Molecular rearrangements isomerization

The formation of enamines from carbonyl compounds and secondary amines usually entails as only questionable structural feature, the possible isomeric position of double bonds in the product. Molecular rearrangements have not presented synthetic limitations. A notable exception is the generation of o-aminophenols on distillation of enamines derived from 2-acylfurans 620,621). 

In order to illustrate some redox-induced molecular rearrangements a little more complicated than simple isomerizations we will consider... 

A chemical reaction in which the principal product is isomeric with respect to the principal reactant. Isomerization can occur in the absence of molecular rearrangements (for example, between conformational isomers). 

Some carbonyl compounds are unreactive in the photocycloaddition reaction because their excited states undergo molecular rearrangement. Illustrative of this point is the isomerization process involved upon irradiation of 2-methylbenzophenone. This ketone is known to be stable to irradiation in isopropanol,45 and irradiation in the presence of isobutylene yields a complex mixture which contains little, if any,... 

W. M. Gelbart and S. A. Rice, Some Formal Results in a Theory of Molecular Rearrangements Photo-Isomerism, in J. Chem. Phys. (in press). 

R. U. Lemieux, Reamangements and isomerization in carbohydrate chemistry. Molecular Rearrangements, Part II (P. de Mayo, ed.). Wiley-Intersdence, New York, 1963, p. 723. 

Rearrangements and Isomerizations in Carbohydrate Chemistry, R. U. Lemieux, In Molecular Rearrangements, P. de Mayo (Ed.) Interscience Publishers (1964) p. 709. 

The Ni(IV) and Pd(IV) species appear to be Lewis acids, since crystalline adducts have been obtained with 7r-bases and Lewis bases such as pyrene, phenanthrene, A Af-dimethylaniline halide ions, and SCN- (18, 42). It has been briefly mentioned (42) that the substituted complexes [B9H9C2(CH3)2]2Ni", n = 0, —1, —2, exhibit a series of molecular rearrangements to form isomeric systems. 

Strange to state, the methylated uric acids, when reduced, do yield purons which (excepting tetramethylpuron) can be molecularly rearranged into isomeric isopurons, but the corresponding hydrated uric acids are not produced (Tafel2). 

Figure 7-1. Three-dimensional potential energy surfaces (a) Energy hypersurface for FSSF SSF2 isomerization (detail). Reproduced with permission [19] copyright (1977) American Chemical Society (b) Potential energy surface of the molecular rearrangement of Agl3, with the corresponding structures indicated on the sides [20], Copyright (2005) American Chemical Society.

Persistence is the net result of many interacting factors, but chemical properties of the pesticide can play a dominant role. Volatility, solubility, stability to ultraviolet irradiation, tendency to adsorb onto or dissolve into tissue surfaces, ease of hydrolysis, sensitivity to humidity, potentiality to polymerize with or without ultraviolet irradiation, possible isomerization or other molecular rearrangement are all important chemical properties which play a significant role in persistence. Weather conditions such as wind, rain, amount and intensity of light, air temperature, and humidity play important roles. Rainfall is usually con-... 

The redistribution of metal valence electrons created by the valence isomerization would support this molecular rearrangement. 

These examples and investigations on azobenzene moieties in polymers show that the photochromic behavior is mainly Gontrolled by the free volume distribution around the chromophore. Tb obtain LBK films in which azobenzene moieties can undergo photoisomerization, therefore, the free volume around the azobenzene chromophore must be controlled precisely to allow for the molecular rearrangement inherent in the reversible tran to cis photo-isomerization. This is possible by (1) mixing with other amphiphiles, (2) adjusting the architecture of the amphiphile, or (3) attaching the chromophore to a polymer either by coulomb interaction or covalently. 

The reaction of sodium diorganodithioarsinates with diphenylarsenic chloride provides a mixture of isomeric compounds, identified by IR and H NMR spectroscopy as Ph2AsSAs(S)R2, Ph2(S)AsSAsR2, and Ph2AsSSAsR2 (R = Me, Ph) as a result of another sulfotropic molecular rearrangement (Scheme 10).173... 

Vitamin B12 is known for its ability to catalyze molecular rearrangements. A variety of cobalt chelates are logical models for vitamin B12, and their stoichiometric and catalytic activities in a variety of reactions,403 particularly olefin isomerizations, were studied intensively.404-411 Noncatalytic isomerization reactions based upon the synthesis of alkylcobalt chelates as model intermediates were favored. A variety of catalytic oxidations of substrates such as hydroquinone, azo compounds, phosphines, and olefins were also investigated.412-415 Copolymerization of a-methylstyrene and other monomers with oxygen in the presence of CoTPP led to alternating polyperoxides.416 418 Cobaloximes were found to catalyze... 

Isomerization does not necessarily imply molecular rearrangement (e.g., in the case of the interconversion of conformational isomers). 

RU Lemieux. Rearrangements and isomerizations in carbohydrate chemistry. In P De Mayo, ed. Molecular Rearrangements, Part 2. New York Wiley Interscience, 1964, pp 709-769. 

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