Nitrogen Imines Oximes

Carb-16.5. Replacement of carbonyl oxygen by nitrogen (imines, oximes, hydrazones, osazones etc.)... 

Oxidation of imine, amidine and guanidine nitrogens yields oximes, oxidative attack at the nitrogen centers in alkyl-, aryl- or cyclic imines affords nitrones. The oxidation state of nitrogen in these functions11 is defined as —1. Both oximes and nitrones exist as a mixture of two geometric isomers, Z and E, previously termed syn and anti, respectively... 

Nitrogen.—Amines, Enamines, Imines, Oximes, Isocyanates, Cyanides, and Related Compounds. Perfluorotributylamine, (C4F9)sN (FCM3 ), has continued to feature in work on medical applications (blood oxygenation etc.) of fluorocarbon-type... 

The other classes of compounds containing sp nitrogen atoms that we will meet are shown in Figure 4.5, imines, oximes, hydrazones, and semicarbazones. In terms of the types of orbitals used in the bonding, all the classes are the same as far as the carbon-nitrogen double bond is concerned, so we ll look at the oxime as an example. Some parts of this are straightforward and already known to us. The carbon of the double bond is sp hybridized and forms three a-bonds to... 

Figure 3.7 Several other examples of tautomerization. Top Phenol-keto tautomeriza-tion strongly favors the phenol form because it is aromatic. Middle Imine-enamine tautomerization favors the imine form unless there is no hydrogen on the nitrogen. Bottom Oxime-nitroso tautomerization favors the oxime form. Aliphatic nitroso compounds are not stable unless the carbon with the N=0 group has no hydrogen.

Carbon—nitrogen double bonds in imines, hydrazones, oximes, nitrones, azines, and substituted diazomethanes can be cleaved, yielding mainly ketones, aldehydes and/or carboxyHc acids. Ozonation of acetylene gives primarily glyoxal. With substituted compounds, carboxyHc acids and dicarbonyl compounds are obtained for instance, stearoHc acid yields mainly azelaic acid, and a smaH amount of 9,10-diketostearic acid. 

The formation of oximes, hydrazones, and related imine derivatives is usually catalyzed by both general acids and general bases. General base catalysis of dehydration of the tetrahedral intermediate involves nitrogen deprotonation concerted with elimination of hydroxide ion. ... 

Compounds containing carbon-nitrogen double bonds can be hydrolyzed to the corresponding aldehydes or ketones. For imines (W = R or H) the hydrolysis is easy and can be carried out with water. When W = H, the imine is seldom stable enough for isolation, and hydrolysis usually occurs in situ, without isolation. The hydrolysis of Schiff bases (W = Ar) is more difficult and requires acid or basic catalysis. Oximes (W = OH), arylhydrazones (W = NHAr), and, most easily, semicarbazones (W = NHCONH2) can also be hydrolyzed. Often a reactive aldehyde (e.g., formaldehyde) is added to combine with the liberated amine. 

Several functional groups containing carbon-nitrogen double bonds can participate in radical cyclizations. Among these are oxime ethers, imines, and hydrazones.337 Hydrazones and oximes are somewhat more reactive than imines, evidently because the adjacent substituents can stabilize the radical center at nitrogen.338 Cyclization at these functional groups leads to amino- substituted products. 

The scheme required to prepare the potent tri-fluoro corticoid cormethasone acetate (292) illustrates the synthetic complexities involved in some of this work. Sequential acetylation of the pregnenolone derivative 278 with first acetic anhydride in pyridine and then acetic anhydride in the presence of tosic acid affords diacetate 279. Reaction of that intermediate with nitrosyl fluoride results initially in addition of the reagent to the 5,6-olefin moiety to afford the fluoro oxime reaction with a second mole of reagent at nitrogen gives the nitroimine derivative 280 passage over alumina serves to hydrolyze the imine function to the corresponding 6-ketone (281). 

Nitro compounds are versatile precursors for diverse functionalities. Their conversion into carbonyl compounds by the Nef reaction and into amines by reduction are the most widely used processes in organic synthesis using nitro compounds. In addition, dehydration of primary nitro compounds leads to nitrile oxides, a class of reactive 1,3-dipolar reagents. Nitro compounds are also good precursors for various nitrogen derivatives such as nitriles, oximes, hydroxylamines, and imines. These transformations of nitro compounds are well established and are used routinely in organic synthesis. 

Titanium silicate molecular sieves not only catalyze the oxidation of C=C double bonds but can be successfully employed for the oxidative cleavage of carbon-nitrogen double bonds as well. Tosylhydrazones and imines are oxidized to their corresponding carbonyl compounds (243) (Scheme 19). Similarly, oximes can be cleaved to their corresponding carbonyl compounds (165). The conversion of cyclic dienes into hydroxyl ketones or lactones is a novel reaction reported by Kumar et al. (165) (Scheme 20). Thus, when cyclopentadienes, 1,3-cyclohexadiene, or furan is treated with aqueous H202 in acetone at reflux temperatures for 6 h in the presence of TS-1, the corresponding hydroxyl ketone or lactone is obtained in moderate to good yields (208). 

In some very early work the conformation of a-methyl groups in cyclohexanone oximes (76) was assigned from solvent shifts. Results are summarized in Table 17. In solutes bearing a lone pair of electrons on nitrogen, the benzene-solute collision complex is likely to occur at a site as far as possible from the nitrogen. Shifts have been summarized for aziridines, oximes and imines, and for the latter a complex of type 77 was proposed. 

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