Big Chemical Encyclopedia

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

Articles Figures Tables About

Active catalyst with allyl alcohols

For the calculations we used a simplified model system in which all substituents were replaced by methyl groups (Scheme 4). Experimentally, the methyl substituted catalyst and methanol as nucleophile are active, but the enantiomeric excesses obtained fall below those obtained with the tert-XmcinQ amide-derived catalyst in combination with allyl alcohol (Scheme 3). [Pg.7]

In the presence of a cationic Rh[((/ )-binap)(cod)] complex, geranyl or neryl amides isomerize slowly to give a mixture of the corresponding enamide and dienamide (Scheme 20) (2). The optical purity of the chiral enamide is high, but the chemical yield is low. Certain cyclic allylic amides give the enamide isomers in a high ee. With a DIOP-Rh catalyst, prochiral allylic alcohols are converted to optically active aldehydes with low ee (31). [Pg.262]

Moiseev and coworkers showed [10,13] that giant palladium clusters with an idealized formula Pd56iL5o(OAc)igo (L = phenanthroline or bipyridine) are highly active catalysts for allylic oxidation of olefins. The catalytically active solution was prepared by reduction of Pd(OAc)2, e. g. with H2, in the presence of the ligand, L, followed by oxidation with O2. The giant palladium cluster catalyzed the oxidation of propylene to allyl acetate under mild conditions. Even in 10% aqueous acetic acid, allyl acetate selectivity was 95-98 % [10]. Oxidation catalyzed by Pd-561 in water afforded a mixture of allylic alcohol (14%), acrolein (2%), and acrylic acid (60%), and only 5% acetone [10]. [Pg.521]

Alternatively, dioxygen can be used as the primary oxidant in combination with TEMPO, using either copper salts, i.e., Cu Cl as reported by Semmelhack et al. 96 or laccase (see above) as co-catalysts. The Cu Cl catalytic system, however, is only effective with activated benzylic and allylic alcohols 96). [Pg.265]

Magnetisation transfer experiments in and P nmr suggest that in the catalytic cycle of hydrogenation by Wilkinson s catalyst, [RhCl(PPhg)3], a cis-(PPha) Rh conformation occurs at key stages . Wilkinson s catalyst hydrogenates allyl alcohol with an activation energy of 19.7 kcal mol but under certain... [Pg.376]

The direct ruthenium catalysed allylation with allylic alcohol derivatives of various aromatic compounds and heterocycles such as furans and thiophenes was performed by Nishibayashi with cationic thiolate-bridged diruthenium(III, II) catalysts. The reaction is consistent with an electrophilic aromatic substitution by the electrophilically activated allyl moiety [68]. Allylation also takes place with the alkene metathesis Grubbs catalyst [69]. More importantly using (phosphine-sulfonate)ruthenium(II) catalyst Bmneau et al. have recently shown that allyl alcohols are activated generating an allyl-ruthenium(IV) intermediate leading to C3-allylation of indole with high regioselectivity in favour of the branched allyl derivative [(Eq. 84)] [167]. [Pg.173]

Activation of an allylic alcohol by metal catalysts can be used to facilitate addition of a variety of nucleophiles. This method has recently been adapted for the synthesis of spiroacetals via hemiacetals substituted with an allylic alcohol side chain (Scheme 17). [Pg.203]

SemmeUiack et al. [104] reported that the combination of CuCl and 4-hydroxy TEMPO catalyzes the aerobic oxidation of alcohols. However, the scope was limited to active benzyhc and allylic alcohols and activities were low (10 mol% of catalyst was needed for smooth reaction). They proposed that the copper catalyzes the reoxidation of TEMPO to the oxoammonium cation. Based on our results with the Ru/TEMPO system we doubted the validity of this mechanism. Hence, we subjected the Cu/ TEMPO to the same mechanistic studies described above for the Ru/TEMPO system [105]. The results of stoichiometric experiments under anaerobic conditions, Hammett correlations and kinetic isotope effect studies showed a similar pattern to those with the Ru/TEMPO system, i.e., they are inconsistent with a mechanism involving an oxoammonium species as the active oxidant. Hence, we propose the mechanism shown in Scheme 4.18 for Cu /TEM PO-catalyzed aerobic oxidation of alcohols. [Pg.107]

Dipolar cydoadditions are one of the most useful synthetic methods to make stereochemically defined five-membered heterocydes. Although a variety of dia-stereoselective 1,3-dipolar cydoadditions have been well developed, enantioselec-tive versions are still limited [29]. Nitrones are important 1,3-dipoles that have been the target of catalyzed enantioselective reactions [66]. Three different approaches to catalyzed enantioselective reactions have been taken (1) activation of electron-defident alkenes by a chiral Lewis acid [23-26, 32-34, 67], (2) activation of nitrones in the reaction with ketene acetals [30, 31], and (3) coordination of both nitrones and allylic alcohols on a chiral catalyst [20]. Among these approaches, the dipole/HOMO-controlled reactions of electron-deficient alkenes are especially promising because a variety of combinations between chiral Lewis acids and electron-deficient alkenes have been well investigated in the study of catalyzed enantioselective Diels-Alder reactions. Enantioselectivities in catalyzed nitrone cydoadditions sometimes exceed 90% ee, but the efficiency of catalytic loading remains insufficient. [Pg.268]

The reason for the efficient epoxidation of explicitly allylic alcohols with this system can be found in the strong associative interactions occurring between the substrate and the catalyst. The [Ti(tartrate)(OR)2]2 dimer 1, which is considered to be the active catalyst in the reaction, will generate structure 2 after the addition of... [Pg.188]

Allylic alcohols can be converted to epoxy-alcohols with tert-butylhydroperoxide on molecular sieves, or with peroxy acids. Epoxidation of allylic alcohols can also be done with high enantioselectivity. In the Sharpless asymmetric epoxidation,allylic alcohols are converted to optically active epoxides in better than 90% ee, by treatment with r-BuOOH, titanium tetraisopropoxide and optically active diethyl tartrate. The Ti(OCHMe2)4 and diethyl tartrate can be present in catalytic amounts (15-lOmol %) if molecular sieves are present. Polymer-supported catalysts have also been reported. Since both (-t-) and ( —) diethyl tartrate are readily available, and the reaction is stereospecific, either enantiomer of the product can be prepared. The method has been successful for a wide range of primary allylic alcohols, where the double bond is mono-, di-, tri-, and tetrasubstituted. This procedure, in which an optically active catalyst is used to induce asymmetry, has proved to be one of the most important methods of asymmetric synthesis, and has been used to prepare a large number of optically active natural products and other compounds. The mechanism of the Sharpless epoxidation is believed to involve attack on the substrate by a compound formed from the titanium alkoxide and the diethyl tartrate to produce a complex that also contains the substrate and the r-BuOOH. ... [Pg.1053]

Asymmetric epoxidation is another important area of activity, initially pioneered by Sharpless, using catalysts based on titanium tetraisoprop-oxide and either (+) or (—) dialkyl tartrate. The enantiomer formed depends on the tartrate used. Whilst this process has been widely used for the synthesis of complex carbohydrates it is limited to allylic alcohols, the hydroxyl group bonding the substrate to the catalyst. Jacobson catalysts (Formula 4.3) based on manganese complexes with chiral Shiff bases have been shown to be efficient in epoxidation of a wide range of alkenes. [Pg.117]


See other pages where Active catalyst with allyl alcohols is mentioned: [Pg.460]    [Pg.224]    [Pg.832]    [Pg.130]    [Pg.232]    [Pg.114]    [Pg.27]    [Pg.106]    [Pg.51]    [Pg.188]    [Pg.112]    [Pg.459]    [Pg.429]    [Pg.416]    [Pg.328]    [Pg.593]    [Pg.54]    [Pg.326]    [Pg.184]    [Pg.95]    [Pg.301]    [Pg.148]    [Pg.400]    [Pg.187]    [Pg.27]    [Pg.80]    [Pg.126]    [Pg.189]    [Pg.212]    [Pg.306]    [Pg.708]    [Pg.265]    [Pg.172]    [Pg.173]    [Pg.223]    [Pg.224]    [Pg.205]   
See also in sourсe #XX -- [ Pg.107 ]




SEARCH



Alcohol activation

© 2024 chempedia.info