Lewis/Bronsted acid catalysts

Some of the catalyst systems used in the asymmetric aldol reaction are also effective in related reactions. Thus, bifunctional catalysts and L-prohne-based organocatalysts have been used to good effect in the nitroaldol reaction and Mannich reaction. The latter process is also effectively catalysed by enantiomeri-cally pure Bronsted acids. Furthermore, much recent progress has been made in the development of a catalytic asymmetric Morita-Baylis-Hillman reaction using Lewis/Bronsted acid catalysts and bifunctional catalysts. 

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. 

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. 

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... 

The various support materials have different effects on potential carbon formation. This seems to go in parallel with the Lewis/Bronsted acidity. The main commercially used catalyst supports can be ranked as follows in decreasing order of carbon forming tendency (and thus in decreasing order of the important minimum practical steam/ carbon ratio) a-alumina > magnesium aluminate (spinel) > calcium aluminate > alkalized calcium aluminate [419],... 

In general, a Lewis acid catalyzed 1,3 dipolar cycloaddition reaction of nitrones with vinyl ethers provides exo isomers predominantly. In fact, the MeAl BINOL catalyzed cycloaddition reaction of ethyl vinyl ether with diaryl nitrone furnished the corresponding product vhth high exo selectivity [121]. However, phosphoramide 6b as the chiral Bronsted acid catalyst exhibited high e-ndo selectivity in the same cycloaddition reaction (see Scheme 3.27). 

Fig. 7 shows the variation of the steady conversion as a function of the integrated areas of the NH/ deformation band. The linear relationship verifies that the Bronsted acidity is directly ruling the isomerization activity of the catalysts. Although no reliable values could be established for the Lewis acidity, the bands in the N-H stretching region suggest a relative decrease of the Lewis/Bronsted acid ratio with increasing sulfate content. 

Montmorillonites are more frequently used as Bronsted acid catalysts even if Lewis acidity plays a role in their catalytic activity. The origin of Bronsted acidity in metal-exchanged montmorillonites is ascribed to the polarizing influence of the cation on the water molecules in spatially restricted interlayers. The exchangeable cations are either protons or polarizing cations [e.g., aluminum, chromium(III), or iron(III)]. 

It must be noted that the phenol/aldehyde reaction can be catalyzed by Bronsted acids (protonation of the carbonyl oxygen) as well as by Lewis acids (coordination of the carbonyl oxygen). In the latter case one Lewis centre (e.g. Al ) can accommodate and activate both the phenol and the aldehyde (cq. the benzyl alcohol, in the consecutive reaction). As a consequence, ortho-substitution is favoured [14,15]. The high 2,2 -dihydroxydiphenylmethane selectivity we obtained with homogeneous Al " -catalysis and with 7-alumina is consistent with these data. Additionally, the finding that the H - US - Y catalyzed toluene/formaldehyde-condensation gives a low 2,2 -selectivity, 19% [16], compared to the 32% we obtained with phenol, also indicates the hydroxyl-group plays a role. However, transalkylation, reported to lead to ortho-substitution in condensations of phenol with methanol on both zeolite- and non-zeolite Bronsted acid catalysts [17], can t be ruled out. 

From a qualitative point of view, both acid-catalyzed mechanisms may account for the observed reactivity, and Bronsted acid catalysts seem to be more efficient than Lewis ones. However, limitations of catalyst modelling prevent us from drawing further conclusions. 

The activation of epoxides for ring opening reactions can be achieved either by Bronsted acidic catalysts via the addition of a proton to the epoxide oxygen or by Lewis acidic catalysts via the coordination of the epoxide oxygen to a multivalent cation. 

FTIR study H3PO4 on silica is the strongest Bronsted acidic catalyst the other supports generate Bronsted acidity and modify Lewis acidity and basicity. Isobutene and but-l-ene oligomerize on all catalysts at 0.027 MPa, 20 °C. Ti-support was most active. 

Furfural is formed by dehydration of pentose. Xylose is a major aldopentose and is involved as a form of xylan in hemicelluloses. Unlike glucose, furfural can be formed from xylose by Bronsted acids alone at high temperature, although the furfural selectivity is low. A variety of Bronsted acid catalysts have been examined for furfural synthesis and they are H-type zeolites such as H-mordenite and H-Y faujasite [183], delaminated zeolite [184], H-MCM-22 [185], ion-exchange resins [186], sulfonated porous silicas [187-189], porous niobium silicate [190], metal oxide nanosheets [51], and sulfated zirconia [191]. Sulfated tin oxide (S04 /Sn02) is an effective catalyst for furfural formation [192] because of the combination of Lewis acid and Bronsted acid properties, as well as HMF synthesis. 

In alcoholic solution, lactic ester, for example, methyl lactate can be produced in the presence of a Lewis add catalyst On the contrary, Bronsted acid catalysts such as ion-exchange resins selectively convert GLA/DHA to pyruvaldehyde dimethylacetal via acetahzation of PA. Strong Br0nsted add sites should thus be diminished to avoid this acetahzation Lewis acid sites are responsible for selective formation of methyl lactate [200-202]. However, the rate-determining step for the reaction is considered to be the first dehydration of GLA/DHA to PA, which is accelerated by weak Bronsted acid sites [203]. A bifunctional catalyst with Lewis acid sites and weak Bronsted acid sites, for example, a composite of carbon (weak Bronsted acid) and Sn-sihca (Lewis add) is reported as a fast and selective catalyst for lactic acid and... 

Friedel-Crafts alkylation is one of the most frequently used and widely studied reactions in organic chemistry. Since the initial discovery by Charles Friedel and James Mason Crafts in 1877, a large number of applications have emerged for the construction of substituted aromatic compounds. Friedel-Crafts alkylation processes involve the replacement of C—H bond of an aromatic ring by an electrophilic partner in the presence of a Lewis acid or Bronsted acid catalyst. Particularly, catalytic asymmetric Friedel-Crafts alkylation is a very attractive, direct, and atom-economic approach for the synthesis of optically active aromatic compounds. However, it took more than 100 years from the discovery of this reaction until the first catalytic asymmetric Friedel-Crafts (AFC) alkylation of naphthol and ethyl pyruvate was realized by Erker in 1990. Nowadays, owing to continued efforts in developing... 

Based on the results, the tiifUc acid is a promising active acid catalyst for isomerization and transalkylation of DIPB isomers in order to produce higher yield of cumene. Its activity is comparable with solid acid catalysts and other Lewis and Bronsted acid catalysts at room temperature. 

Lewis acid catalysts are an interesting option for the fatty acid esters production [16,17], However, Lewis acid compounds that are traditionally used in organic synthesis such as BF3 are more expensive than Bronsted acid catalysts, and are more hardly manipulated, in addition to be few water tolerant. Those unfavorable aspects difficult the use of Lewis acid catalysts in FFA esterification at industrial scale [18],... 

Conversely, in presence of either Lewis or Bronsted acid catalysts, high ethyl oleate yielding (ca. 90-95%) were reached (Figure 6). Noticeably, the initial rate of reactions that were catalyzed by the HPW or H2SO4, which are stronger Bronsted acids, were the highest. 

It is a rather fortunate circumstance that the most easily synthesized calix[n]arenes are those derived from p-tert-butylphenol, as this group can be easily removed using Lewis or Bronsted acid catalysts, opening the way for... 

Arylboron compounds with electron-withdrawing aromatic groups such as triarylborons, diarylborinic acids, and arylboronic acids represent a new class of air-stable and water-tolerant Lewis acid or Bronsted acid catalysts in organic synthesis. In particular, arylboronic acids are showing to be powerful tools in the design of chiral boron catalysts. 

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