Reactions carbocationic

First, we survey the major types of compounds that contain hypercarbon. The relationships that link these apparently disparate species are demonstrated by showing how the bonding problems they pose can be solved by the use of three- or multicenter electron-pair bond descriptions or simple MO treatments. We also show the role played by hypercoordinated carbon intermediates in many familiar reactions (carbocationic or otherwise). Our aim here is to demonstrate that carbon atoms in general can increase their coordination numbers in a whole range in reactions. 

P450 7A1 has also been demonstrated to convert lathosterol to 7-ketolathosterol (the immediate precursor of cholesterol in the normal pathway) to 7-ketoeholesterol and a trace of the 7,8-epoxide [1777]. The reaction with A -dehydrocholesterol is proposed to be responsible for the high level of the oxysterol 7-ketocho-lesterol in individuals with Smith-Lemli-Opitz syndrome [1777], and the ketone is formed in a direct reaction (carbocationic intermediate, with hydride transfer) rather than via rearrangement of the epoxide [1777], The relevance of this reaction has been demonstrated in Smith-Lemli-Optiz syndrome and cerebrotendinous xanthomatosis patients [1778],... 

In their reactions with suitable nucleophiles, such as tt-aromatics or heteroatom donor nucleophiles, the readily polarizable linear acylium ions shift a Tt-electron pair to oxygen, bending the ions and developing an empty p-orbital at the carbocationic center. This enables the reaction with aromatics. The acetylation of benzene can be depicted as... 

In superacidic media, the carbocationic iatermediates, which were long postulated to exist duting Friedel-Crafts type reactions (9—11) can be observed, and even isolated as salts. The stmctures of these carbocations have been studied ia high acidity—low nucleophilicity solvent systems usiag spectroscopic methods such as nmr, ir, Raman, esr, and x-ray crystallography. 

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). 

On the other hand, under superacidic conditions, alkanes are readily alkylated via front-side CJ-iasertion by carbocationic alkylating agents. The direct alkylation of the tertiary C—H CJ-bond of isobutylene with isobutane has been demonstrated (71). The stericaHy unfavorable reaction of tert-huty fluoroantimonate with isobutane gave a Cg fraction, 2% of which was 2,2,3,3-tetramethylbutane ... 

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). 

The first carbocationic photolysis to be investigated in detail was that of the tropylium ion (1) (van Tamelen et al., 1968, 1971), in which generation of the [3,2,0] valence bond isomer (the Dewar tropylium ion ), (2) was the dominant reaction. When irradiated for 10 minutes in 5% aqueous sulfuric acid, two major products were formed in a total... 

The classification fragmentation applies to reactions in which a carbon-carbon bond is broken. One structural feature that permits fragmentation to occur readily is the presence of a carbon that can accommodate carbocationic character (3 to a developing electron deficiency. This type of reaction, known as the Grob fragmentation, occurs particularly readily when the y-atom is a heteroatom, such as nitrogen or oxygen, that has an unshared electron pair that can stabilize the new cationic center.96... 

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) ... 

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. 

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). 

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. 

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