Cyclopropyl-allyl cation system

It is interesting to mention the entirely different situation in the cyclopropyl-allyl cation system the ring-opening reaction is very fast as compared to the isomerization of the allyl cation. In agreement with this situation the disrotatory mode of the cyclopropyl-allyl cation transformation has been much easier to verify ... 

It is possible to view this reaction as involving a ring contraction, because the three-membered ring has formed an allylic cation, i.e. the double bond can be considered as a two-membered ring system, which is then attacked by a solvent molecule. This is an example of a cyclopropyl/allylic cation rearrangement, which we first encountered in the chapter on nucleophilic substitution reactions. 

HB[-(CH2)2-]. The potential energy surface of HB[-(CH2)2 ] was probed using ab initio SCF theory and fourth-order perturbation theory. The results indicate that borirane, shown in Fig, 2-15, is a true minimum on the BC2H5 potential energy surface, and is 102.4 kJ/mol more stable than the open ring form analogous to the allyl cation. Indeed, the results are the reverse of those for the isoelectronic cyclopropyl cation-allyl cation system [16]. 

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 ... 

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 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... 

It was concluded that the stabilization of the positive charge of 63, by delocalization over the three carbons of the mesomeric propargyl allenyl system, entailed a powerful electron releasing substituent at the allenyl end 13). However, although in theory electron releasing substituents might render 1-substituted cyclopropyl cations more stable than their 2-substituted allyl counterparts 36), the expected ring closure of a 2-substituted allyl cation such as e.g. 64 to the cyclopropyl cation 65 has not been observed experimentally, Eq. (19)37). 

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. 

With larger bicyclo[n. 1. OJalkyl cyclopropyl derivatives such as 8 and 9, their solvolytic behavior follows from that of the simple alkyl-substituted cyclopropyl derivatives. With smaller bicyclo[n.l.0]alkyl cyclopropyl derivatives such as 10 and 11, however, where a trans-a y cation cannot be accommodated in the ring, the order of reactivity is reversed. In both the [6.1.0] and [3.1.0] examples mentioned above, the rates are given relative to cyclopropyl tosylate. The much higher reactivity of the endo-[3.1.Qi] system (11) over the endo-[6.1.0] system (9) reflects the stability of the almost strain-free cyclohexenyl allylic cation versus the cyclononenyl allylic cation which possesses both torsional and transannular strain. 

A remarkable transannular phenyl migration occurs in the tricyclic system (380) which hydrolyzes to products derived from the allylic cation (381)3l5 Aryl-assisted ionization gives rise to a cyclopropyl cation which undergoes ring opening to (381). Alkyl shift predominates in the epimeric tosylate (382) although some leakage toward (381) is also found. 

Under the category of kn, k = 4q, electrons system, we can cite the ring closures of allyl anion to cyclopropyl anion, 1,3- and 1,4-pentadienyl cation to cyclopen-tenyl cation, and 1,3,5,7-octatetraene to 1,3,5-cyclooctatetraene. Under the category kn, k = 4q + 2, we can quote the ring closures of allyl cation to cyclopropyl cation and 1,3- and 1,4-pentadienyl anion to cyclopentenyl anion. The chemical equations for these transformations are given below. 

The Woodward-Hoffmann orbital symmetry rules are not limited in application to the neutral polyene systems that have been discussed up to this point. They also apply to charged systems, just as the Htickel aromaticity rule can be applied to charged ring systems. The conversion of a cyclopropyl cation to an allyl cation is the simplest... 

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. 

The Woodward-Hoffmann ru are not limited m application to the neutral systems that have been discussed up roTHis point. They also apply to charged systems. The conversion of cyclopropyl cation to allyl cation has been thoroughly studied and represents the simplest possible case of an electrocyclic transformation, since it involves only two tt-electrons. Because of the restrictions imposed on the internuc-lear angles in cyclopropyl rings, carbonium ions do not form readily, and cyclopropyl halides and arenesulfonates are quite unreactive under ordinary solvolytic conditions. For example, it was found that temperatures of 180°C were necessary for cyclopropyl tosylate to react in acetic acid, and the product was allyl acetate, rather than cyclopropyl acetate. A mechanism was considered in which cyclopropyl cation was formed in the rate-determining step, followed by rapid conversion to allyl... 

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. 

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-... 

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