Anchimeric assistance field effects

For the establishment of anchimeric assistance in tm zs-substituted cyclic compounds, the cis isomer contains essentially the same c-inductive effects and may, from a cursory inspection, seem to be a reasonable model. There are some problems with this model, however (1) Field effects are difficult to determine, and they probably affect many rate comparisons (2) the conformation for reaction may differ from the ground-state-preferred conformation, and hence estimates based on the wrong conformation may lead to errors and (3) hydrogen participation (or facile elimination) in the cis model may introduce errors. We shall now discuss this method and its potential pitfalls with the use of examples. 

Another widely used comparison, especially in bicyclic systems, is the exojendo rate ratio. This comparison is also fraught with pitfalls. Problems arise because the endo and exo isomers of a polycyclic system have unique configurations and hence different steric requirements. This can affect anchimeric assistance by neighboring groups. In addition there are inherent differences in field effects and nucleophilic solvent assistance. The 350-fold difference in rate constant for acetolysis of exo- and endo-2-norbornyl p-bromobenzenesulfonates [(11) and (12), respectively] was taken as evidence for a participation in the solvolysis of (11) by Winstein... 

In summary, the results discussed above reveal that an ether oxygen in a medium or a bicyclic ring system may offer significant anchimeric assistance in reactions in solvolytic displacement reactions. Because of inductive effects, steric effects, field effects, and internal return, anchimeric assistance, when present, is often difficult to demonstrate kinetically. Products formed from bicyclic and tricyclic oxonium ions probably more often result from participation after carbocation formation. Participation by oxygen apparently occurs to the exclusion of transannular hydride shifts in medium ring systems, but other types of participation such as homoallylic or formation of cyclopropyl carbinyl cations will most likely occur to the exclusion of ether oxygen participation. 

The 3,5-dinitrobenzoate (113), however, hydrolyzes at a rate 48,500 times that of cyclooctyl p-bromobenzenesulfonate, after appropriate leaving group conversions. These results dramatically demonstrate transannular anchimeric R2O-5 assistance in this medium ring system. Paquette and Scott discussed inductive, steric, dipolar field, and anchimeric effects that are operative in the medium ring systems. 

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