Allylic substitution folding effect

The allyl cation thus formed may stabilize itself either by readdition of the leaving group — leading to a 2,3-dihalopropene — or by the addition of a nucleophile. The influences of steric and electronic effects on the stereochemistry and on the solvolysis rates of various alkyl-substituted chlorocyclopropanes have been investigated by Parham and co-workers [165, 166], who could show for example that os-2,3-dipropyl-l,l-dichlorocyclopropane solvolyzes 24 times faster than its trans-isomer, in accordance with predictions based on orbital symmetry arguments. When one propyl substituent of the trans-isomer is replaced by an ethoxy group the rate of solvolysis increases 200 fold. 

The effect of the methoxy group at the 4-, 5- or 6-position of the aUyl vinyl ether on the rate of the Claisen rearrangement was investigated by Coates et al. in 1987 using h NMR data for their kinetic analysis [28]. Compared to the unsubstituted allyl vinyl ether, the methoxy substituent can either decrease or dramatically increase the rate of the rearrangement (Scheme 11.20). A very impressive 3800-fold rate acceleration was observed for the 4-methoxy-substituted allyl vinyl ether compared to 5-methoxy-substituted allyl vinyl ether. The rate-accelerating effect of the 6-methoxy substituent was less pronounced but still significant. 

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