Shift, 1,2-hydride

l)-hydride shift rarely occurring in the chemistry of acyclic compounds seems to be common in that of carbocations having a bicycIo[2.2.1]heptyl skeleton. 

Acetolysis of 7-syn-7d and 7-anti-chloro-2-exo-norbomyl tosylate 77 yields identical mixtures (1 1)  

These data have been interpreted by Roberts as an argument to favour the parti- 

Goering however, has shown the ratio of chloroacetates formed from isomeric tosylates 76 and 77 to be different. Therefore, he has suggested, that solvolysis yields isomeric nonclassical ions which turn into one another at a rate comparable to that of capture by the solvent. 

Nortricyclonium ions protonated over the edge and the face cannot be the primary intermediates since otherwise they would be common for both isomers 76 and 77  

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Extensive studies on the Wacker process have been carried out in industrial laboratories. Also, many papers on mechanistic and kinetic studies have been published[17-22]. Several interesting observations have been made in the oxidation of ethylene. Most important, it has been established that no incorporation of deuterium takes place by the reaction carried out in D2O, indicating that the hydride shift takes place and vinyl alcohol is not an intermediate[l,17]. The reaction is explained by oxypailadation of ethylene, / -elimination to give the vinyl alcohol 6, which complexes to H-PdCl, reinsertion of the coordinated vinyl alcohol with opposite regiochemistry to give 7, and aldehyde formation by the elimination of Pd—H. 

Oxidation of ethylene in alcohol with PdCl2 in the presence of a base gives an acetal and vinyl ether[106,107], The reaction of alkenes with alcohols mediated by PdCl2 affords acetals 64 as major products and vinyl ethers 65 as minor products. No deuterium incorporation was observed in the acetal formed from ethylene and MeOD, indicating that hydride shift takes place and the acetal is not formed by the addition of methanol to methyl vinyl etherjlOS], The reaction can be carried out catalytically using CuClj under oxygen[28]. 

FIGURE 5 9 Dehydration of 1 butanol is accompanied by a hydride shift from C 2 to C 1... 

Alkene synthesis via alcohol dehydration is complicated by carbocation rearrangements A less stable carbocation can rearrange to a more sta ble one by an alkyl group migration or by a hydride shift opening the possibility for alkene formation from two different carbocations... 

In the acid catalyzed dehydration of 2 methyl 1 propanol what carbocation would be formed if a hydride shift accompanied cleavage of the carbon-oxygen bond in the alkyloxonium lon" What ion would be formed as a result of a methyl shift" Which pathway do you think will predominate a hydnde shift or a methyl shift" ... 

Addition begins m the usual way by protonation of the double bond to give m this case a secondary carbocation This carbocation can be captured by chloride to give 2 chloro 3 methylbutane (40%) or it can rearrange by way of a hydride shift to give a tertiary carbocation The tertiary carbocation reacts with chloride ion to give 2 chloro 2 methylbutane (60%)... 

A reasonable mechanism for this observation assumes rate determining ionization of the substrate as the first step followed by a hydride shift that converts the secondary carbocation to a more stable tertiary one... 

Why does the carbocation intermediate in the hydrolysis of 2 bromo 3 methylbutane rearrange by way of a hydride shift rather than a methyl shift ... 

We saw rearrangements in volving hydride shifts earlier in Sections 5 13 and 6 7... 

A hydride shift produces a tertiary carbocation a methyl shift produces a secondary carbocation... 

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 conversion of the cationic intermediate of the monomer to the cation of the product occurs by a hydride shift between adjacent carbons ... 

Nucleophilic substitution in cyclohexyl systems is quite slow and is often accompanied by extensive elimination. The stereochemistry of substitution has been determined with the use of a deuterium-labeled substrate (entry 6). In the example shown, the substitution process occurs with complete inversion of configuration. By NMR amdysis, it can be determined that there is about 15% of rearrangement by hydride shift accon any-ing solvolysis in acetic acid. This increases to 35% in formic acid and 75% in trifiuoroacetic acid. The extent of rearrangement increases with decreasing solvent... 

The 2-butyl cation can be observed under stable-ion conditions. The NMR spectrum corresponds to a symmetrical species, which implies either very rapid hydride shift or a symmetrical H-bridged structure. 

Two hydride shifts resulting in interchange of the C(2) and C(3) hydrogens account for the two minor products. 

This result can be explained by a transarmular 1,5-hydride shift. Many similar processes have been documented. ... 

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 ... 

Because of Us high polarity and low nucleophilicity, a trifluoroacetic acid medium is usually used for the investigation of such carbocationic processes as solvolysis, protonation of alkenes, skeletal rearrangements, and hydride shifts [22-24] It also has been used for several synthetically useful reachons, such as electrophilic aromatic substitution [25], reductions [26, 27], and oxidations [28] Trifluoroacetic acid is a good medium for the nitration of aromatic compounds Nitration of benzene or toluene with sodium nitrate in trifluoroacetic acid is almost quantitative after 4 h at room temperature [25] Under these conditions, toluene gives the usual mixture of mononitrotoluenes in an o m p ratio of 61 6 2 6 35 8 A trifluoroacetic acid medium can be used for the reduction of acids, ketones, and alcohols with sodium borohydnde [26] or triethylsilane [27] Diary Iketones are smoothly reduced by sodium borohydnde in trifluoroacetic acid to diarylmethanes (equation 13)... 

Hydride shifts often occur during the dehydration of primary alcohols. Thus, although 1-butene would be expected to be the only alkene fonned on dehydration of 1-butanol, it is in fact only a minor product. The major product is a mixture of cis- and trans-2-butene. 

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