Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Catalysis, enantioselective

In Section 31.6 we mentioned the enantioselective reduction of itaconic acid by a number of entrapped chiral organometallic catalysts [25]. A follow-up and major improvement of that study was reported by Volovych et al. [30]. These authors hydrogenated itaconic acid with sol-gel-entrapped Rh complexed with (2S,4S)-l-tert-butoxycarbonyl-4-diphenylphosphino-2-(diphe-nylphosphinomethyl)pyrrolidine catalyst in methanol solutions. The immobilization process was carried out with different sol-gel precursors TMOS, TEOS, triethoxyphenylsilane PhSi(OEt)3/TMOS, and trimethoxy (octyl)silane OcSi(OMe)3/TMOS. The choice of the precursor was found to influence the enantioselectivity and the rate of the reaction. The immobilized catalyst could be recovered and recycled several times under N2 atmosphere. About 90-99% ee was achieved for the hydrogenation of itaconic acid to (S)-(+)-2-methyl succinic acid. [Pg.977]

Finally, for this section is the study of Zhang et al. [33] who entrapped 5,10,15,20-tetralds[(lS,4R,5R,8S)-l,2,3,4,5,6,7,8-octahydro-l,2 5,8-dimethanoan-thrance-9-yl] porphyrin and showed that it exhibits remarkable enantioselective catalytic activity in alkene epoxidation using 2,6-dichloropyridine-A-oxide (Cl2pyNO) as an oxidant, with 69% ee and a very high turnover. [Pg.977]

Ti(IV). Carreira has reported a novel class of tridentate ligands whose complexes with Ti(IV) 48 serve as catalysts for a variety of enantioselective aldehyde additions [20J. The reactions that have been examined include acetate and dienolate aldol additions as well as ene-like reactions of 2-methoxy propene [21], The salient features of these catalytic systems include the fact that a wide range of aldehyde substrates may be utilized, the ability to carry out the reaction employing 0.2-5 mol% catalyst loading, and the experimental ease with which the process is executed. The typical experimental procedure prescribes the use of an in situ generated catalyst, at -10 to 23 °C in a variety of solvents, employing as little as 0.5 mol% catalyst. [Pg.236]

The catalytic aldol addition process has been extended to include the addition reactions of dienolsilane 49 to a broad range of aldehydes (Eq. (8.12)) [26]. The addition reactions of 49 are conducted at 23 C utilizing 5 mol% of catalyst, giving adducts in up to 94% ee. This dienolsilane is easily prepared by enolization of the commercially available acetone-ketene adduct followed by quenching with chlorotrimethyl silane. The resulting dienolsilane is isolated typically in 78% yield as a clear colorless liquid that can be conveniently purified by distillation. [Pg.237]

As part of a series of studies on the use of BINOL-Ti(IV) complex 53 as a catalyst in a number of C-C bond-forming reactions, Mikami has reported the aldol addition reactions of thioacetate-derived silyl ketene acetals 55, 56 to a collection of highly functionalized aldehydes (Eq. (8.13)) [28]. As little as 5 mol% of the catalyst mediates the addition reaction and furnishes adducts 57 in excellent yields and up to 96% ee. One of the noteworthy features of the Mikami process is the fact that aldehyde substrates containing polar substituents can be successfully employed, a feature exhibited by few other Lewis-acid-catalyzed aldehyde addition reactions. [Pg.238]

In addition to processes involving thioacetate aldols, Mikami has studied the aldol addition reaction of thiopropionate-derived enolsilanes 58, 59 (Eq. (8.14)). The Z-enol silane derived from terr-butyl thiopropionate undergoes addition to benzyloxyacetaldehyde to give products as a 92 8 anti syn mixture of diastereo-mers with the major anti stereoisomer 61 isolated in 90% ee. The additions of E [Pg.238]

The catalytic, enantioselective additions of thioacetate-derived enol silanes has also been studied by Keck (Eq. (8.15)) [29]. In these studies, the active catalyst (62) is readily generated upon mixing binol, TiCl2(0/Pr)2, and 4 A molecular sieves in Et20 at -20 °C followed by an aging period. The addition reactions are best conducted with 10 mol% catalyst in ether at -20 °C the f rt-butyl thioacetate adducts are isolated in up to 98% ee and 90% yield. [Pg.239]

Many catalysts exhibit a decrease in enantioselectivity in the absence of solvent, but there are some examples where stereoselectivity actually increases.Solvent free asymmetric catalysis was recently reviewed, and reactions studied to [Pg.36]

Al catalyst (0.002 nnol%) 30% H2O2, pH 7.4 buffer MeOH or solvent-free 0 °C, 24 h [Pg.38]

A series of organocatalytic solvent free reactions have recently been performed using An asymmetric alkaloid-mediated opening of a cyclic [Pg.38]

Recently, Organon workers have discovered a derivative of y-cyclodextrin (Sugam-madex) that rapidly removes - by inclusion - rocuronium bromide (18) from the receptor sites, thus accelerating surgical after-care [32]. [Pg.113]

Plant sterols such as sitosterol and camposterol, as by-products from vegetable oils at prices of about 15 kg-1, are also important starting materials for the production of steroid hormones. A new application is the cholesterol lowering property of these sterols esterified with fatty acids (with a production of about 10000 t a 1). They can be found in the margarine Becel pro-active of Unilever. A Finnish equivalent is Benecol, which contains stands such as sitostanol and campostanol, sterols having the 5,6-double bound hydrogenated, also esterified with fatty adds [33]. [Pg.113]

An obvious way to target chiral compounds is to start with a compound in which the chiral center is already present. Here natural products and derivatives offer a rich pool of generally inexpensive starting materials. Examples include L-hydroxy and amino adds. Sometimes, just one out of many chiral centers is predestined to remain, as in the synthesis of vitamin C from D-glucose, or in the preparation of (S)-3-hydroxy-y-butyrolactone from ladose. [Pg.113]

The other approach is to apply enantioselective catalysts. First of all, nature s ingenious catalysts, the enzymes, should be mentioned. For a review of this field, including economical considerations, see Chapter 7 of Ref. [19]. In several synthetic areas, e.g., esterification/hydrolysis/transesterification, enzymes now play an important role. Notably, in recent years the performance of some enzymes [Pg.113]

Several examples exist of the application of chiral natural N-compounds in base-catalyzed reactions. Thus, L-proline and cinchona alkaloids have been applied [35] in enantioselective aldol condensations and Michael addition. Techniques are available to heterogenize natural N-bases, such as ephedrine, by covalent binding to mesoporous ordered silica materials [36]. [Pg.114]

Finally, an elegant example of a product derived from renewable raw materials is the bioemulsifier, marketed by Mitsubishi, which consists of a mixture of sucrose fatty acid esters. The product is prepared from two renewable raw materials - sucrose and a fatty acid - and is biodegradable. In the current process the reaction is catalysed by a mineral acid, which leads to a rather complex mixture of mono- and di-esters. Hence, a more selective enzymatic esterification (Fig. 1.43) would have obvious benefits. Lipase-catalysed acylation is possible [126] but reaction rates are very low. This is mainly owing to the fact that the reaction, for thermodynamic reasons, cannot be performed in water. On the other hand, sucrose is sparingly soluble in most organic solvents, thus necessitating a slurry process. [Pg.35]

Another major trend in performance chemicals is towards the development of products - pharmaceuticals, pesticides and food additives, etc. - that are more targeted in their action with less undesirable side-effects. This is also an issue which is addressed by green chemistry. In the case of chiral molecules that exhibit biological activity the desired effect almost always resides in only one of the enantiomers. The other enantiomer constitutes isomeric ballast that does not contribute to the desired activity and may even exhibit undesirable side-effects. Consequently, in the last two decades there has been a marked trend towards the marketing of chiral pharmaceuticals and pesticides as enantiomeri-cally pure compounds. This generated a demand for economical methods for the synthesis of pure enantiomers [127]. [Pg.35]

The same reasoning applies to the synthesis of pure enantiomers as to organic synthesis in general for economic and environmental viability, processes should [Pg.35]

An elegant example of a highly efficient catalytic asymmetric synthesis is the Takasago process [128] for the manufacture of 1-menthol, an important flavour and fragrance product. The key step is an enantioselective catalytic isomerisation of a prochiral enamine to a chiral imine (Fig. 1.44). The catalyst is a Rh-Binap complex (see Fig. 1.44) and the product is obtained in 99% ee using a sub-strate/catalyst ratio of 8000 recycling of the catalyst affords total turnover numbers of up to 300000. The Takasago process is used to produce several thousand tons of 1-menthol on an annual basis. [Pg.36]

An even more impressive example of catalytic efficiency is the manufacture of the optically active herbicide, (S)-metolachlor. The process, developed by Novartis [129], involves asymmetric hydrogenation of a prochiral imine, catalysed [Pg.36]


To our knowledge, the results presented in this chapter provide the first example of enantioselective Lewis-acid catalysis of an organic reaction in water. This discovery opens the possibility of employing the knowledge and techniques from aqueous coordination chemistry in enantioselective catalysis. This work represents an interface of two disciplines hitherto not strongly connected. [Pg.75]

Clearly, there is a need for techniques which provide access to enantiomerically pure compounds. There are a number of methods by which this goal can be achieved . One can start from naturally occurring enantiomerically pure compounds (the chiral pool). Alternatively, racemic mixtures can be separated via kinetic resolutions or via conversion into diastereomers which can be separated by crystallisation. Finally, enantiomerically pure compounds can be obtained through asymmetric synthesis. One possibility is the use of chiral auxiliaries derived from the chiral pool. The most elegant metliod, however, is enantioselective catalysis. In this method only a catalytic quantity of enantiomerically pure material suffices to convert achiral starting materials into, ideally, enantiomerically pure products. This approach has found application in a large number of organic... [Pg.77]

The first example of enantioselective catalysis of a Diels-Alder reaction was reported in 1979 . Since then, an extensive set of successful chiral Lewis-acid catalysts has been prepared. Some selected examples will be presented here together with their mechanistic interpretation. For a more complete... [Pg.77]

In Chapter 2 the Diels-Alder reaction between substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-ones (3.8a-g) and cyclopentadiene (3.9) was described. It was demonstrated that Lewis-acid catalysis of this reaction can lead to impressive accelerations, particularly in aqueous media. In this chapter the effects of ligands attached to the catalyst are described. Ligand effects on the kinetics of the Diels-Alder reaction can be separated into influences on the equilibrium constant for binding of the dienoplule to the catalyst (K ) as well as influences on the rate constant for reaction of the complex with cyclopentadiene (kc-ad (Scheme 3.5). Also the influence of ligands on the endo-exo selectivity are examined. Finally, and perhaps most interestingly, studies aimed at enantioselective catalysis are presented, resulting in the first example of enantioselective Lewis-acid catalysis of an organic transformation in water. [Pg.82]

Effects of L- -amino acid ligands - Stepping on the tail of enantioselectivity The naturally occurring -amino acids form a class of readily available strongly coordinating ligands, which exhibit broad stmctural variation. Moreover, their availability in enantiomerically pure form offers opportunities for enantioselective catalysis. Some derivatives of these compounds have been... [Pg.85]

One example has already been encountered in the form of the binding constants in Table 3.2. These data form one of the first examples of compelling evidence for the involvement of attractive arene -arene interactions in determining the outcome of enantioselective catalysis. These attractive interactions have been frequently invoked as explanations for the observed enantioselectivities of... [Pg.92]

Of all the work described in this thesis, this discovery is probably the most significant. Given the fact that the arene - arene interactions underlying the observed enantioselectivity of ftie Diels-Alder reactions described in Chapter 3 are also encountered in other organic reactions, we infer that, in the near future, the beneficial influence of water on enantioselectivity can also be extended to these transformations. Moreover, the fact that water can now be used as a solvent for enantioselective Lewis-add catalysed reactions facilitates mechanistic studies of these processes, because the number of equilibria that need to be considered is reduced Furthermore, knowledge and techniques from aqueous coordination chemistry can now be used directly in enantioselective catalysis. [Pg.162]

Agbossou E., Carpentier J. E. Hapiot E., Suisse I., Mortreux A. The Aminophos-phine-Phosphinites and Related Ligands Synthesis, Coordination Chemistry and Enantioselective Catalysis Coord. Chem. Rev. 1998 I78-I80 1615-1645 Keywords stereoselective Diels-Alder reaction catalysts, aminophosphine-phosphinites, enantioselective catalysts... [Pg.307]

Brunner H. Right or Left in Chemistry - Enantioselective Catalysis With Transition Metal Compounds Quim. Nova 1995 18 603 07... [Pg.316]

Zang D. -L. and Li P. Enantioselective Catalysis of Diels-Alder Reactions Youji Huaxue 1994 14 581 (in Chinese)... [Pg.318]

Brunner H. Right or Left-that is the Question-Enantioselective Catalysis with Transition Metal Compounds in Adv. Catal. Des., Proc. Workshop, 2nd. 1993 245, Ed. Graziani M., Rao C. N. R., Pb. Wordl Sci. Singapore... [Pg.318]

Brunner H. Enantioselective Catalysis With Transition Metal Componnds. Right or Left - This Is the Qnestion Adv. Chem. Ser. 1992 230 143-152 Keywords homo-Diels-Alder reaction of norbornadiene... [Pg.321]

Enantioselective Catalysis with Chiral-at-Metal Complexes. 283... [Pg.271]

Enantioselective catalysis requires that the distance between the inductor and the substrate is not too long. Basically, the smaller the distance, the better... [Pg.283]

Robertson DE et al. (2004) Exploring nitrilase sequence space for enantioselective catalysis. Appl Environ Microbiol 70 2429-2436. [Pg.333]

Corey, E.J. Helal, C.J. (1998) Reduction of Carbonyl Compounds with Chiral Oxazaborolidine Catalysts A New Paradigm for Enantioselective Catalysis and a Powerful New Synthetic Method. Angewandte Chemie International Edition, 37, 1986-2012. [Pg.188]

Catalytic transformations can be divided on the basis of the catalyst-type - homogeneous, heterogeneous or enzymatic - or the type of conversion. We have opted for a compromise a division based partly on type of conversion (reduction, oxidation and C-C bond formation, and partly on catalyst type (solid acids and bases, and biocatalysts). Finally, enantioselective catalysis is a recurring theme in fine chemicals manufacture, e.g. in the production of pharmaceutical intermediates, and a separate section is devoted to this topic. [Pg.30]

The same reasoning applies to the synthesis of pure enantiomers as to organic synthesis in general processes should be atom efficient and have low E factors, i.e. involve catalytic methodologies. This is reflected in the increasing attention being focused on enantioselective catalysis, using either enzymes or chiral metal complexes. [Pg.53]

A fascinating example of enantioselective catalysis for agrochemicals is the production of a well-known herbicide of Novartis (formerly Ciba Geigy), metolachlor (Trade name Dual... [Pg.176]

Enantioselective Catalysis of the Aldol Addition Reaction. There are also several catalysts that can effect enantioselective aldol addition. The reactions generally involve enolate equivalents, such as silyl enol ethers, that are unreactive toward the carbonyl component alone, but can react when activated by a Lewis acid. The tryptophan-based oxazaborolidinone 15 has proven to be a useful catalyst.148... [Pg.125]

As with aldol and Mukaiyama addition reactions, the Mannich reaction is subject to enantioselective catalysis.192 A catalyst consisting of Ag+ and the chiral imino aryl phosphine 22 achieves high levels of enantioselectivity with a range of N-(2-methoxyphenyljimines.193 The 2-methoxyphenyl group is evidently involved in an interaction with the catalyst and enhances enantioselectivity relative to other A-aryl substituents. The isopropanol serves as a proton source and as the ultimate acceptor of the trimethyl silyl group. [Pg.142]

The a,a,a,a-tetraaryl-l,3-dioxolane-4,5-dimethanol (TADDOL) chiral ligands have also been the basis of enantioselective catalysis of the D-A reaction. In a study using 2-methoxy-6-methylquinone as the dienophile, evidence was found that the chloride-ligated form of the catalysts was more active than the dimeric oxy-bridged form.117... [Pg.512]

Enantioselective catalysis of S alkylation has been achieved.91 A BINOL-phosphoramidite catalyst (o-methoxyphenyl analog) similar to that in Entry 3 in Scheme 8.7 gave good results. [Pg.703]


See other pages where Catalysis, enantioselective is mentioned: [Pg.75]    [Pg.81]    [Pg.86]    [Pg.91]    [Pg.92]    [Pg.101]    [Pg.103]    [Pg.175]    [Pg.177]    [Pg.48]    [Pg.162]    [Pg.3]    [Pg.4]    [Pg.137]    [Pg.90]    [Pg.169]    [Pg.286]    [Pg.205]    [Pg.211]    [Pg.52]    [Pg.176]    [Pg.177]    [Pg.1]    [Pg.185]   
See also in sourсe #XX -- [ Pg.283 , Pg.284 , Pg.285 , Pg.286 ]

See also in sourсe #XX -- [ Pg.1245 ]

See also in sourсe #XX -- [ Pg.273 ]

See also in sourсe #XX -- [ Pg.35 ]

See also in sourсe #XX -- [ Pg.156 ]

See also in sourсe #XX -- [ Pg.235 ]

See also in sourсe #XX -- [ Pg.248 , Pg.270 , Pg.271 ]

See also in sourсe #XX -- [ Pg.97 ]

See also in sourсe #XX -- [ Pg.219 ]

See also in sourсe #XX -- [ Pg.93 ]

See also in sourсe #XX -- [ Pg.143 , Pg.177 ]

See also in sourсe #XX -- [ Pg.261 , Pg.381 , Pg.672 , Pg.808 , Pg.820 , Pg.835 ]

See also in sourсe #XX -- [ Pg.184 , Pg.186 ]

See also in sourсe #XX -- [ Pg.406 ]

See also in sourсe #XX -- [ Pg.64 ]

See also in sourсe #XX -- [ Pg.175 ]

See also in sourсe #XX -- [ Pg.314 ]

See also in sourсe #XX -- [ Pg.207 ]

See also in sourсe #XX -- [ Pg.977 ]




SEARCH



Carbonyl enantioselective catalysis

Catalysis, asymmetric enantioselective

Chiral enantioselective catalysis

Covalent heterogeneous catalysis, enantioselective

Enamine catalysis enantioselective organocatalytic

Enantioselection asymmetric catalysis

Enantioselective Catalysis Using Dendrimer Supports

Enantioselective Catalysis for Enolate Arylation

Enantioselective Catalysis in Alkylations and Allylations of Enolates

Enantioselective Catalysis in Supercritical Carbon Dioxide

Enantioselective Conjugate Addition Reactions via Phase-transfer Catalysis

Enantioselective Fluorous Catalysis

Enantioselective Lewis-acid catalysis

Enantioselective Organocascade Catalysis

Enantioselective Phosphorus and Arsenic Ylide Catalysis

Enantioselective Selenium and Tellurium Ylide Catalysis

Enantioselective acylation, lipase catalysis

Enantioselective heterogeneous catalysi

Enantioselective heterogeneous catalysi immobilization

Enantioselective heterogeneous catalysi support

Enantioselective heterogeneous catalysis

Enantioselective heterogeneous catalysis catalysts

Enantioselective heterogeneous catalysis enzyme catalysts

Enantioselective heterogeneous catalysis immobilization

Enantioselective reduction Lewis-acid catalysis

Enantioselective synthesis amino acids, phase-transfer catalysis

Enantioselective synthesis phase-transfer catalysis

Enantioselective ylide catalysis

Enantioselectivity catalysis

Enantioselectivity zirconium catalysis

Heterogeneous Enantioselective Catalysis Using Inorganic Supports

Heterogeneous Enantioselective Catalysis Using Organic Polymeric Supports

Heterogenous enantioselective catalysis

Homochiral Metal-Organic Coordination Polymers for Heterogeneous Enantioselective Catalysis Self-Supporting Strategy

Homogeneous asymmetric catalysis enantioselective reactions

Immobilization of Transition Metal Complexes and Their Application to Enantioselective Catalysis

Ligand-deficient catalysis enantioselective reaction

Michael addition enantioselective catalysis

Organic synthesis enantioselective catalysis

Oxidation enantioselective metal catalysis

Palladium catalysis enantioselective allylic alkylation

Preparation by Enantioselective Catalysis

Proteins hosting enantioselective catalysi

Rhodium catalysis enantioselective

Zeolites Supported Enantioselective Catalysis

© 2024 chempedia.info