Isomerization and Skeletal Rearrangements

Zeolites have also been described as efficient catalysts for acylation,11 for the preparation of acetals,12 and proved to be useful for acetal hydrolysis13 or intramolecular lactonization of hydroxyalkanoic acids,14 to name a few examples of their application. A number of isomerizations and skeletal rearrangements promoted by these porous materials have also been reported. From these, we can underline two important industrial processes such as the isomerization of xylenes,2 and the Beckmann rearrangement of cyclohexanone oxime to e-caprolactam,15 which is an intermediate for polyamide manufacture. Other applications include the conversion of n-butane to isobutane,16 Fries rearrangement of phenyl esters,17 or the rearrangement of epoxides to carbonyl compounds.18... 

Table II also lists several isomerizations and skeletal rearrangements (examples 4-7) which are related to butadiene-ethylene dimerization. Protonation of phosphorus-containing nickel(O) complexes is sufficient to achieve skeletal rearrangement of 1,4-dienes in a few seconds at room temperature, probably via cyclopropane intermediates (example 6, Table II). For small ring rearrangements see Bishop (69).

In a related series where non-concertedness was also apparent, the reaction of phenyl (l-phenylethyl)triazene with an excess of hydrogen chloride in ether yielded phenyl(1-phenylethyl) amine with 89% retention of configuration, as well as optically active ring-alkylated products and active 1-phenylethyl chloride No reasonable concerted process for forming these compounds comes to mind. Lastly, the isomerizations and skeletal rearrangements characteristic of the nitrosoamide de-... 

In a recently published book [1280] on mass spectrometry, the fragmentation of organic molecules under electron impact, electron capture, and other ionization methods is described not in the traditional way - by classes - but rather by isomerization and fragmentation types with simple bond cleavage, hydrogen- and skeletal rearrangements with systematic thermodynamic approach and separately for positive and negative ions. Our review on mass spectrometry of nitroazoles has been reported in 1998 [1281],... 

Zeolites do not only catalyze isomerizations of pure hydrocarbons. Also for molecules bearing a polar functional group, double bond and skeletal rearrangements can be performed without conversion of the functional group. Suitable zeolites should be rather apolar with a low concentration of acid sites, e.g., HZSM5. The interaction of polar functional groups with the pore walls of these rather apolar zeolites are weak [57]and hence polarization and... 

Sodium on alumina. This high-surface sodium catalyst, prepared by adding sodium to dry alumina with stirring at 150° under nitrogen, is effective for isomerization of butenes and pentene-1. In contrast to acid-catalyzed isomerization, no skeletal rearrangement occurs. The results indicate some stereoselectivity. Thus butene-1 is isomerized initially to about equal amounts of cis- and trani-butene-2 eventually the thermodynamic equilibrium mixture rich in the trans-isomti is obtained. The catalyst was used to effect almost quantitative conversion of methylenecyclobutane into 1-methylcyclobutene. ... 

Pericydic Processes involving Non-concerted Steps.—Synthesis of triquinacenes have been discussed above. The hydrocarbon (388) [=(359)] is remarkably stable to heat. At 600 °C it gives azulene in low conversion with loss of 2H. At 700 °C the azu-lene secondarily isomerizes to naphthalene but t,2-dihydronaphthalene is also formed from triquinacene. At 750 °C some indene is formed with loss of CH2. The formation of azulene is unexpected. It is suggested that initial loss of 2H takes the molecule out of the set of (CH) o isomers and skeletal rearrangement via (389) to azulene is a possibility. At the higher temperatures isomerization via (390)—(393) would provide a path to the other products, each step being known or reasonable. The rates of the thermal conversions (394) (395), (395) - (396X and (394) - (396) at 200 °C are... 

Subsequent to the discovery of skeletal rearrangement reactions on plati-num/charcoal catalysts, the reality of platinum-only catalysis for reactions of this sort was reinforced with the observation of the isomerization of C4 and C5 aliphatic hydrocarbons over thick continuous evaporated platinum films (68,108, 24). As we have seen from the discussion of film structure in previous sections, films of this sort offer negligible access of gas to the substrate beneath. Furthermore, these reactions were often carried out under conditions where no glass, other than that covered by platinum film, was heated to reaction temperature that is, there was essentially no surface other than platinum available at reaction temperature. Studies have also been carried out (109, 110) using platinum/silica catalysts in which the silica is catalytically inert, and the reaction is undoubted confined to the platinum surface. 

A mixture of EtsSiH/TFA in dichloromethane reduces 3-methyl-5-a-cholest-2-ene to give the pure equatorial methyl isomeric product, 3/3-methyl-5o -cholestane, in 66% yield (Eq. 79).126 On the other hand, attempts to reduce cholest-5-ene using the same technique yield neither 5a-cholestane nor 5/3-cholestane, but instead an isomeric mixture of rearranged olefins. This result is presumably because of the inability of hydride attack to compete with carbocation skeletal isomerization and elimination.126... 

Hydrosilation of alkylcyclohexenes illustrates the ability of active catalytic complexes to isomerize olefins and the tendency chlorosilanes have to form primary alkyl adducts even if this requires a skeletal rearrange-... 

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

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. 

A recent study has indicated that the skeletal rearrangement step in the B12-catalysed isomerization of methylmalonyl-CoA to succinyl-CoA occurs not by a radical pathway but by an anionic or organocobalt pathway. A computational study of the isomerization of allyl alcohol into homoallyl alcohol by lithium amide has pointed to a process proceeding via a transition state in which the proton is half transferred between carbon and nitrogen in a hetero-dimer. l,l-Dilithio-2,2-diphenylethene... 

Steric factors are often responsible for skeletal isomerization in ion-radical states. The simple example in Scheme 6.31 illustrates the effect of steric congestion on activation energy of this kind of isomerization and depicts the transition of 2,2,3,3-tetramethylmethylenecyclopropane into 1,1,2,2-tetramethyltrimethylenemethane cation-radical. The rearrangement is brought about by one-electron oxidation of the substrate and represents an entirely barrierless process. Interestingly, methylenecy-clopropane (bearing no methyl groups) is protected from such a spontaneous collapse by a barrier of 7.4 k J mol l (Bally et al. 2005). 

The last synthesis to evolve which is due to Ito and his coworkers is interesting in that it relies on a stereospecific skeletal rearrangement of a bicyclo[2.2.2]octane system which in turn was prepared by Diels-Alder methodology (Scheme XLVIII) Heating of a toluene solution of cyclopentene 1,2-dicarboxylic anhydride and 4-methylcyclohexa-l,4-dienyl methyl ether in the presence of a catalytic quantity of p-toluenesulfonic acid afforded 589. Demethylation was followed by reduction and cyclization to sulfide 590. Desulfurization set the stage for peracid oxidation and arrival at 591. Chromatography of this intermediate on alumina induced isomerization to keto alcohol 592. Jones oxidation afforded diketone 593 which had earlier been transformed into gymnomitrol. 

Except for naphthene dehydrogenation, which only requires a Pt site for catalysis, all the other major reactions require an interaction between sites. Ring and paraffin isomerization require the platinum function for dehydrogenation to olefin, the acid function for carbon skeletal rearrangement, and the metal function again for hydrogenation of the olefin. 

Very little skeletal rearrangement occurs via pyrolysis, a fact inherent in the failure of free radicals to readily isomerize by hydrogen atom or alkyl group migration. As a result, little branched alkanes are produced. Aromatization through the dehydrogenation of cyclohexanes and condensation to form polynuclear aromatics can take place. Additionally, olefin polymerization also can occur as a secondary process. 

Side-Chain Isomerization. Arylalkanes undergo acid-catalyzed isomerization in the side chain in a way similar to the skeletal rearrangement of alkanes.70-72 There are, however, notable differences. Propylbenzene, for instance, yields only a small amount (a few percentages) of isopropylbenzene 73 Similarly, sec-butyl- and iso-butylbenzene are interconverted at 100°C with wet A1C13, but only a negligible amount of tert-butylbenzene is formed.74 In the transformation of labeled propylbenzene the recovered starting material was shown to have equal amounts of labeling in the a and p positions of the side chain, but none in the y position 73... 

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