Percaudal interactions

Shiner and coworkers also studied optically active acyclic secondary sulphonate esters and found rate accelerations of about 100 in solvolysis in TFA at 25 °C compared with the alkyl reference (Table 8)93-95. From the product distribution and the stereochemical outcome of the solvolysis Shiner and coworkers proposed the reaction course outlined in equation 37 for the brosylate 23994. The transition state for the ionization adopts the W conformation and, to a smaller extent, the ewrfo-Sickle conformation and results in the formation of /-silicon carbocation which is stabilized by a percaudal interaction. The incoming nucleophile attacks the bridged ion from the front side94. 

In contrast, the esters 145 and 167 show no significant effects on the C-OR bond distance. These structural effects are consistent with a significant percaudal interaction, as represented by the resonance form 168. 

Figure 9. The transannular percaudal interaction in cf s-5-trimethylstannylcycloocty] mesylate 192.

As discussed in Section III.E, Shiner and coworkers65 67t 68 have recently obtained evidence for y-silylcarbonium stabilization, in favourable cases, through percaudal interaction. 

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