Alkenyl cations

Many alkenyl cations have now been directly observed particularly by Deno, Richey and co-workers,88,217,218 Sorensen,219 Olah and co-workers,220-227 and Carpenter 228 Deno229 has reviewed the chemistry of these ions. Allylic cations particularly show great stability with generally insignificant 1,3-overlap, except in the case of cyclo- 

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

The 2-chloroallyl cation generated by matrix isolation has been studied by FT-IR spectroscopy and calculations [MP2(fc)/6-311G(d,p) level].235 Structure 78 of Cs symmetry with bridging chlorine, proposed earlier,162 was found to be less stable by 7.5 kcal mol 1 than cation 79 of C2v symmetry and could not be found by spectroscopy. Furthermore, in contrast to 1- or 3-chloroallyl cations, the centrally positioned chlorine does not contribute to charge delocalization through back-donation as a consequence of the Ji-orbital noninteraction between the n electrons of Cl and the LUMO of the allyl cation.236 

Allyl cations 80 and 81 have been generated and studied by NMR spectroscopy.237 Although sterically crowded, cation 80 proved to be surprisingly stable up to 80°C. The rotational barrier estimated on the basis of the coalescence temperature of the 13C NMR signals is 16.8 kcal mol-1, in good agreement with the calculated value (MNDO, 16.5 kcal mol-1). In contrast, the rotational barrier of cation 81 was found to be less 

Protonation of allenes also leads to allyl cations, allowing one to obtain ions—for example, the dimethylallyl and tetramethylallyl cations 82 [Eq. (3.36)]—that are otherwise difficult to generate from allylic precursors.238 

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Evidence from a variety of sources however indicates that alkenyl cations (also called vinylic cations) are much less stable than simple alkyl cations and their involve ment m these additions has been questioned Eor example although electrophilic addi tion of hydrogen halides to alkynes occurs more slowly than the corresponding additions... 

It was previously observed that with a catalytic amount of FeCls, benzylic alcohols were rapidly converted to dimeric ethers by eliminating water (Scheme 14). In the presence of an alkyne this ether is polarized by FeCls and generates an incipient benzylic carbocation. The nucleophilic attack of the alkyne moiety onto the resulting benzyl carbocation generated a stable alkenyl cation, which suffer the nucleophilic attack of water (generated in the process and/or from the hydrated... 

These results can be explained by an SN2 mechanism, which can occur both by in-plane and out-of-plane attacks.11 No sign of formation of the primary 1-alkenyl cation was detected. If it were formed, the facile 1,2-hydride shift to give the more stable secondary vinyl cation should have been observed (eq 7). 

Alkyl cations are thus not directly observed in sulphuric acid systems, because they are transient intermediates present in low concentrations and react with the olefins present in equilibrium. From observations of solvolysis rates for allylic halides (Vernon, 1954), the direct observation of allylic cation equilibria, and the equilibrium constant for the t-butyl alcohol/2-methylpropene system (Taft and Riesz, 1955), the ratio of t-butyl cation to 2-methylpropene in 96% H2SO4 has been calculated to be 10 . Thus, it is evident that sulphuric acid is not a suitable system for the observation of stable alkyl cations. In other acid systems, such as BFj-CHsCOOH in ethylene dichloride, olefins, such as butene, alkylate and undergo hydride transfer producing hydrocarbons and alkylated alkenyl cations as the end products (Roberts, 1965). This behaviour is expected to be quite general in conventional strong acids. 

Alkyne Hydrogen halide Alkenyl cation Halide ion = X Alkenyl halide... 

In a like manner, vinyl sulfoxides and sulfones can serve as the equivalents of vinyl and alkenyl cations via a process involving a Michael addition followed by an elimination.43 For example, addition of the enolate of the P-keto ester (168 Scheme 24) to phenyl vinyl sulfoxide (169) furnished, in 50% yield. 

Cracking and disproportionation in the reaction of hexane could be suppressed by the addition of cycloalkanes (cyclohexane, methylcyclopentane, cyclopentane).101 Furthermore, 3-methylpentane and methylcyclopentane also reduced the induction period. These data indicate that reactions are initiated by an oxidative formation of alkene intermediates. These maybe transformed into alkenyl cations, which undergo cracking and disproportionation. When there is intensive contact between the phases ensuring effective hydride transfer, protonated alkenes give isomerization products. 

In alkenyl cations =C —R, n 2 and m 0. The substituents of the divalent central atom lie on a common axis with the central atom and form a bond angle of 180°. Alkenyl cations have not been isolated yet because of their low stability (Section 1.2). However, calculations support the preference for the linear structure. 

Other related systems in which the relative effect of these groups on C-NMR chemical shifts were considered include the dibenzodihydro- and dibenzotropylium ions 137 and 138, respectively " and alkenyl cations 139. Interestingly, in 139 it was concluded that c-Pr equalled or exceeded Ph in its ability to delocalize charge " . 

The addition of H2O to alkynes is catalyzed by Hg(II) under acidic conditions. (The uncatalyzed reaction is extremely difficult to carry out due to the instability of alkenyl cations.)... 

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