Hydroxyallylic cations

Figure 1.3. Frontier orbital energies (eV) and confidents for acrolein and protonated acrolein. In the latter case the upper numbers refer to the situation where bond lengths and angles correspond to those of acrolein. The lower numbers are more suitable for a hydroxyallyl cation. The actual situation is assumed to be intermediate. The data are taken from ref. 104.

Stabilization of 34 by a hydroxy group is smaller, as is suggested by the small energy difference of 1.3 kcal mol between a-hydroxycyclopropyl and 2-hydroxyallyl cation in favour of the latter cation (see Figure 25). Experimentally, the a-methoxycyclopropyl cation appears to be a stable intermediate in substitution reactions in solution, and there is evidence for the independent existence of both the a-methoxycyclopropyl and 2-methoxyallyl cations in the gas phase245. 

It has been demonstrated by Olah et al.420 that a,/3-unsaturated ketones are O-protonated in HF-SbF5 to form hydroxyallylic cations, which were directly observed by NMR spectroscopy. Jacquesy and Coustard have found indirect evidence for diprotonation of a,/3-unsaturated ketones (enones) by trapping the dication with CO.421 The resulting acylium ion centers are then quenched with methanol or benzene. An interesting synthetic method was therefore developed for carboxylation of bicyclic enones in superacid media at atmospheric pressure [Eqs. (5.158) and (5.159)]. 

Upon thermolysis, l.l -disubstituted dicyclopropyl ethers expel cyclopropanone, which oligomerizes rapidly (major pathway). This process is accompanied by some ring opening to a 2-hydroxyallyl cation, thus accounting for the ultimate formation of substituted acetones. 

At first sight, the photochemistry of 4-hydroxy-pyryliums, i.e. of 4-pyrones in acid solution, seems extraordinary, in that they are converted into 2-pyrones, however a rationalisation involving, first, a bicyclic hydroxyallyl cation, second, a bicycUc epoxy-cyclopentenone, and then a second photoexcitation, makes the transformation clear the sequence is shown below. Irradiation at higher pH leads to a trapping of the first-formed photointermediate by solvent and thus the isolation of dihydroxy-cyclopentenones. ... 

What species forms in the RDS The easy answer is that S l loss of chloride from the enolate (enol) produces an oxyallylic (hydroxyallylic) cation that cyclizes in a disrotatory fashion to the corresponding cyclopropanone. The cyclization may be readily reversible, though generally... 

The formation of the protonated zwitterion (the 2-hydroxyallyl cation) has been proved by nmr spectroscopy of the cyclopropanone in super acid medium " see footnote p. S50. 

Cramer and Barrows investigated computationally the intermolecular cycloaddition between s-cis butadiene and various oxyallyl dienophiles [4]. The four studied in order of increasing electrophilicity are oxyallyl (neutral) < sodium oxyallyl cation < lithium oxyallyl cation < 2-hydroxyallyl cation. Computations showed that the more electrophilic oxyallyl cations showed a higher preference for a stepwise pathway, while the weakly electrophilic oxyallyl dienophiles followed concerted cycloadditions. 

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