Carbocations typical reactions

In later studies by various groups, the enyneallene motif was incorporated into more complex hydrocarbon structures, allowing not only a better understanding of the Myers cyclization but also the generation of polycyclic hydrocarbons, some of them resembling the steroid core unit. Conceptually, these latter cyclizations are reminiscent of Johnson s biomimetic cyclization reactions with the main difference that here radical intermediates are involved rather than carbocations. Typical starting materials in these studies are the allenes 221 [87], 222 [88] and 223 [89], their cyclization behavior being discussed in Chapter 20. 

Acidic cleavage is an 8, 2 mechanism with a pseudo-carbocation ion. The reaction produces the trans product (anti addition). Figure 3-37 shows a typical reaction, and Figure 3-38 illustrates the mechanism. 

The typical reactions of carbocation intermediates were discussed in Chapter 7. The solvolysis of alkyl halides is an example of the involvement of carbocations in the SnI mechanism, in other words, where the final outcome is a nucleophilic substitution. The first step is a heterolytic cleavage of the C—X bond. Properties of X which favor heterolytic cleavage, namely electronegativity difference with carbon (the larger, the better) and the degree of overlap of the X orbital with the spn orbital of carbon (the smaller the better), have already been elucidated (Chapter 4). The transition state has partial... 

The reaction of azide ions with carbocations is the basis of the azide clock method for estimating carbocation lifetimes in hydroxylic solvents (lifetime = 1 lkiy where lq, is the first-order rate constant for attack of water on the carbocation) this is analogous to the radical clock technique discussed in Chapter 10. In the present case, a rate-product correlation is assumed for the very rapid competing product-forming steps of SN1 reactions (Scheme 2.24). Because the slow step of an SN1 reaction is formation of a carbocation, typical kinetic data do not provide information about this step. Furthermore, the rate constant for the reaction of azide ion with a carbocation (kaz) is assumed to be diffusion controlled (ca. 5 x 109 M 1 s 1). The rate constant for attack by water can then be obtained from the mole ratio of azide product/solvolysis product, and the molar concentrations of azide (Equation 2.18, equivalent to Equation 2.14) [48]. The reliability of the estimated lifetimes was later... 

The elements of H2O are eliminated. Under acidic conditions, eliminations occur by an El mechanism. First, the leaving group is protonated to convert it into a better leaving group, and then it leaves to give a carbocation. Then fragmentation (one of the three typical reactions of carbocations) of a C-H bond occurs to give a diene. 

Analogies between free radicals and carbocations have been drawn several times, but one of the typical reactions of carbocations, the concerted 1,2-shift, does not occur in free radicals. A concerted 1,2-radical shift is allowed only when one of the components can react antarafacially. The 1,2-hydrogen atom shift is geometrically impossible, and the geometric requirements of the 1,2-alkyl radical shift are so stringent that it is not observed. 

In catalytic cracking many reactions take place simultaneously. Cracking occurs by C-C bond cleavage of paraffins, dealkylation etc. Isomerization and even condensation reactions take place. These reactions occur via positively charged hydrocarbon ions (carbocations). The nature of the carbocations is the subject of debate. For the cracking of paraffinic hydrocarbons it is usually assumed that carbenium ions are the crucial intermediates, which decompose via beta fission into olefins and (smaller) carbenium ions (see Chapter 4, Section 4.4). A typical reaction mechanism for catalytic cracking (and hydrocracking) imder the relatively mild conditions used in FCC is shown overleaf. 

As we look at the reactions shown in Figures 2.9-2.16, we will see the typical reactions of carbocations. The chemical parameters controlling... 

AllylsUanes are reactive towards 1,3-dithienium tetrafluoroborate. The intermediate -silyl carbocation typically suffers elimination of the silyl cation to furnish a 2-allyl-1,3-dithiane (eq 5). This protocol was employed to homologate allylsilane 3 to produce dithiane 4, an intermediate in the synthesis of epiantillatoxin (eq 6). A fert-butyldimethylsilyl ether used as a protecting group was cleaved from 3 during this reaction. 

The reaction of bromocalix[4]arene 28a with alcohols must be conducted in a mixture of the alcohol nucleophile and the ionizing solvent, but for the larger calixarenes the reaction proceeds even in the absence of the additional fluorinated alcohol [30]. Some typical reactions of bromocalix[6]arene 28c are exemplified in Eq. (4.22). Notably, in most cases the reaction proceeds in stereoselective fashion affording mainly the all-cis derivative. The reaction fails for tertiary alcohols while in the reaction with secondary alcohols, a competing reaction was observed yielding reduced bridges. This reaction most likely involves hydride transfer from the secondary alcohol to the benzhydrylic carbocation intermediates [30]. 

Cationic polymerization of unsaturated compounds proceeds through the stage of carbanion cations, called also carbocations. Typical catalysts for this reaction are strong protic acids such as sulfuric acid, perchloric and trifluoroctane or the Lewis acids, which include halides of elements III, IV and V groups of the periodic table (Friedel-Crafts catalysts), such as boron trifluoride, aluminum trichloride, tin tetrachloride and titanium tetrachloride. The activity of Friedel-Crafts catalysts increases significantly the presence of small quantities of cocatalysts, that is, ihe compounds which most often are the source of protons. 

The Ritter reaction is an effective way to form amides by coupling nitriles with alcohols. Typically tertiary, benzylic or allylic alcohols, or substrates which form relatively stable intermediate carbocations by reaction with sulfuric acid, are used. A recent review by Cossy provides an in depth review on the recent advances in metallic and non-metallic Ritter reaction catalysts. ... 

Thus with dihalocarbenes we have the interesting case of a species that resem bles both a carbanion (unshared pair of electrons on carbon) and a carbocation (empty p orbital) Which structural feature controls its reactivity s Does its empty p orbital cause It to react as an electrophile s Does its unshared pair make it nucleophilic s By compar mg the rate of reaction of CBi2 toward a series of alkenes with that of typical electrophiles toward the same alkenes (Table 14 4) we see that the reactivity of CBi2... 

This bicychc carbocation then undergoes many reactions typical of carbocation inter mediates to provide a variety of bicychc monoterpenes as outlined m Figure 26 7... 

Although there is no simple quantitative relationship between the stability of a carbocation intermediate and the rate of its formation, there is an intuitive relationship. It s generally true when comparing two similar reactions that the more stable intermediate forms faster than the less stable one. The situation is shown graphically in Figure 6.13, where the reaction energy profile in part (a) represents the typical situation rather than the profile in part (b). That is, the curves for two similar reactions don t cross one another. 

The S il reaction occurs when the substrate spontaneously dissociates to a carbocation in a slow rate-limiting step, followed by a rapid reaction with the nucleophile. As a result, SN1 reactions are kinetically first-order and take place with racemization of configuration at the carbon atom. They are most favored for tertiary substrates. Both S l and S 2 reactions occur in biological pathways, although the leaving group is typically a diphosphate ion rather than a halide. 

Aromatic alkylations occur in numerous biological pathways, although there is of course no MCI3 present in living systems to catalyze the reaction. Instead, the carbocation electrophile is usually formed by dissociation of an organodiphosphate, as we saw in Section 11.6. The dissociation is typically assisted by complexation to a divalent metal cation such as Mg2+ to help neutralize charge. 

An example of a biological Friedel-Crafts reaction occurs during the biosynthesis of phylloquinone, or vitamin Kl( the human blood-clotting factor. Phylloquinone is formed by reaction of 1,4-dihydroxynaphthoic acid with phytyl diphosphate. Phytyl diphosphate first dissociates to a resonance-stabilized allylic carbocation, which then substitutes onto the aromatic ring in the typical way. Several further transformations lead to phylloquinone (Figure 16.10). 

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