Azomethines reduction

Aromatic nitro compounds are often strongly colored. They frequently produce characteristic, colored, quinoid derivatives on reaction with alkali or compounds with reactive methylene groups. Reduction to primary aryl amines followed by diazotization and coupling with phenols yields azo dyestuffs. Aryl amines can also react with aldehydes with formation of Schiff s bases to yield azomethines. 

Keywords Azomethine compoimds 1,2-Diamines Organometallic compounds Reduction Stereoselectivity... 

The 1,3-dipolar cycloaddition reactions to unsaturated carbon-carbon bonds have been known for quite some time and have become an important part of strategies for organic synthesis of many compounds (Smith and March, 2007). The 1,3-dipolar compounds that participate in this reaction include many of those that can be drawn having charged resonance hybrid structures, such as azides, diazoalkanes, nitriles, azomethine ylides, and aziridines, among others. The heterocyclic ring structures formed as the result of this reaction typically are triazoline, triazole, or pyrrolidine derivatives. In all cases, the product is a 5-membered heterocycle that contains components of both reactants and occurs with a reduction in the total bond unsaturation. In addition, this type of cycloaddition reaction can be done using carbon-carbon double bonds or triple bonds (alkynes). 

Azomethine ylides. Reduction of oxazolium salts (1) with phenylsilane and cesium fluoride provides an unstable 4-oxazoline (2), which can react as an azomethine ylide with a dipolarophile such as DM AD to give a dihydropyrrole (3). ... 

Metal complex pigments are mainly used in paints. The products are fast enough to be applied especially in industrial finishes. Some representatives, particularly azomethine copper complex pigments, are very weatherfast, which makes them suitable candidates for automotive finishes. High transparency in combination with good weatherfastness is an asset for use in metallic finishes. It is not uncommon for metal complexes to lose much of their brilliance in white reductions. Some are also recommended for use in architectural paints, especially for emulsion paints. 

The asymmetric reduction of C=N double bonds in prochiral oximes afforded a maximum of 18% ee [380, 384, 385]. Prochiral azomethines were reduced to the corresponding 1,2-diamines and secondary amines using 36 optically active supporting electrolytes. The dimers were optically inactive, while the monomers showed low optical inductions (<11% ee). The effect of electrolyte, substrate concentration, temperature, pH, and cathode potential on the induction was studied. It was proposed that the enantioselectivity... 

Attaching a Ceo cluster to an [Ru(bpy)3] + core has been achieved by 1,3-dipolar cycloaddition of azomethine ylides to the fullerene. The electrochemistry of the complex is complicated a one-electron reversible oxidation of the Ru center, five one-electron reversible reductions associated with the Ceo cage, and five more reversible reductions centered on the bpy ligands. The photophysical properties of the complex have been discussed. ... 

A series of publications by Vedejs et al. (17-19) outlined the use of oxazolines in the generation of azomethine ylides. Reduction of the oxazolium salts 71 (R = Ph or Me) with PhaSiH/CsF led to formation of the unstable 4-oxazolines (72) and their valence bond azomethine ylide tautomers (17), which could be trapped in situ with DM AD to afford the bicyclic adducts 73, where R and R = Ph or Me, in 75% yield via [2 + 2] cycloaddition (Scheme 3.18). 

Several syntheses of the hepatatoxic alkaloid (+)-retronecine have been reported although the most succinct has utilized a chiral azomethine ylide cycloaddition to construct the bicychc skeleton. The ylide processor 175, which was obtained in five efficient steps from commercially available tran -(l )-4-hydroxy-L-proline, underwent double desilyation in the presence of AgF (described in detail in Section 3.1.1) and in situ cycloaddition with methyl propiolate, to deliver a 3 1 mixture of cycloadducts in favor of the desired regioisomer. DiisobutyMuminum (DIBAL) reduction of 176 furnished enantiopure (-F)-retronecine (Scheme 3.50). 

As previously described, thermolysis of aziridines is one of the standard methods for the generation of azomethine ylides. A diastereomeric mixture of the aziridines 199 possessing an enantiomerically pure N-substituent underwent ylide formation at 280 °C and subsequent cycloaddition to vinylidine carbonate to form a mixture of four separable compounds (d-200, l-201, d-202, l-203) in a 3 3 1 1 ratio (55). Subsequent LiAlH4 reduction and hydrogenolytic N-benzyl cleavage led to all... 

This chapter deals mainly with the 1,3-dipolar cycloaddition reactions of three 1,3-dipoles azomethine ylides, nitrile oxides, and nitrones. These three have been relatively well investigated, and examples of external reagent-mediated stereocontrolled cycloadditions of other 1,3-dipoles are quite limited. Both nitrile oxides and nitrones are 1,3-dipoles whose cycloaddition reactions with alkene dipolarophiles produce 2-isoxazolines and isoxazolidines, their dihydro derivatives. These two heterocycles have long been used as intermediates in a variety of synthetic applications because their rich functionality. When subjected to reductive cleavage of the N—O bonds of these heterocycles, for example, important building blocks such as p-hydroxy ketones (aldols), a,p-unsaturated ketones, y-amino alcohols, and so on are produced (7-12). Stereocontrolled and/or enantiocontrolled cycloadditions of nitrones are the most widely developed (6,13). Examples of enantioselective Lewis acid catalyzed 1,3-dipolar cycloadditions are summarized by J0rgensen in Chapter 12 of this book, and will not be discussed further here. 

For intramolecular 1,3-dipolar cycloadditions, the application of nitrones and nitrile oxides is by far most common. However, in increasing frequency, cases intramolecular reactions of azomethine ylides (76,77,242-246) and azides (247-259) are being reported. The previously described intermolecular approach developed by Harwood and co-workers (76,77) has been extended to also include intramolecular reactions. The reaction of the chiral template 147 with the alkenyl aldehyde 148 led to the formation of the azomethine ylide 149, which underwent an intramolecular 1,3-dipolar cycloaddition to furnish 150 (Scheme 12.49). The reaction was found to proceed with high diastereoselectivity, as only one diaster-eomer of 150 was formed. By a reduction of 150, the proline derivative 151 was obtained. 

Sodium borohydride reduction of the azomethine bond has been noted for 3H-2-benzazepines (75JA4682,76HCA623) and for ll//-dibenz[6,e]azepines (65CCC445). 

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