Bond-shift isomerization, alkanes

The proposed mechanism of the bond shift isomerization of neopentane is shown in Scheme I Cl-3). There are now good models for each step in the proposed sequence, but no simple transition metal complex can accomplish all steps since there cannot be sufficient co-ordination sites. The first steps involve a,y-dinstallation of the alkane, for which there are good precedents in both platinum and iridium chemistry (4, 5, 6). The... 

Several mechanisms were proposed to interpret bond shift isomerization, each associated with some unique feature of the reacting alkane or the metal. Palladium, for example, is unreactive in the isomerization of neopentane, whereas neopentane readily undergoes isomerization on platinum and iridium. Kinetic studies also revealed that the activation energy for chain branching and the reverse process is higher than that of methyl shift and isomerization of neopentane. 

The mechanism of bond-shift isomerization of alkanes is still a subject of controversy. Several suggested mechanisms involved an intermediacy of multiply bonded species [Anderson-Avery (63), Muller-Gault (64), and Garin-Gault (65) mechanisms] (Fig. 2). More recent data (51, 52, 66-68) seem to support the 1,2 bond-shift isomerization involving monoadsorbed alkyl species (69). Figure 2 shows these mechanistic ideas. 

More recently, Finlayson et al. (66) performed deuterium tracer studies using 2,2,4,4-tetramethylpentane as a model reactant, and proved that the bond-shift isomerization to 2,2,5-trimethylhexane may go via a monoad-sorbed intermediate on Pd (as well as on Pt, Ir, and Rh). The authors show that in the mechanisms of nondestructive reactions such as cyclization and bond-shift isomerization of alkanes, only one metal atom is required as the catalytic center. 

The cyclic mechanism was demonstrated by comparing the initial product distributions in the hydrogenolysis of methylcyclopentane and in isomerization of methylpentanes and -hexane. For instance, the ratios 3-methyl-pentane/n-hexane, extrapolated to zero conversion, are the same in hydrogenolysis of methylcyclopentane and in isomerization of 2-methylpentane. Since cyclic type isomerization involves first carbon-carbon bond formation and then carbon-carbon bond rupture, one does not expect hydrocracking of alkanes to occur by this mechanism. In contrast, as suggested early on (55), if bond shift isomerization involves first carbon-carbon bond rupture and then carbon-carbon bond recombination, a common intermediate should exist, leading to both the isomerization and the hydrocracking products. 

The formation of aromatics from alkanes with less than six carbon atoms in their main chain (eg, methylpentanes) is also possible but proceeds much less readily than direct Ce-dehydrocyclization (13,85). This mechanism should involve a sort of dehydrogenative bond shift isomerization. The optimum hydrogen pressure (Fig. 5a) for aromatization of heptane isomers with less than six carbon atoms... 

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. 

Metallacyclobutanes have been proposed as intermediates in a number of catalytic reactions, and model studies with isolated transition metallacyclobutanes have played a large part in demonstrating the plausibility of the proposed mechanisms. Since the mechanisms of heterogeneously catalysed reactions are especially difficult to determine by direct study, model studies are particularly valuable. This article describes results which may be relevant to the mechanisms of isomerization of alkanes over metallic platinum by the bond shift process and of the oligomerization or polymerization of alkenes. 

Low-molecular-weight hydrocarbons (C4 and C5 alkanes) usually undergo isomerization through a simple bond shift. The transformation of [l-l3C]-butane, for instance, yields isobutane via skeletal isomerization and the isotopomer [2-13C]-butane 155... 

In most cases, the two types of mechanisms, the bond shift and cyclic mechanisms, are not exclusive but parallel pathways. With increasing molecular weight, the contribution of the cyclic mechanism increases and may become dominant. The pure selective mechanism on iridium is a unique exception. Hydrogenolysis, however, is the characteristic transformation on this metal. The nature of possible surface intermediates in metal-catalyzed alkane reactions, the role of electronic and geometric effects in their formation, and the relation of isomerization and hydrogenolysis have been reviewed.163... 

Recently, Rooney and co-workers (23,58,59) have questioned the view that triadsorption by loss of 3 hydrogen atoms from the alkane is the minimum requirement for bond-shift reactions. They studied the isomerization of a series of caged hydrocarbons in excess hydrogen on palladium and platinum catalysts. The compounds were chosen in order to render difficult or totally exclude a mechanism involving aory-triadsorbed species. Thus, l,7,7-trimethyl[2,2,l]-heptane interconverts with its endo- and exo-2,3,3-trimethyl isomers, bicyclo-[3,2,2] octane changes to bicyclo[3,3,l] nonane, and protoadamantane to... 

Very recent work (60b) has confirmed that Ir films do not isomerize neopentane most of the transition metals as well as palladium (60c) rearrange isobutane to k-butane but are also inactive for the former conversion. This clearly indicates that isomerization of neopentane on Pt is mechanistically rather special and, in view of the known propensity of Pt to promote ay exchange with deuterium of paraffins (5,49), refocuses attention on the ay species diadsorbed on one metal atom as the precursor for bond shift in simple alkanes. The following mechanism for neopentane isomerization on Pt is feasible, where the shifting... 

Much work has also been carried out on the reactions in the presence of H2 of branched alkanes, and on the mechanism of their skeletal isomerization [6, 7], The use of 13C-labeled molecules permits alternative reaction pathways to be distinguished. Thus, for example, most of the 3-methylpentane formed from 2-methylpentane has followed the bond-shift route, but most of the n-hexane has resulted from the cyclic mechanism. Labeled molecules also allow mechanisms of aromatization of C7 and Cg alkanes to be followed... 

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... 

Investigations which concern the mechanisms of skeletal rearrangements of saturated hydrocarbons induced by heterogeneous transition metal catalysis are of great interest for industrial applications, e.g. for petroleum reforming processes The developments in this field were reviewed recently by Hejtmanek, and by Maire and Garin , who focussed on the probable reaction mechanisms which include bond-shift and cyclic mechanisms for the skeletal isomerization of acyclic alkanes. Scheme 1 summarizes the... 

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