Carbocationic Process

Because of Us high polarity and low nucleophilicity, a trifluoroacetic acid medium is usually used for the investigation of such carbocationic processes as solvolysis, protonation of alkenes, skeletal rearrangements, and hydride shifts [22-24] It also has been used for several synthetically useful reachons, such as electrophilic aromatic substitution [25], reductions [26, 27], and oxidations [28] Trifluoroacetic acid is a good medium for the nitration of aromatic compounds Nitration of benzene or toluene with sodium nitrate in trifluoroacetic acid is almost quantitative after 4 h at room temperature [25] Under these conditions, toluene gives the usual mixture of mononitrotoluenes in an o m p ratio of 61 6 2 6 35 8 A trifluoroacetic acid medium can be used for the reduction of acids, ketones, and alcohols with sodium borohydnde [26] or triethylsilane [27] Diary Iketones are smoothly reduced by sodium borohydnde in trifluoroacetic acid to diarylmethanes (equation 13)... 

The rearrangements of the methyl groups occur via a carbocationic process induced by protonation from the zeolite. 

Catalytic reforming92-94 of naphthas occurs by way of carbocationic processes that permit skeletal rearrangement of alkanes and cycloalkanes, a conversion not possible in thermal reforming, which takes place via free radicals. Furthermore, dehydrocyclization of alkanes to aromatic hydrocarbons, the most important transformation in catalytic reforming, also involves carbocations and does not occur thermally. In addition to octane enhancement, catalytic reforming is an important source of aromatics (see BTX processing in Section 2.5.2) and hydrogen. It can also yield isobutane to be used in alkylation. 

In the dehydrocyclization of alkanes it is clear that ring closure can take place both in a metal-catalyzed reaction and as a carbocationic process. The interpretation of the reforming process proposed by Heinemann and coworkers,123 therefore, is not a complete picture of the chemistry taking place. The scheme they presented (Fig. 2.1) attributes cyclization activity solely to acidic sites. The ample evidence available since requires that metal-catalyzed C5 and C6 ring-closure possibilities be included in a comprehensive interpretation. Additionally, the metal component plays and important role in carbocationic reactions in that it generates carbocations through the formation of alkenes. 

Different catalysts bring about different types of isomerization of hydrocarbons. Acids are the best known and most important catalysts bringing about isomerization through a carbocationic process. Brpnsted and Lewis acids, acidic solids, and superacids are used in different applications. Base-catalyzed isomerizations of hydrocarbons are less frequent, with mainly alkenes undergoing such transformations. Acetylenes and allenes are also interconverted in base-catalyzed reactions. Metals with dehydrogenating-hydrogenating activity usually supported on oxides are also used to bring about isomerizations. Zeolites with shape-selective characteristics... 

Another substrate class, for which the outcomes of a radical and a carbocationic process are opposite, are indoles (Fig. 85) [418], Indeed, when oxaziridines 315a or 315c were treated with indoles 314c in the presence of 2 or 10 mol% of C11CI2/ TBAC oxazolidinoindolines 316c were obtained as the exclusive products in 53-90% yield. The reaction is applicable to 2-, 3-, and 2,3-disubstituted indoles. Chiral indole derivatives acylated with (S)-proline units at nitrogen underwent asymmetric diastereoselective aminohydroxylation reactions with 86-91% de. Tricyclic hemiaminals derived from tryptamine derivatives could be transformed to pyrrolidinoindolines, which are core structures of a number of alkaloids. 

The above results suggest that in the carbocationic process the second 6-endo cyclization is relatively fast but the third 5-exo cyclization is quite slow. Thus, formation of the bicyclic rearranged compound 41 is allowed whereas that of monocyclic products is avoided. In the radical process, however, the second 6-endo cyclization seems to be relatively slow and the third 5-exo cyclization fairly fast, thus allowing the formation of monocyclic achilleol A (9) and avoiding bicyclic products. In other words, there are subtle... 

This representation explains why a carboxylic acid such as DCA, which forms hydrogen bonds with the anion, is a good solvent for carbocationic processes, while 2NP, which has a similarly low basicity but cannot act as a hydrogen bond donor to anions, does not provide a detectable amount of protonated hexamethylbenzene (II). The actual (low) basicity of 2NP can be assessed from the effect of the II on the position of the protonation equilibrium (equation 2 or 4) in TFA solution where anion stabilization is provided by the solvent. 

The solvents that interact with anions and as a consequence favor carbocationic processes are called anion-stabilizing solvents. The anion-stabilizing ability makes, for example, methanol a better solvent for carbocationic solvolysis than acetonitrile or dimethylformamide, although the latter solvents are more polar (50a, 50b). 

Although most attention has focussed on a cationic mechanism in the oxidative cyclization of squalene [20]. Breslow was concerned with the possibility that nature utilizes a free-radical pathway [21]. and studied the addition of benzoyloxy radical to trans, trani-famesyl acetate [22]. The benzoyloxy radicals generated by CuCl-catalyzed thermal decomposition and copper(II) benzoate was added to provide a termination mechanism. Excluding any intervention of a carbocationic process, Breslow obtained a tran -decalin compound (20 30% yield) bearing an exomethylene moiety. As pointed out by Breslow, despite a limited biochemical interest , this work evidenced a new synthetic reaction of considerable potential . An application shortly followed with the first example of a triple cyclization by Julia [23]. Triene isomers 40 were treated by benzoylperoxide in benzene and alforded after saponification alcohol 41 in 12% yield as a single diastereomer (relative stereochemistry confirmed by an X-ray analysis) with a similar tra 5-decalin system (A and B rings. Scheme 14). 

Carbocationic processes for polymerization of al-kenes CH2=CRR (R. R = H. alkyl, aryl, alkoxy. amido) have been known for many years and. because of major commercial applications, have been the subject of intense scrutiny in both academic and industrial laboratories. The active sites are carboca-tions which may be generated in a number of ways including protonation of the alkene (eq 1) in some... 

Strong evidence in support of the carbocationic mechanism in homopolymerizations of isobutene by A, however, is gained by analysis of the product formed on copolymerization of isobutene and isoprene. This commercially important copolymeH is manufactured via a carbocationic process Initiated by AICI3, and contains isoprene ( 1%) incorporated only in a... 

Again, the reaction patterns exhibited the signature of carbocationic processes and again the H NMR spectrum of a poly(ethyl vinyl ether) sample exhibited an aldehyde resonance at d 9.8. 

However, one should also be aware of possibilities that elements of carbocationic processes may directly influence Ziegler-Natta catalytic chemistry. For instance, polymers containing vinylidene end groups of the type CH2=CMe-polymer are often formed during Ziegler-Natta processes and might recoordinate to the metal cation in an fashion, as in I and J for monocyclopentadienyl and metallocene systems, respectively. intermediates of these types were to... 

Recent studies have focused on the use of superacids in isomerization (63,64). An important advantage of these catalysts is that they may be used at lower temperature (at room temperature or below) due to their greatly increased ability to initiate carbocationic processes. These reaction conditions not only allow avoiding side reactions but also favor thermodynamic equilibria with higher proportion of branched isomers. As a result, high branching and, consequently, higher octane numbers are attained. 

The main products are poly- and oligoisobutylenes, poly(butenes), hydrocarbon resins, and poly(vinyl ethers)—these will be discussed in more detail under Industrial Carbocationic Processes. Estimated worldwide production capacities are listed in Table 1. 

Computational, H, and isotope effects on the reaction of 2-methyl-2-butene with formaldehyde catalyzed by Et2AlCl support a stepwise mechanism with either the first or second step being rate-limiting. A carbocationic process for the Lewis acid-catalyzed ene reaction may occur when the optimum geometry for a cyclic transition state is not accessible or the cationic intermediate is initially generated by the use of a strong Lewis acid. 

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