Analogs of Carbonium Ions

The term hetero rearrangement is not limited to those reactions in which only the initially electron-deficient atom is something other than carbon, but may be applied to rearrangements in which any of the carbon atoms of formula L have been replaced with other elements. 

The reader will no doubt have realized that the y-hetero rearrangement is well-known, being none other than the familiar neighboring group effect. The a-hetero rearrangement is known for all of the elements mentioned but boron, aluminum, and silicon. Perhaps a suitably constituted boron compound would rearrange to a zwitterionic product as follows  

The /3-hetero rearrangement is known only for silicon in contrast to the variety of instances of the reverse reaction, the a-hetero rearrangement.295 

Some examples of 0-betero rearrangement in silicon compounds 

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Oxygen and nitrogen electron-deficient intermediates will be discussed as analogs of carbonium ions in Chapter VIII. 

The possible nitrogen analogs of carbonium ions are the cation LIII and its conjugate base LIV. 

These singlet and triplet state species exhibit the important differences in chemical behavior to be expected. The former species, with their analogy to carbonium ions, are powerful electrophiles and the relative rates of their reaction with a series of substrates increases with the availability of electrons at the reaction center their addition reactions with olefins are stereospecific. Triplet state species are expected to show the characteristics of radicals i.e., the relative rates of additions to olefins do not follow the same pattern as those of electrophilic species and the additions are not stereospecific. 

It is also difficult to determine exactly the relative stabilities of vinyl cations and the analogous saturated carbonium ions. The relative rates of solvolysis of vinyl substrates and their analogous saturated derivatives have been estimated to be 10 to 10 (131, 134, 140, 154) in favor of the saturated substrates. These rate differences, however, do not accurately reflect the inherent differences in stability between vinyl cations and the analogous carbonium ions, for they include effects that result from the differences in ground states between reactants, as well as possible differences between the intermediate ions resulting from differences in solvation, counter-ion effects, etc. The same difficulties apply in the attempt to estimate relative ion stabilities from relative rates of electrophilic additions to acetylenes and olefins, (218), or from relative rates of homopropargylic and homoallylic solvolysis. 

An interesting feature is that, although solvated carbonium ions usually afford transition states when the anions are considered in the calculations, their chemistry could be thought of as being analogous to that of carbonium ions. In many cases, these structures could potentially be transition states with interesting properties, such as scrambling and other processes that are characteristic of weakly solvated carbonium ions. 

A somewhat analogous ion-radical reduction of carbonium ions to free radicals has been shown to occur (Conant and Chow, 48) in the reduction of various triphenylmethyl carbonium ions with chromous or titanous ion. 

One cannot distinguish between the analogous copper intermediates involved in oxidative electron-transfer and ligand-transfer reactions. In each the ionization of the ligand to copper(II) has an important role in the formation of carbonium ion intermediates. A reaction analogous to the copper-catalyzed decomposition of peroxides is the copper-promoted decomposition of diazonium salts (178). The diazonium ion and copper(I) afford aryl radicals which can undergo ligand-transfer oxidation with copper(II) halides (Sandmeyer reaction) or add to olefins (Meerwein reaction). 

Recently, Pettit and co-workers (134) have shown that the relative rate of solvolysis of benzyl chloride-chromium tricarbonyl is about a million times faster than for uncomplexed benzyl chloride under analogous aSatI conditions. The area of carbonium ion formation and stabilization in arene-metal systems thus represents another fertile field for future investigation. 

One or two examples of the use of these concepts will illustrate the ideas and help to formulate appropriate rate equations. The acidic catalysts, such as silica-alumina, can apparently act as Lewis (electron acceptor) or Br0nsted (proton donor) acids, and thus form some sort of carbonium ion from hydrocarbons, for example. Note the analogy between this hydrogen deficient entity and a free radical. However, the somewhat different rules for the reactions of carbonium ions apply from organic chemistry and permit miquantitative predictions of the products expected see Table 2.1-2 from Oblad, et al. [11 ]. 

The propagating chain end in an anionic reaction initiated by a reagent such as n-butyl hthium can be thought of as existing in one of the following states, analogous to carbonium ion formation. 

The anions of the type [M(CN)6] " (M = Fe, Ru, Os) react with [Et30][BF4] in acetone solution to give [M(CNCMe2CH2COMe)6][BF4]2. First, aldol condensation of acetone takes place followed by the attack of carbonium ion on coordinated cyanide ligands. The reaction of [Fe(CN)6] with MeCOEt, cyclohexanone, or acetophenone proceeds analogously. 

Carbenes, addition to multiple bonds, 61 addition to olefins, 59, 60 analogy with carbonium ions, 60 co-ordination reactions of, 61 dibromo-, see dihalocarbenes dichloro-, see dihalocarbenes dihalo-, addition to olefins, 59, 61 dihalo-, deoxygenation of aromatic N-oxides by, 77... 

Secondary i-deuterium kinetic isotope effect is caused by hyperconjugation, i.e., by the interactiom of C-H or C-D bond electrons with the emptied p orbital of a-carbon after the formation of carbonium ion. This interaction stabilizes the carbonium ion. The electron shift from C-H bond into the emptied p orbital of C is larger than the analogous shift from Pc-D bond, which means that the former electron shift stabilizes the carbonium ion better than the latter. The better-stabilized carbonium ion has a lower Gibbs energy of activation, which means a faster reaction with protium and slower with deuterium. 

The very small secondary isotope effect arising from replacement cf p-CHs by p-CDs is instructive largely because it shows that full development of carbonium ion character is not necessarily associated with a substantial benzylic deuterium secondary isotope effect. It must be recalled in this connection that the Hammett rho for Ki value is —4.410 and that the effect of one p-methyl group on the equilibrium corresponds to a typical cr+ value, i.e., the reaction is quite sensitive to stabilization of charge and p-methyl shows enhanced participation in stabilization. The observed effect is consistent with the analogous very small effects which have been observed in aromatic substitution (105), including bromination, for which p is —12, but smaller than those which have been reported for solvolyses of benzylic (106) and benzhydryl (107) compounds. It does not appear to be possible to draw a general conclusion from these data. 

This method is suitable only for the preparation of 4-substituted and/or 3,4-disubstituted derivatives, the substituents being only alkyl, aryl or heteroaryl groups. The presence of electron-withdrawing groups in the unsaturated side chain prevents the cyclization step. This is understandable if the influence of such groups on the stability of the intermediate carbonium ion is considered. Of more limited application is the analogous cyclization of diazotized o-aminophenylpropiolic acids, the reaction being referred to as the Richter synthesis (Scheme 70). A related synthesis (also referred to as the Neber-Bossel synthesis)... 

The vinyl cation analog of an allylic carbonium ion is an allenyl cation 242, where the empty p orbital on the unsaturated carbon overlaps with the perpendicular n bond of the allenyl system. Allenyl cation 242 is of course a resonance form of the well known alkynylcarbonium ion,... 

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