Hydride transfer, from alkanes

Estimates of the kinetics of methyl loss from energy-selected CztHg" species have been made by calculation.23 The hydride transfer from alkanes to carbenium ions in the gas phase is calculated to involve a species with a symmetric potential well, which is different from the situation in superacid or zeolite media.24 A correlation between the charge on a carbon and the in-plane tensor component of its 13 C chemical shift has been observed for a number of simple cationic and anionic species.25 High-level calculations... 

In a series of investigations of the cracking of alkanes and alkenes on Y zeolites (74,75), the effect of coke formation on the conversion was examined. The coke that formed was found to exhibit considerable hydride transfer activity. For some time, this activity can compensate for the deactivating effect of the coke. On the basis of dimerization and cracking experiments with labeled 1-butene on zeolite Y (76), it is known that substantial amounts of alkanes are formed, which are saturated by hydride transfer from surface polymers. In both liquid and solid acid catalysts, hydride transfer from isoalkanes larger than... 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

Scheme 5.10), and hydride transfer from the alkane to the incipient carbenium ion (step 3, Scheme 5.10). 

The heterolytic activation of H2 in the above system is particularly interesting in that it may be applicable to reactions in which ionic hydrogenation of hindered substrates from a metal catalyst and H2 is desired. In 1989, Bullock reported the first examples of ionic hydrogenation wherein a mixture of an organometallic hydride such as CpMoH(CO)3 and a strong acid like HO3SCF3 reduces sterically hindered olefins to alkanes via protonation to carbocations followed by hydride transfer from the metal hydride [Eq. (10)] (49). 

Transfer from Alkanes to Methyloxocarbonium Ion. Reel. Trav Chim. Pays-Bas 1973, 92, 689-697. (b) Brouwer, D. M. Kiffen, A. A. Hydride Transfer Reactions III. Rates of Hydride Transfer from Isobutane to Hydroxy-carbonium Ions. Reel Tray Chim. Pays-Bas 1973, 92, 809-813. 

Deuterium is introduced following Markovnikov s rule, that is, corresponding to the most stable carbenium ion intermediate. As desorption only occurs via hydride transfer from a fresh alkane, no D is foimd in the branching position, (or C2 in propane). The mechanistic scheme suggested is shown in Scheme 8. 

When the contact time between iso-butane and deuterated zeolites (or D2SO4) was prolonged to obtain extensive deuteration of the alkane (>90%), deuterium also appeared gradually in the tertiary position.The appearance of deuterium in this position cannot be explained by a simple hydride transfer from iso-butane to the tertio-butyl cation. Alkenyl and polyenyl ions that were previously identified in sulfuric acid as a product... 

Step 3. Hydride transfer from the Alkane to the Incipient Carbenium ion ... 

The relative probabilities of Reactions 24, 25, and 26 were, respectively, 1.00, 0.25, and 0.12 at a hydrogen pressure of about 1 atmosphere (9). These numbers could be derived either by analyzing the stable alkanes formed in the unimolecular decompositions (Reactions 24-26) or from the products of the hydride transfer reactions between C5Hi2 and the alkyl ions. Elimination of H2 from protonated pentane may also occur, but it is difficult (although not impossible) to establish this reaction through neutral product analysis. 

Table I gives the compositions of alkylates produced with various acidic catalysts. The product distribution is similar for a variety of acidic catalysts, both solid and liquid, and over a wide range of process conditions. Typically, alkylate is a mixture of methyl-branched alkanes with a high content of isooctanes. Almost all the compounds have tertiary carbon atoms only very few have quaternary carbon atoms or are non-branched. Alkylate contains not only the primary products, trimethylpentanes, but also dimethylhexanes, sometimes methylheptanes, and a considerable amount of isopentane, isohexanes, isoheptanes and hydrocarbons with nine or more carbon atoms. The complexity of the product illustrates that no simple and straightforward single-step mechanism is operative rather, the reaction involves a set of parallel and consecutive reaction steps, with the importance of the individual steps differing markedly from one catalyst to another. To arrive at this complex product distribution from two simple molecules such as isobutane and butene, reaction steps such as isomerization, oligomerization, (3-scission, and hydride transfer have to be involved.

Theoretically, even the direct alkylation of carbenium ions with isobutane is feasible. The reaction of isobutane with a r-butyl cation would lead to 2,2,3,3-tetramethylbutane as the primary product. With liquid superacids under controlled conditions, this has been observed (52), but under typical alkylation conditions 2,2,3,3-TMB is not produced. Kazansky et al. (26,27) proposed the direct alkylation of isopentane with propene in a two-step alkylation process. In this process, the alkene first forms the ester, which in the second step reacts with the isoalkane. Isopentane was found to add directly to the isopropyl ester via intermediate formation of (non-classical) carbonium ions. In this way, the carbenium ions are freed as the corresponding alkanes without hydride transfer (see Section II.D). This conclusion was inferred from the virtual absence of propane in the product mixture. Whether this reaction path is of significance in conventional alkylation processes is unclear at present. HF produces substantial amounts of propane in isobutane/propene alkylation. The lack of 2,2,4-TMP in the product, which is formed in almost all alkylates regardless of the feed (55), implies that the mechanism in the two-step alkylation process is different from that of conventional alkylation. 

Intermolecular hydride transfer (Reaction (6)), typically from isobutane to an alkyl-carbenium ion, transforms the ions into the corresponding alkanes and regenerates the t-butyl cation to continue the chain sequence in both liquid acids and zeolites. 

The crucial step in self-alkylation is decomposition of the butoxy group into a free Brpnsted acid site and isobutylene (proton transfer from the Fbutyl cation to the zeolite). Isobutylene will react with another t-butyl cation to form an isooctyl cation. At the same time, a feed alkene repeats the initiation step to form a secondary alkyl cation, which after accepting a hydride gives the Fbutyl cation and an -alkane. The overall reaction with a linear alkene CnH2n as the feed is summarized in reaction (10) ... 

A hydrogen-transfer reaction involving the olefin and the addition product of the benzylic carbanion and olefin may accompany the side-chain alkylation reaction. The result is that alkanes and arylalkenes are produced 19). This hydride-tiansfer reaction may take place by elimination of a hydride ion from the carbanion adduct followed by addition of the hydride ion to the olefin [Reaction (6)]. The amount of this side reaction probably depends largely on the severity of reaction conditions used. 

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