Ammonium bromide quaternary

Methyl bromide slowly hydrolyzes in water, forming methanol and hydrobromic acid. The bromine atom of methyl bromide is an excellent leaving group in nucleophilic substitution reactions and is displaced by a variety of nucleophiles. Thus methyl bromide is useful in a variety of methylation reactions, such as the syntheses of ethers, sulfides, esters, and amines. Tertiary amines are methylated by methyl bromide to form quaternary ammonium bromides, some of which are active as microbicides. 

Quaternary ammonium bromides and hydroxides (quats) are applied as templates in the synthesis of zeolites with relatively high Si/Al ratio. Examples will be given of the use of mon-, di-, poly- and associated quats as templates in zeolite growth. Templated zeolites of the MFI-type can be grown in a lateral or in an axial way onto metal supports, providing promising composite systems, for separation and catalysis, respectively. 

Quaternary ammonium bromides play an important role in the synthesis of high silica zeolites... 

The quaternary ammonium bromide (15 mmol) in CH2Cl2 (50 ml) is shaken with three portions (1.2 ml) of aqueous KSCN (sat. soln, 65 mmol). The organic phase is... 

Quaternary ammonium tribromides can also be produced in situ from the quaternary ammonium bromide, sodium hypochlorite and sodium bromide and can be used, for example, in electrophilic addition reactions reaction with alkenes and alkynes. 

Asymmetric induction in the Neber rearrangement was also obtained under phase-transfer conditions with chiral quaternary ammonium bromides 544 as catalysts (equation 243). Moderate enantioselectivities (30-70% ee, 60-95% yield) were observed, but there is still an opportunity for extending the full synthetic utility of this classical rearrangement. 

The addition of bromine to quaternary ammonium bromides to provide the tribromides is most easily achieved in a reaction column with the solid while a stream of air carries the required amount of bromine (Scheme 11). Thus, in the case of tetrabutylammonium bromide (TBAB), a sharp yellow reaction front is obtained while bromine is quantitatively added and the pure bromi-nation agent TBABrg is obtained. Small runs can be quantitatively performed with 0.5 bar bromine vapor and the unground crystals of TBAB [28]. Equally simple is the gas-solid addition of chlorine to triphenylphosphane to give triphenylphosphane dichloride [28]. 

Maruoka introduced a new heterochiral quaternary ammonium bromide PTC... 

Usually, treatment of the crude stannylene with benzoyl chloride for a few minutes at room temperature gives one of the two possible benzoates, in very high yield, in a regioselective manner. Allyl and benzyl halides do not react under these conditions, and they react only sluggishly in refluxing nonpolar solvents. Substitution proceeds in a satisfactory manner in the presence of 5-10 mol% of a quaternary ammonium bromide or iodide. Alternatively, the stannylene may be transferred into N, IV-dimethy lformamide, in which substitution occurs at 80°C. Again, allylation and benzylation of polyhydroxy lated compounds is regioselective by these methods. 

A very efficient catalytic system has been developed by He and coworkers using, under solvent-free conditions, quaternary ammonium bromide-functionalized polyethylene glycol (0.25 mol% 373 K, 8MPa, 5-20min) [68d]. The catalyst worked well for a variety of l-alkyl-2-aryl-substituted aziridines, and could be easily recovered by centrifugation and reused, without any significant loss of catalytic activity and selectivity. 

Monoetherification of polyols.12 Monobenzylation and monoallylation of polyols can be conducted conveniently under mild conditions by conversion to the stannylene derivative (dimeric) by di-n-butyltin oxide (5, 189 9, 141). The stannylene is then treated with benzyl bromide or allyl bromide and tetra-n-butylammonium iodide (1 equiv.) in benzene. The same conditions can be used to prepare monomethoxymethyl ethers. Quaternary ammonium bromides are less efficient catalysts than the iodides. These salts also accelerate reaction of stannylenes with acid anhydrides. The mechanism for this activation is not clear it may involve coordination of I" to tin. 

With this information at hand, Maruoka and coworkers developed a new C2-symmetric chiral quaternary ammonium bromide 12 which incorporated an achiral, conformationally flexible biphenyl subunit (Scheme 5.8) [12]. 

This unique phenomenon provides a powerful strategy in the molecular design of chiral catalysts that is, the requisite chirality can be served by the simple binaphthyl moiety, while an additional structural requirement for fine-tuning of reactivity and selectivity can be fulfilled by an easily modifiable achiral biphenyl structure. This certainly obviates the use of two chiral units, and should be appreciated in the synthesis of a variety of chiral catalysts with different steric and/or electronic properties. Actually, quaternary ammonium bromide possessing a sterically demanding substituent such as (S)-12b can be easily prepared, and benzylation with (S)-12b as catalyst gave 9 in 95% yield with 92% ee. Further, theenantioselectivity was enhanced to 95% ee with (S)-12c as a catalyst [12]. 

Table 5.4 Asymmetric monoalkylation of 21 under phase-transfer conditions catalyzed by chiral quaternary ammonium bromide (R,R)-le or (R)-16Aa.

By using glycine diphenylmethyl (Dpm) amide-derived Schiff base 22 as a key substrate and N-spiro chiral quaternary ammonium bromide lg as an ideal catalyst, a high enantioselectivity was achieved, even in the alkylation with less-reactive simple secondary alkyl halides, as shown in Table 5.5 [21]. This system offers a facile access to structurally diverse optically active vicinal diamines in combination with the subsequent reduction (Scheme 5.14) [21]. 

Since the aldimine Schiff base 21 can be readily prepared from glycine, direct stereoselective introduction of two different side chains to 21 by appropriate chiral phase-transfer catalysis would provide an attractive, yet powerful, strategy for the asymmetric synthesis of structurally diverse a,a-dialkyl-a-amino acids. This possibility of a one-pot asymmetric double alkylation has been realized by using N-spiro chiral quaternary ammonium bromide le (Scheme 5.21). 

Upon facing the difficulty of stereochemical control in peptide alkylation events, Maruoka and coworkers envisaged that the chiral phase-transfer catalyst should play a crucial role in achieving an efficient chirality transfer, and consequently examined the alkylation of the dipeptide, Gly-L-Phe derivative 57 (Scheme 5.28) [31]. When a mixture of 57 and tetrabutylammonium bromide (TBAB, 2 mol%) in toluene was treated with a 50% KOH aqueous solution and benzyl bromide at 0°C for 4h, the corresponding benzylation product 58 was obtained in 85% yield with the diastereo-meric ratio (DL-58 LL-58) of 54 46 (8% de). In contrast, the reaction with chiral quaternary ammonium bromide (S,S)-lc under similar conditions gave rise to 58 with 55% de. The preferential formation of LL-58 in lower de in the reaction with (R,R)-lc indicated that (R,R)-lc is a mismatched catalyst for this diastereofacial differentiation of 57. Changing the 3,3 -aromatic substituent (Ar) of the catalyst 1 dramatically increased the stereoselectivity, and almost complete diastereocontrol was realized with (S,S)-lg. 

Efficient, highly enantioselective construction of quaternary stereocenter on P-keto esters under phase-transfer conditions has been achieved using N-spiro chiral quaternary ammonium bromide lh as catalyst [32]. This system has a broad generality in terms of the structure of P-keto esters 65 and alkyl halides (Scheme 5.31). 

A highly enantioselective alkylation of 3,5-diaryloxazolidin-2,4-diones 70 with N-spiro chiral quaternary ammonium bromide (S,S)-69 was achieved under mild phase-transfer conditions. With this methodology in hand, a wide range of tertiary a-hydroxy-a-aryl carboxylic acid derivatives may easily be obtained in good yields and high enantiomeric excesses (Scheme 5.34) [35]. 

Jew and Park achieved a highly enantioselective synthesis of (2S)-a-(hydroxy-methyljglutamic acid, a potent metabotropic receptor ligand, through the Michael addition of 2-naphthalen-l-yl-2-oxazoline-4-carboxylic acid tert-butyl ester 72 to ethyl acrylate under phase-transfer conditions [38]. As shown in Scheme 5.36, the use of BEMP as a base at —60 °C with the catalysis of N-spiro chiral quaternary ammonium bromide le appeared to be essential for attaining an excellent selectivity. 

Phase-transfer-catalyzed direct Mannich reaction of glycine Schiff base 2 with a-imino ester 79 was achieved with high enantioselectivity by the utilization of N-spiro chiral quaternary ammonium bromide le as catalyst (Table 5.14) [42],... 

Very recently, Maruoka and co-workers described a new N-spiro quaternary ammonium bromide with two chiral biphenyl structures as easily modifiable subunits [37]. These phase-transfer catalysts with biphenyl subunits, containing methyl groups in the 6,6 -position for inducing chirality, and additionally bulky substituents in the 4-position, efficiently catalyzed the alkylation of protected glycinate with high enantioselectivity of up to 97% ee. The substrate range is broad, for example (substituted) benzyl bromide and allylic and propargylic bromides are tolerated [37]. 

Du Y, Wu Y, Liu A-H et al (2008) Quaternary ammonium bromide functionalized polyethylene glycol a highly efficient and recyclable catalyst for selective synthesis of 5-aryl-2-oxazolidinones from carbon dioxide and aziridines under solvent-free conditions. J Org Chem 73(12) 4709-4712... 

Scheme 5.10 Chemical fixation C02 catalyzed by quaternary ammonium bromide functionalized PEG [46]...

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