Azomethynes

An azomethyne ylide is also invoked as an intermediate in the three-component reaction between the dihydro-isoquinoline 10, an alkylating reagent 11, and the dipolarophile 12, which, in a one-pot process, afforded the indolizidine derivatives 13 <2005S2039> (Scheme 2). 

Another common approach consists of the comparison between the experimental rate constants and theoretical values calculated by the procedure developed by Marcus (1956), Marcus and Sutin (1985) as well as Hush (1958). This classical procedure is used widely. Premsingh et al. (2004) gave the relevant references and described a detailed procedure to analyze the ion-radical reaction between anilines and chromium (V) complexes of azomethyne derivatives. Lepage et al. (2003) studied transformation of para-substituted thioanisoles to corresponding methylarylsulfoxides... 

A possibility of formation of 5,7-diaryl-4,7-dihydro-1,2,4-triazolo[l,5-a]pyr-imidines 182 in reactions of 3-amino-l,2.4-triazole 147 with synthetic precursors of unsaturated ketones—aldehydes 179 and ketones 180—or in condensation of acetophenones 180 and azomethynes 183 was established in [176, 177]. It was shown that in the first case the multicomponent procedure was not an independent method but resulted in the initial formation of unsaturated ketones 181 themselves and their further reaction with aminoazol (Scheme 3.53). 

Other reactions of tetrasubstituted azomethyne ylides prepared from tetracyanoethylene oxide are described, I, (5). 

For the azomethyne ylide 86, the presence of the electron-releasing methyl group on the Cl position polarizes the HOMOdipo e through the C3 carbon atom. As a result, the unsubstituted C3 carbon atom presents a larger ff value compared to that at the Cl site. Therefore, along the cycloaddition reaction, the more favourable reactive channel takes place through the C3-C4 bond formation by the nucleophilic attack of the C3 carbon atom of 86 to the more electrophilic C4 site of 14, according to the experiment results. 

Similar poor results in alkylations may be general with organometallic derivatives having the metal linked to the azomethyne carbon atom. 

Synthesis of azomethyne macrocycles by condensation of dicarbonyl compounds with diamines without the use of metal ions as template agents 07UK843. 

These ionic pathways typically occur at temperatures (150-300°C) that are below those of typical free-radical degradation reactions. However, if polymers belonging to the above classes are subjected to higher temperatures, radical reactions are likely to prevail. Furthermore, polymers containing less polar functional groups, like ethers and azomethynes, tend to undergo radical cleavage even when they decompose at relatively low temperatures. ... 

FaiUa, S. and Finocchiaro, P, Mass Spectral Determination of Azomethyne Macrocycle Distribution Formed in Polycondensation Reactions, /. Chem. Soc. Perkin Trans., 2, 701, 1992. 

The formation of azomethynes, from furJher reaction of aldehydes with amino-terminated Ny6 chains, is also supported by the appearance of specific peaks in the MALDl spectra (species 1, Figure 10.35, Table 10.5). ... 

This yellowing can be prevented by blocking the phenol group, for example, by esterification. The actual yellow color of most phenolic resins, however, derives not from quinone methynes, but from a side reaction of the phenolic resins on curing with Hexa. Dehydrogenation, especially of the chain ends, produces azomethyne groups in these cases... 

Taking advantage of the steady increasing techniques for CNT functionalization [7], several routes were explored to attach redox molecules onto SWCNTs. Ferrocene was also attached to MWCNTs by, amide coupling, 7c-stacking interactions [8], aryldiazonium reduction [8] or 1,3 dipolar cycloaddition of azomethyne ylides [9] in order to establish electrical communication between the enzyme and the electrode (Fig. 3.5). 

Of the Other nitrogen-containing stabilizers, F. Yu. Rachinskii etal., have proposed new compounds for stabilizing polyolefins and pol3rvinyl acetals [158, 277, 278] from the class of azomethynes, of the general formula... 

The indicated compounds are produced by condensing o- and p-aminophenols or o- and p-phenylenediamines with aromatic substituted and imsubstituted aldehydes. It is noted [278] that azomethynes, produced by condensation of unsubstituted aniline with benzaldehyde and p-dimethylaminobenzaldehyde, are not stabilizers of polyolefins. 

In addition to the classes of compounds considered, epoxy compounds [58] azomethyne derivatives of o- and p-aminophenols [59], aromatic hydrazones [60], organic zinc compounds [61], tin dibulyl mer-captide, salts of metals of groups II and IV of the Periodic System and organic acids containing from 8 to 24 carbon atoms in the molecule [62], alkaline salts of mercaptans, in particular, mercaptobenzimidazole [28], and a number of others have been recommended and tested as stabilizers of polymers. Most of these recommendations were taken from data on the stabilization of other classes of polymers and are not specific for polyolefins. 

The formation of the azomethyne ylide as an intermediate resulting from the retro-cycloaddition process was proved by carrying out trapping experiments by using Al-phenylmaleimide (NPM). Thus, compound 17 was refluxed in o-DCB in the presence of an excess of NPM(10 equiv). In addition to the obtained 60 (70%), the cycloadduct (18) resulting from the... 

Substituted boron analogues of p-proline were synthesized efficiently via 1,3-dipolar cydoadditions of azomethyne ylides, generated from N-alkyl amino acids and formaldehyde, to alkenyl boronates. Heterocydic cycloadducts were also good precursors of the corresponding pyrrolidin-3-ols by oxidation with triethylamine oxide (Scheme 9.46) [99]. 

Scheme9.46 1,3-Dipolar cycloadditions of azomethyne ylides to alkenylboronates.

Castell P, Sena A, Galia M, Giamberini M, Carfagna C (2003b) Anisotropic thermosets fi om liquid-crystalline azomethynic epoxy resins and primary aromatic diamines. J Polym Sci Polym Chem 41 1-12... 

Electronically excited phthalimides can act as good electron acceptors and carboxylic acids are documented to serve as electron donors in photoinduced SET reactions. It is likely then, that in the excited state, an electron transfer process between the phthalimide system and the carboxylic acid would occur, leading to charge-separated diradical 6. Proton transfer from the carboxylic acid function (iT" is an electrofugal group) would form a carboxy radical 7, which could undergo rapid decarboxylation to azomethyne ylide 8. This reactive species is transformed into the final decarboxylated product 3 by intramolecular proton transfer (Scheme 16.4). 

The formation of an azomethyne ylide during the photodecarboxylation of N-phthaloyl a-amino acid 1 makes easier the interpretation of the results obtained in the presence of methyl aerylate. If we examine the structure of the reaction product 2, we would easily reeognize the fragments corresponding to the acrylate (colored red), the A-phthaloyl a-amino acid 1 that has lost the carboxylic acid function (colored blue) and the new bond formed (in black) (Scheme 16.6). The... 

As we have commented previously (Scheme 16.4), azomethyne ylides 8 are short-lived intermediates that in dre absence of any suitable trapping agent regenerate the phthalimide function by intramolecular proton transfer leading to the decarboxylation product. However, when a dipolarophile like methyl acrylate is present in the reaction medium, they can be trapped to form the corresponding cycloadducts 2 (Scheme 16.7). 

At this point and with the help of a textbook the reader should discuss which are the FMO involved in the 1,3-dipolar cycloaddition between azomethyne ylides and electron poor alkenes and comment the regioselectivity expected for the reaction. 

Azomethyne ylides can be obtained by irradiation of A -phthaloyl a-amino acids. These intermediates can be trapped with electron poor alkenes to yield benzopyr-rolizidines in good yields. 

The formation of azomethyne ylides as intermediates in the photoreactions of N-(silylmethyl)phthalimides and 7V-phthaloyl derivatives of a-amino acids has been confirmed by laser flash photolysis and fluorescence spectroscopy. 

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