Bronsted ring-forming reactions

Since their development in 1974 ZSM-5 zeolites have had considerable commercial success. ZSM-5 has a 10-membered ring-pore aperture of 0.55 nm (hence the 5 in ZSM-5), which is an ideal dimension for carrying out selective transformations on small aromatic substrates. Being the feedstock for PET, / -xylene is the most useful of the xylene isomers. The Bronsted acid form of ZSM-5, H-ZSM-5, is used to produce p-xylene selectively through toluene alkylation with methanol, xylene isomerization and toluene disproportionation (Figure 4.4). This is an example of a product selective reaction in which the reactant (toluene) is small enough to enter the pore but some of the initial products formed (o and w-xylene) are too large to diffuse rapidly out of the pore. /7-Xylene can, however. 

The alkylation of benzene with ethylene is an electrophilic substitution on the aromatic ring. Alkylation reactions are commonly considered as proceeding via carbenium-ion-type mechanisms. On a Bronsted acid site ethylene is protonated to form the active species. The latter can follow two major routes ... 

Xylene isomerization is a test reaction which is claimed to require moderately strong Bronsted acid sites to proceed. One reason for this is the very good stabilization of the formed carbenium ions over the benzene ring. The reaction proceeds via a benzenium ion and the rate-Umiting step in this reaction is the intramolecular methyl transfer. Besides the monomolecular mechanism, xylene isomerization can also proceed via a bimolecular reaction pathway as outlined by Morin et al. [ 172]. They determined the contributions of both pathways and determined the contribution of the monomolecular reaction, which they propose to compare activity and acidity in zeolites. These findings emphasize that the reaction pathway should be known in order to properly estimate acidity. Especially for large pore zeolites, this may be a problem. 

The most rapid ring-opening reactions of epoxides are acid catalyzed. The acid may be a Bronsted or protonic acid or a more generalized Lewis acid. Many of the latter are the well-known Friedel-Crafts catalysts, inorganic halides of the form MX , such as AICI3, BF3, SnCl4, PFg, and ZnCl2. 

The ionization of (E)-diazo methyl ethers is catalyzed by the general acid mechanism, as shown by Broxton and Stray (1980, 1982) using acetic acid and six other aliphatic and aromatic carboxylic acids. The observation of general acid catalysis is evidence that proton transfer occurs in the rate-determining part of the reaction (Scheme 6-5). The Bronsted a value is 0.32, which indicates that in the transition state the proton is still closer to the carboxylic acid than to the oxygen atom of the methanol to be formed. If the benzene ring of the diazo ether (Ar in Scheme 6-5) contains a carboxy group in the 2-position, intramolecular acid catalysis is observed (Broxton and McLeish, 1983). 

One of the problems associated with thermal cyclodimerization of alkenes is the elevated temperatures required which often cause the strained cyclobutane derivatives formed to undergo ring opening, resulting in the formation of secondary thermolysis products. This deficiency can be overcome by the use of catalysts (metals Lewis or Bronsted acids) which convert less reactive alkenes to reactive intermediates (metalated alkenes, cations, radical cations) which undergo cycloaddilion more efficiently. Nevertheless, a number of these catalysts can also cause the decomposition of the cyclobutanes formed in the initial reaction. Such catalyzed alkene cycloadditions are limited specifically to allyl cations, strained alkenes such as methylenccyclo-propane and donor-acceptor-substituted alkenes. The milder reaction conditions of the catalyzed process permit the extension of the scope of [2 + 2] cycloadditions to include alkene combinations which would not otherwise react. 

The reaction is based upon the two components condensation between an aldehyde or ketone 6 (or their synthetic equivalents) and alcohol 95, which contains an allylsilane (or vinylsilane) moiety. The IMSC reaction is mediated by Lewis or Bronsted acids, which activate the carbonyl group of 6 towards nucleophilic attack. After addition of alcohol 95 on the activated carbonyl, the oxonium cation 96 is formed, which is intramolecularly captured by the pendant allylsilane function, leading to oxygen-containing rings 97 (Scheme 13.38). This process typically requires a stoichiometric (or more) amount of Lewis acid. 

Furan itself can be used as the starting material for the synthesis of 1-methylpyrrole <2002MI179>. 7-AI2O3 was found to be an effective catalyst for the dehydration reaction between furan and methylamine to afford 1-methylpyrrole. A yield of 57.6% was achieved under the experimental conditions of a reaction temperature of 400 °C, a methylamine/ furan molar ratio of 1.5, and the molar flow rate of furan approximately 3-3.5 mmol/h. Furan was adsorbed onto Bronsted acid sites on the catalyst, while the methylamine was adsorbed onto Lewis acid sites. With this heterogeneous catalyst, the rate determining step of the mechanism was suggested to be the adsorption of furan on the Bronsted acid sites to form a ring-opened species, which is followed by the insertion of the adsorbed methylamine to form secondary amine intermediates. Further dehydration at the Lewis acid sites would yield 1-methylpyrrole. 

The acidic 10 and 12 membered ring zeolites (H-MOR, ZSM-5, ZSM-11) can also be used to catalyze the condensation of alkenes with aldehydes to form unsaturated alcohols, acetals etc. (Prins reaction)[92]. Chang et a/. [93] showed that this reaction involves in the initial step the activation of the aldehyde by a Bronsted acid site to generate an electrophilic species. The condensation with, e.g., isobutene leads then to a primary alcohol with a positive charge at the tertiary carbon atom. Elimination of water and addition of further aldehyde molecules may lead to a broad variety of products. Some of these reactions can be effectively blocked by chosing zeolites with the appropriate pore size [94,95]. 

However, a totally different catalysis is observed when the protons are not neutralized, so that transition metal clusters and Bronsted sites co-exist in the same catalyst. For such bifimctional catalysts, for instance Pd/HNaY or Pd/HY, ring opening is a minor side reaction, but ring-enlargement becomes the major reaction pathway, with benzene and cyclohexane as the predominant reaction products [28. 29]. Apparently, a carbenium ion has been formed from MCP, it is isomerized via the fused cyclopropane ring to the cyclohexylcarbocation, as depicted in scheme 9 ... 

X-ray crystallographic data), a bicarbonate ion at the active site is shown in red, the zinc cation at the active site is green, a water molecule is shown in blue, and the basic sites that coordinate with the zinc cation (as Lewis bases) or remove the proton from water to form hydroxide (as Bronsted-Lowry bases) are magenta (these bases are nitrogen atoms from histidine imidazole rings). No hydrogen atoms are shown in any of these species. As you can see, a remarkable orchestration of Lewis and Bronsted-Lowry acid-base reactions is involved in catalysis by carbonic anhydrase. 

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