Azomethyne ylide

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). 

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. 

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). 

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.

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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