Carbocations rearrangement path

The mechanism follows the usual path cyclization of linalyl diphosphate, followed by attack of the n electrons of the second double bond, produce an intermediate carbocation. A carbocation rearrangement occurs, and the resulting carbocation reacts with water to form an alcohol that is oxidized to give a-fenchone. 

Scheme 8.66. A representation of the acid-catalyzed elimination of water from trans- or ( )-2-phenylcyclohexanol showing many of the products derived from carbocation rearrangements that intrude and cartoon representations of paths to those products.

These results imply the scenario outlined in path a of Figure 9.49, in which an unstable primary carbocation rearranges to a much more stable tertiary carbocation through the shift of a methyl group. In fact, the presence of the primary ion is so unsettling—they are most unstable and not likely to be formed—that a variant of this mechanism has been proposed in which the methyl group migrates in a concerted fashion as the iodide departs (path b, Fig. 9.49). A concerted reaction is one that has no intermediates. 

FIGURE 9.56 An intermediate carbocation of path b should lead to rearrangements in hydroboration, but no evidence of such can be found, which suggests that the concerted path a is the correct mechanism. 

Kolbe electrolysis is a powerful method of generating radicals for synthetic applications. These radicals can combine to symmetrical dimers (chap 4), to unsymmetrical coupling products (chap 5), or can be added to double bonds (chap 6) (Eq. 1, path a). The reaction is performed in the laboratory and in the technical scale. Depending on the reaction conditions (electrode material, pH of the electrolyte, current density, additives) and structural parameters of the carboxylates, the intermediate radical can be further oxidized to a carbocation (Eq. 1, path b). The cation can rearrange, undergo fragmentation and subsequently solvolyse or eliminate to products. This path is frequently called non-Kolbe electrolysis. In this way radical and carbenium-ion derived products can be obtained from a wide variety of carboxylic acids. 

By anodic decarboxylation carboxylic acids can be converted simply and in large variety into radicals. The combination of these radicals to form symmetrical dimers or unsymmetrical coupling products is termed Kolbe electrolysis (Scheme 1, path a). The radicals can also be added to double bonds to afford additive monomers or dimers, and in an intramolecular version can lead to five-membered heterocycles and carbocycles (Scheme 1, path b). The intermediate radical can be further oxidized to a carbenium ion (Scheme 1, path c). This oxidation is favored by electron-donating substituents at the a-carbon of the carboxylic acid, a basic electrolyte, graphite as anode material and salt additives, e.g. sodium perchlorate. The carbocations lead to products that are formed by solvolysis, elimination, fragmentation or rearrangement. This pathway of anodic decarboxylation is frequently called nonKolbe electrolysis. 

The normal pathway for rearrangement of the substrates (1) is via loss of the heteroatom substituent with migration of an adjacent alkyl (or aryl) group, and concomitant ketone formation (Scheme 1, path a). An alternative migration (path b), is occasionally observed in protic media, i.e. protonation of the alcohol (1) and migration with loss of water generates the stabilized carbocation (2), which is then hydrolyzed. 

The SnI substitution reaction is a two-step process, a slow Dn step to break the carbon-leaving group bond forming a carbocation, followed by a fast An trapping of the carbocation to form the new bond. The Dn and An paths are just the reverse of each other. Carbocations have just three fates They can be trapped by a nucleophile as discussed in this section they can lose a proton to form the alkene (Section 4.3), or they can rearrange to another carbocation of equal or greater stability (Section 4.7). 

Since the product of the carbocation addition to an alkene via path Ag is also a carbocation that can rearrange and/or attack another alkene molecule (polymerization), unwanted product mixtures can result. 

Many rearrangements have been invoked to explain the fragmentation pattern of molecular ions22 (It should be remembered that the fragmenting species are radical cations. Their radical functionality may open to them reaction paths which are not available to simple carbocations). Only a few illustrative examples of well-established alkyl and aryl shifts are given here. The mass spectrum of camphor displays the base peak at m/e 95, corresponding to the loss of ketene and a methyl radical. As shown... 

Reaction conditions that favor elimination by an El mechanism should be avoided because the results can be too variable. The carbocation intermediate that accompanies an El reaction can undergo rearrangement of the carbon skeleton, as we shall see in Section 7.8, and it can also undergo substitution by an S l mechanism, which competes strongly with formation of products by an El path. 

Beckmann rearrangement of oximes to amides can deviate to fragmentation to form nitriles and carbocations, if the latter possess reasonable stability. Both 1-substituted-phenyl-2-propanones and 3-substituted-phenyl-2-butanones in aqueous solvents give both products, and calculations have been used to probe the mechanisms. In borderline cases, a dynamic path bifurcation from a single transition state is claimed. 

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