Salts chiral quaternary ammonium bromide

The ammonium catalyst can also influence the reaction path and higher yields of the desired product may result, as the side reactions are eliminated. In some cases, the structure of the quaternary ammonium cation may control the product ratio with potentially tautomeric systems as, for example, with the alkylation of 2-naph-thol under basic conditions. The use of tetramethylammonium bromide leads to predominant C-alkylation at the 1-position, as a result of the strong ion-pair binding of the hard quaternary ammonium cation with the hard oxy anion, whereas with the more bulky tetra-n-butylammonium bromide O-alkylation occurs, as the binding between the cation and the oxygen centre is weaker [11], Similar effects have been observed in the alkylation of methylene ketones [e.g. 12, 13]. The stereochemistry of the Darzen s reaction and of the base-initiated formation of cyclopropanes under two-phase conditions is influenced by the presence or absence of quaternary ammonium salts [e.g. 14], whereas chiral quaternary ammonium salts are capable of influencing the enantioselectivity of several nucleophilic reactions (Chapter 12). 

Alkylation of phthalimide anion can be carried out under solid-liquid phase-transfer conditions, using phosphonium salts or ammonium salts. In the reaction systems using hexadecyltributylphosphonium bromide, alkyl bromides and alkyl methanesulfonate are more reactive than alkyl chlorides. Octyl iodide is less reactive than the corresponding bromide and chloride. ( )-2-Octyl methanesulfonate was converted into (S)-2-octylamine with 92.5% inversion. Kinetic resolution of racemic ethyl 2-bromopro-pionate by the use of a chiral quaternary ammonium salt catalyst has been reported. Under liquid-liquid phase-transfer conditions, A -alkylation of phthalimide has been reported to give poor results. ... 

Quaternary onium salts. Quaternary ammonium salts include trioctymethylammo-nium chloride (Starks catalyst), Aliquat 336, tricaprylmethylammonium chloride, tetrabutylammo-nium hydrogen sulfate (Brandstrom s catalyst), and benzyltrimethylam-monium chloride (Mqkosza s catalyst) quaternary ammonium salts can also be generated in situ from trialkylamines, etc. Other quaternary onium salts include tetrabutylphospho-nium bromide, tetraphenylphosphonium bromide, triphenylbenzylphosphonium chloride, tetraphenylarsonium chloride, and triphenylsulfonium chloride, etc. Special quaternary salts are 4-aminopyridinium salts, bis-(quaternary ammonium) [R3N -(CH2) -NR3" R3N -(CH2) -0-(CH2) -NRj ] salts, 4,4 -dialkylbipyridinium salts, cluster quaternary ammonium [e.g., P(C6H4S03"NR4)3] salts, crown-quaternary salts [e.g. (18-crown-6)-(CH2)9PBuj Br ], and chiral V-(4-trifluoromethyl)benzylcinchonium bromide, etc. 

The reaction of trimethylsulfonium iodide with benzaldehyde under basic phase transfer conditions catalyzed by chiral quaternary ammonium salts such as (-)-N,N-dimethyl-ephedrinium bromide has been reported to yield styrene oxide in high optical purity [19], which may be somewhat overestimated [20]. 

Benzylic bromination of atropisomers has proven important in the preparation of a variety of dinapththazepine catalysts, like Maruoka s chiral, quaternary ammonium salt used for PTC. Amine 69 is considered a key building block for the preparation of this type of chiral catalyst. Bis-bromination of 67 provided 68 in 54% yield using NBS and catalytic AIBN in cyclohexane. Notably, the pure, bis-bromide 68 precipitates out of solution. Again, the benzyl bromide system was utilized as a leaving group. 

The reaction of carbonyl substrates with oxygen in basic media has been utilized by Shioiri and co-workers to generate optically active a-hydroxy carbonyl compounds when a quaternary ammonium salt such as /V-(4-trifluoromethylphenylmethyl)cinchoninium bromide (2) is employed as a chiral catalyst102. Enantiomeric excesses up to 74% have been realized by using this simple methodology (Table 10). 

Reactions of Phosphonium Salts.— Asymmetric induction is observed on alkaline hydrolysis of the prochiral phosphonium salts (132) under phase-transfer conditions in the presence of an optically active quaternary ammonium salt, forming the chiral oxides (133) with a 0—8 % enantiomeric excess. Alkaline hydrolysis of monobenzyl quaternary salts of a,co-bis(diphenylphosphino)alkanes gives a route to diphosphine monoxides, e.g., (134). Aqueous hydrolysis of (dibromo-fluoromethyl)triphenylphosphonium bromide gives a high yield of dibromo-fluoromethane and triphenylphosphine oxide. When the reaction is carried out in the presence of radiolabelled Br, the evidence points to the involvement of the dibromofluoromethyl carbanion, and not to a carbene intermediate as was observed in the reaction of the related (bromodifluoromethyl)phos-phonium salt. ... 

Several chemocatalytic systems for sulfoxidation that employ other oxidants than hydrogen peroxide, molecular oxygen, or alkyl hydroperoxide have been reported. Manganese-catalyzed oxidation of sulfides with iodosobenzene (PhIO) using chiral Mn(salen) complexes was used to obtain chiral sulfoxides in up to 94% ee [71]. PhIO was also employed as the oxidant in sulfoxidations catalyzed by quaternary ammonium salts [72]. The use of cetyltrimethylammonium bromide (n-Ci6H33Me3N Br ) gave the best result, and with 5-10 mol% of this catalyst, high yields (90-100%) of sulfoxide were obtained from various sulfides. 

An achiral quaternary ammonium salt such as tetrabutylammonium bromide exerts a synergistic effect on addition of diethylzinc to aldehydes catalysed by a chiral phosphoramide. In the presence of 10mol% tetrabutylammonium bromide, the 0 chiral catalyst can be lowered from 10 to 0.5 mol%, while retaining reactivity and ee. 

Cinchona Alkaloid-Derived Quaternary Ammonium Salts The first successful application of cinchona-based quaternary ammonium salts as chiral phase-transfer catalysts was conducted by the Merck research group in 1984 [16]. Dolling and coworkers reported the N-p-trifluoromethylbenzylcinchoninium bromide 11a for the highly enantioselective alkylation of indanone derivatives imder phase-transfer conditions (Figure 12.3). 

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