Bronsted Acid Catalyzed Reactions

The combinahon of a stronger Bronsted acid with an electrophile bearing a basic electrophile would result in a Bronsted-acid-catalyzed reaction. Use of aldehyde in place of aldimine with neutral acid would lead to a hydrogen-bond-catalyzed reaction. 

The DA dimer obtained in the aminium salt reaction differs from that obtained by the PET method and from that obtained using the aminium salt/hindered base method, but is the same as that obtained by the Bronsted acid catalyzed reaction of this diene. The addition of insoluble bases hke sodium carbonate is not sufficient to suppress the extremely facile, acid-catalyzed cyclodimerization of this particular diene, which yield a highly stabilized tetramethylallyl carbocation intermediate. Nevertheless, this results demonstrates the necessity for caution in assigning a cation radical mechanism to a cyclodimerization reaction observed under aminium salt conditions. 

Bronsted acid-catalyzed reactions will be proportional to the proton content of a zeolite as long as the intrinsic acidity of a proton remains unchanged, since the proton concentration is proportional to the Al/Si ratio. Once the Al/Si ratio exceeds a critical value (Al/Si)c, the intrinsic acidity of a zeolitic proton starts to decrease. This effect dominates over the increased number of protons and, as a consequence, the overall rate of the reaction decreases. 

The Fischer esterification, named after the eminent German chemist Emil Fischer, is the Bronsted acid-catalyzed reaction of an alcohol with a carboxylic acid to form an ester (see Table 1). Sulfuric acid is often the catalyst. Esters are widely used in the soap, perfume, and food industries. 

Finally, radiolysis/EPR results forced the reinterpretation of literature spanning more than 25 years on EPR studies of olefin reactions on zeolites in the presence of Lewis acid sites. " Spontaneous oxidation of olefins and products of their Bronsted acid-catalyzed reactions on dehydroxylated (to form Lewis acid sites) ZSM5 and Mordenite vide infra) provides an alternative method of spin labeling, and the technique gives results mostly consistent with those from radiolysis/EPR. However, in the spontaneous oxidation method, the limited degree of control over the ionization process and progress of the catalysis has led to erroneous conclusions (mainly in the earlier work). Interpretation of the radiolysis/EPR experiments is greatly aided by the ability to methodically vary the relative contribution of different products to the EPR spectrum. 

Although SnCl2 being the only Lewis acid studied herein, and consequently it may display a different catalytic action mechanism, it was also included in this topic for comparison. Actually, it seems that the SnCl2 is also an efficient catalyst, taking in account that high yields in ethyl oleate were achieved, similarly to those obtained in the Bronsted acid-catalyzed reactions. 

Many Bronsted acid catalyzed reactions have been reported [21] and they can be classified into two different types depending on the actual role of the Bronsted acid catalyst. One is a proton donor and the other is an electron acceptor. 

Although phosphoric acid has been employed for the activation of carbon-nitrogen double bond, Yamamoto et at. designed a stronger chiral Bronsted acid in order to expand the scope of the chiral Bronsted acid catalyzed reactions. A N-triflyl phosphoramide (50), bearing BINOL backbone, catalyzed the Diels-Alder reaction of a,p-unsaturated ketone with electron-rich diene ((2, )-siloxydiene is major) to give cyclohexene derivatives in high ees (Scheme 2.106) [184]. 

Scheme 38.45 Chiral Bronsted-acid catalyzed reactions with Brassard s diene.

The appHcation of chiral Bronsted acids in asymmetric catalysis has increased in recent years [6], In 2006, Rueping et al. reported a double Bronsted acid catalyzed reaction of imine 1 and cyclohexenone 2, in which the electrophile was achvated by a chiral phosphoric acid 4 and the nucleophile was activated by an achiral Bronsted acid 5 (Scheme 43.1) (10). Various aromahc and heteroaromatic substituted isoquinuclidines with electron-withdrawing and electron-donating substituents could be isolated in good yields and high enantiomeric ratios (82-88% ee), with exo/endo ratios between 1 3 and 1 9. The reachvity of a broad range of achiral Bronsted adds was examined and the effect on the enantiomeric induchon was obvious (52% ee to 86% ee for the product 3). 

Because of the mentioned leveling effect of the solvent (or excess acid itself acting as such) the acidity cannot exceed that of its conjugate acid. In the case of water the limiting acidity is that of HsO. Proton-ated water, H30 (hydronium ion), was first postulated in 1907, and its preeminent role in acid-catalyzed reactions in aqueous media was first realized in the acid-base theory of Bronsted and Lowry. Direct experimental evidence for the hydronium ion in solution and in the... 

This section deals with Bronsted acid and Lewis acid catalyzed reactions, excluding Friedel-Crafts reactions, but including reactions such as nitrations, halogenations, and Claisen rearrangements. Friedel-Crafts reactions are discussed in the subsequent Sections 5.1.2.2 and 5.1.2.3. 

Bronsted-acid-catalyzed Diels-Alder reactions are not frequent because of the proton sensitivity of many dienes and cycloadducts, especially when long reaction times and high temperatures are required. Examples in aqueous medium involving imines activated by protonation as dienophiles and a proton-promoted Diels-Alder reaction of glyoxylic acid with cyclopentadiene are considered in Section 6.1. 

The fluoboric acid-catalyzed aza-Diels-Alder reaction of aldimine and Danishefsky s diene proceeds smoothly to afford dihydro-4-pyridones in high yields [90] (Equation 4.16). Unstable aldimines generated from aliphatic aldehydes can be prepared in situ and allowed to react under one-pot reaction conditions. This one-pot Bronsted acid-catalyzed three-component aza-Diels-Alder reaction affords the adducts in good to high yields. 

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. 

Bronsted acid catalyzed aza-Diels-Alder reaction of Danishefsky s diene with aidimine generated in situ from aldehyde and amine in aqueous media [107]... 

Chiral base catalysis was classified into five sections and reviewed. Although the reactions described herein are promoted by Bronsted or Lewis bases, the Lewis acidic characteristics of metals play important roles in both substrate activation and enantioselection. Compared with chiral Lewis acid-catalyzed reactions,... 

In summary, the differences between Br0nsted-acid catalysis and hydrogen-bond catalysis were discussed. Because there is a gradual transition from hydro-gen-bond catalysis to Bronsted-acid catalysis, it is not always easy to differentiate the two modes of catalysis. However, the combination of a stronger acid and an imine will be a Bronsted-acid-catalyzed reachon, while the combination of a neutral acid and an aldehyde will be a hydrogen-bond-catalyzed reaction. 

Consequently, the processes most relevant to the topic of this chapter, that is, hydrogen bonds in organocatalytic transition states, are (i) transition state stabilization by pure hydrogen bonding (without full proton transfer), and (ii) general Bronsted-acid/Bronsted-based catalyzed reactions which are initiated by hydrogen bonding but move further to distinct proton transfer. 

This novel Bronsted acid catalyzes the Diels-Alder reaction between ethyl vinyl ketone and various acycUc siloxy dienes to furnish adducts in uniformly high yields and ee s. Further, the corresponding chiral phosphoric acid was unable to catalyze this reaction. 

Ionic liquids containing chloroaluminate ions are strong Lewis, Franklin and Bronsted acids. Protons present in [emim][AlCl4] have been shown to be superacidic with Hammett acidities up to —18. Such highly acidic ionic liquids are, nonetheless, easily handled and offer potential as non-volatile replacements for hazardous acids such as HF in several acid-catalyzed reactions. 

Radioactive tracer experiments reported by Lombardo and Hall (4) showed that each butene isomer can be directly interconverted into the other two. These results are consistent with a common intermediate being in operation in this reaction. In Figure 3 the linear relationship between catalytic activity and percentage of Na+ replaced by H+ strongly favors a Bronsted acid catalyzed mechanism in which the common intermediate could be a secondary carbonium ion. This conclusion is also supported by the tracer experiments. 

Dealumination processes which leave residual extraframework aluminum in a Y-type zeolite result in a decrease in the overall number of Bronsted acid sites but an increase in the strength of the remaining acid sites. The net effect is an increase in activity for acid-catalyzed reactions up to a maximum at ca. 32 framework A1 atoms per unit cell. A model for strong Bronsted acidity is proposed which includes (i) the presence of framework Al atoms that have no other A1 atoms in a 4-membered ring and (ii) complex A1 cations in the cages. The essential role of extraframework aluminum is evident from recent studies in which framework A1 has been completely removed from zeolite-Y and by experiments on the related ZSM-20 zeolite. 

Scheme 5.19 The achiral phosphine and chiral binaphthol-derived Bronsted acid-catalyzed MBH reaction of cyclohexenone and 3-phenyl propionaldehyde.

Table 6.20 Chiral Bronsted acid-catalyzed diastereoselective Mannich reactions.

Table 6.21 Bronsted acid-catalyzed asymmetric Mannich reactions.

The proposed mechanism was identical with that in acid-catalyzed reactions except for the initiation step. Photolysis of the iodonium salt yields cations and cation radicals that react with traces of water or the monomer to form HX [23]. The Bronsted acid HX then functions similarly to other Bronsted acids in the polymerization reactions. 1,3-Diisopropenylbenzene has also been polymerized in a photoinitiated cationic reaction using 70 as the initiator [Eq. (14)] [9]. 

Classically, the acylzirconocene chlorides can be converted into aldehydes, carboxyhc acids, esters, and acyl halides. Recently, the usefulness of acylzirconocenes has been extended. In fact, these unmasked acyl anions directly add to aldehydes and imines affording a-hydroxy or a-amino ketones, respectively. The reaction of acylzirconocene chlorides with imines also proceeds under Bronsted acid-catalyzed conditions, even with aqneons acids. ... 

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