Conducted-tour mechanism

This mechanism is exactly analogous to the allylic rearrangement mechanism for nucleophilic substitution (p. 421). The UV spectra of allylbenzene and 1-propenylbenzene in solutions containing NH2 are identical, which shows that the same carbanion is present in both cases, as required by this mechanism. The acid BH protonates the position that will give the more stable product, though the ratio of the two possible products can vary with the identity of BH". It has been shown that base-catalyzed double-bond shifts are partially intramolecular, at least in some cases. The intramolecularity has been ascribed to a conducted tour mechanism (p. 766) in which the base leads the proton from one carbanionic site to the other ... 

Kende, A. S., Riecke, E. E. Allylidenecyclopropane-methylenecyclopentene energy surface. Evidence for a conducted tour mechanism. J. Am. Chem. Soc. 1972, 94, 1397-1399. 

The phrase conducted tour mechanism was coined by Cram to describe the removal of a proton by a base and its subsequent return to a different face of the same molecule from which it was removed [Ij. Originally, the conducted tour mechanism was postulated to explain the observation that rates of racemization of deuterated carbon acids were faster than hydrogen-deuterium exchange in solutions of potassium r rr-butoxide/rerf-butyl alcohol. Thus, the basic catalyst takes hydrogen or deuterium on a conducted tour of the substrate from one face of the molecule to the [other] (ref. [1], p. 101). This process was envisioned as a rotation of the carbanion within the solvent cage. We now recognize that the secondary amine forms a mixed aggregate with the enolate, such that the reprotonation (and perhaps conformational motion) is intrasupramolecular. ... 

It should be noted that isoinversion can also occur without a conducted tour mechanism. This has been observed again with 9-methylfluorene and t rt-butanol solvent, but with n-propylamine replaced by the base penta-methylguanidine. That an alternative mechanism is operative is indicated by the fact that a substituent such as dimethylamide no longer needs to be present. The base can provide a charge delocalization pathway for travel of the deuteron from one side of the planar ring system to the other without solvent exchange. The deuteron can thus add to either side of the plane, as is illustrated below ... 

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