Hydride ions intramolecular migration

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. "... 

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). 

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

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

Hydride transfers are implicated in several acylations of alkynes by saturated acyl chlorides leading to cyclopentenes. Intramolecular [1,5] hydride shifts have been shown to be a common feature in silver-assisted reactions of cyclohexanylcarbonyl chloride with alkynes.The nature of the final product depends on the structure of the resulting cation. Capture of fluoride ion, ring contraction and acyl or alkyl migrations of axial substituents have been observed (Scheme 24). 

An iminium ion-alkene cyclization has been employed to assemble the phenylmorphan ring system (Scheme 26). The conversion of enamine (64) to (66) was suggested to arise by 1,5-hydride migration of an initially formed bicyclic cation (65). Direct intramolecular ene cyclization of the iminium ion (67) produced by protonation of (64) provides an alternative rationale for the net cis addition to the terminal alkene that occurs in this transformation, and avoids postulating the intervention of a relatively unstable fully formed secondary carbocation. 

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