Isomerism tautomerism

The large difference between the nitrogen shieldings in nitroso groups and those molecules that are isomeric, tautomeric or related in any way to nitroso structures facilitates their spectral distinction111. 

Compounds which exhibit photochromism are classified according to the mode of their photochromic reaction, because changes in colour can arise as a result of heterolytic cleavage, homolytic cleavage, isomerization, tautomerization and oxidation. 

Snccino-succinic ester, like phloroglucin, exhibits dynamic isomerism (tautomerism), behaving under some conditions as a lceto-body, and under other conditions as a hydroxylic, or phenolic body having this structure ... 

EXPERIMENT 20A Heats of Formation Isomerism, Tautomerism, and Regioselectivity 179... 

In Scheme 26.24, examples of an RCM/isomerization/tautomerization sequence by Yoshida et al. are shown [30]. After RCM of 4-methylene-l,7-octadien-3-ones 81, synthesis of phenols 83 was accomplished by Rh-catalyzed isomerization of the carbon-carbon double bond of the RCM products 82 from exo to endo, followed by spontaneous tautomerization. The same group also showed that the Mizoroki-Heck reaction could be used as an alternative to the isomerization described above (Scheme 26.25) [30]. In the presence of a catalytic amount of Pd(OAc)2, RCM product 82... 

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although confonnational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thennally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, k = A. The quantity /cg T//i from transition state theory provides... 

The aldehyde or ketone is called the keto form and the keto enol equilibration referred to as keto-enol isomerism or keto-enol tautomerism Tautomers are constitu tional isomers that equilibrate by migration of an atom or group and their equilibration IS called tautomerism The mechanism of keto-enol isomerism involves the sequence of proton transfers shown m Figure 9 6... 

Photochromism Based on Tautomerism. Several substituted anils of saHcylaldehydes are photochromic but only in the crystalline state. The photochromic mechanism involves a proton transfer and geometric isomerization (21). An example of a photochromic anil is /V-sa1icylidene-2-ch1oToani1ine [3172-42-7] C H qCINO. 

Since the electrophilic reagent attacks the multiply-bonded nitrogen atom, as shown for (68) and (69), the orientation of the reaction product is related to the tautomeric structure of the starting material. However, any conclusion regarding tautomeric equilibria from chemical reactivity can be misleading since a minor component can react preferentially and then be continually replenished by isomerization of the major component. 

From a general point of view, the tautomeric studies can be divided into 12 areas (Figure 20) depending on the migrating entity (proton or other groups, alkyl, acyl, metals. ..), the physical state of the study (solid, solution or gas phase) and the thermodynamic (equilibrium constants) or the kinetic (isomerization rates) approach. 

A well-known example of non-prototropic tautomerism is that of azolides (acylotropy). The acyl group migrates between the different heteroatoms and the most stable isomer (annular or functional) is obtained after equilibration. In indazoles both isomers are formed, but 2-acyl derivatives readily isomerize to the 1-substituted isomer. The first order kinetics of this isomerization have been studied by NMR spectroscopy (74TL4421). The same publication described an experiment (Scheme 8) that demonstrated the intermolecular character of the process, which has been called a dissociation-recombination process. 

Tautomerism has been discussed in Section 4.04.1.5.2. It concerns prototropic tautomerism and the decreasing order of stability is (hydrazone) >A (azo)> A (enehydrazine). The isomerization A -> A occurs via a A -pyrazoline (65BSF769). Pyrazolidones and amino-A -pyrazolines exist as such. The only example of non-prototropic tautomerism deals with the isomerization (403) —> (404) (74CJC3474). This intramolecular process is another example (Section 4.04.1.5) of the thermodynamic analogy between prototropy and metallotropy. 

As would be anticipated, amino groups in the homocyclic ring of 1,2-benzisoxazoles behave as normal aromatic amines, forming mono- and bis-acyl derivatives, etc. (67AHC(8)277,p. 296). In th e isomeric 2,1-benzisoxazoles the 3-amino compound exists as such and not in the tautomeric 3-imino form (65CB1562). Amino groups in 3-phenyl substituents behave as normal aromatic amines (67AHC(8)277,p. 331). 

Annular tautomerism does not occur in isothiazoles or benzisothiazoles. Substituent tautomers can sometimes be distinguished by chemical methods, but it is important that reaction mechanisms and the relative rates of interconversion of tautomeric starting materials or isomeric reaction products are carefully investigated. Physical methods only will be considered in this section, and references to original publications can be found in a comprehensive review (76AHC(S1)1). 

N- Aminoaziridines have been converted to alkenes by reaction with a variety of oxidizing agents (70JA1784). Usually, the deamination reaction is stereospecific. The oxidation of l-amino-2,3-diphenylaziridines with manganese dioxide, however, was not stereospecific. The trans compound gives entirely frans-stilbene, whereas the cfs-aziridine forms a mixture of 85% trans- and 15% c -aikene. cw-Stilbene is not isomerized to trans under the reaction conditions, and the results are explained in terms of an azamine intermediate which can isomerize through a tautomeric equilibrium. 

Oxazol-5(2H)-one, 2-benzylidene-4-methyl-tautomerism, 6, 186 Oxazol-5(2ff)-one, 2-methylene-isomerization, 6, 226 Oxazol-5(2H)-one, 2-trifluoromethyl-acylation, 6, 201 Oxazol-5(4ff)-one, 4-allyl-thermal rearrangements, 6, 199 Oxazol-5(4H)-one, 4(arylmethylene)-Friedel-Crafts reactions, 6, 205 geometrical isomerism, 6, 185 Oxazol-5(4ff)-one, 4-benzylidene-2-phenyl-configuration, 6, 185 photorearrangement, 6, 201 Oxazol-5(4ff)-one, 4-benzyl-2-methyl-Friedel-Crafts reactions, 6, 205 Oxazol-5(4ff)-one, 4-methylene-in amino acid synthesis, 6, 203 Oxazol-5(4ff) -one. 2-trifluoromethyl-hydrolysis, 6, 206 Oxazolones... 

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