Corner-protonated cyclopropane

Structures of protonated cyclobutanes have been studied in the same fashion (see Figure 9 B). In the corner-protonated cyclobutane, the structure corresponds essentially to a methyl cation interacting with a trimethylene diyl, and is much less favorable than that for cyclopropane. Similarly, for the edge-protonated ion, the proton must come much closer to the carbons to form a bond than for cyclopropane, and as a result, cyclobutane is much less basic. 

Theoretical calculations of the ab initio type by Radom et a/. (1971, 1972) and at semi-empirical level by Bodor and Dewar (1971 Bodor et al., 1972) did not give consistent results. Recent ab initio calculations including electron correlation by Lischka and Kohler (1978) are inconsistent with earlier ab initio work. Their calculations have confirmed the stability of the 2-propyl cation and the instability of face-protonated cyclopropane [40]. Edge-protonated cyclopropane [41] is found to be a saddle point on the potential energy surface of lower energy than the corner protonated species [42]. 

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). 

Can all of the present results be accommodated without the need of postulating an edge-protonated cyclopropane ring The tracer work of Baird Lee 32) and Deno can be fitted to a scheme in which corner protonation and direct opening with 1,3-hydride shifts are competitive, but an edge protonated cyclopropane ring accommodates the data in a much simpler fashion ). An edge protonated cyclopropane... 

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... 

The problems associated with theoretical calculations of the energies of protonated cyclopropane are readily apparent and the norbornyl skeleton increases the complexity of computation. Early quantum mechanical calculations of the MINDO/3, STO-3G (MINDO/3 geometry)and ab initio STO-3G type show the corner-protonated norbornyl cation less stable (- 9143, i44 (g i3)i45. 5 9144 4.9 kcal mol" ... 

The solvolysis of propyl and bornyl systems and protonolysis of cyclopropane and nortricyclene are two routes to protonated cyclopropanes (Scheme 10). For sufficiently dissymmetric cyclopropanes, corner and edge attack by electrophiles at cyclopropane can be differentiated. This stereochemical division of electrophilic attack is not to be confused... 

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... 

The apparent discrepancy in assignment of structure to the protonated cyclopropanes from the experiments described above could be resolved if corner-protonated and edge-protonated ions were in fact very close in energy and if the preference for one or other then depended on substituents. As it happens, this is exactly the picture that emerges from molecular orbital calculations on protonated cyclopropane. While early calculations seemed to favor the corner-protonated structure later ones involving extensive electron... 

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