Carbonium ions vinyl, stability

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). 

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. 

The ether-catalyst complex (II) splits into a complex anion (III) and a carbonium ion (IV), which rearranges to the configuration of maximum stability (V). This carbonium ion (V) could itself initiate polymerisation, but it is more likely that it attacks the double bond of the closely associated anion (III), giving the double ion (VI) in equilibrium with the aldehyde (VII). Rearrangements of the type (I)-(VII) have been observed for vinyl ethers [7], and a closely parallel isomerisation is that of isobutyl phenyl ether into para-tertiary butyl phenol under the influence of A1C13 [8]. It is unlikely that the steps from (II) to (VI) take place in a well defined succession. The process probably proceeds by a single intramolecular transformation. 

Problem 11.13 Hydrogen iodide adds to vinyl chloride more slowly than.to ethylene, and yields 1-chloro-l-iodoethane. (a) Draw the formula of the carbonium ion formed in the initial step of the addition to vinyl chloride, (b) Of addition to ethylene, (c) Judging from the relative rates of reaction, which w ould appear to be the more stable carbonium ion (d) Account for the difference in stability. 

Carbonium ions in which the cationic carbon is -hybridized are of higher energy because of the greater electronegativity associated with increasing s-character. It has been estimated that vinyl cation, CH2=CH, lies between ethyl cation and methyl cation in its stability. The intermediacy of substituted vinyl cations in solvolysis reactions has been demonstrated, but no evidence has yet been presented for their direct observation by NMR." ... 

When the metal fragment is a poor ir base, the L model (5.4) applies and the vinylic carbons bound to the metal behave as masked, metal-stabilized carbonium ions. In such a case we often see nucleophilic attack (e.g., Eq. 5.10)." This is an example of a more general reaction type—nucleophilic attack on polyenes or polyenyls, and will be discussed in more detail in S tion 8.3. 

Vinyl—silicon bonds are also rather easily cleaved by electrophiles. It appears that the R3Si group is able to stabilize //-carbonium ions. Allylsilanes are prone to attack even by fairly weak electrophiles. Acetals, aldehydes and ketones react in the presence of Lewis acids to form unsaturated alcohols. The reaction is accompanied by an allylic transposition ... 

A final case deals, in some more detail, with the acid catalyzed hydration of ethylenes and acetylenes. The reactivity ratio between two pairs of similarly substituted alkenes and alkynes is very much near unity(18), contrary to what expected from the large difference in stability of carbonium ions and vinyl cations. One explanation maintains that the two intermediates ions 1 and 2 are... 

However, this reaction requires the prior formation of the less probable radical ending, M HR-CH2. On these grounds, therefore, it is to be predicted that head-to-tail linkages will predominate over head-to-head and tail-to-tail linkages in the final polymer. Since the stabilization of carbanions and carbonium ions follows similar principles to those discussed above, it is to be expected that the head-to-tail structure will also be favoured in ionic vinyl polymerizations. 

If the allylic chloride group is particularly favourable to such elimination (and aflylic chlorine is highly reactive in heterolytic reactions because of the stability of the carbonium ion formed) then an unzipping process can be envisaged for poly (vinyl chloride). 

Preliminary accounts of the reactions of HCl with complexes [Pt(C CH)2L2] (reported in Vol. 5, p. 386) have been superseded by a full paper. A feature of the reactions is the intermediacy of platinum-stabilized vinyl carbonium ions [Pt-(C+=CH2)] and another paper by the same authors is concerned with mechanistic studies of the formation and decomposition of alkoxycarbene complexes, reaction (2), which can also be rationalized in terms of a mechanism involving such species... 

A dominant feature of the chemistry of ferrocenes is the ease of stabilization of carbonium ions in which a positive charge is localized at the -position. For example a vinyl substituent in ferrocene is readily protonated, even by acetic acid ... 

PouLTER has demonstrated that chrysanthemyl derivative 16-OPy I asymmetrically deuterated at the carbinyl position ionizes with 91 2% stereoselectivity from a conformation with the leaving group aligned antiparallel to the 1,3-cyclopropyl bond (68). Since rotation about the C1-C2 bond in the cyclopropylcarbinyl carbonium ion intermediate should be slow relative to nucleophilic capture, this selectivity is reflected in the position of the deuterium atom in the vinyl group of the two major products. The stereoselectivity is attributed to stabilization of the transition state for ionization by charge delocalization into the vinyl group which should be more favorable in the a/i/f-parallel alignment. Kinetic data also indicate that the double bond facilitates the ionization process. 

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