Electrocyclic reactions allyl-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 ... 

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]. 

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... 

The product would be a cyclopropyl cation. Now, in fact, it is the cyclopropyl cations that undergo this reaction (very readily indeed—cyclopropyl cations are virtually unobservable) because ring strain encourages them to undergo electrocyclic ring opening to give allyl cations. 

The selection rules can be applied to charged species as well as to neutral molecules. The only requirement is that the reaction be a concerted process involving electrons in overlapping p orbitals. For example, the conversion of a cyclopropyl cation to the allyl cation can be considered as a tt -electrocyclic process. For this process, the selection rules predict a disrotatory process. 

Cyclopropenes are readily cleaved by acids. The reactions are rationalized either by initial protonation of the double bond and subsequent electrocyclic ring-opening of the cyclopropyl cation thus formed (path a, equation 82), or by protonation of a cyclopropene o bond to give the allylic cation more directly (path b, equation 82) The final products of reaction are derived from nucleophilic capture of the allyl ion. 

Problem 29.4 The commonly observed conversion of cyclopropyl cations into allyl cations is considered to be an example of an electrocyclic reaction, (a) What is... 

Cyclopropyl cations and allylic cations can open and close, respectively, by two-electron electrocyclic reactions, as can the corresponding halides. 

If, however, there is a strongly cation-stabilizing substituent at the reaction center, cyclopropanes 10 are observed as byproducts, or even predominant products, in addition to allylic compounds 8. A two-step mechanism with electrocyclic opening of stabilized cyclopropyl cation 9 has been proposed for the formation of allylic products 8 from cyclopropanes 6. In at least one case 9 was unequivocally observed by spectroscopic means. The 1-ferrocenyl-l-cyclopropyl cation has been observed in superacidic solution at — 40 C by H and C... 

Ring strain is important in preventing a reaction that would otherwise change your view of a lot of the chemistry you know. Allyl cations are conjugated systems containing 2k electrons, so if you knew no other chemistry than what is in this chapter you might expect them to cyclize via disrotatory electrocyclic ring closure. The product would be a cyclopropyl cation. 

Woodward—Hoffmann orbital symmetry rules can be applied to the charged systems as well. The conversion of a cyclopropyl cation to an allylic cation is the simplest one, which involves only 27r-electrons (Figure 2.13). This is an electrocyclic reaction of (4n + 2) type (n = 0) and should, therefore, be a disrotatory process. 

Due to the strain imposed by the three-membered ring, the cyclopropyl cation is not a stable intermediate and electrocyclic ring opening occurs readily. Therefore, in the solvolysis of cyclopropyl tosylate in acetic acid, allyl acetate is obtained rather than cyclopropyl acetate. Solvolysis reactions of other cyclopropyl halides, sulphonates, and diazotization of cyclopropylamine in aqueous solution also give the allylic products. 

The usual notion of the coordinate of electrocyclic reactions is associated with the rotation of the end groups about double bonds. The conrotatory motion is thermally allowed for the reaction XXI-XXIII. Semiempirical [2,36-42] and ab initio [43-45] calculations of the critical regions of the PES of this reaction and of the still simpler cyclization of the allyl cation to the cyclopropyl cation have greatly refined the overall picture of the intrinsic mechanism and revealed some important distinguishing features common to all electrocyclic transformations. 

This corresponds to the electrocyclic ring-opening of the cyclopropyl cation the preferred reaction pathways should involve + 2 ] or [ jOa + 2a] interactions, (Equation 6.1). In simple unfused ring systems the evidence for disrotatory scission comes from kinetic measurements (see below). The stereochemical test is not available because of the interception of the allyl cation by a counter-ion, (Equation 6.2). However, when the cyclopropyl cation is part of a bicyclic system, for example (1), it is found that electrocyclic cleavage occurs readily even when the number n has the small value of 3 or 4. In these circumstances the conrotatory mode, which would yield the unstable trans-... 

Initial interest in the solvolyses of cyclopropyl derivatives stemmed from the observation that they underwent solvolysis with concerted ring-opening , and that the reaction was strongly dependent on the nature and stereochemistry of substituents on the ring. This was explained by Woodward and Hoffmann who predicted from orbital symmetry considerations that the electrocyclic transformation in which a cyclopropyl carbocation is converted to an allyl cation should occur in a disrotatory fashion. Also, the particular disrotatory path a given system will take should be dependent on the stereochemistry of the leaving group. This is illustrated as follows. 

According to the postulated reaction pathway, after the initial addition of the olefin to the activated diene, the cyclopropyl unit undergoes a ring enlargement reaction. The gold stabilized allyl cation participates then in a Nazarov-type electrocyclization leading to the final angular tricyclic compound. 

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