Rearrangements, carbocations hydrogen shifts

These carbocation rearrangements are similar to those that occur dur- ing electrophilic additions to alkenes (Section 6.12). For example, the relatively unstable primary butyl carbocation produced by reaction of 1-chloro-butane with AICI3 rearranges to the more stable secondary butyl carbocation by shift of a hydrogen atom and its electron pair (a hydride ion, H ) from C2 to Cl. 1... 

As a result of carbocation rearrangement, two alkyl halides are formed—one from the addition of the nucleophile to the unrearranged carbocation and one from the addition to the rearranged carbocation. The major product is the rearranged one. Because a shift of a hydrogen with its pair of electrons is involved in the rearrangement, it is called a hydride shift. (Recall that H is a hydride ion.) More specifically it is called a 1,2-hydride shift because the hydride ion moves from one carbon to an adjacent carbon. (Notice that this does not mean that it moves from C-1 to C-2.)... 

This kind of ions with a bridged hydrogen can be generated from cycloalkanes with 8-11 carbon atoms, i.e, those for which direct transannular hydrogen shifts in carbocation reactions was postulated Ions with Cg, and -rings as distinct from a cyclodecyl cation, rearrange to contract the cycle and form tertiary ions, e.g. ... 

Schiess examined the 1,5-shift of carbonyl groups in 1,3-cyclohexadiene and found that the rate of rearrangement for the formyl material is roughly 125 times faster than that for the hydrogen shift while that for the acetyl derivative is only 4 times faster. Further, the carbomethoxy derivative is substantially slower (by a factor of 70). Here it is likely that there is substantial bond formation to the carbonyl group recognizing that it is carbocation-like so the interaction is not unlike that in transannular tt participation however, it is unclear as to the extent of ionic character in this transition state, so (Scheme 7.42) is merely meant to be provocative. 

As shown in the following mechanism, reaction is initiated by heterolytic cleavage of the carbon-chlorine bond to form a 2° carbocation, which rearranges to a considerably more stable 3° carbocation by shift of a hydrogen with its pair of electrons (a hydride ion) from the adjacent benzylic carbon. Note that the rearranged carbocation is not only tertiary (hyperconjugation stabilization) but also benzylic (stabilization by resonance delocalization). 

Less stable carbocations rearrange to more stable carbocations by shift of a hydrogen atom or an alkyl group. Besides rearrangement, carbocations also react with nucleophiles (7.13) and bases (8.6). 

Hydride shift (Section 5 13) Migration of a hydrogen with a pair of electrons (H ) from one atom to another Hydnde shifts are most commonly seen in carbocation rearrange ments... 

The rearrangement of the intermediate alkyl cation by hydrogen or methyl shift and the cyclization to a cyclopropane by a CH-insertion has been studied by deuterium labelling [298]. The electrolysis of cyclopropylacetic acid, allylacetic acid or cyclo-butanecarboxylic acid leads to mixtures of cyclopropylcarbinyl-, cyclobutyl- and butenylacetamides [299]. The results are interpreted in terms of a rapid isomerization of the carbocation as long as it is adsorbed at the electrode, whilst isomerization is inhibited by desorption, which is followed by fast solvolysis. 

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