Cyclopropyl-allyl ring-opening

The reaction of cycloprop-l-enylpropan-2-ols with thionyl chloride is also highly dependent on substitution, presumably reflecting changes in the rate of cyclopropyl-allyl ring opening 165) ... 

Much of the initial work on the solvolyses of non-ring-fused cyclopropyl derivatives was carried out to determine the effects of the nature and stereochemistry of substituents on their rates of reaction and direction of cyclopropyl-allyl ring-opening. The reviews by DePuy , Schollkopf, Wendisch and especially by Aksenov, and coworkers summarize this early work which agreed with the theoretical predictions of Woodward and Hoffmann . Examples of some of the kinetic results are shown in Scheme 1 ... 

Thus with the cyclopropyl anions 331- and 340 - it has been established that the disrotatory mode, as predicted by Woodward and Hoffmann , is the preferred one. It is however not clear whether a photochemical cyclopropyl-allyl anion or a thermal cyclopropyl radical ring-opening (the latter caused by photochemical electron ejection) takes place. It has also been realized that systems with X = vinyl or Br (342- and 343-, respectively) do not open photochemically . 

Allyl and Cyclopropyl Radicals Ring Opening, Tunneling and IVR... 

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 ... 

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. 

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. 

Cyclopropyl ions and radicals (23) can undergo conversion to allyl (24) by typical electrocyclic ring opening processes we have carried out calculations for ring opening by both conrotatory and disrotatory paths. Table 5 shows calculated activation energies for the various processes. 

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... 

Cyclopropyl silyl ethers undergo ring opening to give allylic fluorides when treated with DAST at room temperature (see Table 5).35... 

Solvolysis of cyclopropyl derivatives leads directly to the allyl cation the ring opening is disrotatory as predicted. The most direct demonstration is the transformation of the 2,3-dimethyl-1-chlorocyclopropanes at — 100°C in strong acid... 

The equilibria between the allyl and the cyclopropyl cations and the corresponding equilibria of the anions, are not seen as such, but they are found in disguise. Thus the cationic reaction is seen when cyclopropyl halides are heated or treated with silver ions. The cyclopropyl cation itself is not an intermediate, because the ring opening (4.23, arrows) takes place at... 

The ring-opening reaction of cyclopropyl radical [Equation (7)] was shown to occur at 174°C to give ally radical, but the product stereochemistry was unclear. Ab initio direct dynamics study was carried out to clarify the stereochemical course of the reaction.40 Trajectories were initiated at the ringopening TS obtained at CASSCF(3,3)/6-31G(d), with quasiclassical normal sampling at the experimental temperature of 174°C. ZPE was included, and thermal vibrational energy was sampled from the normal-mode Boltzmann distribution. A rotational energy of RT/2 was added toward the allyl radical product. 

The direct observation of the cyclopropyl cation 44 has evaded all attempts, owing to its facile ring opening to the energetically more favorable allyl cation 45 [Eq. (3.29)].158... 

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

On the other hand, cation formation by decarboxylation of an acyloxonium cation RC02+ is supported by the partial stereospecificity observed in the electrolysis of cis- and trans-bicyclo [3.1.0] hexane-3-carboxylic acid 2°5 and the electrocyclic ring opening in the anodic oxidation of 3-methyl-2-phenylcyclopro-panecarboxylate (22, 23) to cyclopropyl methyl ether (24, 25) and allylic ethers (26, 27) (Eq. (97)) 206). 

Fig. 9. Conversion of oyclopropyl species to allyl species. Conrotatory changes are on the left disrotatory changes are on the right. Upper section alternative modes of ring opening and orbital correlation of cyclopropyl species. Middle section symmetry state correlation of radical. Bottom section symmetry state correlation of anion.

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