Comer-protonated cyclopropane

The alkyl-bridged structures can also be described as comer-protonated cyclopropanes, since if the bridging C—C bonds are considered to be fully formed, there is an extra proton on the bridging carbon. In another possible type of structure, called edge-protonated cyclopropanes, the carbon-carbon bonds are depicted as fully formed, with the extra proton associated with one of the bent bonds. MO calculations, structural studies under stable-ion conditions, and product and mechanistic studies of reactions in solution have all been applied to understanding the nature of the intermediates involved in carbocation rearrangements. 

These results indicate an energy profile for the 3-methyl-2-butyl cation to 2-methyl-2-butyl cation rearrangement in which the open secondary cations are transition states, rather than intermediates, with the secondary cations represented as methyl-bridged species (comer-protonated cyclopropanes) (Fig. 5.10). 

Mechanism a involves a comer-protonated cyclopropane (26) we have already seen examples of such ions in the 2-norbomyl and 7-norbomenyl cations (pp. 409, 414). Mechanism b involves an edge-protonated cyclopropane (27). Mechanism c consists of a one-step SE2-type attack by to give the classical cation 28, which... 

Some systems show NMR coalescence consistent with a rearrangement to an intermediate cation that is less stable. The NMR of the isopropyl cation 9, for example, shows a coalescence of the two proton signals between 0 and 40 °C. The activation barrier is 16 kcal/mol, close to the difference between a secondary and a primary cation. Calculations show that a comer protonated cyclopropane (11) is the only other minimum on the C3H energy surface. " The 1-propyl cation 10 is a transition state. 

Calculations have shown that there are two ways in which cyclopropane may be protonated at one of the carbons (comer protonated) and at one of the C—C bonds (edge protonated). They have about the same energy, and are involved in the movement of a proton around the ring. The calculations indicate that cyclopropane is unusually basic for a hydrocarbon. The reason can be seen in the stmctures of the protonated cyclopropanes (Fig. 15.4). The bent bonds permit a proton to bond to a C C bond without coming close to the positively charged carbon nuclei. 

Another mechanism, that shown in Scheme 7, has also been suggested for the label scrambling of Equation 6.44. In each of the comer-protonated cyclopropanes (96a-97c), all three participants in the three-center bond are carbon. 

Sometimes intermediates like [6] undergo hydride shift to another comer protonated cyclopropane (7). This is another mechanism by which carbon scrambling can be achieved, i.e. through a combination of methide- and hydride shifts. Together with 1,2-hydride shifts, such a mechanism accounts for the complete degeneracy of the 2-butyl cation [7]. Vinyl cations substituted... 

A great deal of sense can be made of the experimental data if we assume that there is a competition in any cyclopropane protonation reaction between comer protonation and direct one-step opening with the latter pathway the more favored the greater the driving force for ring-opening. We therefore expect the one-step mechanism to predominate for 4, and for other systems in which a good carbonium ion is formed. When the carbonium ion which would be formed is not especially stable... 

Transient Three-and Four-membered Ring compoonds.—Protonated Cyclopropanes. The C3H7 energy surface has been re-examined by MO theory with an improved basis set. The conclusion remains that the 2-propyl cation is the most stable, but the second most stable structure is now found to be a corner-protonated cyclopropane. Furthermore, the energy gap between the edge- and comer-protonated species is now calculated to be only 6 kcal mol ... 

The relationship between stabilization in substituted comer-protonated cyclopropanes and the charge on the LUMO has been discussed. ... 

It is believed that a more correct description of the process involves migration through a pentacoordinate carbon species referred to as a protonated cyclopropane. A comer-protonated cyclopropane is a structure in which an alkyl group acts as a bridge in an electron-deficient carbocation structure. The cyclohexyl methylcyclopentyl rearrangement is postulated to occur by rearrangement between two such structures. 

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. ... 

The energy surface for C3H7 has been calculated at the 6-311G /MP4 level. The 1-and 2-propyl cations and comer- and edge-protonated cyclopropane structures were... 

In 1969 Lee and Hahn studied the scrambling upon solvolysis of 2-(3-cyclo-pentenyl)-2-r C-ethyl nosylate the radioactivity of the atom in the formed products was found to exceed that of the atom and this was accepted as an argument in favour of the formation of intermediate, both comer- and edge-protonated, cyclopropanes... 

A choice between the structure of comer- or edge-protonated cyclopropanes for the stable 2-norborayl ion can be made, according to Olah from PMR spectrum recorded at — 150°C. In the lowest field there are two signals 3.05 (2H) and 6.6 ppm (2H). These data deny the structure of the edge-protonated cyclopropane since in the latter case there would be three signals in the low field with a 1 2 1... 

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