Electronic Substituent Effects for the Fine-Tuning of the Regioselectivity in the Diastereoselective Rearrangement of 1,3-Cyclopentanediyl Radical Cations Generated from Tricyclo[3.3.0.02,4]octanes (Housanes) by Chemical Electron Transfer

Abstract

1,3-Cyclopentanediyl radical cations 5•+ were generated from tricyclo[3.3.0.02,4]octanes (housanes) 5 through chemical oxidation with tris(4-bromophenyl)aminium hexachloroantimonate (TBA•+SbCl6-) and shown to afford the regioisomeric olefinic products 6 and 7 on methyl 1,2 migration. A complete reversal in the regioselectivity of the 1,2 shift was observed, which reflects the electronic character of the X substituent at the migration terminus in the radical cation 5•+. The regioselectivity is rationalized in terms of a simple MO interaction diagram by considering the eSOMO orbital energies (AM1 method) of the X-substituted radical fragments in the intermediary 1,3 radical cations 5•+ relative to that of the cumyl radical fragment. The excellent correlation between the calculated orbital energy differences (Δe) and the experimentally observed regioisomeric ratios allows a quantitative assessment of the electronic substituent effects. The diastereoselectivity of the 1,2 shift is controlled by the steric fac...