Allyl cations configuration

The same reaction in the opposite direction—the ring closure of an allyl cation—is also known. The remarkable formation of the thermodynamically less favoured cw-di-t-butylcyclopropanone 4.72 from the zwitterion 4.71, which probably has the W configuration, is evidence of its being disrotatory. 

The electronic configuration of the allyl cation (Figure 15-12) differs from that of the allyl radical it lacks the unpaired electron in tt2, which has half of its electron density on Cl and half on C3. In effect, we have removed half an electron from each of Cl and C3, while C2 remains unchanged. This MO picture is consistent with the resonance picture showing the positive charge shared by Cl and C3. 

Q Show how to construct the molecular orbitals of ethylene, butadiene, and the allylic Problems 15-35 and 36 system. Show the electronic configurations of ethylene, butadiene, and the allyl cation, radical, and anion. 

In a more polar solvent, Favorskii reactions cease to be stereospecific, and presumably take place by ionisation of the chloride to give the same cation from each diastereoisomer. Whether the reaction takes place by way of the cation or with concerted loss of the chloride ion, this reaction presented a serious puzzle before its pericyclic nature was recognised. The a overlap of the p orbital on C-2 of the enolate with the p orbital at the other end of the allyl cation 6.340 or with the orbital of the C—Cl bond 6.341 looked forbiddingly unlikely—it is 3-endo-trig at C-2. It is made possible by its pericyclic nature, where the tilt of the orbitals can begin to sense the development of overlap. The torquoselectivity in the development of overlap 6.341, however improbable it looks, corresponds to inversion of configuration at the carbon atom from which the chloride departs. 

The formation of a mixture of 3a,6/S- and 3j8,6j9-diacetoxy cholest-4-enes, mentioned above, is a consequence of equilibration of C(3) substituents in the acidic medium, presumably via an allylic cation [24], The ratio (3jS/3 ca. 1.8 1) represents a slight preference for the pseudo-equatorial conformation at C(3). The 3,6a-diacetates undergo similar equilibration in acetic anhydride-HBF4, but with the unexpected result that the 3a-epimer is very slightly favoured in this case (ga/sjS ratio I 0.94). The dependence of the equilibrium position upon the C 6) configuration has not been explained. 

The deamination of the allylic 3-aminocholest-4-enes is rather remarkable in that the products are not derived from an intermediate delocalised allylic cation 14) [20]. Other cholest 4-en-3-yl derivatives solvolyse to give the allylic cation (14) irrespective of the original C(3)"Configuration, and the cation affords a characteristic mixture of cholesta-3,5-diene and cholest-4-en-3a yl and -SjS-yl derivatives (p. 381). The 3(S-amine (10), however, gave only cholest-4-en 3 -ol (ii) on deamination, whereas the 3a-amine (12) gave cholesta-2,4-diene (13) [20], Both amines therefore react in the same... 

Figure 29.7. Allyl system. Configuration of tt electrons in cation, free radical, and anion.

Fig. 7a and b. Interconversion of cyclopropyl and allyl cation (1), radical (2), and anion (3) correlation diagrams for electron configurations and states a) conro-tatorymotion b) disrotatory motion... 

A beautiful illustration of a delicate balance between a stepwise and a concerted reaction has been found in the reactions of 1,1-dimethylbutadiene 6.133.716 This diene rarely adopts the s-cis conformation necessary for the Diels-Alder reaction with tetracyanoethylene giving the cyclohexene 6.136. However, it can react in the more abundant s-trans conformation in a stepwise manner, leading to a moderately well stabilised zwitterion 6.134. The intermediate allyl cation is configurationally stable, and a ring cannot form to C-l, because that would give a trans double bond between C-2 and C-3 in the cyclohexene 6.137. Instead a cyclobutane 6.135 is formed. All this is revealed by the solvent effect. In the polar solvent acetonitrile the stepwise ionic pathway is favoured, and the major product (9 1) is the cyclobutane 6.135. In the nonpolar solvent hexane, the major product (4 1) is the cyclohexene 6.136 with the Diels-Alder reaction favoured. 

Again, the diene does not need to be in the s-cis conformation—as long as the substituents stabilise the radicals well enough, as they do here, the first bond can form while the diene is still in its more abundant s-trans conformation. The allyl radical 6.139 produced from the s-trans diene is configurationally stable, just as the allyl cation 6.134 was, and it will not be able to cyclise to give a frara.v-cyclohexene. Rotation about the bond between C-2 and C-3 is evidently too slow to compete with the radical combination giving the cyclobutane 6.140. 

This result hardly looks worth reporting and, anyway, how do we know that equilibration or S l reactions aren t happening Well, here the mechanism must be 8 2 because the corresponding Z-allylic alcohol preserves its alkene configuration. If there were equilibration of any sort, the Z-alkene would give the -alkene because E and Z allylic cations are not geometrically stable. 

There is a third kind of orbital called a non-bonding orbital. Electrons in these orbitals neither strengthen nor weaken the bonding between atoms. The 7r-electron configurations for the ground states, and first excited states of the allyl cation, free radical, and anion are ... 

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