Bond-shift isomerization

An extremely wide variety of catalysts, Lewis acids, Brmnsted acids, metal oxides, molecular sieves, dispersed sodium and potassium, and light, are effective (Table 5). Generally, acidic catalysts are required for skeletal isomerization and reaction is accompanied by polymerization, cracking, and hydrogen transfer, typical of carbenium ion iatermediates. Double-bond shift is accompHshed with high selectivity by the basic and metallic catalysts. 

In general side reactions are rare. In a few cases an isomerization by shift of the double bond favored by formation of a conjugated system can be observed ... 

Slow double-bond shifts and little skeletal isomerization H-transfer is minor and nonselective for tertiary olefins only small amounts of aromatics formed from aliphatics at 932°F (500°C)... 

Rapid double-bond shifts, extensive skeletal isomerization, H-transfer is major and selective for tertiary olefins large amounts of aromatics formed from aliphatics at 932°F (50t) O... 

No matter which of the electrophilic methods of double-bond shifting is employed, the thermodynamically most stable alkene is usually formed in the largest amount in most cases, though a few anomalies are known. However, there is another, indirect, method of double-bond isomerization, by means of which migration in the other direction can often be carried out. This involves conversion of the alkene to a borane (15-16), rearrangement of the borane (18-11), oxidation and hydrolysis of the newly formed borane to the alcohol (12-28), and dehydration of the alcohol (17-1) ... 

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,... 

With the evidence available at the moment, it is the author s opinion that Pines and Goetschel s free-radical mechanism is well founded, and thus this mechanism is quite distinct from the bond shift reaction occurring over metals. In order to preserve this distinction, we shall retain the term vinyl insertion for the type of isomerization exemplified in (24) and (25). 

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. 

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... 

Olefins react secondarily for isomerization and hydrogenation (on cobalt sites that are not active for chain growth lower scheme in Figure 9.15). There is a first reversible H-addition (at the alpha- or beta-C-atom of the double bond) to form an alkyl species, and a slow irreversible second H-addition to form the paraffin (lower scheme in Figure 9.15). Thus, double-bond shift and double-bond hydrogenation are interrelated by a common intermediate to produce olefins with internal double bonds or paraffins from the primary FT alpha-olefins. Experimental results1018 are presented in Figures 9.16 and 9.17. 

In summary, we may say that the NBO/NRT description of partial proton transfer in the equilibrium H-bonded complex(es) is fully consistent with the observed behavior along the entire proton-transfer coordinate, including the transition state. At the transition state the importance of partial co valency and bond shifts can hardly be doubted. Yet the isomeric H-bonded complexes may approach the TS limit quite closely (within 0.2 kcal mol-1 in the present example) or even merge to form a single barrierless reaction profile (as in FHF- or H502+). Hence, the adiabatic continuity that connects isomeric H-bond complexes to the proton-transfer transition state suggests once more the essential futility of attempting to describe such deeply chemical events in terms of classical electrostatics. 

Not all acids are equally active isomerization catalysts. With zeolite H-BEA, nearly identical selectivities are achieved when the feed is 1-butene instead of 2-butene (48). In general, even mildly acidic zeolites are excellent catalysts for double-bond shift isomerization. Sulfuric acid also produces nearly identical... 

The numerous transformations of cyclooctatetraene 189 and its derivatives include three types of structural changes, viz. ring inversion, bond shift and valence isomerizations (for reviews, see References 83-85). One of the major transformations is the interconversion of the cyclooctatetraene and bicyclo[4.2.0]octa-2,4,7-triene. However, the rearrangement of cyclooctatetraene into the semibullvalene system is little known. For example, the thermolysis of l,2,3,4-tetra(trifluoromethyl)cyclooctatetraene 221 in pentane solution at 170-180 °C for 6 days gave three isomers which were separated by preparative GLC. They were identified as l,2,7,8-tetrakis(trifluoromethyl)bicyclo[4.2.0]octa-2,4,7-triene 222 and tetrakis(trifluoromethyl)semibullvalenes 223 and 224 (equation 71)86. It was shown that a thermal equilibrium exists between the precursor 221 and its bond-shift isomer 225 which undergoes a rapid cyclization to form the triene 222. The cyclooctatetraenes 221 and 225 are in equilibrium with diene 223, followed by irreversible rearrangement to the most stable isomer 224 (equation 72)86. 

JJor chemists interested in modem theories of chemical bonding, the most useful data obtainable by the Mossbauer technique are the magnitude and sign of the electric quadrupole field gradient tensor and the magnitude of the shift, 8, (which we prefer to call the chemical isomeric. Cl, shift), of the center of the Mossbauer spectrum relative to some standard absorber. Although a considerable amount of chemical and structural information is potentially available from quadrupole data on iron compounds, relatively little use has been made of such data in the literature, and we will not discuss this parameter here. We will instead restrict ourselves to two main points review of the explanations put forth to explain Cl shift data in iron compounds, and a survey of some of the correlations and generalizations which have been found. 

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

Isomer formation from dimethylbutanes is much lower, and hardly any acceleration from increasing hydrogen pressure is observed (78). Small amounts of 2,2-dimethylbutane produced over platinum black from 3-meth-ylpentane indicate a slow bond shift isomerization at any hydrogen pressures (25). 

Benzene formation from all isohexanes had a similar energy of activation value. With platinum this was nearly twice as high as that of n-hexane aromatization (62) with palladium black, however, nearly the same values were found for -hexane and isohexanes (97a). This indicates a common rate-determining step for aromatization with skeletal rearrangement. This is not the formation and/or transformation of the C5 ring. We attribute benzene formation to bond shift type isomerization preceding aromatization. It requires one step for methylpentanes and two steps for dimethyl-butanes this is why the latter react with a lower rate, but with the same energy of activation. 

---