Vinyl cation stabilized

In addition to electron-deficient alkenes, under the catalysis of TiCLt, 1,2-allenylsi-lanes can react with aldehydes or N-acyliminium ion to afford five-membered vinylic silanes 71 and 72. Here the carbocations generated by a Lewis acid regiospecifically attack the C3 of the 1,2-allenylsilanes to produce a vinyl cation stabilized by hyper-... 

Synthesis of Furans and Isoxazoles. Electrophilic species of the general form Y=X+ serve as heteroallenophiles, combining with allenylsilanes in a regiocontrolled [3 + 2] annulation method. As illustrated in the mechanism shown in eq 13, addition of the heteroallenophUe at C-3 of the allenylsilane produces a vinyl cation stabilized by hyperconjugative interaction with the adjacent carbon-silicon tr-bond. A 1,2-trialkylsilyl shift then occurs to generate an isomeric vinyl cation, which is intercepted by nucleophilic X. Elimination of H" furnishes the aromatic heterocycle. 

Both experimental studies on gas-phase ion stability and MO calculations indicate that the two vinyl cations shown below benefit fiom special stabilization. Indicate what stmctural features present in these cations can provide this stabilization. 

The stereochemistry of addition is usually anti for alkyl-substituted alkynes, whereas die addition to aryl-substituted compounds is not stereospecific. This suggests a termo-iecular mechanism in the alkyl case, as opposed to an aryl-stabilized vinyl cation mtermediate in the aryl case. Aryl-substituted alkynes can be shifted toward anti addition by including bromide salts in the reaction medium. Under these conditions, a species preceding the vinyl cation must be intercepted by bromide ion. This species can be presented as a complex of molecular bromine with the alkyne. An overall mechanistic summary is shown in the following scheme. 

According to part 4.1.4 of this article, an increase in cation stability leads to an increase in cationic polymerizability. The latter order agrees satisfactorily with the well-known fact that vinyl ethers (which have an oxygen atom in the neighboring position... 

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. ... 

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. 

A limited amount of information is available on vinyl cations in the gas phase. These mass spectral data suggest that the heat of formation and stability of simple alkylvinyl cations, such as CH2=8h and CH3CH=6h, is in between those of methyl and ethyl cations (2). The bulk of the evidence for the existence of vinyl cations comes from mechanistic studies in the liquid phase. Although vinyl cations have not yet been prepared in solution with lifetimes adequate for direct spectral observation, sufficient, increasing evidence has been presented for the existence of such species as transient intermediates. 

In the hydration of compounds 2f and 2g, besides the expected ester, three other products (acetic acid, an alkene, and alcohol) were observed. These products were postulated to arise via a fragmentation of the intermediate vinyl cation, 6, as shown in Scheme II. The importance of the fragmentation path is presumably determined by the stability of the alkyl cation formed by the alkyl oxygen fission. 

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

However, the observations of Ward and Sherman need not rule out triple-bond participation and vinyl cations in the systems studied by Hanack and co-workers (75-79). Presumably, the enol formate 61 itself arises via a transition state involving a rate-determining protonation and vinyl cation 62 (see previous section). A vinyl cation such as 62 with an adjacent phenyl group is considerably more stable and hence more accessible than a vinyl cation such as 63, stabilized only by a neighboring alkyl group. Hence, formation of enol formate 61 and its... 

The second reason for the lack of early investigations into vinyl cations was the seemingly extreme unreactivity of vinyl halides in solvolytic processes. The unreactivity of vinyl chloride, for instance, even in the presence of silver nitrate, has been almost a legend in organic chemistry (102). This lack of reactivity of simple alkylvinyl halides has been attributed to the low stability of simple vinyl cations or to the very strong carbon-halogen bond, or both. 

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... 

A reasonable idea of the stability of the stereoisomeric trigonal vinyl cations can be gained from the behavior of vinyl anions and radicals. It is known that the interconversion between stereoisomeric vinyl anions is fairly slow, with an activation energy of the order of 18-24 kcal/mole (171). On the other hand, inversion of stereoisomeric vinyl radicals is reasonably rapid, even at fairly low temperatures, with an activation energy of the order of 2-8 kcal/mole (172). Hence, extrapolating from the electron-rich vinyl anion through the neutral vinyl radical to the electron-deficient vinyl cation, one would expect rapid interconversion between stereoisomeric vinyl cations and only a small amount (if any) of stereospecificity. To put it differently, the vinyl cation should be mostly linear with an empty p orbital and very little trigonal character. 

Perhaps it is not unexpected and surprising that there is no /3-aryl participation in the solvolysis of triarylvinyl substrates, as the intermediate vinyl cation is especially stable because of charge delocalization into the a-aryl ring and has no need for extra stabilization. 

An indication of the great stability of an allenyl cation relative to a vinyl cation may be gained from the fact that triphenylchloroallene, 243a, reacts in... 

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. 

An important contribution to silylium ion chemistry has been made by the group of Muller, who very recently published a series of papers describing the synthesis of intramolecularly stabilized silylium ions as well as silyl-substituted vinyl cations and arenium ions by the classical hydride transfer reactions with PhjC TPEPB in benzene. Thus, the transient 7-silanorbornadien-7-ylium ion 8 was stabilized and isolated in the form of its nitrile complex [8(N=C-CD3)]+ TPFPB (Scheme 2.15), whereas the free 8 was unstable and possibly rearranged at room temperature into the highly reactive [PhSi /tetraphenylnaphthalene] complex. ... 

These substituent effects are due to the stabilization of the carbocations that result from protonation at the center carbon. Even if allylic conjugation is not important, the aryl and alkyl substituents make the terminal carbocation more stable than the alternative, a secondary vinyl cation. 

Considering the above-mentioned facts, according to which simple diazoketones yield dihydrofurans with ketene acetals but cyclopropanes with enol ethers, one exports an interlink between these clear-cut alternatives to exist, i.e. substrates from which both cyclopropanes and dihydrofurans result. In fact, providing an enol ether with a cation-stabilizing substituent in the a-position creates such a situation The Rh2(OAc)4-catalyzed decomposition of -diazoacetophenone in the presence of ethyl vinyl ether produces mainly cyclopropane 82 (R=H), but a small amount of dihydro-... 

Allenyl cations 1 are a stabilized form of vinyl cations1-3 in which the /1-carbon atom of the vinylic structure is part of the substituent which effects the stabilization of the ion via its electron-donating ability. This leads to a resonance hybrid having formally the alkynyl cation structure 2. Allenyl cations should be distinguished from the allenyl substituted carbenium ions 3 formulated as the mesomeric structures of the vinyl cations 4 (dienyl cations) stabilized by an w-vinyl group (equation 1). 

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