Carbenium ions initiation mechanisms

Stereoisomeric alcohols (93) and (94) yielded identical ring-expansion products [e.g. (97)] on formation of carbocations.168 This is evidence of a stepwise reaction in sterol biosynthesis, whereby a tertiary cation [e.g. the model (95)] rearranges to a secondary cation (96)-an anti-Markovnikov rearrangement . The synthetic aspects of biomimetic cyclizations of isoprenoid polyenes were reviewed.169 Included was a detailed discussion of carbenium ion-initiated cyclizations, with a discussion of the different mechanisms that have been proposed. A novel biomimetic carbocation polyene cyclization of a daurichromenic ester was reported an unusual 2 + 2-carbocation cyclization occurred as a side reaction.170... 

The reaction occurring during alkylation can be explained by carbenium ion chain mechanism initiated by the protonation of the olefin. The main steps of the mechanism are [7] ... 

The positive chain end is assumed to be a carbenium ion. This mechanism is not applicable to polymerizations initiated with strong alkylating agents such as triethyl-oxonium tetrafluoroborate because with these initiators no zwitter ions can be produced. Since no difference was observed between the products resulting from BF3 or from oxonium initiators (6), it is unlikely that tetramer would be formed by two different mechanisms in these two cases and consequently the proposed mechanism is dubious. 

Sommer and coworkers have made important observations with respect to the activation of alkanes over sulfated zirconia, a new type of solid superacid. Whereas isotope exchange of small alkanes occurs with the involvement of the corresponding pentacoordinate ions, a classical carbenium ion-type mechanism was found to be operative for larger homologs (propane, isobutane). The exception is the isomerization of n-butane over sulfated zirconia promoted by Pt and alumina, where the initiation step for isomerization was suggested to be the protolysis of the C-H bond. ... 

As shown by the data in Fig. 31, the chain transfer constant of this initiator, Q = 1.0. In this context it is of interest to remember that the effect of initiator concentration on the molecular weight of HSi-PaMeSt was negligible, probably because of unfavorable thermodynamics (Sect. III.B.3.b.iv.). In contrast, with isobutylene chain transfer from the propagating carbenium ion to initiator is thermodynamically favorable (see Sect. IH.B.4.b.i.). Thus it is not surprising to find a large Q. The chain transfer mechanism has been illustrated in Scheme 5. 

The reactions proceed via carbenium ions in a chain mechanism, initiated by the reaction between an olefin and an acid to C-C -C, which then reacts with iso-butane to give C-C C)-C. This carbenium ion is the central species in propagation steps to alkylated products such as 2,2-dimethylpentane and related products (Fig. 9.14). 

The hydrocarbon catalytic cracking is also a chain reaction. It involves adsorbed carbonium and carbenium ions as active intermediates. Three elementary steps can describe the mechanism initiation, propagation and termination [6]. The catalytic cracking under supercritical conditions is relatively unknown. Nevertheless, Dardas et al. [7] studied the n-heptane cracking with a commercial acid catalyst. They observed a diminution of the catalyst deactivation (by coking) compared to the one obtained under sub-critical conditions. This result is explained by the extraction of the coke precursors by the supercritical hydrocarbon. 

My last kinetic work was aimed at determining the kp+ of a range of monomers by what I believed to be a reliable method. For kinetic and electrochemical reasons I chose nitrobenzene as the solvent, and I chose carbenium and carboxonium salts as initiators so as to achieve a clean and fast initiation. The rate-constants were adequately reproducible, but it turned out that they were not the kp+. The project was flawed because I had been unaware of the reversible cationation of the solvent by the carbenium ions. A careful analysis of the kinetic, analytical and thermochemical results gave a new insight into the reaction mechanisms in nitrobenzene, but the main objective had eluded me. 

The Ritter reaction [6] proceeds by the electrooxidation of alkyl iodides (56) in an MeCN-(Pt) system to form Ai-alkyl acetamides (58) (Scheme 21). Attack of carbenium ion intermediate - from dissociation of the initially formed alkyl cation radical - to acetonitrile would give the iminium cation (57). However, a different mechanism is proposed, whereby the alkyl iodide reacts with the electrogenerated iodo cation [I]" " [73]. 

Reactions involving esters and secondary or tertiary alcohols proceed by a different mechanism as demonstrated by the experiments using lsO-enriched alcohols. This mechanism entails essentially that the ester act as the nucleophile on a nascent carbenium ion produced from the protonated alcohol (Scheme 1). The initial association is described as a proton-bound cluster. 

The fact that only trans-1,2- and cis- 1,4-glycols are obtained implies that they cannot actually be formed by the simplified mechanism in Scheme 9. The carbenium ions 99-101 should give a mixture of cis and trans glycols. However, the reaction can be neither entirely concerted, as shown for a 1,5-hydride shift in Equation 6.46, nor involve initial formation of a carbonium ion, as shown in Equation 6.47 The kHlkD isotope effects are too small for C—H bond breaking... 

Two alternative mechanisms have been suggested, (i) The reactions are initiated by hydride abstraction from the alkane by the Lewis acid to form a carbenium ion, and not by protonation of the C—C bond of n-butane. (ii) n-Butane is protonated by the Brpnsted acid to form a carbonium ion intermediate and either the hydrogen formed is used up to reduce SbF5 or it loosely remains bound to the ion during the isomerization process.111... 

A comparison of the reactivity of SbF5-treated metal oxides with that of HS03F-, SbCl5-, and HS03F-SbF5 (magic acid)-treated catalysts showed that the former was by far the best catalyst for reaction of alkanes (31, 32). Tracer studies of conversion of alkanes catalyzed by the superacids were performed it was suggested that the reactions proceeded by carbenium ion mechanisms in which the reactions were initiated by abstraction of H from the reactants (33). 

The polymerization of trioxane in solution has been studied by Okamura (26) and his co-workers and by Kern and Jaacks (56, 58, 63). The initiators were borontrifluo-ride or its complexes and anhydrous perchloric acid. During the polymerization the eight-membered ring, tetroxane, is formed rapidly but this compound takes part in the polyoxymethylene formation. This results in an equilibrium concentration of tetroxane when the rate of formation becomes equal to the rate of consumption (27). Minor amounts of the ten-membered ring, pentoxane, are also formed (28). The authors conclude that tetroxane and pentoxane are formed by a back-biting mechanism. It is assumed that the active species in the reaction is a carbenium ion. Although this ion... 

Under very mild conditions (0-20°C, 200 Torr ethylene pressure), ethylene was shown to be selectively dimerized to n-butenes over RhY (140). As shown in Fig. 14, 1-butene was formed initially but further isomerized to an equilibrium composition of -butenes with increasing reaction time. In a comparative experiment using HY as a typical solid-acid catalyst, no ethylene conversion was measurable up to 200°C, and at higher temperatures unselective polymerization and cracking reactions occurred. This provided good evidence that the selective dimerization over RhY did not proceed via a carbenium ion mechanism. 

It was suggested73 that the most probable mechanism of this reaction is an initial aldol condensation of the starting ketone leading to the a,/ -unsaturated ketone 100 or to the /Miydroxyketone 101 which serve as precursors to the tertiary carbenium ions 102, which reacts in turn with nitriles by an acid-catalyzed Ritter reaction to give 103 (equation 36). This suggestion is confirmed by the results of a cross-reaction experiment of benzaldehyde and diethyl malonate with acetonitrile to give 14 (equation 37). 

Several classes of compounds initiate cationic polymerizations of alkenes, including protonic acids, Lewis acids (usually in combination with a cation or proton source), stable carbenium ions, oxidizing reagents, and other strong electrophiles. This section attempts to explain the mechanism of initiation with quantitative information when available physical means of initiation (electric current, y-rays, field ionization and emission, nuclear chemical initiation) will not be discussed. 

Acid adds slowly to styrene to form covalent esters, which have been detected directly by, 9F NMR [101]. Carbenium ions capable of propagation are then generated by ionization of the covalent esters with excess acid. However, the concentration of carbenium ions is too low to detect directly by spectroscopy, and propagation was initially proposed to occur by a nonionic mechanism. 

Even very small amounts of anhydrous perchloric and triflic acids (<10-3 M) polymerize styrene rapidly and quantitatively [123,126-130], Carbenium ions were initially not detected in these systems, and they were therefore proposed to proceed by a pseudocationic mechanism in which covalent esters react directly with styrene in a concerted muticenter rearrangement [128], However, short-lived carbenium ions have since been detected directly by stopped-flow UV [17-19,131]. The mechanism of the propagation step in these systems is discussed in more detail in Section lV.D.2.a. 

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