Aromatic azomethine

Selenium-containing aromatic azomethines are ligands in whose chelates five- (493) [269,883] and six-member (494) [269,884] metal-cycles are formed. The latter also exist in complexes of (3-aminovinylselenoketones 495 [885] ... 

The tetrahedrizating influence of the heteroannelation is so high that it exceeds the m-planar effect, as observed (Sec. 2.2.5A [133,135,157]) in nickel complexes of aromatic azomethines with N2S2-ligand environment. 

Unsymmetrical azines of an aromatic and an aliphatic carbonyl compound, such as benzophenone cyclohexanone azine, are reduced in slightly acidic solution with cleavage of the nitrogen-nitrogen bond and saturation of the aromatic azomethine group [Eq. (12)], whereas the aliphatic imine is then hydrolyzed [Eq. (13)] [48] unsymmetrical cyclic azines, such as 5-i/-2,3-benzodiazepines, are reduced similarly, usually with ring contraction. 

In 2006 Chen et al. [128] reported the first organocatalyzed stereoselective [3 + 2] dipolar cycloaddition of azomethine imines with aliphatic a,p-unsaturated aldehydes using chiral secondary amine catalyst 59. The desired cycloaddition products were obtained with good yields, good diastereoselectivities and good enantioselectivities for both electron-rich and electron-poor aromatic azomethine imines. In contrast, aliphatic azomethine imines (R = -Pr) led to poorer results (40% yield and 77% ee). Investigation of a variety of solvents and additives revealed that tetrahydrofurane/HaO mixture and 10 mol% of trifluoroacetic acid were preferred. No reaction was observed when aromatic a,p-unsaturated aldehydes were used. The authors proposed an iminium ion mechanism that could proceed via the transition states shown in Scheme 11.47. 

The enantioselective 3 + 3-cycloaddition reaction of aromatic azomethine imines with donor-acceptor cyclopropane diesters produced 6,6,6-tricyclic dihydroquinolines in high yields (99%) and up to 98% ee. The side-arm-modified In-TOX/Ni(II) complex was shown to be an effective stereoselective catalyst for this reaction. The formal 3 + 3-cycloaddition reaction of allylic phosphonium ylides (127) to a, -unsaturated carbonyls (128) formed multi-substituted benzenes (129) in high yields (36-93%) (Scheme 38). °... 

Schreiber and coworkers (Scheme 7.10) described the [3 + 2] cycloaddition of an immobilized aromatic azomethine ylide with tcrt-butyl acrylate in the presence of silver acetate/(S)-QUINAR Reaction gave rise to the endo isomer (53) with 79% overall yield and 90% ee. However, to achieve the results as good as in solution-phase chemistry, a three times more catalyst loading was required. 

Scheme 7.10. Enantioselective [3 + 2] cycloaddition of an immobilized aromatic azomethine...

The dimerization of carbonyl-, thiocarbonyl-, imidoyl- and thiocarbamoyl isothiocyanates proceeds in a [4-1-2] cycloaddition reaction (see Section 3.3.2.2). Carbonyl isothiocyanates also readily undergo [4-1-2] cycloaddition reactions with aliphatic and aromatic azomethines, the former being considerably more reactive. For example, phenylcar-bonyl isothiocyanate reacts with benzylidenemethylamine 184 to give a [4-1-2] cycloadduct 185... 

Compounds 39 were studied by electronic and heteronuclear H, Li NMR spectroscopy (Figure 10.27) [123]. They exist in the form E, which is the most characteristic for aromatic azomethines. Addition of Zn to acetonitrile solutions of 39 leads to the formation of chelates 40. Imines 39 E exhibit weak fluorescence in the range 530-550 nm with an abnormally large Stokes shift... 

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. 

Aroylaziridines (32) and aromatic aldehydes react to give oxazolidines (33), the stereochemistry of which suggests reaction very largely through the trans-azomethine ylide, irrespective of the aziridine configuration (70JCS(C)2383). 

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. 

Another example of a microwave-assisted 1,3-dipolar cycloaddition using azomethine ylides and a dipolarophile was the intramolecular reaction reported for the synthesis of hexahydrochromeno[4,3-fo]pyrrolidine 105 [70]. It was the first example of a solvent-free microwave-assisted intramoleciflar 1,3-dipolar cycloaddition of azomethine ylides, obtained from aromatic aldehyde 102 and IM-substituted glycinate 103 (Scheme 36). The dipole was generated in situ (independently from the presence of a base like TEA) and reacted directly with the dipolarophile present within the same molecifle. The intramolecu-... 

Dipolar addition to nitroalkenes provides a useful strategy for synthesis of various heterocycles. The [3+2] reaction of azomethine ylides and alkenes is one of the most useful methods for the preparation of pyrolines. Stereocontrolled synthesis of highly substituted proline esters via [3+2] cycloaddition between IV-methylated azomethine ylides and nitroalkenes has been reported.147 The stereochemistry of 1,3-dipolar cycloaddition of azomethine ylides derived from aromatic aldehydes and L-proline alkyl esters with various nitroalkenes has been reported. Cyclic and acyclic nitroalkenes add to the anti form of the ylide in a highly regioselective manner to give pyrrolizidine derivatives.148... 

Simple criss-cross cycloadditions described so far are in fact limited to aromatic aldazines and cyclic or fluorinated ketazines. Other examples are rather rare, including the products of intramolecular criss-cross cycloaddition. The criss-cross cycloadditions of hexafluoroacetone azine are probably the best studied reaction of this type. It has been observed that with azomethine imides 291 derived from hexafluoroacetone azine 290 and C(5)-C(7) cycloalkenes < 1975J(P 1)1902, 1979T389>, a rearrangement to 177-3-pyrazolines 292 competes with the criss-cross adduct 293 formation (Scheme 39). 

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. 

Stereochemistry was controlled by the stereodirecting phenyl group at position 3 and by the ortho-substituents of the aromatic ring at position 10 in the azomethine imines (Equation 101) <2007T991>. 

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