Cyclopropyl chlorides, solvolysis

That maximum acceleration occurs when the vacant p orbital is parallel to the plane of the cyclopropyl ring can be seen from the solvolysis of spiro[cyclo-propane-l,2 -adamantyl] chloride (71). The carbocation formed by departure of Cl is unable to adopt the geometry of the bisected cyclopropylcarbinyl cation, but can orient its empty p orbital properly to form the bicyclobutonium ion. This compound solvolyzes 103 times more slowly than 1-adamantyl chloride.82 On the other hand, 72 solvolyzes 10s times faster than 73. The cation from 72 does have its p orbital parallel to the plane of the ring as in the bisected cyclopropylcarbinyl cation.83... 

The rates of solvolysis of substituted cumyl chlorides (equation 3) are used to define and (Tp parameters, and this reaction has been studied in several different laboratories These constants reflect the enhanced stabilization when direct conjugation of the p-substituent with the cationic center is possible and clearly demonstrate the importance of this effect for cyclopropyl. 

The conformational dependence of cyclopropyl conjugation was illustrated in solvolysis of methyl-substituted cyclopropylcumyl chlorides. Thus for solvolysis of 2-aryl-2-propyl chlorides (14) there was an acceleration by a factor of 154 for replacement of the p-H by cyclopropyl, whereas for the 2-(3, 5 -dimethylphenyl)-2-propyl chloride (15) the acceleration was decreased to a factor of 9.0. ... 

This concerted ionization and disrotatory ring-opening of cyclopropyl substrates has been an area of continuing interest . It can be prohibited by steric or conjugative interactions . For instance, 1-cyclopropylcyclopropyl chloride (3a) or tosylate (3b) lead to unrearranged solvolysis products, i.e. 1-cyclopropylcyclopropanol (4), and to cyclopropylethyl ketone (5) arising from homoketonization (equation 4). 

One case where C—1C bonds are exceptionally effective in hyperconjugation is in the stabilisation provided by a cyclopropyl substituent to an empty p orbital. The cyclopropylmethyl cation is actually better stabilised than an allyl cation, as judged by the 41 times more rapid solvolysis in a good ionising solvent of cyclopropylmethyl chloride 2.42 than of crotyl chloride 2.43.96... 

Another system, which has attracted much interest, is that of the cyclopropyl-methyl cation, for similar reasons to the 2-norbomyl systems namely, the great ease of solvolysis of cyclopropyhnethyl substrates accompanied by formation of rearrangement products. Of particular interest is that regardless of the method of generation of the cationic species the ratio of products is always very similar. Both hydrolysis of cyclopropylmethyl chloride under conditions favoring 1 substitution and diazotization of cyclopropylamine produce cyclobutanol and but-3-en-l-ol as well as cyclopropyhnethanol in approximately 47%, 5%, and 48% yields respectively. Moreover, diazotization of cyclobutylamine also produces the same three alcohols in the same ratios (Scheme 2.38). 

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