Unstable carbocations

Certain aliphatic diazonium species such as bridgehead diazonium ions and cyclo-propanediazonium ions, where the usual loss of N2 would lead to very unstable carbocations, have been coupled to aromatic substrates. ... 

When 4-/-butylcyclohex-1 -enyl(phenyl)iodonium tetrafluoroborate (3) is heated at 60 °C in chloroform, 1-fluorocyclohexene 4, 1-chlorocyclohexene 5 and l-(o-iodophenyl)cyclohexene 6 are formed with accompanying iodobenzene leaving group (eq 2).3 These three substitution products are best accounted for by formation of an ion pair involving cyclohexenyl cation 7. The cyclohexenyl cation 7 formed picks up fluoride from tetrafluoroborate and chloride from chloroform solvent, and recombines with the iodobenzene generated (eq 3). This kind of reactions with a counteranion and solvent are characteristic of unstable carbocations and are known in the case of phenyl cation generated from the diazonium salt in the Schiemann-type reaction.4... 

Ritter reactionThis reaction involves the condensation of nitriles with an alcohol in a strongly acidic medium to form an amide.J Yields are generally low with primary and secondary alcohols, which form relatively unstable carbocations. 

In practice, extrapolations of p fR in water have usually used the older acidity function based method, for example, for trityl,61,62 benzhydryl,63 or cyclopropenyl (6) cations.66,67 These older data include studies of protonation of aromatic molecules, such as pKSi = —1.70 for the azulenium ion 3,59 and Kresge s extensive measurements of the protonation of hydroxy- and methoxy-substituted benzenes.68 Some of these data have been replotted as p fR or pKa against XQ with only minor changes in values.25,52 However, for more unstable carbocations such as 2,4,6-trimethylbenzyl, there is a long extrapolation from concentrated acid solutions to water and the discrepancy is greater use of an acidity function in this case gives pA 2° = —17.5,61 compared with —16.3 (and m = 1.8) based on X0. Indeed because of limitations to the acidity of concentrated solutions of perchloric or sulfuric acid pICs of more weakly nucleophilic carbocations are not accessible from equilibrium measurements in these media. 

For such unstable carbocations, an alternative approach to pAR can be developed, by recognizing the relationship that exists between pATR and pAa implied in Equation (15) (p. 30). For carbocations with [3-hydrogen atoms, loss of a proton normally yields an alkene. Then, as discussed by Richard, pATR and pAa form two arms of a thermodynamic cycle, of which the third is the equilibrium constant for hydration of the alkene, pAH2o, as already indicated in Scheme 1. The relationship between these equilibrium constants is shown for the t-butyl cation in Scheme 4. In the scheme the equilibria are... 

Choride ion is considerably less reactive than the azide ion. Thus, although values of kc 1/ kn2o have been quite widely available from mass law effects of chloride ion on the solvolysis of aralkyl halides, normally the reaction of the chloride ion cannot be assumed to be diffusion controlled and the value of kn2o cannot be inferred, except for relatively unstable carbocations (p. 72). Mayr and coworkers251 have measured rate constants for reaction of chloride ion with benzhydryl cations in 80% aqueous acetonitrile and their values of logk are plotted together with a value for the trityl cation19 in Fig. 7. There is some scatter in the points, possibly because of some steric hindrance to reaction of the trityl cations. However, it can be seen that chloride ion is more... 

K. Okamoto, Generation and Ion-Pair Stmctures of Unstable Carbocation Intermediates in Solvolytic Reactions", in Advances in Carbocation Chemistry (X. Creary, Ed.) 1989,1, JAI Press, Greenwich, CT. 

A second limitation is that aromatic compounds substituted with moderately or strongly deactivating groups cannot be alkylated. The deactivated ring is just too poor a nucleophile to react with the unstable carbocation electrophile before other reactions occur that destroy it. 

By these methods, solutions of highly stabilized (e.g., trityl cations) as well as of relatively unstable carbocations (e.g., sec-alkyl cations) have been produced. Although the precision of the calorimetric measurements is smaller than that of most equilibrium determinations, it is an advantage of Arnett s approach that very different types of carbocations can be studied by the same method (Scheme 3). Error propagations, which may be introduced when a series of equilibrium constants or overlapping scales are connected, are thus eliminated. 

The hybridization state of the carbon atom that is attacked changes from sp to sp and the planar aromatic system is destroyed. An unstable carbocation is simultaneously produced and so it is clear that this step is energetically unfavourable. It is therefore the slower step of the sequence. 

Heterolysis of the C-X bond forms a very unstable carbocation, making the ratedetermining step very slow. 

This unstable carbocation slows the first step, making an aikyne less reactive than I an alkene towards HX. 

The AdgS avoids the unstable carbocation and rationalizes the stereochemistry of the product. The attack is anti, the bromide attacks the bottom face since the pi complex of the alkyne with HBr blocks the top attack. In this case the addition surface has folded so that the AdgS is the lowest-energy route proceeding up through a pass between the two... 

Two of the factors that determine the reactivity of tethered ir-nucleophiles in Mannich-type cycliza-tions have been emphasized stereoelectronic effects and reaction medium effects. The stereoelectronics of orbital overlaps between the ir-nucleophile and the iminium electrophile are best evaluated by considerations such as antiperiplanar addition trajectories and Baldwin s rules for ring formation. The critical importance of the reaction medium has received serious attention only recently. However, it already appears clear that Tr-nucleophiles that would lead, upon cyclization, to relatively unstable carbocations can have their reactivity markedly increased by carrying out the cyclization in the presence of a nucleophilic solvent or additive which, by nucleophilic participation, can obviate the formation of high energy cyclic carbenium ion intermediates. 

A key to the success of the living cationic polymerization of vinyl ethers is the stabilization of the unstable carbocations via suitable nucleophilic counterion. There are two ways to stabilize the carbocations (1) generation of suitable nucleophilic counterion resulted from the initiator and the catalyst, and (2) addition of nucleophilic agents to the polymerization media. In the first way, Bronsted acids such as hydrogen iodide are employed as the initiators, while Lewis acids such as zinc iodide are employed as the catalysts (Scheme 11.41) [140-143],... 

From these examples we see that the mechanistic and stereochemical details of hydrogen halide addition depend on the reactant structure. Alkenes that form relatively unstable carbocations are likely to react via a termolecular complex and exhibit anti stereospecificity. Alkenes that can form more stable cations can react via rate-determining protonation and the structure and stereochemistry of the product are determined by the specific properties of the cation. 

However, if departure of the leaving group would create an unstable carbocation (e.g., a methyl, vinyl, aryl, or primary carbocation), the leaving group cannot depart. It has to be displaced by the halide ion. In other words, an Sn2 reaction occurs. The halide ion preferentially attacks the less sterically hindered of the two alkyl groups. 

Condensed tannins in grapes and wine are more-or-less complex polymers of flavan-3-ols or cate-chins. The basic structural units are (-l-)-catechin and (—)-epicatechin (Figure 6.14). Heating these polymers in solution in an acid medium releases highly unstable carbocations that are converted... 

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