The Search for a Protonated Cyclopropyl Ring

The presence of a protonated cyclopropyl ring distinguishes the two norbomyl ions and one mi t hope to characterize a stabilized ion by its proton exchange behaviour. To do this one would have to investigate the exchange behaviour of protonated alkylcyclopropanes and tertiary and secondary cations in an acid sufficiently strong to stabilize all species for some time. 

Protonated cyclopropane has been considered to be a real intermediate since the work of Aboderin and Baird (1964), and protonated alkylcyclopropanes have also recently come to be considered intermediates as opposed to transition states in many rearrangements occurring in strong acid media where SbFs is the Lewis acid. Thus Brouwer and Oelderik (1968a) suggested that protonated methyl-cyclopropane is an intermediate in the isomerization of sec-butyl-1- C to sec-butyl-2- cations in the HF-SbFj system, and Saunders et al. (1968) implicated this species in the rearrangement of 

The norbomyl system has been subjected to an analogous series of experiments with the results shown in Table 4. When exo-2-chloro-norbornane was solvolysed in the presence of a hydride donor good yields of norbornane were obtained. The product had not acquired any protons from the acid and thus an edge-protonated intermediate had not formed before reaction with the hydride donor occurred. 

Trapping the Norbornyl Ion by Hydride Transfer from Methylcyclopentane in 2M SbFs/H(T)S03F. ([MCP]/[norbornyl ion] = 40) 

A—The ion was trapped by solvolysing in the presence of methylcyclopentane. B—Methylcyclopentane was added 10 minutes after ionization. 

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