Azomethines reactivity

These compounds are soluble in ether, are comparatively stable, and exhibit many of the reactions of Grignard reagents but are more reactive. Because of their greater reactivity, organohthium compounds can often be used where Grignard reagents fail thus they add to the azomethine linkage in pyridines or... 

As the sp nitrogen atom in many heterocycles can be alkylated and aminated, the construction of an azomethine ylide or azomethine imine dipole is readily attainable as shown in Scheme 13. These ylides are very reactive and undergo cycloaddition with a... 

Photochemical elimination of carbon dioxide from suitable precursors has given a variety of reactive intermediates at low temperatures where they are often stable and can be studied further. This approach has been utilized in attempts to generate new 1,3-dipolar species, and photolysis of (515) gave an azomethine nitrene intermediate (516) (see Section 4.03.6)... 

The high reactivity of azomethine ylides allows addition to aromatic systems (71TL481). For example, trans-aziridine (30) adds to phenanthrene to give the fran5-phenanthropyr-rolidine (31). The reversal of expected stereochemistry is again attributed to azomethine ylide interconversion being allowed by the low reactivity of the aromatic system. 

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. 

Diethylamino-4-(4-methoxyphenyl)-isothiazole 5,5-dioxide 6 is (95T(51)2455) a highly reactive partner in 1,3-dipolar cycloadditions with several dipoles. Azomethine yhdes, such as oxazolones 7 and miinchnones 8, afforded with 6 bicychc pyrrolo[3,4-d]isothiazole 5,5-dioxides 9, 10, 11 in satisfactory yield. The regioselectivity of the reaction was excellent. The thermal behavior of these new bicychc systems was investigated. When heated at their melting point or shghtly above, triarylpyrroles 12, 13 were obtained through SOj and AtiV-diethylcyanamide ehmination. 

H(65)1889, 2005EJO3553>. Starting dihydro[l,2,4]triazolo[3, 4-4]benzo[l,2,4]triazines 482 readily react with aromatic aldehydes to yield iminium salts 483. These salts treated with a base (e.g., triethylamine) are deprotonated to reactive 1,3-dipolar azomethine imines 484. In contrast to related five-membered heterocycles, these compounds are relatively unstable on storage in the solid form and particularly in solution. Fortunately, this obstacle can be easily circumvented by their in situ preparation and subsequent 1,3-dipolar cycloaddition. These compounds can participate in 1,3-dipolar cycloadditions with both symmetric and nonsymmetric dipolarophiles to give the expected 1,3-cycloadducts in stereoselective manner. Selected examples are given in Scheme 82. 

Other 1,3-dipolar reagents show the same mode of reactivity towards cyclopropenones. Thus, the Munchnones 412 serving as potential azomethine ylides259-261 or the nitrile ylids 41 3262 effect expansion of the three-membered ring to the 4-pyridone systems 411/414 as a result of (2 + 3) cycloaddition to the C /C2 bond. 

Kiss [8] examined various techniques for the efficient separation and preconcentration of boron from marine sediments. Alkaline fusion with potassium carbonate was used to render boron reactive, even in the most resistant silicate minerals. Fusion cakes were extracted with water and borate was isolated by Amberlite XE-243 boron-selective resin. Borate was determined spectrophotometrically, following elution with 2 mol L 1 hydrochloric acid. Either the carminic acid complex (620nm), formed in sulphuric acid (94%) or sulphuric acetic acid (1 4), or the azomethine hydrogen ion association complex (415nm) formed at pH5.2, were used for borate measurement. 

A typical 1,3-dipolar cycloaddition involves the use of azomethine ylides as the reactive species. Ylides are formed in situ by thermal condensation of a-amino acids and aldehydes, which then react to form the pyrrolidine-CNT system [32]. The R substituent on the 5-membered heterocycle attached to the SWCNT can be varied by the selection of the amino acid and the aldehyde, giving access to a range of different functional groups on the nanotube sidewalls. 

The reaction of 410 with dimethyl acetylenedicarboxylate yields 411 (78TL1291). Since 410 can be represented as the azomethine ylide 412 or the azomethine-imine ylide 413, this result may indicate that the azomethine ylide is more reactive in cycloadditions with acetylenes than azomethine-imine ylide s. 

Many different types of 1,3-dipoles have been described [Ij however, those most commonly formed using transition metal catalysis are the carbonyl ylides and associated mesoionic species such as isomiinchnones. Additional examples include the thiocar-bonyl, azomethine, oxonium, ammonium, and nitrile ylides, which have also been generated using rhodium(II) catalysis [8]. The mechanism of dipole formation most often involves the interaction of an electrophilic metal carbenoid with a heteroatom lone pair. In some cases, however, dipoles can be generated via the rearrangement of a reactive species, such as another dipole [40], or the thermolysis of a three-membered het-erocycHc ring [41]. 

This ready nucleophilic substitution at the 6-position is surprising since this position is electron-rich in both dihydrodiazepines and dihydrodiaze-pinium salts and is the site at which electrophilic substitution occurs. The likely explanation is that in the presence of base some prototropic rearrangement of the normal dihydrodiazepine base into a bis-imino form takes place. Although the equilibrium concentration of the bis-imine is likely to be very small (it has not been observed spectroscopically) it would be strongly electrophilic at the 6-position owing to the combined effects of the bromine atom and the two azomethine groups, and could well be the reactive species in the nucleophilic substitution of the bromine atom ... 

In synthetic efforts toward the DNA reactive alkaloid naphthyridinomycin (164), Gamer and Ho (41) reported a series of studies into the constmction of the diazobicyclo[3.2.1]octane section. Constmction of the five-membered ring, by the photolytic conversion of an aziridine to an azomethine ylide and subsequent alkene 1,3-dipolar cycloaddition, was deemed the best synthetic tactic. Initial studies with menthol- and isonorborneol- tethered chiral dipolarophiles gave no facial selectivity in the adducts formed (42). However, utilizing Oppolzer s sultam as the chiral controlling unit led to a dramatic improvement. Treatment of ylide precursor 165 with the chiral dipolarophile 166 under photochemical conditions led to formation of the desired cycloadducts (Scheme 3.47). The reaction proceeded with an exo/endo ratio of only 2.4 1 however, the facial selectivity was good at >25 1 in favor of the desired re products. The products derived from si attack of the ylide... 

Due to the increased reactivity of the reaction in the presence of a Lewis acid, the reaction scope was extended to singly activated alkenes. Previous results had shown either no reaction or extremely poor yields. However, under the Lewis acid catalyzed conditions, acrylonitrile furnished a 1 1, endo/exo mixture of products. The addition of the catalyst gave unexpected regiochemistry in the reaction, which is analogous with results described in Grigg s metal catalyzed reactions. These observations in the reversal of regio- and stereocontrol of the reactions were rationalized by a reversal of the dominant, interacting frontier orbitals to a LUMO dipole-HOMO dipolarophile combination due to the ylide-catalyst complex. This complex resulted in a further withdrawal of electrons from the azomethine ylide. 

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