Isopropyl cation labeled

Our study of propene-J-13C, -2-13C, and -5-13C reacting on zeolite HZSM-5 clearly shows that the isopropyl cation is not formed in measurable concentration as a persistent species (45). Furthermore, there is no label scrambling of the 2 position, although 1,3-label scrambling is facile on the zeolite. This strongly argues against a free isopropyl cation—even as a transient intermediate At low temperature, the equilibrium structure of propene is a 77 complex 22 with the Brpnsted site. This mode of coordination... 

Fig. 23. 75.4-MHz 13C MAS spectra of 2-bromopropane-2-13Creacting on Lewis superacids AlBr3 and SbF5. Spectrum a shows the formation of an adsorption complex (89 ppm for C2 and 29 ppm for the methyl carbons) with AlBr3 at 298 K. Note that 13C label scrambling from C2 to Q is complete in the adsorption complex as indicated by the 2 1 intensity ratio of the 29 ppm to 89 ppm peaks. Spectrum b shows a completely 13C scrambled isopropyl cation (320 and 52 ppm) and a partially 13C scrambled adsorption complex with SbF5 (92 and 26 ppm) at 233 K. (c) Upon raising the temperature, the adsorption complex was completely converted to the isopropyl cation.

Strong support for the involvement of protonated cyclopropane intermediates came from the work of Olah and White.94 The isopropyl cation obtained from [2-13C]2-chloropropane (50% 13C) was studied by H NMR. The 13C label scrambled uniformly over 1 and 2 positions at 60°C within a few hours (half life 1 h). 

The relative rates of hydrogen and carbon interchange have been measured by Saunders et al. (1973c) using a mixture of (1,1,1- H3)- and (2- C)-labelled isopropyl cations at —88°C. The changes in the relative areas of different peaks as well as C-satellites were observed, and the time dependence of the concentrations of different labelled isomers were simulated by computer [using mechanisms (34)-(36)]. A combination of mechanisms (34) and (35) or mechanisms (34) and (36) could match the measurements. The rate for (34)... 

Propyl cationic intermediates have also been proposed by Smith et al. (1975) in crossed beam studies of methyl cation with ethylene to explain the scrambling of isotopes that results from reaction with labelled substrates (e.g. or C2D4). Linear ions (e.g. n-propyl and isopropyl cations) are favoured over protonated cyclopropanes for these high energy species which dissociate into CH4 + C,H+... 

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