Degenerate rearrangement of 9-isopropenyl-9,10-dimethyl-phenanthren-9-yl and 9,10-dimethyl-9-(trans-1-methylprop-1-en-1-yl)phenanthren-9-yl cations

¹H NMR study has shown that long-lived 9-R-9,10-dimethylphenanthren-9-yl cations (R = isopropenyl, trans-1-methylprop-1-en-1-yl) generated in the system HSO₃F-SO₂ClF-CD₂Cl₂ at ?130°C undergo degenerate rearrangement via 1,2-vinyl shifts (ΔG’ = 37 and 39 kJ/mol, respectively, at ?88°C). Analysis of the geometric parameters of the initial structures and transition states calculated by the DFT method indicates that unfavorable steric factors are responsible for the sharp deceleration of 1,2-shifts of the isopropenyl and trans-1-methylprop-1-en-1-yl groups as compared to vinyl and cis-1-methylprop-1-en-1-yl groups, respectively.