigations into stereochemical aspects of these reactions were initiated,7 and at the end of the 1980s, the development of chiral Lewis acids and thus the development of catalytic, enantioselective versions of the Mukaiyama reaction started.8 The reaction mechanism has not been explained yet. The most important fact is that Lewis acid enolates are not involved in this reaction.7 No transmetalation occurs. In this reaction, the Lewis acids coordinate with the carbonyl function leading to its activation.9 Two works published by Carreira and Shibasaki suggest the involvement of chiral metal enolates during the aldol addition (for copper enolates, see ref 10; for palladium enolates, see ref 11). Moreover, there is a marked stereochemical differRainer Mahrwald obtained his M.S. degree in chemistry from the MartinLuther-University, Halle, in 1973. In 1975 he joined the Institute “Manfred von Ardenne”, Dresden, where he obtained his Ph.D. in 1979 in the field of the synthesis of nuclesides. In 1980 he joined the Academy of Sciences in Berlin. There he worked in the field of total synthesis of prostaglandins. He pursued his formation as a postdoctoral fellow at Philipps-University, Marburg, in the group of Prof. M. T. Reetz (1991). Since 1994 he has been a lecturer at the Humbold-University. His main research interests have been associated with the development of catalytic stereoselective C−C bond formation. 1095 Chem. Rev. 1999, 99, 1095−1120

C2-Symmetric Copper(II) Complexes as Chiral Lewis Acids. Scope and Mechanism of Catalytic Enantioselective Aldol Additions of Enolsilanes to (Benzyloxy)acetaldehyde

Abstract: C2-Symmetric bis(oxazolinyl)pyridine (pybox)−Cu(II) complexes have been shown to catalyze enantioselective Mukaiyama aldol reactions between (benzyloxy)acetaldehyde and a variety of silylketene acetals. The aldol products are generated in high yields and in 92−99% enantiomeric excess using as little as 0.5 mol % of chiral catalyst [Cu((S,S)-Ph-pybox)](SbF6)2. With substituted silylketene acetals, syn reaction diastereoselection ranging from 95:5 to 97:3 and enantioselectivities ≥95% are observed. Investigation into the reaction mechanism utilizing doubly labeled silylketene acetals indicates that the silyl-transfer step is intermolecular. Further mechanistic studies revealed a significant positive nonlinear effect, proposed to arise from the selective formation of the [Cu((S,S)-Ph-pybox)((R,R)-Ph-pybox)](SbF6)2 2:1 ligand:metal complex. A stereochemical model is presented in which chelation of (benzyloxy)acetaldehyde to the metal center to form a square pyramidal copper intermediate accounts for the obser...

Lewis Acid Mediated Hydrosilylation on Porous Silicon Surfaces

Abstract: Lewis acid mediated hydrosilylation of alkynes and alkenes on non-oxidized hydride-terminated porous silicon derivatizes the surface with alkenyl and alkyl functionalities, respectively. A very broad range of chemical groups may be incorporated, allowing for tailoring of the interfacial characteristics of the material. The reaction is shown to protect and stabilize porous silicon surfaces from atmospheric or direct chemical attack without compromising its valuable material properties such as high porosity, surface area and visible room-temperature photoluminescence. The reaction is shown to act on alkenes and alkynes of all possible regiochemistries (terminal and internal alkynes; mono-, cis- and trans-, di-, tri-, and tetrasubstituted alkenes). FTIR as well as liquid- and solid-state NMR spectroscopies show anti-Markovnikov addition and cis stereochemistry in the case of hydrosilylated terminal alkynes. Material hydrosilylated with long-chain hydrophobic alkynes and alkenes shows a substantially slower surface oxidation and hydrolysis rate in air as monitored by long-term FTIR monitoring and chemography. BJH and BET measurements reveal that the surface area and average pore size of the material are reduced only slightly after hydrosilylation, indicating that the porous silicon skeleton remains intact. Elemental analysis and SIMS depth profiling show a consistent level of carbon incorporation throughout the porous silicon which demonstrates that the reaction occurs uniformly throughout the depth of the film. The effects of functionalization on photoluminescence were investigated and are shown to depend on the organic substituents.

Chiral bis(oxazoline)copper(ii) complexes as lewis acid catalysts for the enantioselective diels-alder reaction

Abstract: Bis(oxazoline)copper(II) complexes are highly enantioselective catalysts in Diels−Alder reactions involving bidentate dienophiles. Cationic [Cu((S,S)-t-Bu-box)]X2 complexes with different counterions have been used as catalysts, revealing a profound influence of the counterion on the rate and stereoselectivity of the catalyst. A square-planar catalyst−substrate complex is proposed to account for the high diastereo- and enantioselectivities observed. Three bis(oxazoline)−Cu(II) X-ray structures have been obtained that support this model. Double-stereodifferentiating experiments, performed employing chiral dienophiles, afforded results that are fully consistent with the proposed square-planar transition-state assemblage. In addition to imide-based substrates, α,β-unsaturated thiazolidine-2-thiones have been introduced as a new class of dienophiles with enhanced reactivity. Kinetics experiments were performed to quantify the role that product inhibition plays in the course of the reaction. Rate and equilibri...

Facile, Novel Methodology for the Synthesis of Spiro[pyrrolidin‐3,3′‐oxindoles]: Catalyzed Ring Expansion Reactions of Cyclopropanes by Aldimines

Abstract: The role of the bifunctional catalyst is decisive: The magnesium ion as Lewis acid and its nucleophilic iodide counterion contribute in synergy to the successful ring expansion of the cyclopropane 1 by aldimine 2 [Eq. (1)]. This reaction offers a novel route to spiro[pyrrolidin-3,3'-oxindoles] 3.

C2-Symmetric Copper(II) Complexes as Chiral Lewis Acids. Scope and Mechanism of the Catalytic Enantioselective Aldol Additions of Enolsilanes to Pyruvate Esters

Abstract: The C2-symmetric (S,S)-tert-butyl-bis(oxazolinyl)Cu(OTf)2 complex (1a) has been shown to catalyze the enantioselective aldol reaction between α-keto esters and silylketene acetals or enolsilanes with enantioselectivities ranging from 93 to 99%. With substituted silylketene acetals, syn reaction diastereoselection ranging from 90:10 to 98:2 and enantioselectivities ranging from 93 to 98% are observed. High levels of carbonyl regioselectivity (98:2), diastereoselectivity (93:7), and enantioselectivity (97% ee) are also observed in the aldol addition to 2,3-pentanedione. In all instances, the aldol adducts are generated in high yield and in excellent enantiomeric excess using as little as 1 mol % of the chiral complex 1a. Mechanistic insight into the pyruvate aldol reaction has also been gained. Silyl crossover experiments demonstrate that the silyl-transfer step is intermolecular. Based upon these results, TMSOTf has been identified as an addend to accelerate these reactions. Furthermore, solvent was shown ...

Immobilization of a Soluble Metal Complex in an Organic Network. Remarkable Catalytic Performance of a Porous Dialkoxyzirconium Polyphenoxide as a Functional Organic Zeolite Analogue

Abstract: Treatment of anthracenebisresorcinol 1 (a tetraphenol) with Zr(OtBu)4 in THF results in polycondensation to give an O−Zr−O network and affords a poly(dialkoxyzirconium phenoxide), 14-·2[Zr(OtBu)2] (Zr host), in quantitative yield as an insoluble, amorphous, microporous powder with a particle size of ∼0.7 μm, a pore size of ∼0.7 nm, and a specific surface area of ∼200 m2/g. The powder exhibits reversible Langmuir-type adsorption/desorption of N2 at 77 K and hexane at 298 K. Adsorption and coadsorption of ethyl acetate, benzene, and other polar and apolar guests also occurs readily at 298 K. The Zr host catalyzes the Diels−Alder reaction of acrolein with 1,3-cyclohexadiene in a remarkable manner. As a solid metal−organic catalyst, it has a formula-based turnover rate constant of 40 h-1, which far exceeds those of its components, i.e., the soluble Lewis acid Zr(OtBu)4 (0.1 h-1) and the hydrogen-bonded insoluble organic network 1 (0.3 h-1). The solid catalyst can be easily separated from the organic product, ...

Ester hydrogenolysis over promoted Cu/SiO2 catalysts

Abstract: A series of Cu/SiO2 catalysts promoted with manganese, iron, cobalt, nickel, molybdenum, magnesium and yttrium was prepared by homogeneous deposition precipitation. For most catalysts a copper metal surface of about 15 m2 g−1 catalyst is measured. TPR reveals that copper–promoter contact is good for most samples. Despite similar Cu0 surface areas, ester hydrogenolysis activity varies considerably with the type of promoter, in the following order: Mo>Co≥Zn≥Mn>Fe≥Y>Ni≫Mg. As a function of metal–oxygen bond strength, a vulcano type catalyst activity plot is observed. With respect to alkanes formation, a correlation with normalised electronegativity is observed, inferring that the formation of alkanes proceeds on Lewis acid sites, which is in agreement with literature results. When both activity and selectivity are taken into account, manganese and zinc are the promoters of choice.

The role of K2O in the selective reduction of NO with NH3 over a V2O5(WO3)/TiO2 commercial selective catalytic reduction catalyst

Abstract: To elucidate the nature of the acid sites of the V2O5(WO3)/TiO2 catalyst upon K2O addition and its relation to the selective reduction of NO with NH3, measurements were made by means of infrared and Raman spectroscopy, NH3 chemisorption, and NO reduction measurements as a function of the K2O loading. The catalytic activity was found to decrease rapidly with the K2O loading, irrespective of the similar textural properties of all samples. Addition of K2O modified the vanadium species on the catalyst surface. For large additions of K2O, the potassium partially reacted with V2O5 to form KVO3. The amount of NH3 chemisorbed on the catalyst was observed to decrease with both the loading of K2O and the temperature. The adsorption of NH3 on both Bronsted and Lewis acid sites was confirmed. The strength and the number of Bronsted acid sites decrease largely with the loading of K2O in parallel with the decrease of the SCR activity, suggesting that the SCR reaction involves NH3 adsorption on the Bronsted acid sires. At low surface coverage of NH3, the isosteric heat of NH3 chemisorption was determined to be 370 kJ/mol for 0 wt.% K2O addition. With increasing K2O amount, the heat of adsorption decreased and was 150 kJ/mol for the catalyst with higher amounts of K2O addition. The results obtained imply that potassium disturbs the formation of the active ammonia intermediates, NH4+, resulting in deactivation of the catalyst. (C) 1999 Elsevier Science B.V. All rights reserved.

Acidity and active sites of Al-MCM-41

Abstract: The influence of the aluminum contents of MCM-41 (Si/Al ratio varied between 2.7 and 69) on the coordination of Al, on the acidity, and on the catalytic properties is studied by 27Al MAS NMR, temperature programmed desorption of ammonia, and the conversion of acetone. Based on TPDA results, an assignment of the peaks of desorption of ammonia is proposed. With low Al contents, the concentration of strong Bronsted acid sites, which are attributed to tetrahedral aluminum, increases with growing Al amounts. At higher Al contents, however, the number of strong acid sites decreases again. Besides usual strong Bronsted sites, TPDA reveals the existence of weak Bronsted and Lewis sites and of Lewis sites of a high acidic strength. Separation between the two types of sites of weak acidity is incomplete. In the temperature programmed decomposition of NH4-exchanged MCM-41 samples, strong Lewis acid sites temporarily re-adsorb ammonia generated by the thermal decomposition of the NH4+ ions located at the Bronsted sites. Ammonia re-adsorption at Lewis sites results in a shift of the maximum of the TPDA peak to a higher temperature. Calcination of samples leads to the formation of strong Lewis sites at the expense of Bronsted sites. To evaluate the acidity of Al-MCM-41, recording of decomposition profiles has been extended to Ga- and Fe-MCM-41. Catalytic activity in the conversion of acetone reaches its maximum with the sample based on Al-MCM-41 with the molar Si/Al ratio of 6.85. Highly favored formation of isobutene points to a comparably high acidic strength of the active sites.

C2-symmetric cu(ii) complexes as chiral lewis acids. catalytic enantioselective michael addition of silylketene acetals to alkylidene malonates

Abstract: 2016-present Professor Department of Chemistry, Columbia University, New York, NY 2013 Chair, Organometallic Chemistry Directed Towards Organic Synthesis (OMCOS 17) July 28-August 1, 2013 Fort Collins, CO 2012-2018 Permanent Member, Synthetic and Biological Chemistry B Study Section, NIH (SBC-B) 2012-present Associate Editor for the Americas, Synlett 2010-2015 Member, Editorial Board, Organic Reactions 2010-present Member, Editorial Board, electronic Encyclopedia of Reagents for Organic Synthesis 2008-2016 John K. Stille Chair Department of Chemistry, Colorado State University, Fort Collins, CO 2008-2016 Professor Department of Chemistry, Colorado State University, Fort Collins, CO 2005-2008 Associate Professor Department of Chemistry, Colorado State University, Fort Collins, CO 2000-2005 Assistant Professor Department of Chemistry, Colorado State University, Fort Collins, CO 1998-2000 NSERC Post-Doctoral Research Fellow Harvard University, Cambridge, Massachusetts with Prof. David A. Evans 1993-1998 Ph.D. Organic Chemistry University of Toronto, Toronto, Ontario with Prof. Mark Lautens 1986-1990 B.Sc. Human Biology University of Toronto, Toronto, Ontario Awards and Honors

Catalyst activators comprising expanded anions

Abstract: Catalyst compositions comprising a group 4 metal complex and an activator corresponding to the formula: (A* +a ) b (Z*J* j ) −c d , wherein: A* is a cation of charge +a, selected from the group consisting of ammonium-, sulfonium-, phosphonium-, oxonium-, carbonium-, silylium-, ferrocenium-, Ag + , and Pb +2 ; Z* is an anion group of from 1 to 50 atoms, not counting hydrogen atoms, further containing two or more Lewis base sites as defined in the claims; J* independently each occurrence is a Lewis acid coordinated to at least one Lewis base site of Z*, and optionally two or more such J* groups may be joined together in a moiety having multiple Lewis acidic functionality, j is a number from 2 to 12 and a, b, c, and d are integers from 1 to 3, with the proviso that a×b is equal to c×d.

A systematic study of ligand effects on a Lewis-acid-catalyzed Diels-Alder reaction in water. Water-enhanced enantioselectivity

Abstract: The influence of a series of diamine ligands and α-amino acid ligands on the rate and enantioselectivity of the Ni2+- and Cu2+-catalyzed Diels−Alder reaction between 3-phenyl-1-(2-pyridyl)-2-propen-1-ones and cyclopentadiene in water has been investigated. Equilibrium constants and enthalpies and entropies for binding of the dienophile to the catalyst−ligand complex as well as rate constants, activation enthalpies, and entropies for the subsequent reaction with the diene have been determined using UV−vis spectroscopy. Ligand-accelerated catalysis is observed for several aromatic α-amino acid ligands. This is likely to be a consequence of arene−arene interaction between the aromatic ring of the α-amino acid ligand and the pyridine ring of the dienophile and for which quantitative evidence is provided. The same interaction also induces up to 74% enantioselectivity in the Diels−Alder reaction. This is the first example of enantioselectivity in a Lewis-acid-catalyzed organic reaction in water. Most importantl...

Catalytic enantioselective inverse-electron demand 1,3-dipolar cycloaddition reactions of nitrones with alkenes

Abstract: A general reaction protocol for catalytic enantioselective 1,3-dipolar cycloaddition reaction of nitrones, activated by chiral Lewis acids, with electron-rich alkenes has been developed. The nitrones are activated by various chiral 2,2‘-dihydroxy-1,1‘-binaphthyl (BINOL)-AlMe complexes, and it has been found that 3,3‘-diaryl-BINOL-AlMe complexes catalyze a highly regio-, diastereo-, and enantioselective 1,3-dipolar cycloaddition reaction of aromatic nitrones with vinyl ethers, giving the exo-diastereomer of the isoxazolidines with de's up to >90% and up to 97% ee. The reaction has been investigated under various conditions with different nitrones and vinyl ethers (and alkenes), and a general synthetic procedure is presented. The mechanism for the reaction is discussed on the basis of a linear stereochemical effect of the catalyst, the diastereoselectivity, and absolute stereochemistry of the isoxazolidines formed, and theoretical calculations of the 3,3‘-diphenyl-BINOL-AlMe−nitrone intermediate.

Lewis Acid Catalyzed Highly Regio- and Stereocontrolled Trans-Hydrosilylation of Alkynes and Allenes

Abstract: Lewis acids such as AlCl3 or EtAlCl2 dramatically catalyzed the hydrosilylation of alkynes 1 with trialkylsilanes to produce the corresponding cis-vinylsilanes 2 in a regio- and trans-stereoselective manner. For example, the hydrosilylation of 1-dodecyne 1a with triethylsilane in the presence of 0.2 equiv of AlCl3 gave cis-1-(triethylsilyl)-1-dodecene in 93% yield. Other alkyl- and phenyl-substituted terminal and internal acetylenes also underwent trans-hydrosilylation very smoothly. In the case of alkoxy- or silyloxy-substituted acetylenes, the use of 1.2 equiv of AlCl3 or EtAlCl2 was essential to obtain the corresponding trans-hydrosilylation products in high yields. Moreover, AlCl3 catalyzed the hydrosilylation of aromatic allenes 11, producing the alkenylsilanes 12 with high regio- and stereoselectivities in moderate to high chemical yields. Not only the simple monosubstituted, but also the disubstituted and trisubstituted allenes, underwent the hydrosilylation reaction smoothly, serving as a useful t...

Trigonal-Bipyramidal Lewis Base Adducts of Methyltrioxorhenium(VII) and Their Bisperoxo Congeners: Characterization, Application in Catalytic Epoxidation, and Density Functional Mechanistic Study

Abstract: Pyrazole and pyridine adducts of methyltrioxorhenium(VII) form bisperoxo complexes with excess H2O2 (see picture). Density functional calculations reveal that the increased efficiency of these complexes as catalysts for olefin epoxidation originates from their stability and from the moderate energies of the corresponding transition states. Nonaromatic nitrogen-base ligands reduce the catalytic performance of the adducts, in agreement with the computational results.

A Stable and Recoverable Chiral Ru Lewis Acid: Synthesis, Asymmetric Diels–Alder Catalysis and Structure of the Lewis Acid Methacrolein Complex

Abstract: Ease of generation, stablity in solution at ambient temperature, high enantioselectivity in Diels-Alder reactions, efficient catalyst recovery, and large rate differences on variation of the anion are all characteristics of the new Ru Lewis acid [CpRu((S,S)-biphop-F)]+ (biphop-F=(C6F5)2POCH2(Ph)CH2(Ph)OP(C6F5)2). The structure of complex 1 (L=methacrolein, Y=SbF6) provides evidence for a cooperative binding of the dienophile by both the Lewis acid and the anion.

Bismuth(III) Trifluoromethanesulfonate: An Efficient Catalyst for the Sulfonylation of Arenes

Abstract: Sulfonylation reactions are one of the most important groups of aromatic electrophilic substitutions.1 The sulfonyl group is a widely used synthon for synthetic organic chemists,2 and sulfones have many industrial applications.3 In Friedel-Crafts (FC) acylation and sulfonylation, a consequence of the complexation of the Lewis acid with the reaction product implies that a stoichiometric amount of the activator is often required. However, some metal halides such as iron(III) chloride,1 Bronsted acids, for example polyphosphoric acid4 or triflic acid,5 and zeolites6,7 have been reported to catalyze the sulfonylation of arenes. In recent years, rare earth trifluoromethanesulfonates (triflates) have been reported for the catalysis of the FC acylation8 but not for the catalysis of the FC sulfonylation. Also, our laboratory reported that bismuth(III) chloride (1a) and particularly bismuth(III) triflate (1b) were catalysts for acylation reactions,9 especially for FC acylation.10 In a stoichiometric process from arylsulfonyl bromides, silver triflate gives trifluoromethanesulfonic arenesulfonic anhydrides (ArSO2OTf), which are strong sulfonylating reagents.11 We present here a study of the catalytic activity of 1a and 1b for the sulfonylation reaction of arenes (eq 1), for which 1b proved to be a metal triflate capable of realizing an outstanding catalytic process.

Catalytic Approach for the Formation of Optically Active Allyl alpha-Amino Acids by Addition of Allylic Metal Compounds to alpha-Imino Esters.

Abstract: A new catalytic enantioselective approach for the formation of allyl α-amino acid derivatives by reaction of N-tosyl α-imino esters with allyl stannanes and silanes catalyzed by chiral copper(I) complexes has been developed. A series of different BINAP and phosphine−oxazoline (P,N) ligands have, in combination with various Lewis acids, been tested as chiral catalysts for allylation of N-tosyl α-imino esters. It has been found that both type of ligands, in combination with copper(I) salts, give highly valuable unsaturated α-amino acid derivatives. The reaction has been investigated for different allyl stannanes and silanes, and it has been found that tri-n-butyl allyl stannane gives the best results of the simple allyl compounds tested, leading to γ,δ-unsaturated α-amino acid derivatives in up to 94% yield and with up to 83% ee, which can be improved to be >95% ee by recrystallization. The reaction has also been investigated using different acyclic and cyclic allyl stannanes leading to various types of uns...

Skeletal isomerization mechanism of alkanes over solid superacid of sulfated zirconia

Abstract: Skeletal isomerizations of n-butane and n-pentane were performed over solid superacids of sulfated zirconia and Pt-promoted sulfated zirconia. Changes in apparent activation energy and product selectivity were observed during the reactions. Isomerization proceeded by a monomolecular mechanism on Lewis acid sites in the initial period; then it changed to a bimolecular mechanism on Bronsted acid sites by forming surface alkenes. By addition of hydrocarbons or hydrogen, the monomolecular reaction became predominant. n-Pentane was converted into isopentane with very high selectivity. The overall mechanism of acid-catalyzed skeletal isomerization of alkanes is discussed.