Catalysts Bronsted acid

Reaction of 1 mole of aminals 352 with 4 mol of methyl 3-aminocrotonate in the presence of the solid acids montmorillonte clay (Kio) and ZF520 zeolite as strong Bronsted acidic catalysts, gave 1,4-dihydropyridines 353 and 2-methyl-4//-pyrido[l, 2-n]pyrimidin-4-one (99MI8). 

Benzene can be alkylated in the presence of a Lewis or a Bronsted acid catalyst. Olefins such as ethylene, propylene, and C12-C14 alpha olefins are used to produce benzene alkylates, which have great commercial value. Alkyl halides such as monochloroparaffms in the C12-C14 range also serve this purpose. 

Another conceptually unique approach in alkene aziridination has come from Johnston s labs. These workers shrewdly identified organic azides as nitrene equivalents when these compounds are in the amide anion/diazonium resonance form. Thus, when a range of azides were treated with triflic acid and methyl vinyl ketone at 0 °C, the corresponding aziridines were obtained, in synthetically useful yields. In the absence of the Bronsted acid catalyst, cycloaddition is observed, producing triazolines. The method may also be adapted, through the use of unsaturated imi-des as substrates, to give anti-aminooxazolidinones (Scheme 4.25) [32]. 

Nafion-H (144), a perfluorinated resin-sulfonic acid, is an efficient Bronsted-acid catalyst which has two advantages it requires only catalytic amounts since it forms reversible complexes, and it avoids the destruction and separation of the catalyst upon completion of the reaction [94], Thus in the presence of Nafion-H, 1,4-benzoquinone and isoprene give the Diels-Alder adduct in 80% yield at 25 °C, and 1,3-cyclohexadiene reacts with acrolein at 25 °C affording 88 % of cycloadduct after 40 h, while the uncatalyzed reactions give very low yields after boiling for 1 h or at 100 °C for 3.5 h respectively [95], Other examples are given in Table 4.24. In the acid-catalyzed reactions that use highly reactive dienes such as isoprene and 2,3-dimethylbutadiene, polymerization of alkenes usually occurs with Nafion-H, no polymerization was observed. 

In some cases the effect of the nature of aromatic hydrogen substituent has been also observed (Table 48.2). These results contrast with those obtained using typical homogeneous Bronsted acid catalysts (e.g., sulphuric) for the acylation of the same substrate with acetic anhydride, under the same experimental conditions, where the yields (98%, 95%, 91% for R=N02, H, OMe, respectively) do not significantly depend on the nucleophile s substituent nature [23]. These data imderline the contribution of the heterogeneous catalyst. 

Scheme 10 Plausible catalytic mechanism for alkyne-carbonyl coupling as supported by the effect of chiral Bronsted acid catalyst and deuterium-labeling...

On treating diisobutene with acetic anhydride and anhydrous zinc chloride, A. C. Byrns and T. F. Doumani had isolated in 1943 a crystalline compound to which they had ascribed the structure of a zinc complex with a 1,3-diketone 40 the correct pyrylium chlorozincate structure was established by A. T. Balaban et al.41 in 1961, after extended investigation on the formation of pyrylium salts by alkene diacylation.42 This formation again had remained undetected for many decades during which alkenes had been acylated but only the water-insoluble monoacylation products had been investigated, whereas the water-soluble pyrylium salts went into the sink with the Lewis or Bronsted acid catalysts that had been used in the acylation. 

Ground-state alkenes generally undergo electrophilic addition with alcohols in the presence of a Bronsted acid catalyst, yielding the Markovnikov product ... 

Inspired by the reaction mechanism of Noyori s catalytic enantioslective transfer hydrogenation of ketones (32) using a chiral Ru-amido complex 31, Dcariya et al. reported that 31 can also function as a unique Bronsted base-Bronsted acid catalyst... 

Williams JT, Bahia PS, Snaith JS (2002) Synthesis of 3,4-disubstituted piperidines by carbonyl ene and prins cyclizations a switch in diastereoselectivity between Lewis and Bronsted acid catalysts. Org Lett 4 3727-3730... 

Examples of the Bronsted-acid catalysts and hydrogen-bond catalysts are shown in Figure 2.1. We have recently reported the Mannich-type reaction of ketene silyl acetals with aldimines derived from aromatic aldehyde catalyzed by chiral phosphoric acid 7 (Figure 2.2, Scheme 2.6) [12]. The corresponding [5-amino esters were obtained with high syn-diastereoselectivities and excellent enantioselectivities. 

In the above example, although the hydrogen-bond complex was formed initially, the nucleophilic addihon reaction was supposed to proceed via protonated form CPli, which clearly indicated that the phosphoric acid worked as a Bronsted-acid catalyst. 

The results demonstrate that hydrogensulfate and tetrakis(hydrogensulfato)borate ionic liquids are highly interesting additives to mineral acids to form new, highly Bronsted-acidic catalysts. For example, it was found that a mixtures of sulphuric acid with only 2.2 mol% of [0MIM][B(HS04)4] ionic liquid yielded 90% more monoalkylbenzene product than the neat sulphuric acid catalyst under identical reaction conditions. This and related results are explained by an interplay of solubility and acidity effects caused by the ionic liquid additive. 

The allylation of carbonyl compounds with allyltin reagents is still an active area of organotin chemistry from the methodological point of view, and also for synthetic applications. For completeness we should add several alternative techniques, such as the development of trifluoromethanesulphonic acid as a Bronsted acid catalyst for the allylation of aldehydes in water123, or the design of fluorous allyltin reagents for the platinum-catalysed allylation of aldehydes124. 

In a continuation of their work, however, Heilmann and Maier found that their imprinted silica gels exhibited selective transesterification and lactonisation that was no better than control materials containing phosphoric acid [55]. In fact, the catalytic activity could be removed by rinsing the gel in water. The TSA apparently turns into phosphoric acid during the high temperature treatment and then remains on the gel as a Bronsted acid catalyst (Fig. 8.15). The enhancement of selectivity... 

Recently [43] Gao et al. applied a zeolite-fiOlled polyvinyl alcohol (PVA) membrane in esterification and acetalization reactions. Zeolites NaA, KA and CaA as well as NaX were loaded into PVA up to 27 wt% and the composites tested in selective water removal during reaction. A pervaporation cell with a membrane area of 22.9 cm was coimected to a collection system kept at a vacuum of 0.1 mm Hg. A sulfonated resin was used as Bronsted acid catalyst in the esterification mixture (120 ml). Figure 28 shows the progress of the esterification of salicylic acid and methanol at 60°C. The reaction is accelerated considerably as a result of the water removal. 

Zeolite beta has been widely studied as a Bronsted acid catalyst and has been shown to be highly active for reactions such as alkylation and acylation [8,9]. The effect of the crystallite size on the catalytic activity of beta has been investigated [10] but since betas often have very high external surface areas, it is possible that acid sites associated with framework aluminium close to the outer surface will contribute to the overall catalytic activity of the zeolite. This may adversely affect the shape-selectivity of the reaction. In this study a series of beta zeolites with differing Si/Al ratio and ESA were investigated by means of non-contact AFM and N2 absorption measurements and the catalytic activity was tested by an acylation reaction capable... 

The use of Bronsted acid catalysts such as HF and H2SO4 is discouraged in favor of mild solid acid catalysts such as zeolites, montmoriUonite and Nafion-H vide infra). Triflic acid and /7-toluenesulfonic acid can also be used as convenient catalysts for the alkylation of phenols. 

Because of their high acidity, sulfonic acids are extensively used as Bronsted acid catalysts for esterification. 

Enantioselective Synthesis of Amines by Chiral Bronsted Acid Catalysts... 

The desirable features of phosphoric acids as chiral Bronsted acid catalysts are summarized as follows (Figure 3.1) ... 

Figure 3.1 Chiral phosphoric acids as chiral Bronsted acid catalysts.

In this chapter, we focus on recent achievements in the enantioselective synthesis of chiral amines using 1,1 bi 2 naphthol (BINOL) derived monophosphoric acid (1) or related phosphoric acids as chiral Bronsted acid catalysts 2, 3], The contents are arranged according to the type of bond forming reaction, including carbon carbon, carbon hydrogen, and carbon heteroatom bond forming reactions, followed by specific reaction types. 

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