Vinylcyclopropane probe

FIGURE 1.29 Vinylcyclopropane probe to distinguish between radical and cation intermediates. 

On the other hand, the CIDNP approach and recent ab initio calculations have provided what we consider unambiguous evidence for the nature of the vinylcyclo-propane radical cation as well as for some simple derivatives. We have carried out ab initio calculations on the prototype 19 and several simple derivatives, and probed by CIDNP the structures of three simple vinylcyclopropane radical cations in which the two functionalities are locked in a syn configuration. 

In contrast to the isomerization of 39, the retention of optical activity for the conversion of 41 to 42 is very small. However, the loss of activity cannot be explained by a degenerate vinylcyclopropane rearrangement, since the recovered starting material ([(X]54g = +32 ) shows essentially complete retention of chirality. An alternative explanation involves competing stereoselective processes in a chiral intermediate (41a ). For 41a a hydride shift from the allylic position, C2, might compete with the migration from C6 to Cl. This possibility can be probed by... 

Homoconjugative interactions between a cyclopropane ring and a double bond were studied for the three bicyclic compounds 7-9, each possessing a locked vinylcyclopropane fragment [84], The polarization patterns in the chloranil-sensitized photoreactions of these substrates were used to probe the spin density distribution in the radical cations. A strong dependence of electronic structure of the radical cations on steric effects was found (see Chart IV). 

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