Long-Lived Carbocations: Generation from 5,5,10,10-Tetramethyl-5,10-dihydroindeno[2,1-a]indene, Structure, and Rearrangements

According to the NMR data, protonation of 5,5,10,10-tetramethyl-5,10-dihydroindeno[2,1-a]indene in the system HSO₃F-SO₂ClF-CD₂Cl₂ at -80°C gives rise to long-lived 5,5,10,10-tetramethyl-2H-5,10-di- hydroindeno[2,1-a]indenium ion which undergoes isomerization into 4b,5,10,10-tetramethyl-4b,5,9b,10-tetra- hydroindeno[2,1-a]inden-5-yl cation on raising the temperature. On the basis of the data obtained for rear- rangements of such carbocations an alternative mechanism has been proposed for the rearrangements of struc- turally related carbocations having a bicyclo[3.3.0]octane skeleton.