Bond-shift mechanism

Anderson and Avery 24, 128) have proposed a bond shift mechanism based upon a 1-3 adsorbed species which, when formed from neopentane, is... 

Anderson and Avery s bond shift mechanism has the consequence of predicting that a quaternary carbon atom cannot be generated in the hydrocarbon product. In fact, Anderson and Avery (24) showed that in the isomerization of isopentane over platinum films, only a very small amount (<1%) of neopentane was produced (although the equilibrium constant for isopentane <= neopentane is 0.16 at 278°C). Furthermore,... 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

Two main pathways of metal-catalyzed skeletal rearrangement have been distinguished bond shift mechanism and C5 cyclic isomerization (7, 8). 

The rearrangement and formation of 2,2-dimethylbutane—with a quaternary carbon atom—is only possible via the above mechanism. Over platinum, 2,3-dimethylbutane (75) gave more benzene over palladium, 2,2-dimethylbutane (97a) gave more benzene. This is the opposite selectivity, as reported by Muller and Gault, for ring expansion of 1,1,3-trimethyl-cyclopentane (57). This may be more evidence that at least two different types of bond shift mechanism can occur. 

Ring Enlargement. The ring enlargement of five-membered cycles to six-membered ones is possible over pure metallic sites. This reaction is, however, generally slower than any of the cyclization processes. We believe that it proceeds by some kind of radical type bond shift mechanism (see Section III,A) rather than via ionic intermediates owing to some kind of acidic properties of platinum assumed by Lester (108). 

Because of historic reasons, the mechanism employing the 3Cay complexes is often called a bond shift mechanism and the mechanism with 5C complexes— a cyclic mechanism. However, both mechanisms involve cyclic intermediates at certain stages and for both mechanisms bonds are shifted. Therefore, notation specifying the number of carbon atoms involved seems to be preferable. 

Figure 20.11 Mechanistic scheme for n-heptane isomerization according to a bond-shift mechanism with a metallocyclobutane intermediate.

The results presented in this paper show that the molybdenum oxycarbide catalysts are significantly different from the platinum catalysts and this is ascribed to differences in mechanisms operating over these catalysts. The bond shift mechanism involving a metallacyclobutane intermediate accounts for the results obtained over molybdenum oxycarbide and a scheme for this mechanism is shown for the isomerization of n-heptane in Figure 20.11. 

The 13C-labelling experiments allowed one to determine the relative contributions of cyclic and of bond-shift mechanisms in the isomerization and cracking reactions of 2-methylpentane and hydrogenolysis of methylcyclopentane over Pt TiC>2 catalysts prepared by different methods566. 

Figure 17 Bond shift mechanism (after Gault and co-workers347 348)...

This experimental result is, in fact, readily explained by the bond-shift mechanism described in the following paragraphs. A vinyl shift (Diagram 5) is predicted to be much easier... 

Reference will be made again to bond-shift mechanisms in the following section in considering ring enlargement. 

To illustrate this method of approach, results of three of the experiments by Corolleur et al. are shown in Tables XIII and XIV. In the examples selected here, a 13C-labeled 3-methylpentane and a labeled 2-methylpentane are reacted, in turn, and distribution of 13C is shown for the methylpentane products in Table XIII and for the n-hexane products in Table XIV. Distributions expected for a pure cyclic mechanism (C) and for a methyl shift (T) are indicated. Detailed discussion by the authors of abnormal products (i.e., those not predicted by the single-stage purely carbocyclic mechanism) led to the conclusion that these are formed on a second, less numerous, type of surface site by the action of which a succession of several rearrangements, according to a cyclic or a bond-shift mechanism, takes place. In Table XIV it can be seen that assumption of a simple skeletal rearrangement of the type... 

It is found that Mode E behaves similarly to the zeolite free Pt-Re/Al203 Both catalysts have a relatively high proportion of isomer products which could be formed over the metal surface via a bond-shift mechanism [8]. Isomers are formed by doublebond isomerization and skeletal isomerization reactions at both the acid sites of the alumina support and the metal sites. The later provides a dehydrogenation-hydrogenation function and the acid sites an isomeiization function for the olefins to dehydrogenate from paraffins over the metal function, since it is known that olefin isomerization proceeds much quicker than the respective paraffin isomerization [8]. On the other hand, branched paraffins are less easily cracked than linear ones [10]. Therefore, once isomers are formed over conventional reforming catalysts, they are likely to be the final products. Evidently, the isomerization of paraffin requires the metal function in the bimetallic catalyst, and so does the paraffin aromatization. This can also explain the obseiwed decrease in the isomers and aromatics production with time-on-Hne since it is well- known that coke preferentially deposits on a metal surface first [14]. 

The bond shift mechanism (Scheme 6), which corresponds to a simple carbon-carbon bond displacement, accounting for the isomerization of short-chain paraffins 34). 

In order to examine the possible participation of adsorbed cyclopropanes in the bond shift mechanism, the relative contributions of Paths A and B in chain lengthening were determined for a series of 2-methylalkanes (40, 43, 54). The contribution of Path B was found to decrease regularly from isopentane to 2-methylpentane and 2-methylhexane. The decreasing contribution of Path B from isopentane to 2-methylpentane is readily explained by the decreasing number of methyl substituents in the cyclopropane intermediate, but the difference between 2-methylpentane and 2-methylhexane cannot be accounted for by the cyclopropane mechanism (43). 

Further evidence for the existence of two bond shift mechanisms and also for the metallocyclobutane mechanism are provided by the isomerization of C labeled C7 hydrocarbons 2,3-dimethylpentane (3S), 2-methyl-hexane (45), and 3-methylhexane (47) on Pt/AljOj catalysts. 

All these results are readily interpreted by assuming the existence of two bond shift mechanisms. The first one, which accounts for methyl shift, may be ascribed to the metallocyclobutane mechanism responsible for the group III reactions of n-pentane and isopentane. The second one, which accounts for chain lengthening (and chain shortening) is the same as the mechanism of higher activation energy (group II) responsible for the interconversion between n-pentane and isopentane. The first is very sensitive to alkyl substitution, while the latter seems relatively insensitive to structural effects. 

Mechanism C, in our opinion, is analogous to the metallocyclobutane bond shift mechanism (Scheme 25). Since the latter accounts for not only bond shift isomerization, but also C-C bond rupture, it might be operative for the hydrogenolysis of cyclic hydrocarbons. 

In Mechanism C, which is identical to one of the bond shift mechanisms already discussed (Scheme 25), the key intermediate is believed to be a... 

On platinum, the a, -dicarbene mechanism which accounts for the hydrogenolysis of cycloalkanes (Scheme 34) is no longer predominant in the hydrocracking of acyclic alkanes. It has already been emphasized that the internal fission of isopentane and n-pentane is related to the metallocyclobutane bond shift mechanism of isomerization (see Section III, Scheme 29), and that in more complex molecules, the favored rupture of the C-C bonds in a p position to a tertiary carbon atom is best explained by the rupture of an a,a,y-triadsorbed species (see Section III, Scheme 30). The latter scheme can account for the mechanism of hydrocracking of methylpentanes on platinum. Finally, the easy rupture of quaternary-quaternary C-C bonds in... 

On the other hand, according to the classification of the active sites into two groups, A for the nonselective cyclic mechanism and B for selective cyclic and bond shift mechanisms, it may be erroneous to identify A sites with edge and B sites with face atoms. Indeed, the constancy of the bond shift mechanism throughout the 0-0.5 range of dispersion is understandable on this... 

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