Cyclopropane isotope scrambling

Saunders and co-workers have recently reported results strongly supporting the protonated cyclopropane intermediate in the isotope scrambling process.114 They found no isomerization of the 2-butyl-1,2-13C2 cation (19a) at —78°C, whereas the isotopomer 2-butyl-2,3-13C2 cation (19b) showed rapid formation of all other iso-topomers except 19a (Scheme 3.2). These results are consistent with the involvement of protonated cyclopropane with the interchange of either C(l) and C(2) or C(3) and C(4) with the breaking of only the C(2)-C(3) bond. 

Such a mechanism obviously cannot account for all of the rearrangements for the isotopic scrambling observed when cyclopropanes are reacted with electrophiles and so cannot be the sole pathway operative. 

M. V. Frash, V. B. Kanzansky, A. M. Rigby, and R. A. van Santen, /. Phys. Chem. B, 101, 5346 (1997). Density Functional and Hartree-Fock Calculations on the Cyclopropane Ring Intermediates Involved in the Zeolite-Catalyzed Skeletal Isomerization of Hydrocarbons and in the Carbon Isotope Scrambling in 2-Propyl Cation. 

Recent results by Saunders and coworkers with the double-labeled 2-butyl-1,2- C2 and 2-butyl-2,3- C2 cations strongly support the protonated cyclopropane intermediate in the isotope scrambling process. 

Unlike the isomerization of l-chloro[l- C]butane with aluminium trichloride, which proceeds without major participation of protonated cyclo-propanes, the isotopic scrambling from the reaction of [l- C]-l-propyl-mercuric perchlorate in trifluoroacetic acid does require intermediate protonated cyclopropanes. The isolated 1-propyl trifluoroacetate (565) was shown by degradation to have the label distributed as shown in Scheme 76. The greater scrambling from C-1 to C-3 than from C-1 to C-2 requires an edge-rather than a comer-protonated ion. Full details have appeared of the liquid-phase thermolyses of cycloalkyl and cycloalkylmethyl chloroformates which take place via carbonium ions. Protonated cyclopropanes are believed to be intermediates for 5-10% of the products. The alkylation of benzene and toluene by cyclopropane with acidic catalysts also involves initial formation of a protonated cyclopropane. ... 

Walker GE, Kronja O, Saunders M. Isotopic scrambling in Di-C-13-labeled 2-butyl cation evidence for a protonated cyclopropane intermedia. J Org Chem 2004 69 3598-601. 

Dianions of the above types may not fall into the category of homoenolate in a strictly formal sense. Nevertheless the amide dianion does show a behavior typical of the homoenolate. Thus, the reaction of the isotopically labeled stannylpropionate results in scrambling of the label probably via a cyclopropane intermediate Eq. (47) [44]. As the result of such an equilibration, isomerization of a-methyl and a-phenyl substituted propionate homoenolates may occur to give the thermodynamically more favorable isomers, respectively. 

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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