Edge-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. 

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... 

Along with the minimal barrier for H shift, the 2-butyl to t-butyl rearrangement gives the energy surface shown in Fig. 5.9. This diagram indicates that the mechanism for C-3/C-4 scrambling in the 2-butyl cation involves the edge-protonated cyclopropane intermediate. 

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... 

The fact that only one proton exchanges therefore indicates that [23] is an edge protonated compound. The behaviour of the butyl system is typical of the pentyl system and hexyl system in all of which exchange studies indicate that edge-protonated cyclopropanes form during many ionic rearrangements in this acid (again with the assumption that a protonated cyclopropane is a bona fide intermediate) (Kramer, 1970). 

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. 

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. 

Aboderin and Baird suggested the mechanism of Scheme 6 for the label scrambling in Equation 6.44.120 In each of the edge-protonated cyclopropanes (96a-... 

Figure 6.17 Orbital picture of edge-protonated cyclopropane.

Ab initio theoretical calculations by Pople and co-workers101 and at the semiem-pirical level by Dewar and co-workers102 103 did not give consistent results. Subsequent ab initio calculations (SCF level including electron correlation) by Lischka and Kohler104 are inconsistent with earlier ab initio work. Their calculations have confirmed the stability of the isopropyl cation 2 and the instability of face-protonated cyclopropane 15. Edge-protonated cyclopropane 16 is found to be a saddle point on the... 

The existence of protonated cyclopropanes in solution was postulated by a number of workers, often in connection with the problem of 1,3-elimination (Skell and Starer, 1960 cf. also Nickon and Werstiuk, 1967). Such intermediates were preferably represented as rapidly equilibrating edge-protonated cyclopropanes, rather than as a face-protonated moiety (Baird and Aboderin, 1964 Benjamin et al., 1966 Deno and Lincoln,... 

Finally, it is noteworthy that direct n.m.r. evidence on the formation of edge-protonated cyclopropanes in cold solutions of magic acid containing c-CsHg was recently reported (Olah and Lukas, 1968). 

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... 

The preference for the edge-protonated structure emerges from interesting microwave spectroscopy experiments conducted on the cyclopropane-hydrogen fluoride and cyclopropane-hydrogen chloride complexes which reveal a structure like that shown in Figure 54. 

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