Structural Isomerization Reactions

Structural Isomerization Reactions.—In earlier work Kropp and his co-workers19 suggested that the unsaturated and cyclopropane products formed by the irradiation of olefins in solvents of low nucleophilicity were produced by initial rearrangement to carbenes. In a later study Kropp and Fields20 have demonstrated that carbene intermediates are indeed formed in the irradiation of olefins 

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The Arrhenius equations for the various reactions are shown in Table 1. The geometrical isomerizations of 1,2,3-trimethylcyclopropane and l-ethyl-2-methylcyclopropane have also been studied. In both cases geometrical isomerization is faster than the structural isomerization reactions to yield olefins. The Arrhenius equations obtained were ... 

One further complication concerning the origin of selectivity changes during structural isomerization reactions has been indicated by the work of Kramer and Zuegg.352 They observed that the percentage of n-hexane produced from methyl cyclopentane increases with the amount of interface between Pt and the support. They propose that isomerization occurs by two parallel routes, one on the metal the other at the metal/oxide interface. Such effects, if confirmed, should be more important for small metal particles, and may have influenced the selectivity observed in the other work quoted above. 

Figure 13. A typical time evolution of the three gyration radii, a, a2, and CI3 (a > ct2 > 03) (a), and the three hyper-angles, 3( ) (b). Structural isomerization reaction has taken place two times in this example by crossing the planar structures as indicated by the condition 3 = 0. Total internal energy is E = —4.1e.

Structural isomerization reactions, such as the interconversion of normal and isopropyl radicals... 

Trialkyl esters of phosphonic acid exist ia two structurally isomeric forms. The trialkylphosphites, P(OR)2, are isomers of the more stable phosphonates, 0=PR(0R)2, and the former may be rearranged to resemble the latter with catalytic quantities of alkylating agent. The dialkyl alkylphosphonates are used as flame retardants, plasticizers, and iatermediates. The MichaeUs-Arbusov reaction may be used for a variety of compound types, including mono- and diphosphites having aryl as weU as alkyl substituents (22). Triaryl phosphites do not readily undergo the MichaeUs-Arbusov reaction, although there are a few special cases. 

The following reactions share only the feature that they are structural isomerizations. They incorporate a variety of reagents and procedures from acid catalysts to photolysis. 

The possible occurrence of such major rearrangement of a compound s carbon skeleton, during the course of apparently unequivocal reactions, is clearly of the utmost significance in interpreting the results of experiments aimed at structure elucidation particularly when the actual product is isomeric with the expected one. Some rearrangements of this type are highly complex, e.g. in the field of natural products such as terpenes, and have often made the unambiguous elucidation of reaction pathways extremely difficult. The structure of reaction products should never be assumed but always confirmed as a routine measure lH and 13C n.m.r. spectroscopy have proved of enormous value in this respect. 

In a very thorough investigation of the reactions of hexaric acid lactones, and particularly dilactones, with sodium methoxide, F. Smith138,139 invariably obtained unsaturated products, at that time thought to be formed by isomerization and enolization. Hence, for alkyl D-glucofuranosidurono-6,3-lactones, also of bicyclic structure, similar reactions were believed24,25 to account for their unusual behavior in alkaline solutions. 

This means that the isopropyl group stabilises the secondary ion sufficiently (compared to but-l-ene) for hydride transfer reactions to be suppressed, at least at low temperature. At about -120 °C a high polymer of structure (IV) is formed. This is a true phantom polymer, since there exists no corresponding monomer. Evidently, at the very low temperature the propagation reaction which would lead to structure (III) becomes much slower than the isomerization reaction ... 

Another isomerization reaction of arene oxides is equilibrium with oxe-pins [5], Here, the fused six-membered carbocycle and three-membered oxirane merge to form a seven-membered heterocycle, as shown in Fig. 10.2. An extensive computational and experimental study involving 75 epoxides of monocyclic, bicyclic, and polycyclic aromatic hydrocarbons has revealed much information on the structural factors that influence the reaction rate and position of equilibrium [11], Thus, some compounds were stable as oxepins (e.g., naphthalene 2,3-oxide), while others exhibited a balanced equilibrium... 

The rate constants were determined at a series of pressures in the fall-off region, and the fall-off curve was very similar to that obtained for the structural isomerization to propylene. The similarity of the two sets of data suggests that both reactions may proceed through similar reaction paths. One obvious possibility is that once again the trimethylene biradical is formed, which can undergo internal rotation followed by recyclization. An alternative transition state has been suggested which involves, as an activated complex, a much expanded cyclopropane ring in which hindered internal rotation occurs (see also Smith, 1958). 

The cis-trans isomerization of cyclopropanes is not restricted to the deuterium-substituted molecules, cis- and traws-l,2-Dimethylcyclo-propane have been shown to imdergo reversible geometrical isomerization as well as slower structural isomerization. All the processes are homogeneous and kinetically first order, and almost certainly unimolecular. The reaction scheme is shown below. 

The structural isomerization of a number of alkyl substituted cyclopropanes has been investigated. In all cases the reaction is probably unimolecular. In general several olefins are formed. The results obtained are shown in Table 2. It is to be expected (especially if a diradical is an... 

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