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Catalysts salts

The bimolecular termination reaction can be neglected at low conversions since no linear sulphide residues are present initially, enabling a simpler interpretation of initial conversion/time curves to be made. In the earlier work the concentration of active centres was equated with the initial catalyst salt concentration, but later an XH NMR method of analysis was employed (137). As in the polymerisation of tetrahydrofuran it was anticipated that both free ions and ion pairs were likely to contribute to the propagation reaction and the calculated rate constant kP.pp e t, was described by... [Pg.34]

A new catalyst salt (20) that consists of an achiral ammonium ion and a chiral phosphate anion and which catalyses highly enantioselective transfer hydrogenations of ,/J-unsaturated aldehydes to the corresponding saturated derivatives has been developed. The underlying principle, namely asymmetric counteranion-directed catalysis, is claimed to be a new strategy for highly enantioselective synthesis.357... [Pg.138]

Catalyst Salt Decrease of [NjF/g"] after addition of salt Decrease of k... [Pg.35]

A mild and efficient method for the aminolysis of oxiranes in aprotic solvents using metal ion salts of Li+, Na+, Mg2+, Ca2+, and Zn2+ as Lewis acid has been reported <90TL4661). The reaction rates depend, in addition to the nature of the amine and epoxide, also on the type of the metal ion of the catalyst salt. The stereoselectivity observed in these reactions is complete inversion of configuration. Epoxy carboxylic acids are cleanly ring-opened by primary amines at C-2 to provide a-amino-/ -hydroxy acids <92TL2497>. [Pg.106]

The solubility of the catalyst salt is improved by the use of 3,5-bis(trifluoromethyl)phenylborate or triflate anions. Unsaturated fatty acids can be reduced in the same way.173 Hydrogenations in sc carbon dioxide can be more selective than in the gas phase while using 35 times less catalyst.174 Cyclohexene was reduced with hydrogen and a polysilox-ane-supported palladium catalyst in in a continuous-flow reactor in 95-98% yields. Epoxides, oximes, nitriles, aldehydes, ketones, and nitro compounds can also be reduced. By varying the temperature, the products from the reduction of nitrobenzene can be selected from aniline, cyclo-hexylamine, dicyclohexylamine, and cyclohexane. In the... [Pg.212]

According to Nitta the introduction of chlorides or bromides into the catalyst salt mass during the formation of deposited catalyst, promotes an increase in ee by virtue of the inerease in D, but the overall rate of the reaetion decreases under these eonditions. Additions of NiCfy NaCl, FeCfy and NaBr or of HCl or HBr to the eatalyst precursor mass prepared by method A (see Table 4.8.) increases ee from 36.1% to 49.7-57.5%, and also increases D from 13 to 20-30 nm while narrowing the size distribution of the niekel crystallites... [Pg.103]

Melchiore et al. reported highly stereoselective aziridinations of linear and cyclic a,p-unsaturated ketones with excellent diastereo- and enantio-selectivity (up to 99% ee). Aziridination was performed using N-protected hydrojylamines and a primary amine salt catalyst, made by combining 9-amino-9-deo)y-ep/-dihydroquinine (9-epz-DHQA) with o-N-Boc-phenylglycine (Scheme 15.8). The first step of the reaction is the nucleophilic addition of Af-protected hydro>ylamine to the iminium intermediate (formed from the enone and amine catalyst salt) followed by an intramolecular cyclisation of the resulting enamine to the aziridine product. [Pg.48]

Melchiorre et al. accomplished the highly enantioselective otganocatalytic p-hydro)qrlation of a,p-unsaturated ketones by using oximes as the oxygen nucleophile. Catalyst salt, made by combining 9-amino-9-deoxy-epr-hydroquinine with d-AI-Boc phenylglycine, functions as an efficient catalyst, and optically active products are obtained with enantioselectivily up to 94%. [Pg.63]

The same catalyst (93) was applied with success to the first organocatalytic multi component asymmetric Biginelli reaction providing medicinally relevant chiral 3,4-dihydropyrimidini-2-(l//)-ones (94) (Scheme 29)7 Chiral acid-catalyzed inverse electron-demanding aza Diels-Alder reaction of aldimines (95) with electron-rich alkenes (Scheme 30). A new catalyst salt (96) that consist of an achiral ammonium ion and chiral phosphate anion has been developed that catalyzes highly... [Pg.136]

Table 1. Symmetrically substituted onium salts (e.g. tetrabutyl, tetraheptyl) are the best catalysts. Salts containing methyl or benzyl groups (such as Aliquat 336) are relatively poor catalysts in these reactions, presumably because of competing side reactions involving nucleophilic attack of these groups to give methyl or benzyl ethers or alcohols. Tetrabutylammonium bromide (TBAB) can be used with either sodium or potassium hydroxide whereas Aliquat 336 is best used only with potassium hydroxide. Finally, the onium salts are superior to the two crown ethers which were used and, as expected, 18-crown-6 gave better results with potassium hydroxide while 15-crown-5 gave better results with sodium hydroxide. Table 1. Symmetrically substituted onium salts (e.g. tetrabutyl, tetraheptyl) are the best catalysts. Salts containing methyl or benzyl groups (such as Aliquat 336) are relatively poor catalysts in these reactions, presumably because of competing side reactions involving nucleophilic attack of these groups to give methyl or benzyl ethers or alcohols. Tetrabutylammonium bromide (TBAB) can be used with either sodium or potassium hydroxide whereas Aliquat 336 is best used only with potassium hydroxide. Finally, the onium salts are superior to the two crown ethers which were used and, as expected, 18-crown-6 gave better results with potassium hydroxide while 15-crown-5 gave better results with sodium hydroxide.
General Procedure for One-Pot Diels-Alder/Benzoin Condensation A vial equipped with a Teflon-coated stir bar and a plastic screw cap was charged with (f )-2-(diphenyl((trimethylsilyl)oxy)methyl)pyrrolidine (0.02 mmol, 6.5 mg, 20 mol%). Then, 2,4,6-trimethylbenzoic acid (TMBA, 0.02 mmol, 3.2 mg, 20mol%) and toluene (0.2 mL) were added in one portion, and the resulting solution was stirred at ambient temperature for 10 min to allow the catalyst salt formation. The reaction was started by the sequential addition of the aldehyde (0.12 mmol, 1.2 equiv) and dibenzoylethylene (0.1 mmol). The vial was sealed and kept in a water bath (ther-mostated at 40 °C). After 48 h, the vial was removed from the water bath and cooled to room temperature. Then, 0.3 mL of toluene, sodium acetate (0.2 mmol, 16.4 mg. [Pg.198]


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See also in sourсe #XX -- [ Pg.76 ]




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