Ring opening of cyclopropyl cations

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... 

All these facts are perfectly consistent with solution chemistry results showing the essentially barrierless electrocyclic disrotatory ring opening of cyclopropyl cations... 

Formation of allylic products is characteristic of solvolysis reactions of cyclopropyl halides and arenesulfonates. Similarly, diazotization of cyclopropylamine in aqueous media gives only allyl alcohol. In a few instances, products containing an intact cyclopropyl ring have been reported, but these are special cases in which either the cyclopropyl cation is especially stabilized or the ring-opened allyl cation especially destabilized. Apart from these, the ring-opening of cyclopropyl cations to... 

Other factors which affect the case of electrocyclic ring opening include the nature of substituents which can stabilize or destabilize the development of possible charge and the release of strain in small cyclic systems. Thus different stereochemistries have been observed in the ring opening of cyclopropyl derivatives. All cis derivatives generate an all-cis allyl cation but the anti derivatives will form the all trans cation. 

The relative rates for electrocyclic ring openings of cyclopropyl ions [200] are shown in Eqs (173,174). For the cations the faster rates are exhibited by compounds in which an acceptor is directly bonded to the electron deficient center (a-a arrangement), whereas precursors with a donor substituent at the center open most slowly. 

Treatment of cyclopropyl sulfides bearing a hydroxy group in the side chain with ceric ammonium nitrate (CAN) in methanol gives five- and six-membered cyclic ethers.In this reaction, the formation of cyclic ethers is understood by assuming a single electron transfer mechanism, which involves ring opening of the cation intermediate followed by intramolecular nucleophilic addition. 

Computational studies on the ring openings of cyclopropyl radical and cyclopropyl cation ... 

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 ... 

Another interesting cationic domino process is the acid-induced ring opening of a-cyclopropyl ketones and subsequent endocyclic trapping of the formed carboca-... 

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. 

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 ... 

The formation of allyl cations from halocyclopropanes via ring opening of the unstable cyclopropyl cations also has been investigated159,161,162 [Eq. (3.35)]. 

The instability of cyclopropyl cations means that, even as they start to form as intermediates, they spring open to give allyl cation-derived products. Try nucleophilic substitution on a cyclopropane ring and this happens. 

Although the initial product of the ring opening is a cation, and therefore a hard-to-observe reactive intermediate, some nice experiments in superacid media (Chapters 17 and 22) have proven that cyclopropyl cation ring openings are indeed disrotatory. 

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