13C scrambling

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.

An explanation that would also account for the similar statistical scrambling of the 13C label that is found to occur (over several hours)... 

The ratio of the amount of n-butane-2-13C to the amount of isobutane produced was, provided measurements were made under conditions where secondary reactions were unimportant (i.e., initial reaction products), constant and independent of temperature, and this ratio was 1/4. At the same time, no scrambling of the 13C occurred i.e, all of the isotopically substituted molecules remained singly labeled. Anderson and Baker (68) speculated that the butane isomerization might have occurred by a recombination of adsorbed surface residues produced by fragmentation of the... 

Figure 14. Principle for measuring bidirectional fluxes by 13C metabolic flux analysis. In a carbon labeling experiment, 1 13C glucose is provided in the medium, and the culture is grown until a steady state is reached. Glucose can either go directly via the hexose phosphate pool (Glu 6P and Fru 6P) into starch, resulting in labeling hexose units of starch only at the Cj position, or it can be cleaved to triose phosphates (DHAP and GAP), from which hexose phosphates can be resynthesized, which will result in 50% labeling at both the Ci and the C6 position (assuming equilibration of label by scrambling at the level of triose phosphates). From the label in the hexose units of starch, the steady state fluxes at the hexose phosphate branchpoint can be calculated for example, if we observe 75% label at the Ci and 25% at the C6 position, the ratio of vs to V7 must have been 1 to 1. All other fluxes can be derived if two of the fluxes of Vi, V6, and V7 are known (e.g., V2 vi V3 V5 + v6).

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... 

The intramolecular nature of most carbocationic isomerization was proved by means of labeling experiments. [l-13C]-Propane was isomerized in the presence of aluminum bromide promoted by hydrogen bromide to form [2-13C]-propane. None of the propane product contained more than one l3C atom per molecule.64 Similarly, very little label scrambling was observed in the isomerization of labeled hexanes over SbF5-intercalated graphite.65 Thus simple consecutive 1,2-methyl shifts can account for the isomerization of l3C-labeled methylpentanes (Scheme 4.3). 

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 Ce isomer distribution—2-methylpentane (28%), 3-methylpentane (14%), and w-hexane (32%)—is very far from thermodynamic equilibrium, and the presence of these isomers indicates that both isopropyl cation 31 [Eq. (5.62)] and n-propyl cation 34 [Eq. (5.63)] are involved as intermediates [as shown by 13C(2)—13C(1) scrambling in the stable ion147]. 

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