Pathways elimination/isomerization

A new catalytic elimination/isomerization pathway of stereodefined enol triflates capable of providing the corresponding functionalized, highly substituted, 1,3-dienes in synthetically useful yields has been discovered. Preliminary mechanistic studies support a distinct catalytic pathway that rationalizes the stark reactivity differences between E and Z enol triflates through stereoisomeric cationic vinyl palladium(II) complexes. 

Elimination degradation pathways are also possible. Decarboxylation, in which a carboxylic acid releases a molecule of CO2, occurs for p-aminosalicylic acid.24 Oxidation is very common as well, largely due to the presence of oxygen during manufacture and/or storage. Several examples can be found in Yoshioka and Stella.14 Isomerization and racemization reactions are other degradation pathways. Two compounds which undergo isomerization reactions are amphotericin B25 and tirilizad.26... 

For the hydride elimination in Heck reactions of substituted thiophenes, where the iyn-mechanism is structurally precluded (from the 2-position), three mechanistic pathways can be envisaged (i) isomerization followed by syn-/i-H elimination (ii) a-H elimination, and 1,2-H shift and (iii) anti-P-H elimination. According to DFT calculations, the base-assisted anti-P-H elimination (third pathway) is the most energetically... 

The electron impact mass spectrometric fragmentations of (E)-3- and ( )-4-styryl-pyridazines show that the intensity ratio of the M and (M -1)" ions, the general degree of fragmentation and the elimination pathways of nitrogen are the most characteristic features distinguishing between the two isomeric compounds (81JHC255). 

The key pathway branching competition in this reaction is the isomerization of the initial adduct (I) to either the resonantly stabilized four-member ring intermediate (III), or the three-member ring intermediate (II). Formation of the four-member ring has a lower energy barrier (saddle point 4), but is more entropically constrained, as all the internal rotors of the system are eliminated. 

SCHEME 10.2 Common pathways of QM formation in biological systems, (a) Stepwise two-electron oxidation by cytochrome P450 or a peroxidase, (b) Enzymatic oxidation of a catechol followed by spontaneous isomerization of the resulting n-quinone. (c) Enzymatic hydrolysis of a phosphate ester followed by base-catalyzed elimination of a leaving group from the benzylic position. 

Essentially different from the reactions described in (2) are radical eliminations in which the radical X is already-present in the ionized molecule Rt — X)+ but where the actual dissociation of the R, — X bond is preceded or accompanied by an isomerization of the charge carrying part R. This situation (3) may occur in cases, in which the direct cleavage 11- 12 is energetically less likely than the two-step (or generally multi-step) pathway 11-+13-+14. 

The reaction mechanism was considered to be oxidative cyclization, and pal-ladacyclopentene 32 was formed. Reductive elimination then occurs to give cyclobutene 33, whose bond isomerization occurs to give diene 28. The insertion of alkyne (DMAD) into the carbon palladium bond of 32 followed by reductive elimination occurs to give [2+2+2] cocyclization product 27. Although the results of the reactions of E- and Z-isomers of 29 with palladium catalyst 26a were accommodated by this pathway, Trost considered the possibility of migration of substituents. Therefore, 13C-labeled substrate 25 13C was used for this reaction. 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

The observed differences in the fragmentation pathways of the isomeric pairs were further enhanced under chemical ionization conditions. Thus, elimination reactions became even more pronounced in the o/t/ o-nitro-substiluted compounds than in the para isomers83. 

Fig. 2.2 Key features of the computational pathway for rhodium-catalyzed hydroboration, with energies in kcal/mol. Similar results were obtained for the related dioxaborolidine pathway. The isomeric intermediate C does not have access to a low energy H-migration pathway but rather eliminates B-H.

Phenylperoxy radical, originally assumed to be a factor in low-temperature combustion only, has actually been shown to play a substantial role in dictating the overall combustion trends of benzene. Just as the isomerizations and eliminations of the alkylperoxy radicals significantly affected their overall combustion pathways, rearrangements and other intramolecular pathways available to phenylperoxy radical similarly impact the overall progress of benzene combustion. This knowledge can be extrapolated to more complex aromatic species. 

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