Rearrangement bridge flipping

It should be noted that all the described rearrangements of 7-norbomadienyl ions — 5C-rearrangements, bridge-flipping, 1,2-bond shift — do not, as a rule, compete markedly with ion trapping by the nucleophile under the solvolytic conditions the latter process is far faster than the above rearrangements. 

The H-nmr spectrum of the methyl-substituted ion [419] exhibited four bands with area ratios 2 3 2 1 besides the methyl absorption. This spectrum showed reversible temperature dependence consistent with a degenerate bridge-flip rearrangement (266) which exchanges H(2), H(3) with H(6), H(7) and H(4) with H(5). At the coalescence temperature (—8°C) the free energy of activation was estimated to be 13.0 kcal mol . The C-nmr spectrum of [419] displayed at —80°C seven peaks. 

To determine the value of the energetic barrier of the degenerate rearrangement of the bridge-flipping type Winstein generated the 7-norbomadienyl cation from... 

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 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. 

As stiown recently by Jovanovich and Lambert syn-endo-6-tricyclo[3,2,l,0 ]-octenyl-8-p-nitrobenzoate 576 is solvolysed 10 times faster than compound 220 but 10 times more slowly than 573. There are no reasons for the accelerating effect of the syn-endo ring the solvolysis is not accompanied by skeletal rearrangements and, hence, there is no relief in the steric strain. The authors detected no bridge-flipping in the intermediate ion A which might lead to a more stable ion B. 

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