HYPERCONJUGATION IN STRAINED BRIDGEHEAD CYCLOBUTYL CATIONS - AN AB-INITIO STUDY OF BICYCLO[1.1.1]PENT-1-YL CUBYL AND NORCUBYL CATIONS

Ab initio molecular orbital calculations (MP2/6-31G**) provide strong evidence that the bicyclo[1.1.1]pent-1-yl, cubyl and tricyclo[3.1.1.0(3,6)]hept-6-yl (6-norcubyl) cations are principally stabilized by hyperconjugative interaction of the strained alpha-beta and beta-gamma carbon-carbon bonds with the (vacant) cationic p-orbital; the degree of interaction is strongly dependent on the geometry of the charged four-membered ring in each case.