Corner-protonated cyclopropan

This ion appears to have a classical structure, which is in equilibrium with a bridged ion under conditions of low nucleophilicity, but undergoes extremely fast 1,2-hydride shifts which have not been frozen out. It also shows complete hydrogen and carbon degeneracies which most probably involve corner protonated cyclopropane intermediates. At higher temperatures it rearranges to t-butyl cation. 

Saunders and Rosenfeld (1969) extended their H-nmr investigation to temperatures above 100°C and discovered another, slower process which exchanges the two methylene protons with the nine methyl protons, resulting in coalescence of these bands above 130°C. The band shape analysis gave an activation energy of 18.8 1 kcal mol" for this new process. Since any mechanism involving primary alkyl cations is expected to have a barrier of ca 30 kcal mol" (the enthalpy difference between tertiary and primary carbo-cations), the formation of a methyl-bridged (corner protonated cyclopropane)... 

The results of Baird and Aboderin cannot be accommodated by assuming that corner protonated cyclopropanes are the only reactive intermediate. If they were, 1- and 2-deuteriopropanol would of necessity be formed in equal amounts (except for a small isotope effect) no matter what the acid concentration (Eq. (24)). 

However, recent studies on long-lived carbocations in super acid solutions are beginning to give additional data. Thus Olah 6) has obtained evidence, by the use of a number of spectroscopic techniques, that the norbomyl cation is best represented as a corner protonated cyclopropane 31). 

Mechanism a involves a corner-protonated cyclopropane (28) we have already seen examples of such ions in the 2-norbornyl and 7-norbornenyl cations (pp. 453, 460). Mechanism b involves an edge-protonated cyclopropane (29). Mechanism c... 

Solution phase reaction of an electrophile with cyclopropane(s) requires favorable interaction of the LUMO of the electrophile with the HOMO degenerate, symmetric or anti-symmetric, orbitals of cyclopropane . The Is orbital of H or one lobe of the p-orbital of a carbocation or electrophile can react with the anti-symmetric (3e ) orbital (Figure 2) with consequent reduction in bonding in adjacent carbon-carbon bonds and relief of C(2)-C(3) antibonding. Completion of this carbon-proton or carbon-electrophile bond results in a corner-protonated cyclopropane (39). Reaction of an electrophile, e.g. proton or carbocation, with the degenerate symmetric 3e orbital (Figure 3) will give an... 

Early calculations at the STO-3G level by Pople and coworkers indicated the 1-propyl cation (42) to be lower in energy than edge (47) and corner-protonated cyclopropanes 45 and 46. Higher level 4-3IG calculations reduced both the energy of corner-protonated cyclopropane (46) and 1-propyl cation in conformations 42 and 43. Calculations at the STO-3G, 4-3IG and at the 6-3IG levels favor a conformation of the 1-propyl cation (42) where the C(2)-C(3) cr-bond can hyperconjugate effectively with the vacant p orbital at C(l). This cation at the HF/6-31G level was suggested to convert without activation to corner-protonated cyclopropane (45, 46) on optimization. 

Calculated values for cyclopropane + MeOH2 ->edge- or corner-protonated cyclopropane cf. propene-h MeOHa, 3.9 kcal mol ... 

The corner-protonated cyclopropane configuration 45 is analogous to the non-classical norbornyl ion at the center of controversy for the past three decades. The non-classical cation concept had its origin with Wilson and coworkers in 1939 who depicted structure 58 as a possible intermediate in the camphene hydrochloride-isobornyl chloride... 

A biosynthetic pathway involving intramolecular cyclization to form a non-classical carbocation (corner-protonated cyclopropane) or its equivalent between C(l) and the terminal 14,15-double bond of geranylgeranyl pyrophosphate (66), followed by proton elimination from the original C(l) to yield casbene (65) with its fused cyclopropane and macrocyclic ring system, has been proposed (equation 4) This process is mechanisti-... 

Roberts seems to have been the first to propose the existence of a carbonium ion intermediate with a bridging methyl group (a structure that we would now call a corner-protonated cyclopropane), in part as a result of a study on the deamination of propylamine-1-in an aqueous medium, in which it was believed that a small amount of label migration to both C(2) and C(3) of the 1-propanol product was observed (Figure 50). In fact, the analysis of the label position was later shown to be in error but, by then, other experiments had suggested the existence of some kind of protonated cyclopropane. Among these were studies showing the formation of cyclopropanes from deamination of alkylamines in non-aqueous media. This process was subjected to further scrutiny... 

I.4. Alkyl Shifts from Secondary to Secondary Carbon. Corner-protonated cyclopropanes should be stabilized by alkyl substituents at the non-protonated corners. In terms of simple resonance theory, a hybrid of two secondary cations is lower in energy than a hybrid of secondary and primary cations. Therefore, bridged cations are more likely to intervene in 1,2-alkyl shifts between secondary carbons than in the 2°->-l0 rearrangements discussed above. 

Nucleophilic capture of the spirooctadienyl cation opens the 3-member ring. This behavior characterizes many reactions of many other cyclopropane-containing carbocations, as well, y-radiolysis of perdeuterated propane forms CsD ions, most of which either transfer D or form isopropyl adducts. As the propane pressure is raised from 1000 mbar to 2000 mbar, however, the isopropyl/ -propyl adduct ratio falls from 30 1 to about 5.5 1. This implies the formation of corner-protonated cyclopropane, which reacts with nucleophiles as though it were an -propyl cation. With increased pressure, vibrationally excited protonated cyclopropane experiences more frequent nonreactive collisions, which deactivate it and slow down its rate of unimolecular isomerization to isopropyl cation. 

These results indicate an energy profile for the 3-methyl-2-butyl cation to 2-methyl-2-butyl cation rearrangement in which different rotamers of the open secondary cations are transition structures rather than intermediates, with the secondary cations represented as methyl-bridged C (corner-protonated cyclopropanes), as shown in Figure 4.12 The computational investigation of this system was extended to include the effect of a polar medium (dielectric constant = 39) and the effect of the proximity of anions... 

NMR, Raman and ESCA spectra the long-lived 2-norbornyl ion corroborate reliably its structure as that of corner-protonated cyclopropane (see Chapter 3.8). 

Finally, the ab initio calculations made in 1971-72 by Pople and co-workers clearly confirm the ions of type 5 to be more stable than those of type 71 since the former are homoaromatic, the latter are not also most of the strain energy of the norbomane molecule is due to the distortion of normal angles. In the classical ion 6 and in Traylor s ion 71 with unchanged geometry this strain is mainly retained whereas ion 5 does not preclude the relief of angular strain owing to rather long (1.803 A) C—C bonds in corner-protonated cyclopropane. 

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