Carbocationic intermediate

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Carbocations derived from the alcohol are probably the reactive species, but data concerning by-products expected with carbocationic intermediates are lacking. Rearrangement of 2-alkylaminothiazoles to 2-amino-5-alkylthiazoles may also explain the observed products 2-aminothiazole with benzyl chloride yields first 2-benz Iaminothiazole (206). which then rearranges to 2-amino-5-benzvlthiazole (207) (Scheme 130) (163. 165. 198). 

Preformed Carbocationic Intermediates. Propargyl cations stabilized by hexacarbonyl dicobalt have been used to effect Friedel-Crafts alkylation of electron-rich aromatics, such as anisole, /V, /V- dim ethyl a n il in e and 1,2,4,-trimethoxybenzene (24). Intramolecular reactions have been found to be regio and stereo-selective, and have been used ia the preparatioa of derivatives of 9JT- uoreaes and dibenzofurans (25). 

The P-halo ketone intermediates formed in the foregoing reactions arise from the capture of carbocationic intermediates by halide of the gegenions. In some cases, solvents such as acetonitrile can act as the competing nucleophilic species. For example, P-amido ketones could be obtained by the acylation of alkenes in acetonitrile (172). 

A large variety of salts of triflic acid formed both from metals and nonmetals are known Many of these salts are versatile reagents for organic synthesis because of such properties of the tnflate anion as very low nucleophilicity and low coordinating ability However, despite low nucleophilicity, the triflate anion can combine with carbocationic intermediates under appropriate conditions to form triflate esters [116, 117, II8. ... 

It is found in practice that in (highly polar) Sjvl reactions attack takes place on the carbocationic intermediate, R , through the atom in the nucleophile on which electron density is the higher. With, for example, halides that do not readily undergo SN1 attack this can be promoted by use of the silver salt of the anion, e.g. AgCN, as Ag promotes R formation by precipitation of AgHal (cf. p. 102) ... 

A very similar situation is encountered in the electrophilic addition of unsymmetrical adducts (e.g. HBr) to vinyl halides (e.g. CH2=CHBr), where the inductive effect of halogen controls the rate, but relative mesomeric stabilisation of the carbocationic intermediate controls the orientation, of addition (p. 185). 

The formation of the carbocationic intermediate (37), either directly or via an initial it complex, appears to be rate-limiting, and the overall orientation of addition is Markownikov. There is evidence of some ANTI stereoselectivity, but this is not very marked and is dependent on the alkene and on the reaction conditions. 

Acids that have weakly nucleophilic anions, e.g. HS04e from dilute aqueous H2S04, are chosen as catalysts, so that their anions will offer little competition to H20 any R0S03H formed will in any case be hydrolysed to ROH under the conditions of the reaction. Rearrangement of the carbocationic intermediate may take place, and electrophilic addition of it to as yet unprotonated alkene is also known (p. 185). The reaction is used on the large scale to convert cracked petroleum alkene fractions to alcohols by vapour phase hydration with steam over heterogeneous acid catalysts. Also under acid catalysis, ROH may be added to alkenes to yield ethers, and RCOzH to yield esters. 

Initial attack will always take place on a terminal carbon atom of the conjugated system, otherwise the carbocationic intermediate (64), that is stabilised by delocalisation, would not be obtained. It is because of this stabilisation that a carbocation intermediate is formed rather than a cyclic trromonium ion (cf. 66). Completion of overall addition by nucleophilic attack of Bre on (64) can then take place at C2 [1,2-addition, (a) ->(68)] or C4 [1,4-addition, (b) — (69) J ... 

Exactly the same considerations apply to the esterification of hindered acids (182) in the reverse direction. It will be noticed that this mechanism requires protonation on the less favoured (cf. p. 240) hydroxyl oxygen atom (185) to allow the formation of the acyl carbocationic intermediate (184). Apart from a number of R3C types, a very well known example is 2,4,6-trimethylbenzoic (mesitoic) acid (186), which will not esterify under ordinary acid-catalysis conditions—and nor will its esters (187) hydrolyse. Dissolving acid or ester in cone. H2S04 and pouring this solution into told alcohol or water, respectively, is. found to effect essentially quantitative esterification or hydrolysis as required the reaction proceeds via the acyl cation (188) ... 

Thus in the above case the elimination product is found to contain 82 % of (7). Unexpected alkenes may arise, however, from rearrangement of the initial carbocationic intermediate before loss of proton. El elimination reactions have been shown as involving a dissociated carbocation they may in fact often involve ion pairs, of varying degrees of intimacy depending on the nature of the solvent (cf. SN1, p. 90). 

This intermediate is then attacked in a fast, non rate-limiting step by any one of the series of electrophiles—G2, Br2, I2, H20, D20, etc.—to yield end-products such as (92), (93), etc. all of which will necessarily be produced at the same rate. This process has a formal resemblance to slow, rate-limiting formation of a carbocationic intermediate, followed by rapid nucleophilic attack, in the SN1 pathway it is therefore referred to as an SE1 process. 

Solvolysis of the p-MeO and p-Me chlorides is found to be faster (p-MeO 800 times) than would have been predicted from their op. values. This stems from the stabilisation, by through-conjugation, of the carbocationic intermediates (21a and 21b) which are developing during the slow, rate-limiting step of the overall reaction ... 

Loss of MeCH the ethyl cation (376), leads to a marked decrease in +ve charge adjacent to the reaction centre (had it actually been from the reaction centre itself the +ve value of p would have been much larger) this carbocationic intermediate (37 b) will then react rapidly with any available water to yield ethanol. 

There seems little doubt that the overall reaction follows a four-step pathway, the first two steps constituting an El (p. 247) elimination of water to yield a carbocationic intermediate (40), which then, in the last two steps, effects internal electrophilic... 

The observed stereoselectivity with the alkenes utilized in this particular study led the authors to postulate that the reaction proceeded via a synchronous mechanism. However, there was also the possibility of a stepwise electrophilic mechanism involving carbocationic intermediates, as represented in Scheme 15. 

Polymeric hydrocarbon by-products accompany the products of the latter two reactions. The structures of the products are clear evidence of the occurrence of 1,2-alkyl shifts leading to more stable carbocationic intermediates. ... 

The changes of stereochemistry in these systems are due to changes in the preferred direction of nucleophilic attack upon a carbocationic intermediate or a related ion pair. These results therefore indicate that the substrates or reaction intermediates have preferred orientations at the micellar surface. 

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