Acid-catalyzed

As an example, experimental kinetic data on the hydrolysis of amides under basic conditions as well as under acid catalysis were correlated with quantitative data on charge distribution and the resonance effect [13]. Thus, the values on the free energy of activation, AG , for the acid catalyzed hydrolysis of amides could be modeled quite well by Eq. (5)... 

Alkylation combines lower-molecular-weight saturated and unsaturated hydrocarbons (alkanes and alkenes) to produce high-octane gasoline and other hydrocarbon products. Conventional paraffin-olefin (alkane-alkene) alkylation is an acid-catalyzed reaction, such as combining isobutylene and isobutane to isooctane. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Acid-catalyzed isomerization reactions of alkanes as well as alkylation and condensation reactions are initiated by protolytic ionization. Available evidence indicates nonlinear but not necessarily triangular... 

A fundamental difference exists between conventional acid-catalyzed and superacidic hydrocarbon chemistry. In the former, trivalent car-benium ions are always in equilibrium with olefins, which play the key role, whereas in the latter, hydrocarbon transformation can take place without the involvement of olefins through the intermediacy of five-coordinate carbocations. 

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... 

Acid-catalyzed alkylation of aromatics with alcohols themselves is widely used. Whereas tertiary (and secondary) alcohols react with rel-... 

Some people think that 48% aq, HBr is good enough and, until recently, some underground chemistry texts agreed. But it is not The 48% is only strong enough to promote what is called acid catalyzed hydration (don t ask) when the medium it is in is water. That means that a water molecule (an OH) will add instead of a Br. 

The usual base or acid catalyzed aldol addition or ester condensation reactions can only be applied as a useful synthetic reaction, if both carbonyl components are identical. Otherwise complicated mixtures of products are formed. If two different aldehydes or esters are to be combined, it is essential that one of the components is transformed quantitatively into an enol whereas the other component remains as a carbonyl compound in the reaction mixture. 

Bromomethyl)- or (hydroxymethyl)cycIopropane derivatives undergo acid-catalyzed homoallyiic rearrangements to yield trans-olefins (J.P. McCormick, 1975 S.F. Brady, 1968 M. Julia, 1974). This rearrangement is the basis of Julia s terpene synthesis (see. p. 70). 

A simple acid-catalyzed cyclization transforms i//-ionone into n-ionone (W. Kimel, 1957, 1958). Further treatment with protic acids transforms the tr-ionone to the thermodynamically more stable -ionone. 

In an intramolecular aldol condensation of a diketone many products are conceivable, since four different ends can be made. Five- and six-membered rings, however, wUl be formed preferentially. Kinetic or thermodynamic control or different acid-base catalysts may also induce selectivity. In the Lewis acid-catalyzed aldol condensation given below, the more substituted enol is formed preferentially (E.J. Corey, 1963 B, 1965B). 

The most commonly used protected derivatives of aldehydes and ketones are 1,3-dioxolanes and 1,3-oxathiolanes. They are obtained from the carbonyl compounds and 1,2-ethanediol or 2-mercaptoethanol, respectively, in aprotic solvents and in the presence of catalysts, e.g. BF, (L.F. Fieser, 1954 G.E. Wilson, Jr., 1968), and water scavengers, e.g. orthoesters (P. Doyle. 1965). Acid-catalyzed exchange dioxolanation with dioxolanes of low boiling ketones, e.g. acetone, which are distilled during the reaction, can also be applied (H. J. Dauben, Jr., 1954). Selective monoketalization of diketones is often used with good success (C. Mercier, 1973). Even from diketones with two keto groups of very similar reactivity monoketals may be obtained by repeated acid-catalyzed equilibration (W.S. Johnson, 1962 A.G. Hortmann, 1969). Most aldehydes are easily converted into acetals. The ketalization of ketones is more difficult for sterical reasons and often requires long reaction times at elevated temperatures. a, -Unsaturated ketones react more slowly than saturated ketones. 2-Mercaptoethanol is more reactive than 1,2-ethanediol (J. Romo, 1951 C. Djerassi, 1952 G.E. Wilson, Jr., 1968). 

If one heats acetone and pyrrole in the presence of catalytic amounts of acid, so-called acetone pyrrole is formed in over 80%i yield. This colorless, macrocyclic compound contains four pyrrole units which are connected by dimethylmethylene bridges, ft is formed by electrophilic-a-substitution of pyrrole by acetone, acid-catalyzed oligomerization, and spontaneous. 

The following acid-catalyzed cyclizations leading to steroid hormone precursors exemplify some important facts an acetylenic bond is less nucleophilic than an olelinic bond acetylenic bonds tend to form cyclopentane rather than cyclohexane derivatives, if there is a choice in proton-catalyzed olefin cyclizations the thermodynamically most stable Irons connection of cyclohexane rings is obtained selectively electroneutral nucleophilic agents such as ethylene carbonate can be used to terminate the cationic cyclization process forming stable enol derivatives which can be hydrolyzed to carbonyl compounds without this nucleophile and with trifluoroacetic acid the corresponding enol ester may be obtained (M.B. Gravestock, 1978, A,B P.E. Peterson, 1969). 

Finally a general approach to synthesize A -pyrrolines must be mentioned. This is tl acid-catalyzed (NH4CI or catalytic amounts of HBr) and thermally (150°C) induced tea rangement of cyclopropyl imines. These educts may be obtained from commercial cyan> acetate, cyclopropyl cyanide, or benzyl cyanide derivatives by the routes outlined below. Tl rearrangement is reminiscent of the rearrangement of 1-silyloxy-l-vinylcyclopropancs (p. 7 83) but since it is acid-catalyzed it occurs at much lower temperatures. A -Pyrrolines constitut reactive enamines and may be used in further addition reactions such as the Robinson anei lation with methyl vinyl ketone (R.V. Stevens, 1967, 1968, 1971). 

Two synthetic bridged nitrogen heterocycles are also prepared on a commercial scale. The pentazocine synthesis consists of a reductive alkylation of a pyridinium ring, a remarkable and puzzling addition to the most hindered position, hydrogenation of an enamine, and acid-catalyzed substitution of a phenol derivative. The synthesis is an application of the reactivity rules discussed in the alkaloid section. The same applies for clidinium bromide. 

In his cephalosporin synthesis methyl levulinate was condensed with cysteine in acidic medium to give a bicyclic thiazolidine. One may rationalize the regioselective formation of this bicycle with the assumption that in the acidic reaction mixture the tMoI group is the only nucleophile present, which can add to the ketone. Intramolecular amide formation from the methyl ester and acid-catalyzed dehydration would then lead to the thiazolidine and y-lactam rings. The stereochemistry at the carboxylic acid a-... 

Synthesis of (A) started with the combination of 2,4,6-trimethylphenol and allyl bromide to give the or/Ao-allyl dienone. Acid-catalyzed rearrangement and oxidative bydroboration yielded the dienone with a propanol group in porlactone ring were irons in the product as expected (see p. 275). Treatment with aqueous potassium hydroxide gave the epoxy acid, which formed a crystalline salt with (R)-l-(or-naphthyl)ethylamine. This was recrystallized to constant rotation. 

Cyclobutadiene itself is not stable at room temperature. Several derivatives with stabilizing groups have been prepared by the acid-catalyzed dimerization of alkjmes (R. Gompper, 1975). Less substituted cyclobutadienes could be obtained by photolytic reactions in solid matrix at low temperatures (G. Maier, 1973, 1974). 

Spherical, pentagonal dodecahedrane is the thermodynamically most stable CjoHjo-polycycloalkane. It is the so-called CjjHjo stabilomer . It should therefore be available by thermod5mamically controlled, e.g. acid-catalyzed, isomerization of less stable C24H20-isomers. Experiments along this line, e.g. treatment of the basketene photo-dimer with Lewis... 

PdCb-CuCb catalyzes the condensation of branched-chain alkenes with formaldehyde to give the l,3-dioxanes 96a and 96b (Prins reaction)[73]. The yields are much higher than in the conventional acid-catalyzed Prins reaction. 

Toth et al. have thoroughly studied the rearrangement their kinetic determinations suggest a general acid-catalyzed mechanism (Scheme 115) (1578). Some points remain unclear, however why is the intermediate (181) written as a transition state when it is known that a tetrahedral intermediate (181b or 181b ) could as well be postulated How does this... 

The general pattern of alkylation of 2-acylaininothiazoles parallels that of 2-aminothia2ole itself (see Section III.l). In neutral medium attack occurs on the ring nitrogen, and in alkaline medium a mixture of N-ring and N-amino alkylation takes place (40, 43, 161. 163). In acidic medium unusual behavior has been reported (477) 2-acetamido-4-substituted thiazoles react with acetic anhydride in the presence of sulfuric acid to yield 2-acetylimino-3-acetyl-4-phenyl-4-thiazolines (255) when R = Ph. but when R4 = Me or H no acetylation occurs (Scheme 151). The explanation rests perhaps in an acid-catalyzed heterocyclization with an acetylation on the open-chain compound (253), this compound being stabilized... 

An interesting class of 2-imino-3-amino-4-thiazolines (408) has been described (578, 701, 726). These 3-amino derivatives of 4-thiazoiine may also be prepared from 2,3-diaminothiazolium salts (406) in basic medium (101) or through the acid-catalyzed rearrangement of 2-acylaminoimino-3-phenyl-4-phenyl-4-thiazolines (407) (Scheme 233) (99, 724). 

The use of a reagent bearing a basic center or the addition of a base to the reaction mixture was recognized as necessary to prevent the acid-catalyzed elimination of the elements of water from the intermediates. Since the publication of this work, a number of similar intermediates have been isolated from thioamides and a-halogeno carbonyl compounds (608, 609, 619, 739, 754, 801), and as a result of kinetic studies, the exact mechanism of this reaction has been well established (739, 821). 

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