Hydroxamic acids oxidation mechanism

Three papers from Ghosh s group deal with the hydrolysis of benzohydroxamic acids in acidic and alkaline conditions. A pre-equihbrium protonation followed by a slow A-2 type nucleophilic attack by water is seen as the mechanism of the acid-catalysed hydrolysis of j -chlorophenylbenzohydroxamic acid (107 R = / -ClCgEU) by mineral acids (HCl, HCIO4) in 20% aqueous dioxane. An A-2 mechanism was also supported for the reaction of (107 R = Me) imder comparable conditions. The alkaline hydrolysis imder micellar conditions of (107 R = Ph) and a series of para-substituted derivatives has been investigated in the presence of cationic and anionic micelles in 5% dioxane-water medium at 55 °C. Cationic surfactants exerted a catalytic effect and anionic surfactants were inhibitory. The rate-surfactant profiles were analysed in terms of the pseudophase and Piszkiewicz models. The detection of N2O in the products of the oxidation of hydroxamic acids suggests the intermediacy of nitroxyl, HNO, in the process. Scheme 9 may represent the pathway followed. 

Oxidation Mechanism of Hydroxamic Acids Forming HNO and NO Implications for Biological Activity... 

In terms of solvent choice, DMF, DMA and toluene proved to be inferior to tet-rahydrofuran (THE) or THF/hexane solvent mixtures. It is speculated that the mechanism of this copper-mediated reaction proceeds via the oxidative addition of copper(I) to the hydroxamic acid, thereby generating a copper(III) amido species which slowly undergoes transmetallation. 

A unique example of direct olefination of a cyclopropane was also disclosed by the Yu lab [21]. An electron-deficient arylamide was employed as directing group, as the previously employed oxazoline or hydroxamic acid was unreactive in the alkenylation. The proposed mechanism for the reaction involves an amide-directed C-H insertion of the Pd(II) catalyst into the cyclopropane methylene C-H bond of 9, followed by olefin carbopalladation and p-hydride elimination to provide intermediate 10 (Scheme 3a). Pd(0) is re-oxidized back to Pd(II) by Ag(I)/Cu(II), and a tandem 1,4-addition between the amide moiety of 10 and the acrylate provides the corresponding y-lactam 11 as the sole isolated product, fii the presence of an... 

Diaryl-l,2,3-thiadiazole 1,1,2-trioxides (24) are photolysed to nitriles (25), small amounts of diarylalkynes (26), and intermediates (27), which, in the presence of aqueous methanol, react further to yield hydroxamic acids (28). A mechanism has been proposed correlating these findings with the photolysis of other 1,2,3-thiadiazole oxides. ... 

---