Vinyl cations, also

Vinyl cations also have been invoked as intermediates in the electrophilic addition of HCl to 1-phenylpropyne in acetic acid. Fahey and Lee (30,31) have shown that the reaction is first order in 1 -phenylpropyne and between first and second order in HCl. The observed products were ds-( 75%) and frans-( 10%) 1-chloro-l-phenylpropene, along with about 15% propiophenone. Control experiments demonstrated that the propiophenone arose from cis- and trans-1-acetoxy-l-phenylpropene. The results were best explained by rate-determining... 

Vinyl cations also have been invoked as intermediates in the reaction of HCl with r-butylacetylene (49). With neat mixtures in the liquid phase at ambient temperatures, the following four products were observed ... 

Vinyl cations also have been invoked as intermediates in the addition of carbonium ions generated in strong acid to acetylene (50-53). Sasaki et al (50) observed 1-adamantyl methyl ketone, 25, as the sole product in the reaction of acetylene with 1-bromoadamantane in concentrated H2SO4 at 5°. Bott (51), on the other hand, reported a mixture consisting of 75% 1-adamantylacetaldehyde,... 

Vinyl cations also have been proposed as intermediates in the deamination of vinylamine, 135(113). When diphenylvinylamine,, 135, was dissolved in boiling benzene and isoamylnitrite was added, diphenylacetylene formed in 50-85% yield in 3-5 h. The sequence of reactions shown in Scheme IX has been proposed as a possible route to the observed product (113). 

Stabilized vinyl cations also can be formed by allylic double-bond participation in vinyl halide solvolysis. Grob and co-workers (151) have investigated the solvolysis of a number of substituted 2-bromo-l,3-butadienes, 165. Bromodienes 165 solvolyzed via first-order rates in 80% aqueous ethanol... 

Trivalent ( classical ) carbenium ions contain an, vp2-hybridized electron-deficient carbon center that tends to be planar in the absence of constraining skeletal rigidity or steric interference. (It should be noted that ip-hybridized, linear acyl cations and vinyl cations also show substantial electron deficiency on carbon.) The carbenium carbon contains six valence electrons, and thus it is highly electron-deficient. The stmcture of trivalent carbocations can always be adequately described by using two-electron, two-center bonds (Lewis valence bond structures). 

The relative stabilities of vinyl cations also can be given by the gas-phase basicity AAG(cc)h+ of alkynes, namely as the standard free energy change for the protonation reaction (28). 

The TT-donor and (3-hyperconjugation effects are present for both alkyl and vinyl carbocations. The vinyl carbocations exhibit somewhat larger stabilizations because they are inherently less stable. The vinyl cations also appear to be more sensitive to the a-polar effect. Note that the F, OH, and NH2 substituents are less stabilizing of vinyl cations than of alkyl cations. This is believed to be due to the greater sensitivity to the polar effect of the more electronegative sp carbon in the vinyl cations. The NPA and bond order characterizations of the carbocations are shown in Table 3.13. The values give the charge associated with the cationic carbon and the bond order between the cationic carbon and the substituent X. 

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... 

Highly unstable vinyl cations, generated in situ from vinyl triflates have also been arylated (the triflate group is replaced by the aromatics) to give vinyl aromatics under Friedel-Crafts conditions (28). 

The stabilities of most other stable carbocations can also be attributed to resonance. Among these are the tropylium, cyclopropenium, and other aromatic cations discussed in Chapter 2. Where resonance stability is completely lacking, as in the phenyl (CeH ) or vinyl cations, the ion, if formed at all, is usually very short lived. Neither the vinyl nor the phenyl cation has as yet been prepared as a stable species in solution. ... 

Solutions of aiyl-substituted vinyl cations have been reported to be stable for at least a short time at low temperatures. The NMR spectra have been obtained Abram, T.S. Watts, W.E. J. Chem. Soc., Chem. Commun., 1974, 857 Siehl, H. Carnahan Jr., J.C. Eckes, L. Hdnack, M. Angew. Chem. Int. Ed. Engl., 1974, 13, 675. The 1-cyclobutenyl cation has been reported to be stable in the gas phase Franke, W. Schwarz, H. Stahl, D. J. Org. Chem., 1980, 45, 3493. See also Siehl, H. Koch, E. J. Org. Chem., 1984, 49, 575. 

As in the case of the base-catalyzed reaction, the thermodynamically most stable alkene is the one predominantly formed. However, the acid-catalyzed reaction is much less synthetically useful because carbocations give rise to many side products. If the substrate has several possible locations for a double bond, mixtures of all possible isomers are usually obtained. Isomerization of 1-decene, for example, gives a mixture that contains not only 1-decene and cis- and franj-2-decene but also the cis and trans isomers of 3-, 4-, and 5-decene as well as branched alkenes resulting from rearrangement of carbocations. It is true that the most stable alkenes predominate, but many of them have stabilities that are close together. Acid-catalyzed migration of triple bonds (with allene intermediates) can be accomplished if very strong acids (e.g., HF—PF5) are used. If the mechanism is the same as that for double bonds, vinyl cations are intermediates. 

Product studies, general acid catalysis, and kinetic data indicate that hydration of compounds 7 and 8 also proceed by way of a vinyl cation (12,13). 

In all of the above cases involving decompositions of vinyl diazonium ions, the observed products are consistent with a vinyl cation formulation, but extensive mechanistic studies of these reactions have not been reported. It is difficult, for instance, to establish to what extent reaction proceeds through the diazonium ion via a backside nucleophilic attack and concerted loss of nitrogen rather than through the free vinyl cation. In the absence of kinetic data, it is also difficult to rule out competing or alternative mechanisms not involving vinyl cations. 

As indicated in this Scheme, triflate 184 presumably ionizes to vinyl cation 185, which can eliminate a proton and give acetylene 187 or react with solvent to give pinacolone 188 it can also undergo a methyl migration to give the tertiary... 

Finally, participation by a remote double bond in the solvolytic generation of a vinyl cation has also been observed recently (190). Solvolysis of cis and trans triflate, 234a and 234b, in trifluoroethanol buffered with lutidine gave, besides acyclic products, 20% cyclic products 235abc in the case of the cis triflate 234a and 35% cyclic products in the case of the trans isomer 234b (190). 

Other than the stabilized allenyl 242 and related 248 cations, there is also to date no report on the solvolytic generation of a primary 256 vinyl cation. Such species have, however, been implicated in the decomposition of some... 

It is also difficult to determine exactly the relative stabilities of vinyl cations and the analogous saturated carbonium ions. The relative rates of solvolysis of vinyl substrates and their analogous saturated derivatives have been estimated to be 10 to 10 (131, 134, 140, 154) in favor of the saturated substrates. These rate differences, however, do not accurately reflect the inherent differences in stability between vinyl cations and the analogous carbonium ions, for they include effects that result from the differences in ground states between reactants, as well as possible differences between the intermediate ions resulting from differences in solvation, counter-ion effects, etc. The same difficulties apply in the attempt to estimate relative ion stabilities from relative rates of electrophilic additions to acetylenes and olefins, (218), or from relative rates of homopropargylic and homoallylic solvolysis. 

Last but not least it should be mentioned that the process of H loss from ionized ethylene 184 is a reaction which not only follows a simple (C—H) dissociation giving rise to the formation of the classical vinyl cation 31 (47). From the careful analysis of extensive ab initio calculations it has to be concluded73 that the stretching and dissociation of the (C—H) bond of ionized 184 is also coupled with a molecular... 

Butatrienyl cations 61 were also implied in the solvolysis of the w-propynyl vinyl triflate 6243. The vinyl triflate solvolyzes 35-70 times faster than the corresponding simple vinyl analogues and gives products resulting mainly from the a-alkynyl vinyl cation 63. A small amount of the allenic ketone 64 derived from the butatrienyl cation is also detected (equation 22). 

Despite the successful prediction of chemical shifts for a great structural variety of carbocations some difficulties have been encountered for vinyl cations.47 The effect of electron correlation, basis sets and geometry on calculated NMR spectra of vinyl cations has been studied in some detail also for the parent vinyl cation in its linear form.48 Comparative experimental and computational NMR studies, however, have... 

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