Cyclopropyl halides/tosylates

We have previously seen (p. 345) that this is the reason nucleophilic substitutions are not feasible at a cyclopropyl substrate. The reaction is often used to convert cyclopropyl halides and tosylates to allylic products, especially for the purpose of ring expansion, an example being1211... 

The Woodward-Hoffmann ru are not limited m application to the neutral systems that have been discussed up roTHis point. They also apply to charged systems. The conversion of cyclopropyl cation to allyl cation has been thoroughly studied and represents the simplest possible case of an electrocyclic transformation, since it involves only two tt-electrons. Because of the restrictions imposed on the internuc-lear angles in cyclopropyl rings, carbonium ions do not form readily, and cyclopropyl halides and arenesulfonates are quite unreactive under ordinary solvolytic conditions. For example, it was found that temperatures of 180°C were necessary for cyclopropyl tosylate to react in acetic acid, and the product was allyl acetate, rather than cyclopropyl acetate. A mechanism was considered in which cyclopropyl cation was formed in the rate-determining step, followed by rapid conversion to allyl... 

Due to the strain imposed by the three-membered ring, the cyclopropyl cation is not a stable intermediate and electrocyclic ring opening occurs readily. Therefore, in the solvolysis of cyclopropyl tosylate in acetic acid, allyl acetate is obtained rather than cyclopropyl acetate. Solvolysis reactions of other cyclopropyl halides, sulphonates, and diazotization of cyclopropylamine in aqueous solution also give the allylic products. 

Another interesting application of allyl halides in Pd-catalyzed allylic alkylations utilizes substituted 1-alkenylcyclopropyl chlorides (Scheme 12.38). These allylic chlorides, as well as the more reactive, analogous tosylates, react with soft nucleophiles, such as malonates, in a highly regioselective manner [17d,e, 72]. The attack at the ii-allylpalladium intermediate takes place preferentially at the allylic position distal to the cyclopropyl substituent, providing access to a variety of synthetically useful methylenecydopropane derivatives. 

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