Methylcyclopropane intermediate protonated

If the intermediate were corner-protonated, however, and undergoing exchange with the acid, it would be most reasonable to expect all the protons at the comer to exchange with the acid before ring cleavage occurred. Thus, corner protonated methylcyclopropane would be expected to exchange three protons with the acid if it exchanged any. 

C-labeled t-butyl carbocation (CH3)3 C, obtained at 195 K in FS03H/SbF5/S02FCl from labeled t-BuCl, exhibits complete scrambling of the label after 20 h at 343 K. No deprotonation occurs, and the scrambling is thought to involve a delocalized protonated methylcyclopropane intermediate or transition state. "... 

Under mild conditions, the label scrambling was also observed in oct-l-ene on H-ZSM-5 [131] and in propane on H-ZSM-5 at523-573K [132]. Becausethe scrambling of the label between the 1- and 2-positions is impossible without the formation of protonated methylcyclopropane, which is in the equilibrium with the butyl cation [127] (Scheme 1), one can conclude that carbenium ions as transient intermediates are formed on solid acid catalysts. Recent theoretical estimates of the stability of some cations in zeolites proved this possibility [133-135]. 

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