Hydride-ion shifts

In effect we postulate that the olefin ion is formed by a 1-3 hydride ion shift accompanied by a beta homolytic bond fission. The fact that olefin ions are formed only at branch points (except methyl branch points) could be explained on an energetic basis if it were not for the contrary fact that the over-all energetics are highly unfavorable. Thus in Reaction 20 we see that a disubstituted olefin ion is formed, and this will be true for any branch other than a methyl branch. Thus ... 

The result is not totally surprising, because hydride ion shifts are known in many methylations. Thus, it was proposed that the methyl carbinol is formed by the sequence methylation of a double bond - hydride shift - formation of terminal methylene - epoxidation - opening of the epoxide to aldehyde - reduction to carbinol (Scheme 6). The pathway can explain well the loss of two original hydrogens in methionine methyl group. 

In the 1960s, after Kennedy and Thomas [25] had established the isomerisation polymerisation of 3-methylbutene-l, this became a popular subject. From Krentsel s group in the USSR and Aso s in Japan there came several claims to have obtained polymers of unconventional structure from various substituted styrenes by CP. They all had in common that an alleged hydride ion shift in the carbenium ion produced a propagating ion different from that which would result from the cationation of the C C of the monomer and therefore a polymer of unconventional structure the full references are in our papers. The monomers concerned are the 2-methyl-, 2-isopropyl-, 4-methyl-, 4-isopropyl-styrenes. The alleged evidence consisted of IR and proton magnetic resonance (PMR) spectra, and the hypothetical reaction scheme which the spectra were claimed to support can be exemplified thus ... 

Hint The secondary carbocation formed in step 11 rearranges to a more stable tertiaiy carbocation by a hydride ion shift from 3rd carbon atom. 

The oxidation and reduction steps are coupled through the binding of G3P to a sulfhydryl group on an enzyme that already has bound a NAD+ molecule. The enzyme facilitates a H (hydride ion) shift from G3P to reduce the NAD+ to NADH. The oxidation of G3P results in the formation of a high-energy thioester bond that is subsequently replaced by an orthophosphate group to yield the 1,3-BGP. 

Once the carbonium ions are formed, the modes of interaction constitute an important means by which product formation occurs during catalytic cracking. For example, isomerization either by hydride ion shift or by methyl group shift, both of which occur readily. The trend is for stabilization of the carbonium ion by movement of the charged carbon atom toward the center of the molecule, which accounts for the isomerization of a-olefins to internal olefins when carbonium ions are produced. Cyclization can occur by internal addition of a carbonium ion to a double bond which, by continuation of the sequence, can result in aromatization of the cyclic carbonium ion. 

This rearrangement is called a 1,2-hydride ion shift. A hydride ion isH". 

In basic solution there is convincing evidence, at least for one reaction, that an intramolecular hydride ion shift occurs. When phenylglyoxal is treated with barium deuteroxide, mandelic acid is formed which contains no deuterium attached to a carbon atom.24 Furthermore, the reaction is first order in hydroxide ion and the glyoxal,25 and there is no rearrangement in the carbon skeleton during the transformation.24 26 This can mean only that the transfer of the hydrogen atom with its pair of electrons to the carbonyl group must take place by an intramolecular path. 

The oxidation-reduction process resembles a Cannizzaro reaction and probably proceeds by a similar hydride ion shift involving as acceptor the Schiff base coi jugate acid ... 

Consider, for example, the polymerization of 3-methyl-l,2-butene. Isomerization occurs by a 1,2-hydride ion shift in the first-formed carbocation... 

This result shows that a hydride-ion shift from cyclohexene to the corresponding perchlorobenzyl cation takes place. This process is related to the aromatization of cyclohexadiene with tetrachlorobenzoquinone (chloranil), which is assumed to occur through hydride-ion transfer (March, 1985, p. 1053). When cyclohexene is replaced by toluene no reaction takes place. Hydrogen bromide is also an efifective hydride-ion donor (Ballester el al., 1967). 

Shifts of hydride ions comparable to the NIH-shift have also been found during the methylation of compounds with isolated double bonds (see the methy-lation of fatty acids, D 3.2.1 or sterols, D 6.4.1). The intermediates originating by addition of the methyl cation (Figs. 58 and 123) correspond to intermediate I in Fig. 20 and are stabilized according to the. rules mentioned above. Corresponding intermediates and a hydride ion shift may also be expected during substitution of products by the isopentenyl cation, but so far have not been detected. 

The 1,2-hydride ion shift in the first intermediate gives a T-carbo-cation which cyclizes into tetrahydrofuran derivatives. This process is observed during the acylation of decaline (vide infra). 

Reactions between primary or secondary polyfluoroalkyl phosphines with nucleophiles have been discussed in terms of either a phosphaalkene intermediate or a nucleophilically initiated hydride ion shift from phosphorus to carbon. Evidence for the correctness of the former possibility has been presented for the alkaline hydrolysis of (CF3)2PH by the trapping of the intermediate CF3P==CF2 [equation (28)] ... 

Investigations of the mechanism of this reaction suggest that there is a hydride ion shift, a hydroxide ion transfer, loss of a proton, and the reduction of palladium, Eq. (96). The palladium is then oxidized by CuCh, Eq. (97), which is in turn regenerated by oxidation with air, Eq. (98). 

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