Carbonium ions as intermediates

The term Kolbe electrolysis is sometimes exclusively used to denote the formation of alkyl dimers R-R, path a) in Eq. (94) (see Sect. 12.1), while the reaction leading to substituted alkyls, R-Nu, via carbonium ions as intermediates, path b) in Eq. (94), is named non-Kolbe electrolysis, abnormal Kolbe electrolysis,or... 

Cabaleiro and Johnson (1967) report that the addition of chlorine to -methyl cinnamate in chloroform or acetic acid is syn-selective, SS 0-75, in chloroform and acetic acid acetoxychloro derivatives are produced as well. Again, Dewar and Fahey (1964) argue that the normal course of addition of hydrogen halides onto olefins is a polar electrophilic process involving classical carbonium ions as intermediates and leading mostly but not exclusively to cis-adducts. A syn-preference was found in the additions of deuterium bromide to acenaphthylene, indene, and cis- and fraws-phenylpropene. In the case of indene, phenylpropene and methyl cinnamate, which are styrene analogs, concerted syn addition is symmetry-allowed (see bottom of p. 273). 

The inherent plausibility of metastable bridged carbonium ions as intermediates is supported by two independent types of observation. One is the extensive rearrangements which can occur in allylic systems under conditions in which ionic displacement reactions are possible. A second is the existence of stable bridged compounds, including, in the case of boron compounds, pentavalent atoms. Thus diborane and substituted diboranes have stable bridged structures. 

The nature and role of carbonium ions as intermediates in molecular rearrangements has been the subject of a vast literature including many reviews [i]. Reduced to its simplest terms, the overall reaction may be represented as ... 

Oxymercuration involves electrophilic addition to the carbon-carbon double bond, with the mercuric ion acting as electrophile. The absence of rearrangement and the high degree of stereospecificity (typically anti)—in the oxymercuration step—argues against an open carbonium ion as intermediate. Instead, it has been proposed, there is formed a cyclic mercurinium ion analogous to the bromonium... 

We have depicted the pinacol rearrangement as a two-step process with an actual carbonium ion as intermediate. There is good evidence that this is so, at least when a tertiary or benzylic cation can be formed. Evidently the stability of the incipient cation in the transition state permits (SNl-lihe) loss of water without anchimeric assistance from the migrating group. This is, we note, in contrast to what happens in migration to electron-deficient nitrogen or oxygen. 

G. M. Kramer, G. B. McVicker, and J. J. Ziemiak, On the Question of Carbonium Ions as Intermediates Over Silica-Alumina and Acid Zeolites, Journal of Catalysis 92 355-363 (1985). 

Isomerization may accompany nucleophilic substitution during the solvolysis of carboxylate and arenesulfonate esters of unsaturated alcohols. Most of the substrates for which such isomerization reactions have been observed are esters of allylic alcohols. If such an ester is optically active by virtue of having an asymmetric a-carbon atom, three experimentally observable processes occur simultaneously racemization, solvolysis, and allylic isomerization. All three of these processes involve formation of planar, achiral allylic carbonium ions as intermediates, viz-... 

Sen, A. Lai, T.-W. Catalysis by solvated transition-metal cations. Novel catalytic transformations of alkenes by tetrakis( acetonitrile )paUadium ditetrallnoroborate. Evidence for the formation of incipient carbonium ions as intermediates. J. Am. Chem. Soc. 1981,103, 4627- 629. 

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. 

Carbonium Ions as Reaction Intermediates.—The properties of electron-deficient substances may be expected to be of great importance in the theory of chemical reactions. For example, a positively charged (and hence electron-deficient) carbon atom in a complex carbonium ion would be expected to cause adjacent atoms to increase their ligancies, as by the formation of a three-membered ring and by the use of bridging hydrogen atoms. The analysis of the mechanisms of chemical reactions may in the course of time permit much more precise principles to be formulated than are now at hand. 

Tphe isomerization of olefins over acidic catalysts has been carefully A studied in the past few years. Hightower and Hall (1, 2) have studied the isomerization of n-butenes over silica-alumina. They were able to interpret their results in terms of a simple model involving the 2-butyl carbonium ion as a common intermediate. More recently Lombardo and Hall studied the isomerization of the same olefins over Na-Y-zeolite. They showed that the reaction was first order in conversion as well as time (3), that the isomers could be directly interconverted (4), and that activity sharply increased with water addition reaching a saturation value (5). There are, however, reports in the literature which are at variance with this idea. Dimitrov et al. (7, 8) explained their results for n-butene isomerization on Na-X-zeolite in terms of a free radical type mechanism. As discussed more thoroughly elsewhere (4) others have argued about the nature of catalytic activity on zeolites (9-13). 

A central feature of the mechanism that accounts for the catalytic cracking of hydrocarbons by appropriately cation exchanged zeolites is the formation of carbonium ions (also designated carbocations and alkylcarbenium ions) as intermediates. Many other reactions for which aluminosilicates, be they clay-or zeolite-based, also predicate (320) the existence of carbonium ion intermediates, formed usually by proton donation from Bronsted acid sites, have been discussed earlier (Section III,K). 

On the other hand, the n-pentylbenzene - methylnaphthalene reaction proceeds through C6H5-CH2-CH2-CH2-C+H-CH3, a secondary carbonium ion. As a consequence, acid-catalyzed cyclization produces both five- and six-membered rings. The possible carbonium ion intermediates leading to five- or six-membered ring closure may have similar structures (e.g., both are secondary carbonium ions, as in the case of n-pentylbenzene). If so, acid-catalyzed cyclization favors six-membered ring products, as shown by the k5/k6 ratios (Table IV). 

The isolation of stable vinylidene complexes and elucidation of many of their reactions have given substance to speculations concerning their intermediacy in many reactions. Indeed, the reactions of many alkynes with a series of platinum(II) complexes were explained several years ago by considering the formation of metal-stabilized carbonium ions as nonisolable intermediates (10). Summarized below are several reactions that may reasonably be assumed to proceed via vinylidene complexes. 

Treatment of polyolefinic ketal 230 with stannic chloride in pentane gave a mixture (30% yield) of about equal amounts of the two racemic D-homoster-oidal tetracyclic isomers 231 (88). In this cyclization, the first cationic intermediate is not chiral and the two faces of the 5,6-double-bond can react with equal facility with the carbonium ion as a consequence, the product obtained (231) is necessarily racemic. The conversion of the open-chain tetraenic acetal 230 having no chiral centers into a tetracyclic system having seven such centers and producing only two (231) out of a possible 64 racemates is a striking tribute to the power of stereoelectronic effects. 

The possible existence of 7r-complex isomers of carbonium ions as stable reaction intermediates was predicted88 on the basis of simple MO calculations before their existence had been demonstrated unambiguously. Intermediates of this kind are now recognized to play a very important role and their prediction... 

In their protonated forms, zeolites are widely employed in the oil and petrochemical industries, in processes such as the conversion of alcohols to gasoline, catalytic cracking, isomerization and alkylations of hydrocarbons [3]. These chemical reactions most probably involve proton transfer from the acidic site of the zeolite to the organic substrate. In the case of hydrocarbons, this transfer gives rise to carbenium (I) or carbonium (II) ions as intermediates or transition states. 

The high oxidation potentials of alkanes, however, make it difficult to carry out the oxidation in solvents such as acetonitrile since the first intermediates generated in these oxidations are carbonium ions, as illustrated by equations (4) and (S), Their stabilization with strongly acidic solvents like anhydrous fluorosulfonic acid often lowers the oxidation potentials of these hydrocarbons. ... 

Unimolecular solvolyses of allylic secondary halides and similar derivatives are often favoured by the capacity of the double bond to stabilise the intermediate carbonium ion as the mesomeric "allylic cation 4). This extra stability of the intermediate is reflected in the energy of the transition state for its formation whenever the path of departure of the anion... 

Catalysis by acid suggests that here, as in dehydration, the protonated alcohol R0H2" is involved. The occurrence of rearrangement suggests that carbonium ions are intermediates—although not with primary alcohols. The idea of carbonium ions is strongly supported by the order of reactivity of alcohols, which parallels the stability of carbonium ions—except for methyl. 

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