Allyl cations rearrangement

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. 

Scheme 1.7. Allyl cation-initiated cyclization/rearrangement.

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

Iron(II) salts, usually in conjunction with catalytic amounts of copper(II) compounds, have also been used to mediate radical additions to dienes91,92. Radicals are initially generated in these cases by reductive cleavage of peroxyesters of hydroperoxides to yield, after rearrangement, alkyl radicals. Addition to dienes is then followed by oxidation of the allyl radical and trapping by solvent. Hydroperoxide 67, for example, is reduced by ferrous sulfate to acyclic radical 68, which adds to butadiene to form adduct radical 69. Oxidation of 69 by copper(H) and reaction of the resulting allyl cation 70 with methanol yield product 71 in 61% yield (equation 29). 

Inherent instability, which can manifest itself by a unimolecular rearrangement to a form which has a more negative free energy of formation, e.g., the isomerisation of a secondary to a tertiary cation or to an allylic cation. 

Thus, fluorination of 1,3-dienes proceeds through an allylic ion, while weakly bridged halonium ions are the intermediates in chlorination and bromination of dienes (vide infra). Furthermore, starting from the experimental evidence that 13 is produced under kinetic conditions and not from subsequent rearrangement of the 1,2- and 1,4-adducts, the authors suggested that 13 arose from rearrangement of the allyl cation intermediate, 17. Consistent with an open ion pair intermediate is also the stereoselective formation of the threo isomer from both 1,3-pentadienes, as well as the preference for the addition to the 1,2-bond observed in the reaction of both isomeric pentadienes. This selectivity may indeed... 

The signihcant peaks in the mass spectrum that are consistent with our assignment are mie = 102 - 15 (CH,) = 87. which is (CH,),CHOCHCH, 43 is (CHj)2CH 41 would be the allyl cation H7C=CHCH formed from fragmentation of (CH,)2CH. The most abundant peak, m/e = 45, probably comes from rearrangement of fragment ions. It could have the formula C HjO. The presence of this peak convinces us that O was indeed present in the compound. The peak cannot be from a fragment with only C and H C,HJ is impossible. 

Homoallyl rearrangements occur in the solvolyses of allenic /j-toluenesulfonates 1 (5-tosy-loxypcnta-1,2-dienes, ethenylidene p-toluenesulfonates). Hydrolysis or acetolysis of acyclic and cyclic compounds indicates that the allene group participates strongly to give allyl cations which rearrange to methylenecyclobutanols 2 (see Houben-Weyl, Vol. 4/4, p63). 

In gas-phase hydrobromination, where a radical mechanism is operative, the bromine atom always adds to the central carbon atom of the allenic system. As a result, vinylic bromides are formed through the stable allylic radical. In the solution phase under ionic addition conditions, either the vinylic or the allylic cation may be the intermediate, resulting in nonselective hydrobromination. Allylic rearrangement or free-radical processes may also affect product distributions. 

Reaction of the optically active allene (205) with bromine leads to the inactive 3-bromodihydropyran (208 X = Br), whereas activity is retained in the reaction with 2,4-dinitrobenzenesulfenyl chloride, giving (208 X = SC6H3(N02)2 Scheme 39) (67JA7001). The different behaviour has been attributed to the varying stabilities of the cyclic intermediates (206 X = Br or SC6H3(N02)2), the latter showing less tendency to rearrange to the achiral allylic cation (207). 

The dienylic cations 44 with cyclopropyl and phenyl groups were also prepared and characterized by the protonation of respective fulvenes74. Other cyclopropyl substituted allyl cations include acyclic 1,3- and 1,4-disubstituted allyl cations 45 and 46. The charge in these cations is localized mainly on the carbon adjacent to the cyclopropyl group. The rearrangement of these cations at higher temperatures was also studied76. 

The 3-spirocyclopropy 1-2-cyclopropyl-2-norbomyl cation 97, R = c-C,H5 is stable even up to -20 °C, whereas the phenyl and methyl analogues rearranged to the allylic cations 98 at -70 °C and -90 °C, respectively (equation 58). 

Reductive alkene isomerizations can also be induced by photochemical excitation. Geometric isomerization and rearrangement can be observed upon electron transfer sensitization with molecules with inverse electron demand. Thus, a substituted cinnamyl alcohol in the presence of excited p-dimethoxybenzene gave geometric isomerization and rearrangement characteristic of a free allyl cation, eq. 32 (94) ... 

Presumably the radical anion generated by the primary electron transfer fragments to a radical anion pair. Back electron transfer to the sensitizer cation radical generates the allyl cation from which geometric isomerization and structural rearrangement can be easily accomplished. 

In a bicyclo[4.1.0]heptadiene, there should be a walk-around rearrangement analogous to the one we discussed above in the bicyclic allylic cations.143... 

The cyclopropyl-allyl rearrangement has been shown to proceed with nucleophilic assistance,87i232<233 and the intermediate allyl cation can be trapped by nucleophiles leading to synthetically useful derivatives. An example is the formation of an unsaturated acetal and the propiolic acid ortho ester (equations 92 and 93).232... 

Many cyclopropyl cation precursors indeed readily rearrange to allyl cations under stable ion conditions.159-162 However, a distinct cyclopropyl cation 46 showing a significant 2n aromatic nature has been prepared by the ionization of 11-methyl-ll-bromotricyclo[4.4.1.01,6]undecane in SbF5-S02ClF solution at — 120°C163 [Eq. (3.30)]. 

Diprotonated 2,4-pentanediol 10 loses water and rearranges to form 1,3-dimethyl allyl cation [Eq. (4.6)]. Diprotonated 2,5-hexanediol 11, above — 30°C, rearranges to a mixture of protonated cis- and tra .v-2,5-dimethyltetrahydrofurans [Eq. (4.7)]. This would seem to indicate that there is a significant amount of the monoprotonated form present or that the carbocation formed can easily lose a proton before ring formation occurs. 

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