Hydroximation

Arylsydnone-4-carbonitrile oxides, which are generated in situ by thermal dehydrochlorination of the corresponding hydroximic acid chlorides, undergo 1,3-dipolar cycloadditions with sydnone-4-carbonitriles to give 3-aryl-4-[5-(3-arylsydnonyl)-l,2,4-oxadiazol-3- yl]sydnones 228 (392). 

The pentacoordinate silicon compounds 81, 8,54 82,54 83,54 and 8455 are spirocyclic zwitterionic A5S7-silicates with an Si04C skeleton. The chiral zwitterions contain two diolato(2—) ligands that formally derive from aceto-hydroximic acid and benzohydroximic acid (tautomers of acetohydroxamic acid and benzohydroxamic acid). 

Szymanowski, J. Hydroximes and Copper Hydrometallurgy, CRC Press Boca Raton, Florida, 1993. 

For hydroxamic acids, it is generally assumed that it is the Af-hydroxyamide/keto form, as opposed to the hydroximic/hydroxyoxime form, that predominates in acid medium, the environment usually required for most precipitates or colors to form . It is in general unknown what is the stoichiometry and structure of most metal hydroxamate complexes in solution. Nevertheless, the reaction of the majority of hydroxamic acids with metal ions can be written schematically as shown in equation 2. 

The formation of oximes and hydroximic acids from hydroxylamine, and of, say, ethylnitrolic acid, CH3.C(N02)N0H, from dibromonitroethane, CH3.CBr2(N02), and hydroxylamine, observed by V. Meyer ... 

The discoveiy that a mixture of (Z)-(94a) and (Z)-(94b), on treatment with LDA, forms only (Z)-(95a) and (Z)-(95b) is consistent with an intramolecular mechanism (e.g. Scheme 6) for this new imino-Wittig rearrangement of hydroximates alternative radical pathways have also been discussed for reactions of Z- and. E-isomers.169... 

A diastereoselective imino [1,2]-Wittig rearrangement of allyl hydroximates provides for the preparation of optically active a-hydroxy oxime ethers when R3 is designed (g) as shown to participate in the reaction (Scheme 5).8... 

The lactim/lactam tautomerism of hydroxamic acids and their O-alkyl and O-acyl derivatives has also been studied [146], Hydroxamic acids exist in the solid state and in polar solvents as the lactam tautomer only, whereas in nonpolar solvents the hydroximic tautomer is also present. Further analogous solvent-dependent lactim/lactam equilibria have been observed for certain Schiff bases (prepared from anilines and 2(4)-hydroxybenzaldehyde [256] or 2-hydroxynaphthaldehyde [257]), for 3-hydroxypyrazole [258], and for 3-methyl-l-phenylpyrazolin-5-one [259]. 

The zwitterionic X Si-silicate 28 is, to the best of our knowledge, the first pentacoordinate silicon species containing diolato(2-) ligands derived from a hydroximic acid (Scheme 8). 

The results presented here suggest that a rich complex chemistry of pentacoordinate silicon with ligands derived from a-hydroxycarboxylic acids (including tartaric acid), hydroximic acids, and oxalic acid may be developed. As most of these ligands derive from natural products and as some of these X Si-silicon(IV) complexes were shown to exist in aqueous solution, compounds of this formula type are of particular interest it has been speculated in the literature [16] that silicon transport in biological systems might be based on higher coordinate Si species, and complexes such as the title compounds could be of interest as model systems in this respect. 

Method D Action of ammonia on hydroximic acid chlorides... 

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