Carbenium ions, acid catalysis

Methanol dehydrogenates to methyl formate over fresh WC and P-W2C powders with selectivities higher than 90% (109,110). The dominant side reaction is the decomposition to synthesis gas. Over WC and P-W2C modified with oxygen, methanol selectively dehydrates to dimethylether at 473 K and at higher reaction temperatures, C2-C4 olefins are produced (47). Thus, the dehydrodimerization of methanol apparently requires WC sites. These sites are titrated by chemisorbed oxygen. Thus, oxygen on the surface inhibits the formation of methyl formate and introduces a surface acid function WO that catalyzes dehydration by carbenium-ion type catalysis. 

Reaction of glycosylmethylamines with aryldiazonium salts gives a class of compounds which, by acid catalysis or unknown factors of enzymic catalysis, generate glycosylmethyldiazonium ions. These, in turn, lose nitrogen, to yield highly electrophilic carbenium ions, as illustrated for the ) -d-galactosyl derivative 38 (see Scheme 8). 

This reaction type has been intensely studied °. The application of highly polar solvents, catalysis with tertiary amines" or with acids mesomeric stabilization of intermediate carbenium ions " (allylic and benzylic systems propargylic systems" ) as well as derivatives of sulfinic acids with increasing acidity - usually indicate an ionic pathway (intra- and/or inter-molecular) ... 

Carbonyl reactions are extremely important in chemistry and biochemistry, yet they are often given short shrift in textbooks on physical organic chemistry, partly because the subject was historically developed by the study of nucleophilic substitution at saturated carbon, and partly because carbonyl reactions are often more difhcult to study. They are generally reversible under usual conditions and involve complicated multistep mechanisms and general acid/base catalysis. In thinking about carbonyl reactions, 1 find it helpful to consider the carbonyl group as a (very) stabilized carbenium ion, with an O substituent. Then one can immediately draw on everything one has learned about carbenium ion reactivity and see that the reactivity order for carbonyl compounds ... 

Carbocations are central to hydrocarbon chemistry (/). Much of this chemistry is based on acid catalysis, which leads to generation of positive ions of carbon. The resulting intermediates are classified as carbenium and carbonium ions, as proposed by Olah (2-4). Carbonium ions are the penta- or higher coordinate carbocations that maintain 8 valence electrons via 2-electron/3-center bonding, quite different from carbenium ions that possess only 6 valence electrons. Figure 1 shows a systematic classification of carbocations. 

We wish to emphasise that the formation of esters (E) from alkenes (M) and acids (HA), the catalysis of the reactions of E by HA or MtXn, and the activation of E, such as organic chlorides, by the co-ordination of a Lewis acid, such as A1C13, are all very familiar chapters in conventional organic chemistry. It follows that the pseudo-cationic theory is nothing more than a generalisation of conventional organic-chemical ideas and a revival of some pre-Whitmore interpretations which had become occulted by the usefulness and novelty of the carbenium ion concept. 

Carbenium ions, 42 115, 143 acid catalysis, 41 336 chemical shift tensors, 42 124-125 fragments in zeolites, 42 92-93 history, 42 116 superacids, 42 117 Carbide catalysts, 34 37 Carbidic carbon, 37 138, 146-147 Carbidic intermediates, 30 189-190, 194 Fischer-Tropsch synthesis, 30 196-197, 206-212... 

Prins reaction, heteropolyacid catalysis, 41 156 Probe molecules, 42 119 acidic dissociation constant, 38 210 NMR solid acidity studies, 42 139-140 acylium ions, 42 139, 160 aldehydes, 42 162-163 alkyl carbenium ions, 42 154-157 allyl cation, 42 143-144 ammonia, 42 172-174 arenium ions, 42 150-154 carbonium ions, 42 157-160 chalcogenenonium ions, 42 161-162 cyclopentenyl cations, 42 140-143 indanyl cations, 42 144-147 ketones, 42 162,163-165 nitrogen-containing compounds, 42 165-170... 

Traditionally, the same overall mechanisms of acid catalysis invoking carben-ium ions have been assumed to prevail both in heterogeneous (2) and in liquid homogeneous (3) systems. But these mechanisms do not adequately take into account the fact that adsorbed, rather than free, carbenium ions are formed in the pores of solid catalysts. Consequently, a quantum-chemical model that demonstrates how the interaction of carbenium ions with the sites of their adsorption can influence the reaction mechanism has been formulated by Kazansky (4), taking double-bond-shift reactions in olefins as a particular example. According to this view, adsorbed carbenium ions are best regarded as transition states rather than reaction intermediates, a notion that had also been proposed earlier by Zhidomirov and one of us (5). 

For our final example of theoretical NMR in catalysis, we again turn to carbenium ion chemistry. Here we study the formation of the isopropyl cation on frozen SbF5, a strong Lewis acid (27). In contrast to the studies presented earlier, with this system we were able to experimentally measure the chemical shift tensor. Because the full tensor is naturally obtained from NMR calculations, a comparison can readily be made. In addition, for the isopropyl cation we also studied the effect the medium (in this case, the charge balancing anion) had on the chemical shift tensor. 

Theory helps the experimentalists in many ways this volume is on chemical shift calculations, but the other ways in which theoretical chemistry guides NMR studies of catalysis should not be overlooked. Indeed, further theoretical work on two of the cations discussed above has helped us understand why some carbenium ions persist indefinitely in zeolite solid acids as stable species at 298 K, and others do not (25). The three classes of carbenium ions we were most concerned with, the indanyl cation, the dimethylcyclopentenyl cation, and the pentamethylbenzenium cation (Scheme 1), could all be formally generated by protonation of an olefin. We actually synthesized them in the zeolites by other routes, but we suspected that the simplest parent olefins" of these cations must be very basic hydrocarbons, otherwise the carbenium ions might just transfer protons back to the conjugate base site on the zeolite. Experimental values were not available for any of the parent olefins shown below, so we calculated the proton affinities (enthalpies) by first determining the... 

EPR experiments have shown that the redox ability of WZ catalysts is sufficient to initiate a homolytic cleavage of C-H bonds in alkanes. Exposure of a WZ catalyst to n-pentane at 523 K led to the formation of W5+ species and organic radicals on the surface.27 The formation of organic radicals also occurred when WZ catalysts interacted with other hydrocarbons, including benzene.31 We therefore infer that one-electron transfer, although it is not regarded as a step in the catalytic cycle, can initiate catalysis by a process that leads to the formation of the carbenium ion chain carriers,27 as also occurs in acidic solutions.32 We emphasize that a strong redox reactivity is necessary but not sufficient for the catalytic activity of WZ. 

It was concluded from these results that ketones are incapable of intermolecular reactions with nitriles. Starting from a general concept on the stability and reactivity of carbenium ions75,76 it can be explained that the carbenium ion intermediates 105 formed from ketones under acid catalysis are more stable than those (106) formed from aldehydes and, therefore, the former are less reactive in the reactions with the weak nucleophilic nitriles. 

DBr, DC1, and CH3SO3D additions to E- and Z-2-butene proceed without diastereoisomerization, H/D exchange, or positional isomerization [108,109]. Although this suggests that carbenium ions may not develop completely, carbenium ion intermediates are apparently involved when the reaction is catalyzed by triflic acid. That is, triflic acid catalysis greatly increases the rate, and both stereo- and positional isomerization occur in its presence [110]. 

For example, many catalytic cycles involve the transfer of protons. Common intermediates are carbenium ions and carbanions, and the catalysts include soluble and solid acids and bases and enzymes. The catalytic cycles may be similar, whether the proton donor (or acceptor) is a soluble molecule or ion or a functional group on a surface. Similarly, catalysis proceeding via organometallic intermediates may involve soluble transition metal complexes, metalloenzymes, or metal surfaces. Catalysis by metals is, however, much more complicated than acid-base catalysis, and the analogies between soluble metal complexes and surfaces cannot yet be developed beyond a few selected examples. 

Aryl but not alkyl tetrahydropyranyl acetals show general acid catalysis, for the same reason [13] but aryl methyl acetals do not, because the methoxymethyl car-benium ion is not sufficiently stable. (This situation can lead to enforced general acid catalysis, when the specific acid catalyzed reaction requires nucleophilic assistance if the nucleophile is the conjugate base of the general acid this will be observed as general acid catalysis.) At the other extreme, sufficient stabilization of the carbenium ion can have the same effect, as shown by the observation of general acid catalysis of tropolone diethyl acetal 2.1 (Scheme 2.10) [14]. And even... 

Although true carbenium ions have never been detected on solid surfaces, catalysis over solid and liquid acids displays many similarities, but also some characteristic differences. 

Catalysts which contain reduced transition metal clusters besides acid sites are able to catalyze reactions that are not observed on catalysts exposing one type of site only. The reaction network is inadequately described by models which assume only additivity of catalytic functions and shuttling of intermediates between sites. There is strong evidence that metal clusters and Bronsted sites form metal-proton adducts. These act as "collapsed bifunctional sites" all alkane isomerization steps can take place on such sites during one single residence of the adsorbed molecule. At low temperature, adsorption in a mode reminiscent of a carbenium ion can suppress pure metal catalysis. 

Acid-base catalysis produces mostly carbenium ions by electron or by proton transfer. Among the solid acids, microporous, crystalline alumina silicates (zeolites) are utilized most frequently. 

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