Cation cyclopropyl

Fonnation of allylic products is characteristic of solvolytic reactions of other cyclopropyl halides and sulfonates. Similarly, diazotization of cyclopropylamine in aqueous solution gives allyl alcohol. The ring opening of a cyclopropyl cation is an electrocyclic process of the 4 + 2 type, where n equals zero. It should therefore be a disrotatory process. There is another facet to the stereochemistry in substituted cyclopropyl systems. Note that for a cri-2,3-dimethylcyclopropyl cation, for example, two different disrotatory modes are possible, leading to conformationally distinct allyl cations ... 

The disrotatory mode, in which the methyl groups move away from each other, would be more favorable for steric reasons. If the ring opening occurs through a discrete cyclopropyl cation, the W-shaped allylic cation should be formed in preference to the sterically less favorable U-shaped cation. This point was investigated by comparing the rates of solvolysis of the cyclopropyl tosylates 6-8 ... 

This interpretation is supported by results on the acetolysis of the bicyclic tosylates 9 and 10. With 9, after three months in acetic acid at 150°C, 90% of the starting material was recovered. This means that both ionization to a cyclopropyl cation and a concerted ring opening must be extremely slow. The preferred disrotatory ring-opening process would lead to an impossibly strained structure, the /ran -cyclohexenyl cation. In contrast, the stereoisomer 10 reacts at least 2x10 more rapidly because it can proceed to a stable cis-cyclohexenyl cation ... 

The elusive diazoalkenes 6 and 14 are unlikely to react with methanol as their basicity should be comparable to that of diphenyldiazomethane. However, since the formation of diazonium ions cannot be rigorously excluded, the protonation of vinylcarbenes was to be confirmed with non-nitrogenous precursors. Vinyl-carbenes are presumedly involved in photorearrangements of cyclopropenes.21 In an attempt to trap the intermediate(s), 30 was irradiated in methanol. The ethers 32 and 35 (60 40) were obtained,22 pointing to the intervention of the al-lylic cation 34 (Scheme 10). Protonation of the vinylcarbene 31 is a likely route to 34. However, 34 could also arise from protonation of photoexcited 30, by way of the cyclopropyl cation 33. The photosolvolysis of alkenes is a well-known reaction which proceeds according to Markovnikov s rule and is, occasionally, associated with skeletal reorganizations.23 Therefore, cyclopropenes are not the substrates of choice for demonstrating the protonation of vinylcarbenes. 

In this connection the case of cyclopropylalkyl cation and substituted cyclopropyl cations is interesting. Cyclopropylmethyl cation has been found to be more stable than benzyl cation and the stability increases with each addition of cyclopropyl groups. Thus... 

The most simple electrocyclic reaction involving a 2k electron system is the formation of an alkyl cation from a cyclopropyl cation... 

The change of cyclopropyl cation to allyl cation is an irreversible reaction because only the ring opening is observed. It is irreversible cyclopropyl cation, is thermodynamically unstable with respect to allyl cation and also because the ring is under strain. 

For example in cyclopropyl cations, it can be seen that groups situated on the same side of the ring as the leaving group will move inwardly. The same process will impose an ontoward disrotatory motion to the groups situated in anti as follows. 

Note that we predict ring opening of the cyclopropyl cation to require activation this at first sight seems to be at variance with evidence that rearrangement occurs as a concerted process in the solvolysis of cyclopropyl esters and indeed acts as a driving force 44). Moreover the evidence shows very clearly that this is so only for one of the possible disrotatory processes, i.e. that indicated in 25 ... 

A number of gem-dibromocyclopropane derivatives has been reacted with aromatic compounds in the presence of aluminum chloride or ferric chloride providing indenes in yields up to 80%. To rationalize this interesting anellation process it has been proposed that the cyclopropyl cation formed under the influence of the Lewis acid collapses to an allylic ion, which then functions as the alkylating agent [179],... 

The phenomena are explainable in terms of destabilization of the former series of cyclopropyl cations by the contiguous a-a array as well as stabilization of the resulting allyl cations by a beneficial polarity alternating sequence. The stabilization/destabili-zation situations are reversed in the least reactive compounds. 

The electrocyclic reactions of 3-membered rings, cyclopropyl cation and cyclopropyl anion, may be treated as special cases of the general reaction. Thus the cyclopropyl cation opens to the allyl cation in a disrotatory manner (i.e., allyl cation, n = 0), and the cyclopropyl anion opens thermally to the allyl anion in a conrotatory manner (i.e., allyl anion, m = 1). Heterocyclic systems isoelectronic to cyclopropyl anion, namely oxiranes, thiiranes, and aziridines, have also been shown experimentally and theoretically to open in a conrotatory manner [300]. 

W. T., J. Am. Chem. Soc., 1998, 120, 6187-6188). Explain why these compounds would be expected to behave analogously to cyclopropyl cations. 

Figure B14.2. Orbital correlation diagram for electrocyclic opening of cyclopropyl cation S, A and solid lines indicate disrotatory opening S, A and dotted lines indicate conrotatory opening.

The ring opening of cyclopropyl cations (pp. 345, 1076) is an electrocyclic reaction and is governed by the orbital symmetry rules.389 For this case we invoke the rule that the o bond opens in such a way that the resulting/ orbitals have the symmetry of the highest occupied orbital of the product, in this case, an allylic cation. We may recall that an allylic system has three molecular orbitals (p. 32). For the cation, with only two electrons, the highest occupied orbital is the one of the lowest energy (A). Thus, the cyclopropyl cation must... 

Perfluoro(alkoxycyclopropanes) 33 in the presence of a Lewis acid rearrange to perfluoro(2-alkoxypropenes) 34, presumably via the alkoxy-stabilized cyclopropyl cation.24-25... 

TABLE 26. Relative energies (kcal moL1) of cyclopropyl cation and other C3HS+ cations238... 

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