Hydride ion migration

Intramolecular hydride ion migration in a substituted 9-fluorenyl carbocation 287... 

Intramolecular Hydride Ion Migration in a Substituted 9-Fluorenyl Carbocation... 

Similarly if tlris electrolyte is made into a composite with SrS, SrC2 or SrH2, the system may be used to measure sulphur, carbon and hydrogen potentials respectively, tire latter two over a resuicted temperamre range where the carbide or hydride are stable. The advantage of tlrese systems over the oxide electrolytes is that the conductivity of the fluoride, which conducts by F ion migration, is considerably higher. 

Hydride ion can migrate. In example c, it was hydride that shifted, not bromine ... 

Rearrangements occur almost invariably when the migration of an alkanide ion or hydride ion can lead to a more stable carbocation. 

Hydride transfer or hydride migration is initiated by the electrophilic attack of the poly(oxymethylene) cation from the methylene bridge of its own or of a neighboring macromolecule. A hydride ion is thus split off, and a methoxy end group is formed. 

Attention may be drawn, moreover, to acid-catalyzed rearrangements of pyrethrosin described by Barton and oo-worltera involving migration of a double bond -electrons) instead of the mere customary hydride ion or alkyl group (o-electronB). The transformation shown in Eq. (486) illustrates this point. 

The intramolecular condensation of 341 would be expected to give dihydroquinoline 342 (Fig. 133), but a dismutation reaction takes place, affording the tetrahydro-quinoline 343 along with approximately equal amounts of the aromatic derivative 344, as is frequently observed when the reaction is performed in the absenc-e of an oxidizing agent. In polyphosphoric acid medium the proposed mechanism involves migration of the hydride ion from position 2 of 342 toward the cationic site of a protonated dihydroquinoline molecule. "... 

Transfer of two hydride ions and one proton would result in DMH. Since the methyl groups could migrate on the chain, DMH s other than 2,5-DMH could be produced. Some t-butyl cations dissociate into isobutylene and protons hence this method could occur during alkylation with olefins other than isobutylene. Reaction N is probably only of minor Importance in most cases, however, since only small concentrations of free isobutylene are thought to occur at the acid-hydrocarbon interface most isobutylene quickly protonates to form t-butyl cations. 

The common feature in such reactions is the attack of a hydride ion on a relatively electron-deficient carbon atom. The hydride may migrate from another carbon atom in the same molecule, from a carbon atom on another molecule, or even from a Lewis-type site (—Al—H ) on the catalyst. Whatever the source, the structural environment to which the hydride adds becomes more hydrogen-rich, and that from which it derives, more hydrogen-deficient. In Scheme 6, some of these reactions are tabulated. 

The pathway involving cyclization of a protonated migrating group provides a very appealing alternative to the fragmentation-recombination pathway. Given the lack of evidence for imine formation, it is interesting to note that the formation of a protonated imine (14-H ) in the model system can alternatively arise formally as the result of removal of a hydride ion from the parent (saturated) system (12), aminopropyl (path e. Scheme 9). 

We have already seen that in acid solution many carbonium ion rearrangements may be interpreted as involving the intramolecular migration of an alkyl group with a pair of electrons to an electronically deficient atom (Chapter 3). A number of other reactions appear to involve a similar migration of a hydrogen atom with a pair of electrons (i.e., a hydride ion). Not only do these reactions proceed by the internal shift of a Hydride ion (pp. 68, 145), but others are known in which an mtermolecular transfer is involved (pp. 60, 146). 

Wagner-Meerwin rearrangement. Carbon-to-carbon migration of alkyl, aryl, or hydride ions. The original example is the acid-catalyzed rearrangement of camphene hydrochloride to isobornyl chloride. 

Migration of a hydride ion from the branching carbon of the primary (terminal) carbonium ion so formed can then produce a relatively stable tertiary carbonium ion. An unchanged olefinic product may be formed from this by loss of a further proton (Eq. 18.21). 

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