Bronsted or Lewis acids

In principle, complex hydrides (NaBHj, LiAlH ) ought to react similarly with 4-pyrones and lead after treatment with Bronsted or Lewis acids to 4-unsubstituted pyrylium salts. This reaction has not been reported the reduction of 2-pyrones with LiAlH4 results in ring opening. " ... 

Other methods of nitration that Laali investigated were with isoamyl nitrate in combination with a Bronsted or Lewis acid in several ionic liquids, with [EMIM][OTf] giving the best yields (69 %, 1.0 1.0 o p ratio). In the ionic liquid [HNEt( Pr)2] [CE3CO2] (m.p. = 92-93 °C), toluene was nitrated with a mixture of [NH4][N03] and trifluoroacetic acid (TEAH) (Scheme 5.1-37). This gave ammonium trifluoroacetate [NH4][TEA] as a by-product, which could be removed from the reaction vessel by distillation (sublimation). 

Friedel-Crafts reactions have been studied in detail by Olah [74, 75]. These reactions result in the formation of carbon-carbon bonds and are catalyzed by strong Bronsted or Lewis acids. 

Particularly, some newly developed drags, which incorporate the N-acyl sulfonamide moiety [8-10], are synthesized from the parent sulfonamides, by their coupling with acid chlorides or carboxylic anhydrides in basic conditions [11-15]. Unfortunately all these methods lead to substantial waste products. Less common reports mentioning this transformation under acidic conditions (Bronsted or Lewis acids) do not systematically examine the purpose and limitations of the reaction [16]. 

Scheme 43) [92]. Reaction of dienophiles such as 4-nitrobenzaldehyde with linker 80 at high temperature gave Diels-Alder products. Dihydro-pyrans were released from the support by Bronsted or Lewis acid-nucleo-phile combinations in moderate to good yield with stereoselectivity for the anti isomer. 

Adsorption enthalpies and vibrational frequencies of small molecules adsorbed on cation sites in zeolites are often related to acidity (either Bronsted or Lewis acidity of H+ and alkali metal cations, respectively) of particular sites. It is now well accepted that the local environment of the cation (the way it is coordinated with the framework oxygen atoms) affects both, vibrational dynamics and adsorption enthalpies of adsorbed molecules. Only recently it has been demonstrated that in addition to the interaction of one end of the molecule with the cation (effect from the bottom) also the interaction of the other end of the molecule with a second cation or with the zeolite framework (effect from the top) has a substantial effect on vibrational frequencies of the adsorbed molecule [1,2]. The effect from bottom mainly reflects the coordination of the metal cation with the framework - the tighter is the cation-framework coordination the lower is the ability of that cation to bind molecules and the smaller is the effect on the vibrational frequencies of adsorbed molecules. This effect is most prominent for Li+ cations [3-6], In this contribution we focus on the discussion of the effect from top. The interaction of acetonitrile (AN) and carbon monoxide with sodium exchanged zeolites Na-A (Si/AM) andNa-FER (Si/Al= 8.5 and 27) is investigated. 

Perhydrooxazolo[3,2- ]pyridines 338 are excellent precursors of iminium ions 339 obtained after treatment of the oxazolidine with either a Bronsted or Lewis acid. Trapping of these intermediate iminium ions with nucleophiles then allows for substitution at the C-8a position together with ring opening, yielding functionalized piperidines 340 (Scheme 93). 

In anionic polymerization, the initiators are either Bronsted or Lewis acids. In Bronsted acid, H+ is the effective catalyst while in Lewis, co-catalyst is involved to give catalytically active species, e.g. 

If the hydroformylation of olefins is conducted in the presence of aromatic hydrazines and Bronsted or Lewis acids indoles can be obtained directly in one pot [91-93,95]. Hydroformylation of the olefin gives an intermediate aldehyde, which is trapped immediately by the present aromatic hydrazine as an aromatic hydrazones similar to the formation of imines under hydroformylation conditions. Under acid mediation these aromatic hydrazones undergo a Fischer indolization, consisting of a [3,3]-sigmatropic rearrangement followed by a cyclization and elimination of ammonia (Scheme 38). 

A number of reactions that are traditionally carried out using either Bronsted or Lewis acids as catalysts have been shown to take place in the presence of EG A formed either in situ or ex situ. The reactions do not involve charge-consuming conversions of the substrates. 

Both mechanisms discussed above involve an acidic species, and differ primarily in whether that species behaves as a Bronsted or Lewis acid. The vanadic acid approach treats the Y species as a proton donor whereas the oxygen abstraction implies electrophilic attack by V. In either case, it is easy to see how oxygen rich, basic oxides such as MgO function as Y passivators. 

Many other cationic carbon electrophiles cannot be isolated but can be generated insitu in a reaction mixture by Bronsted or Lewis acid-base reactions. In... 

Moreover, as highlighted in Table 5.4, the use of Bronsted or Lewis acid additives increases the reaction rate, yield and selectivity, but these results are normally substrate-dependent. 

Tertiary alcohols, tertiary ethers, or carboxylic acid esters of tertiary alcohols can undergo El eliminations, but only in the presence of Bronsted or Lewis acids. Anyone who has prepared a tertiary alkoxide by a Grignard reaction and treated the crude reaction mixture with HC1 and obtained the alkene knows that tertiary alcohols can be converted into alkenes even with dilute hydrochloric acid. 

Under the influence of a Bronsted- or Lewis acid the dienol ether B in the example of Figure 12.23 reacts with an acetal. As far as the dienol ether is concerned the reaction that occurs corresponds to an electrophilic substitution reaction, and it corresponds to an SN1 reaction with respect to the acetal, which is illustrated at the very bottom of Figure 12.23. Initially, cleavage of a methoxy residue from the acetal A in an equilibrium reaction produces the oxo-carbenium ion F, which is the active C electrophile. The latter reacts with the dienol ether B, forming a new oxocarbenium ion G. G picks up the equivalent of methanol originating from the acetal A and thus delivers the methoxy-substituted acetal C. Why the methanol and not B ... 

Catalysts prepared via the latter method were found to be more rapidly deactivated during reaction. This is almost certainly due to the formation of Bronsted or Lewis acid sites during the reduction step 175, 176), i.e.,... 

Which catalyst should be chosen for a given reaction will depend upon chemical, steric, and mechanistic factors. The application of Pearson s soft and hard acid-base (SHAB) principle has often proved a valuable qualitative guide as to suitable surface sites for a particular reactant. In fact, certain solids actually owe their catalytic power to attached Bronsted or Lewis acid and base groups as exemplified by weak acid ion exchange resins (Sect. 2.3), alumina (Sect. 3.2), and sometimes charcoals. Steric aspects can be con-... 

Epoxide rearrangements are key steps in the manufacture of numerous synthetic intermediates in the fine chemical industry. They are generally performed with conventional Bronsted or Lewis acids or strong bases as catalysts, often in stoichiometric amounts. Here again, replacement by recyclable solid catalysts has obvious benefits [69]. 

Both benzoxazepine 158 and 160 and benzthiazepines 161 and 163 and have been formed by the ring opening of the fused gem-dihaloaziridines 159 and 162 with either a Bronsted or Lewis acid <07S225>. The substrate gave selectively the lactam derivatives with TFA and the iminyl chlorides with BFj.OEtj. 

While catalysts are also used in the production of other types of polymers, the properties of most of these materials are not particularly dependent on the type of catalyst employed. Many poly condensation reactions, e. g. the formation of polyesters, polyamides or urea-formaldehyde resins, are speeded up by addition of some Bronsted or Lewis acids. Since relevant properties of these polymer products, such as their average chain lengths, are controlled by equilibrium parameters, primarily by the reaction temperatures and molar ratios of the monomers employed, and since their linkage patterns are dictated by the functional groups involved, addition of a catalyst has little leverage on the properties of the resulting polymer materials. 

However there are several major hurdles. The most common catalysts are based on acid catalysis with Bronsted or Lewis acid sites these sites promote the formation of propylene rather than ethylene as is witnessed by conventional FCC operations. Ethylene is promoted by free radical processes. Catalysis of free radical reactions is rare, but not unknown . One route is to take a conventional acid catalysis and to neutraUse the acid sites with alkaline metals (magnesium, calcium) or phosphorus or a mixture of such. This can generate a further problem, in that the catalyst promotes the formation of carbon (coke) and hydrogen which are thermodynamically favoured at the reaction temperatures. 

Dixon and coworkers recently demonstrated that strong Bronsted or Lewis acids catalyze the AB dehydrogenation in glymes [82]. The results of the investigations performed at 60 °C are summarized in Table 8.4. 

Supported metal oxide catalysts are a new class of catalytic materials that are excellent oxidation catalysts when redox surface sites are present. They are ideal catalysts for investigating catalytic molecular/electronic structure-activity selectivity relationships for oxidation reactions because (i) the number of catalytic active sites can be systematically controlled, which allows the determination of the number of participating catalytic active sites in the reaction, (ii) the TOP values for oxidation studies can be quantitatively determined since the number of exposed catalytic active sites can be easily determined, (iii) the oxide support can be varied to examine the effect of different types of ligand on the reaction kinetics, (iii) the molecular and electronic structures of the surface MOj, species can be spectroscopically determined under all environmental conditions for structure-activity determination and (iv) the redox surface sites can be combined with surface acid sites to examine the effect of surface Bronsted or Lewis acid sites. Such fundamental structure-activity information can provide insights and also guide the molecular engineering of advanced hydrocarbon oxidation metal oxide catalysts such as supported metal oxides, polyoxo metallates, metal oxide supported zeolites and molecular sieves, bulk mixed metal oxides and metal oxide supported clays. 

Reactions in SCCO2 have also been used for the production of minor lipid components, such as tocopherols and sterol esters. The synthesis of D, L, ot-tocopherol in SCCO2 and nitrous oxide by condensation of trimethyUiydroquinone with iso-phytol in the presence of various Bronsted or Lewis acids as catalysts resulted in... 

The acidic properties of some hydroxyl groups have been demonstrated by the existence of a 1540 cm" band in all the spectra of adsorbed pyridine on solids activated at low temperature. Further dehydrated samples behave either as Bronsted or Lewis acid solids the latter are created by dehydroxylation of the zeolite, which occurs in two different ways First... 

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