Carbenium ions reduction

Tertiary Alkyl Alcohols. Tertiary alkyl alcohols generally undergo facile reduction when treated with acids in the presence of organosilicon hydrides.127,136 This comparative ease of reduction reflects the enhanced stability and ease of formation of tertiary alkyl carbenium ions compared with primary and secondary carbenium ions. Thus, treatment of 1-methylcyclohexanol with mixtures of triethylsilane and aluminum chloride in dichloromethane produces near quantitative yields of methylcyclohexane with or without added hydrogen chloride in as little as 30 minutes at room temperature, in contrast to the more vigorous conditions needed for the reduction of the secondary alcohol cyclohex-anol.136... 

Camphor reduction, a-bromo ketone to ketone, 127 Carbenium ions ... 

According to further papers from Gronowitz etal. [70ZC389 73ACS2257, 73CS(3)165 78JHC285], the reduction of other tricyclic tropones does not require the presence of a Lewis acid. This might be due to the stability (see Section III,B,3,b) of the carbenium ions (formed from the intermediate tropols), which are further reduced to the tropilidenes by hydride transfer. For example, tropylium ions 237 and 238 can be prepared by this method. 

If the reduction step is the rate-determining one, path 1 (see Scheme 14), which appears to be the plausible one, fails to explain the different selectivity for the exo- and endo-oxolane acetals, because these would then proceed through the same oxo-carbenium ion intermediate. 

Consequently, the reverse reaction of protolytic ionization of hydrocarbons to carbenium ions—that is, the reduction of carbenium ion by molecular hydrogen — can be considered as alkylation of H2 by the electrophilic carbenium ion through a pentacoordinate carbonium ion. Indeed, Hogeveen and Bickel have experimentally proved this point by reacting stable alkyl cations in superacids with molecular hydrogen [Eq. (5.7)]. 

More highly stabilized l,8-bis(diarylmethyl)naphthalene dications have been prepared, including the p -methoxyphenyl derivative 53.20 This dication is generated from ionization of the diol in HBF4 and (CF3CO)2O.20a Dication 53 has been characterized by experimental studies (single crystal X-ray analysis and NMR) and theoretical calculations. The carbenium ion centers are found to be separated by just 3.076 A (X-ray and ab initio results) and show 13C NMR resonances at A3C 191.8. Two electron reduction is also shown to give the acenaphthene derivative 54. 

Fig. 2.20. C,C bond-length reductions and elongations in the cumyl cation (compared to cunene) confirming the stabilization of the carbenium ion center through conjugation, and H3d -C bond-length reduction (compared to a-methylstyrene) due to the additional stabilization of the carbenium ion center caused by hyperconjugation (cf. Figure 2.19).

The [l,2]-alkyl migration A —> B of Figure 14.7 converts a cation with a well-stabilized tertiary carbenium ion center into a cation with a less stable secondary carbenium ion center. This is possible only because of the driving force that is associated with the reduction of ring strain a cyclobutyl derivative A is converted into a cyclopentyl derivative B. 

Guaiazulene reacts with thiophene 2-carbaldehyde in methanol in the presence of hexafluorophosphoric acid to give the stabilized carbenium ion 183 in 98% yield <2005T10349>. Reduction of this with zinc powder gives a mixture of stereoisomeric dimers (Equation 83). 

Oxidation is the microscopic reverse of reduction, and electron transfer in equation (la) has its counterpart in reduction, i.e. equation (lb). Accordingly, for every organic oxidation there is a conjugate process involving reduction, as simply illustrated by the electron-transfer equilibria among carbenium ions, free radicals and carbanions (equation 2). - ... 

Direct Alkyl Chloride Reductions. It was previously pointed out that at 40° in HF-TaF5 methyl- and ethyl-chloride do not form the expected alkylation product with alkanes, expected that is, assuming traditional carbenium ion chemistry, but rather underwent an independent reaction. The second part of this paper will describe in more depth what reactions do take place when alkyl chlorides react in the presence or absence of the lower alkanes under our reaction conditions of 40°C. 

From these considerations we can see an outline of the kinetics and mechanism of catalyst decay. While the catalyst remains in the presence of the reactant-product stream, on each active site the processes which dominate are the "fiuitful" processes of the attached carbenium ions, involving protolysis, P-cracking, disproportionation, and the reversible adsorption-desorption of product olefins. These events, in combination, constitute the dKiin mechanism of cracking 4) and yieldthe major products of the "cracking" reactioa None of these processes results in an irreversible reduction of catalyst activity, although the various carbenium ions present will undergo various mainline reactions at different rates. 

A similar investigation from Szwarc s laboratory showed that 1,1-di-p-methoxy-phenylethylene can be transformed into its carbenium ion by reaction with an excess of antimony pentachloride. However, in this study it was postulated that the ionisation process involved the reduction of antimony throu one of the following alternative mechanisms ... 

Althou the evidence presented in support of such reaction is not entirely convincing, in its favour is the existence of a lower oxidation state of antimony which can be readily reached by smooth reduction. In the case of other Lewis acids a lower oxidation state is either unavailable or less easily obtained and the likelyhood of hydride abstraction seems therefore more remote, at least from saturated hydrocarbons. Kennedy s scheme implies however the removal of the allylic hydride ion from an olefin. Although plausible in certain cases (but never proved), this mechanism is obviously impossible with styrene and 1,1 -diphenylethylene With 3-phenylindene it would yield an aryl-substituted allylic carbenium ion which would not be expected to be in equilibrium with its precursor yet, this equilibrium was observed With 2,3-dimethylindene in the same conditions initiation did not take place yet Kennedy s mechanism shouldhave operated without impediments. Finally, with 1,1-diphenylpropene hydride abstraction would have produced an allylic ion incapable of giving back the precursor by reacting with methanol yet Bywater and Worsfold showed that this reversible reaction takes place. 

For PtCu/NaY, however, most of the coke is presumably not in intimate contact with metal clusters, its oxidation therefore takes place at a higher temperature. Since splll -over of active oxygen from the oxidized metal over finite distance can be excluded, the oxidation of this coke will not be catalyzed by Pt or Cu. In the reduced form of the catalysts the Ft ensembles at the surface of the encaged bimetal particles are diluted with Cu and additional protons of hi Brensted acidity were formed in the reduction of each Cu ion to Cu°. The combination of both phenomena, small Pt ensembles and hl concentration of protons of strong Brensted acidity, results in an Increase of the RE/RO activity ratio as observed and reported(S). It is therefore reasonable to attribute the TPO peak at higher temperature to the coke deposited on the acid sites of the zeolite via carbenium ion formation and polymerization. The results are thus quite similar to those observed and... 

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