Influence of the 7-Methyl Group

While in Winstein s works the bicyclo[3,2,0]heptadienyl ions were assumed to be intermediates in the rearrangements of the observed bicyclo[2,2,l]heptadienyl ions, the opposite situation was foxmd by Hogeveen in polymethylated analogues the bicyclo[2,2,l]cations are assumed to be intermediates in the rearrangements of the observed bicyclo[3,2,0] cations. 

The simplest mechanism of the conversion 439- 440 is the shift of the unbonded vinyl C from to C. Another way may be 5C-rearrangement with the subsequent bridge-flipping. The structure of ion 440 is confirmed by its PMR spectrum and conversion into compound 441 when quenching with methanol. The spectra of 439 and 440 show the greater part of their positive charge to be concentrated on and 

The a- and P-protons of a-cyclopropylcarbinyl ions have close chemical shifts (t 7). The signal of H (x 3.19) for ion 440 is shifted 3.4 ppm to a lower field than that of H and 3 ppm to a lower field than the a-proton of cyclopropylcarbinyl ions Thus, one can add the spectral data to the above-discussed chemical and stereochemical reasons against considering norbomadienyl ions as rapid equilibria of tricyclic cations. 

Subsequently Winstein directly observed 1-methyl-444 and 5-methyl-substituted 445 norbomadienyl ions, to elucidate the rearrangement mechanism of ion 439 into ion 440. At —105 °C cation 446 is rearranged into a mixture (55 45) of 1-methyl-444 and 5-methyl-cation 445. At —75 °C ion 444 is rearranged irreversibly into ion 445 which at —45 °C is decomposed but not converted into ion 440. Ion 445 was obtained from another precursor — quadricyclanoi 447 with intermediate rearrangement of the 7-methyl-7-quadricyclyl ion. TTie structure of ions 444 and 445 is established by PMR analysis and by conversion of ion 445 into syn-methyl ether 448 which is different from the anti-isomer 441 formed from ion 440. 

The isomerization of ion 444 into ion 445 results from the contraction and expansion of the cycle here one can exclude one of the two earlier suggested mechanisms for the conversion of ion 439 into ion 440, namely contraction-expansion of the cycle and then rearrangement of the bridge-flipping type. 

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