Allyl ion

The 3-cyclohexeityl ion provides another example of the dependence of the extent of rearrangement on reaction conditions. The major product is the result of direct solvent displacement. There is also some product resulting from Itydride shift to the more stable allylic ion as well as a trace of the bicyclo[3.1.0]hexane product arising from participation of the double bond. ... 

An immediate presumption that the more stable allyl ion will be formed overlooks the stereoelectronic facets of the reaction. Protonation at the center carbon without rotation of one of the terminal methylene groups leads to a primary carbocation... 

The allylic ion (XI) is probably too stable to propagate the reaction. The inhibiting (poisoning) effect of oxygen, nitrogen, and sulphur compounds is also due to formation of stable ions (in so far as it is not due to sequestration of metal halide by complex formation), e.g., the reaction [98] ... 

In some cases in which the rate of polymerization is of first order in monomer and for which there are reasons for believing a stationary state of the First Kind to prevail, it has been argued that Vt must be independent of [PJ (without, indeed, much evidence) and that therefore V must also be independent of [PJ. However, a termination reaction with monomer, though unlikely with most monomers, is not impossible, and in the Mayo plots it would be indistinguishable from monomer transfer. One possible mechanism for such a termination reaction is the formation of an allylic ion by abstraction of a hydride ion from the monomer [112] ... 

An entirely different type of termination mechanism has been proposed to account for the cessation of polymerization in the system propene-AlBr3-HBr through exhaustion of the catalyst [9]. It is suggested that by the transfer of a hydride ion, and then of a proton, an allylic ion ... 

In view of the reports of the instability of solutions in which the formation of tert-butyl cations had been attempted and of the formation of allylic ions from cyclic condensation... 

In order to find out what R+ is, we consider first the common experience that when a tert- mty halide is treated with an aluminium halide under ordinary conditions, there is a brisk evolution of hydrogen halide and a coloured solution containing oligo-isobutylenes and condensed allylic ions is formed. In the present experiments the solutions were colourless, no hydrogen halide was evolved, and the conductivity was stable and behaved reversibly. Further, the rectilinearity of the k-[A1X3] plots in Figures 9 and 10, and the smallness of the intercepts on the K-axis, showed that ions generated by reactions other than those of type (viii) must have been very scarce, and for the experiment with tert-butyl bromide this was borne out by the absence of any byproducts. 

We report spectroscopic and conductimetric studies on the reactions of styrene in the presence of perchloric acid (most in methylene dichloride but some in ethylene dichloride), both during and after the polymerisation. During polymerisation no ions are detectable, but at the end aralkyl (and subsequently allylic) ions are formed. Quantitative results show that ion formation sets in when the styrene concentration has fallen to four times the acid concentration we interpret this finding as showing that the ester (oligo-)styryl perchlorate requires four molecules of styrene for stabilisation in solution. 

Figure 1 A typical sequence of visible spectra for a spectroscopic experiment, a At the end of the ion formation (only SD ions), b Some 10-20 min later (mixture of SD and allylic ions), c About 1 h later (more allylic ions), d A few hours later (dienic polymer molecules + allylic ions), e 20-30 h later (more dienic polymer molecules and less allylic ions), f Same as e after addition of a few drops of ethanol (only dienic...

The bathochromic shifts with increasing chain concentration are compatible with the mechanism proposed above, since an increase in the concentration of unsaturated chains will favour hydride abstraction and will therefore give allylic ions with higher degrees of conjugation, which will absorb at wavelengths greater than 450 mp. The only serious chemical (as opposed to mechanistic) uncertainty in this scheme is whether route III—> IB or III — IC, or both, or perhaps some other process, adequately represent the removal of the ions by addition of monomer. Some reaction path of this kind seems to exist since there is no evidence that either route II —> III or route II — VI is reversible. 

Our studies have confirmed that the oligomerisation of styrene by perchloric acid is both chemically and kinetically simple. However, the reactions which follow completion of the polymerisation, and during which carbonium ions are formed and destroyed, are complicated. They can be rationalised in terms of equilibria involving ions, acid, double bonds, and esters cyclisation of olefinic oligomers and formation of polyenes by way of allylic ions add further complications. We have thus shown in some detail just why such systems must be treated with the greatest circumspection if they are to yield valid information. 

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... 

If the allyl alkali metal compounds are ionic compounds, then if the allyl ion is delocalized the terminal carbons would be equivalent and contain most of the charge, and all three... 

A number of gem-dibromocyclopropane derivatives has been reacted with aromatic compounds in the presence of aluminum chloride or ferric chloride providing indenes in yields up to 80%. To rationalize this interesting anellation process it has been proposed that the cyclopropyl cation formed under the influence of the Lewis acid collapses to an allylic ion, which then functions as the alkylating agent [179],... 

Since the products are optically active, this excludes the symmetric, resonance-stabilized allylic ion intermediate. The main (treats) product arises from the preferential attack of Br- on the more stable, less strained, partially bridged 30 intermediate (as compared to 31). Similar conclusions were drawn from the results of the iodination and haloiodination of 2,3-pentadiene256 and from bromination of 1,2-cycloalkadienes.257... 

Carbocation 7, which is an allylic ion, is quite reactive and undergoes intramolecular self-condensation. 

The nuclear framework, pn. AOs, and a set of reference axes for the allyl ion are shown below ... 

The structure of (88) is remarkable two of the xanthato ligands are bonded in a normal, terminal way whereas the other two ligands are terminally bonded to one molybdenum and tf bonded to the other other molybdenum atom as in (89)." The latter bonding type shows that a xanthate can act like an allyl ion. 

A second difficulty arises from consideration of allylic systems. Because the resulting cationic center is stabilized by interaction with the -it electrons, allylic halides ionize readily to produce the delocalized allylic ion, 2. The free ipn theoiy predicts that isomeric allyli halides that give the samp intermediate,-upon ionization should yield a product distribution independent of the isomeric origin of thp ion Scheme 2 illustrates the argument. The prediction is sometimes, but... 

Allylic systems have also provided fertile ground for investigation of ion-pair phenomena. Young, Winstein, and Goering established the importance of ion pairs in solvolysis of these compounds. They showed that ion pairs are responsible for the rearrangement of a,a-dimethylallyl chloride to y,y-dimethyl-allyl chloride (Equation 5.8).24 Goering s labeling methods have subsequently supplied a number of details about allylic ion-pair structure.25... 

The cryoscopic method is also applicable to other triarylmethyl systems, to some diarymethyl and allylic ions, and, when ortho substituents are present, to aryl acylium ions (20) (Equation 5.15) 73 unfortunately, side reactions frustrate most attempts to generate carbocations in sulfuric acid. 

The observation of stable alkyl cations in antimony pentafluoride solutions also opened up the possibility of investigating the electronic spectra of these solutions. It has been reported73 that solutions of alkyl cations in HSC F-SbFs solution at — 60°C showed no absorption maxima above 210 nm. In view of this observation, it was resolved that previous claims relating to a 290-nm absorption of alcohols and alkenes in sulfuric acid solutions were due to condensation products or cyclic allylic ions and not to simple alkyl cations.74... 

The carbocation is allylic and stabilized by resonance. This allylic ion can react with the nucleophile Cl at either end, giving the observed products ... 

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