Isomerism intramolecular rearrangements

Photolysis. Photolysis of a chemical can proceed either by direct absorption of light (direct photolysis) or by reaction with another chemical species that has been produced or excited by light (indirect photolysis). In either case photochemical transformations such as bond cleavage, isomerization, intramolecular rearrangement, and various inter-molecular reactions can result. Photolysis can take place wherever sufficient light energy exists, including the atmosphere (in the gas phase and in aerosols and fog/cloud droplets), surface waters (in the dissolved phase or at the particle-water interface), and in the terrestrial environment (on plant and soil/mineral surfaces). 

We define repeating unit isomerization as a process subsequent to polymerization, in which an intramolecular rearrangement of the repeating unit leads to a thermodynamically preferred structure ... 

Dynamic intramolecular rearrangements are observed for a variety of diene-metal complexes at, or near, ambient temperature. This stereochemical non-rigidity may be detected by variable temperature NMR experiments40 in which the signals observed for a static structure coalesce into time averaged signals for the fluxional process. For purposes of this section, processes with activation energies > ca 25 kcal mol 1 or which are irreversible will be considered to be isomerization phenomena and will be discussed in Section IV. 

Isomerizations are important unimolecular reactions that result in the intramolecular rearrangement of atoms, and their rate parameters are of the same order of magnitude as other unimolecular reactions. Consequently, they can have significant impact on product distributions in high-temperature processes. A large number of different types of isomerization reactions seem to be possible, in which stable as well as radical species serve as reactants (Benson, 1976). Unfortunately, with the exception of cis-trans isomerizations, accurate kinetic information is scarce for many of these reactions. This is, in part, caused by experimental difficulties associated with the detection of isomers and with the presence of parallel reactions. However, with computational quantum mechanics theoretical estimations of barrier heights in isomerizations are now possible. 

Internal rearrangements, isomerizations, and eliminations Another common type of cellular reaction is an intramolecular rearrangement, in which redistribution of... 

Isomerization reactions of organo-fluorine compounds are included in Houben-Weyl, Vol. 5/3, pp 346-353, where five types of isomerization reactions are covered (1) intramolecular cycli-zation of dienes, (2) opening of cyclobutene rings, (3) opening of cyclobutane rings, (4) isomerization of polyfluorocyclohexadienes, and (5) intramolecular rearrangement of chlorofluoro-and bromofluorocarbons. Much of the work reported is still of fundamental significance and is covered here, where appropriate. Several more later reviews also deal with various aspects of this area.1... 

The formation of 5-hexenal (reaction 18) is believed to be an intramolecular rearrangement since the addition of oxygen does not cause its suppression. At least in a methyl substituted cyclohexanone the analogous process has been shown to occur by the transfer of a hydrogen atom from the beta position to the carbonyl group before the fission of the six-membered carbocyclic ring (29) as only one of the two possible isomeric heptenals is formed. 

In connection with the above discussion, formation of 3,3-disubsti-tuted 2 (3 H)-oxepinones (73) in the dye-sensitized photooxygenation of 6,6-disubstituted fulvenes is of special interest. 57>58> The reaction may be pictured in terms of an allene oxide intermediate which, as shown in Scheme 11, isomerizes to a cyclopropanone, followed by intramolecular rearrangement. 

A further example (the oldest recorded) of isomerization, which differs from the preceding two examples, is credited to Fischer," who attempted the hydrolysis of the chlorine atom of 6-amino-2-chloro-7-methylpurine (54, R = H) with alkali but obtained 7-methylguanine (55, R = H) instead of 7-methylisoguanine. Some insight into the intramolecular rearrangement may be gained from the fact that the... 

Class 5. Isomerases interconvert isomeric structures by intramolecular rearrangements. They include racemases, epimerases, cis- and trans-isomerases, intramolecular transferases (mutases), and intermolecular lyases. 

Intramolecular rearrangement of germylenes, stannylenes and plumbylenes into a doubly bonded isomer can be another cause of their intrinsic instability. For example, in the matrix IR study of l-germacyclopent-3-en-l,l-ylidene, direct experimental evidence was obtained for a photochemical germylene-germene isomerization . ... 

In contrast, phenylimidoylcarbodiimides PhN=C(R)N=C=NR, isomerize in solution to fom aminoquinazolines. Other imidoylcarbodiimides also undergo intramolecular rearrangements. For example, RR CHN=C(CCl3)N=C=NAr 1 undergoes a rearrangement to form dihydro-1,3,5-triazines 2 ... 

Winter, R.E.K., and Lindauer, R.J. 1967. The photochemical isomerization of l-acetylbicyclo[4.1.0] heptane an intramolecular rearrangement. Tetrahedron Lett 2345-2348. 

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