SUBSTITUENT EFFECTS ON CYCLOBUTYL AND CYCLOPROPYLCARBINYL CATIONS

The acetolyses of 3-substituted cyclobutyl tosylates (X = A Cl, and OEt) were examined giving rate constants and product distributions. With the alkyl- and aryl-substituted compounds, the rate-determining step leads to the formation of a bridged cyclobutyl cation which then rearranges to a cylopropylcarbinyl/homoallyl ion. The value of rho for the solvolysis of the 3-aryl derivatives was -1.5, suggesting some charge transfer to the 3-position. The 3-chlorocyclobutyl tosylates react to give only the inverted 3-chlorocyclobutyl acetates. The 3-ethoxy derivatives give approximately 1:1 mixtures of the corresponding acetates. Further information concerning these reactions was obtained via ab initio MO calculations at the MP2/6-31G* theoretical level. They showed that except for the silyl substituents, all of the other 3-substituents led to a decrease in energy on rearrangement to the corresponding cyclopropylcarbinyl ion. The latter was found to have considerable homoallyl cation character when substituted at the 2-position. All of the results are in accord with the hypothesis that the rate-determining step with X = alkyl or aryl is the formation of a, bridged cyclobutyl cation that is a transition state for a stereospecific rearrangement to the corresponding cyclopropylcarbinyl ion. The reaction products are then formed from the latter ion.