Hydride shift halides

An alternative view of these addition reactions is that the rate-determining step is halide-assisted proton transfer, followed by capture of the carbocation, with or without rearrangement Bromide ion accelerates addition of HBr to 1-, 2-, and 4-octene in 20% trifluoroacetic acid in CH2CI2. In the same system, 3,3-dimethyl-1-butene shows substantial rearrangement Even 1- and 2-octene show some evidence of rearrangement, as detected by hydride shifts. These results can all be accoimted for by a halide-assisted protonation. The key intermediate in this mechanism is an ion sandwich. An estimation of the fate of the 2-octyl cation under these conditions has been made ... 

In addition to 1,2-hydride shifts, the 1,5-hydride shift is favoured [24] in the decomposition of radical-cations of long chain alkyl halide such as 10. 

Reaction between acetonitrile and the radical-cations of secondary alkyl halides is almost entirely S l in character. Both direct substitution and 1,2-hydride shift reactions occur and the products from a chiral alkyl halide such as 2-iodooctane, are almost totally racemised [25]. 

Despite the structural similarity of this alcohol to the alkyl halide in the preceding part of this problem, its dehydration is more complicated. The initially formed carbocation is secondary and can rearrange to a more stable tertiary carbocation by a hydride shift. 

Silver is very efficient at removing halides, resulting in generation of carbocations. Because, once a carbocation is formed, a 1,2-hydride shift applied to the illustrated secondary carbocation can only generate a less stable primary carbocation or an identical secondary carbocation, therefore, there is only one product formed in this reaction. 

Attempts to perform the cationic polymerization of vinylcyclohexane have been reported. While coordination-type polymerization of vinylcyclohexane monomer gives isotactic PCHE, polymerization under conditions that lead to the formation of carbocationic intermediates leads to polymers with a differentiated backbone structure. Instead of propagating via the vinyl carbons, the cationic polymerization proceeds via hydride shift to a tertiary carbocation, which then propagates to provide the polymer shown in Scheme 23.3 [32]. Conditions for these polymerizations typically involved the use of aluminum halide catalysts in halogenated hydrocarbon solvents at low temperatures [32,38]. For the most part, molecular weights are relatively low. 

The result in Equation [1] is explained by a carbocation rearrangement involving a 1,2-hydride shift the less stable 2° carbocation (formed from the 2° halide) rearranges to a more stable 3° carbocation, as illustrated in Mechanism 18.8. 

The indirect reduction of many organic substrates, in particular alkyl and aryl halides, by means of radical anions of aromatic and heteroaromatic compounds has been the subject of numerous papers over the last 25 years [98-121]. Many issues have been addressed, ranging from the exploration of synthetic aspects to quantitative descriptions of the kinetics involved. Saveant et al. coined the expression redox catalysis for an indirect reduction, in which the homogeneous reaction is a pure electron-transfer reaction with no chemical modification of the mediator (i.e., no ligand transfer, hydrogen abstraction, or hydride shift reactions). In the following we will consider such reactions and derive the relevant kinetic equations to show the kind of kinetic information that can be extracted. 

The same rearrangement pattern is obtained when chlorine instead of bromine is at the bridgehead as well as with the bicyclo[5.1.0] and [6.1.0] skeletons, but not with the [3.1.0] skeleton. 1,2-Hydride shifts can occur simultaneously or subsequent to halide extrusion. Thermolysis of 13 gave 3-chloromethylene-2-methylchroman-4-one 14. This product contrasts what that obtained on solvolysis of the 2-phenyl analog of 13 which occurs with ring expansion to yield a benzoxepine derivative (Section 2.4.1.3.1.1.2.). 

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.)... 

A shift involves only the movement of a species from one carbon to an adjacent electron-deficient carbon 1,3-shifts normally do not occur. Furthermore, if the rearrangement does not lead to a more stable carbocation, then a carbocation rearrangement does not occur. For example, when a proton adds to 4-methyl-1-pentene, a secondary carbocation is formed. A 1,2-hydride shift would form a different secondary carbocation. Because both carbocations are equally stable, there is no energetic advantage to the shift. Consequently, rearrangement does not occur, and only one alkyl halide is formed. 

A carbocation intermediate is formed in an SnI reaction. In Section 4.6, we saw that a carbocation will rearrange if it becomes more stable in the process. If the carbocation formed in an SnI reaction can rearrange, SnI and Sn2 reactions of the same alkyl halide can produce different constitutional isomers as products, since a carbocation is not formed in an Sn2 reaction and therefore the carbon skeleton cannot rearrange. For example, the product obtained when HO is substituted for Br in 2-bromo-3-methylbutane by an SnI reaction is different from the product obtained by an Sn2 reaction. When the reaction is carried out under conditions that favor an SnI reaction, the initially formed secondary carbocation undergoes a 1,2-hydride shift to form a more stable tertiary carbocation. 

Because the reaction of a secondary or a tertiary alcohol with a hydrogen halide is an SnI reaction, a carbocation is formed as an intermediate. Therefore, we must check for the possibility of a carbocation rearrangement when predicting the product of the substitution reaction. Remember that a carbocation rearrangement will occur if it leads to formation of a more stable carbocation (Section 4.6). For example, the major product of the reaction of 3-methyl-2-butanol with HBr is 2-bromo-2-methylbutane, because a 1,2-hydride shift converts the initially formed secondary carbocation into a more stable tertiary carbocation. 

All three conformations can give rise to I -butene and although this is not the major product it is formed in much greater abundance than from El reactions of 2-butyl halides or esters. Methyl rearrangement does not occur in this case as this would give rise to a less stable primary carbonium ion. A hydride shift is possible to give the same but essentially a new ion which subsequently breaks down, but this possibility can be excluded by the use of carbon-labelling experiments. 

Mechanism 8.3 for this reaction assumes rate-determining ionization of the alkyl halide (step 1), followed by a hydride shift that converts a secondary carbocation to a more stable tertiary one (step 2). The tertiary carbocation then reacts with water to yield the observed product (steps 3 and 4). 

In the absence of a good nucleophile, carbocation rearrangements may occur following addition of an electrophile to the alkene double bond (Section 9-3). Rearrangements are favored in electrophilic additions of acids whose conjugate bases are poor nucleophiles. An example is trifluoroacetic acid, CF3CO2H. Its trifluoroacetate counterion is much less nucleophilic than are halide ions. Thus, addition of trifluoroacetic acid to 3-methyl-1-butene gives only about 43% of the normal product of Markovnikov addition. The major product results from a hydride shift that converts the initial secondary cation into a more stable tertiary cation before the trifluoroacetate can attach. 

Eq 3 shows an unusual reaction in which the nature of the reaction depends on the amount, acidity, and alkyl group of the alkylaluminum halide. Use of 1 equiv of Me2 AlCl leads to a concerted ene reaction with the side chains cis. Use of 2 equiv of Me2AlCl produces a more electrophilic aldehyde complex that cyclizes to a zwitterion. Chloride transfer is the major process at —78 °C at 0 °C, chloride transfer is reversible and a 1,5-proton transfer leads to an ene-type adduct with the side chains trans. Use of 2 equiv of MeAlCl2 forms a cyclic zwitterion that undergoes two 1,2-hydride shifts to form the ketone. A similar zwitterion forms with EtAlCl2, but p-hydride transfer leading to the saturated alcohol is faster than 1,2-hydride shifts. 

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