Hydroxamic acids, effect

The use of dehydrating agents such as sulfuric or phosphoric acid on (555 X = OH) was also successful, and these closures may proceed via mixed anhydrides 67AHC(8)277, 75MIP41600). Carbonyldiimidazole effected the conversion of hydroxamic acid (557) into a 3-hydroxy-1,2-benzisoxazole derivative (79JHC1277). The mixed anhydride (558) where... 

The protonation equilibria for nine hydroxamic acids in solutions have been studied pH-potentiometrically via a modified Irving and Rossotti technique. The dissociation constants (p/fa values) of hydroxamic acids and the thermodynamic functions (AG°, AH°, AS°, and 5) for the successive and overall protonation processes of hydroxamic acids have been derived at different temperatures in water and in three different mixtures of water and dioxane (the mole fractions of dioxane were 0.083, 0.174, and 0.33). Titrations were also carried out in water ionic strengths of (0.15, 0.20, and 0.25) mol dm NaNOg, and the resulting dissociation constants are reported. A detailed thermodynamic analysis of the effects of organic solvent (dioxane), temperature, and ionic strength on the protonation processes of hydroxamic acids is presented and discussed to determine the factors which control these processes. 

When Jencks reacted hydroxylamine with p-nitrophenyl acetate, p-nitrophenolate ion was released at a rate faster than that at which acetohydroxamic acid was formed. This burst effect is evidence for a two-step reaction. In this case the intermediate is O-acetylhydroxylamine, which subsequently reacts with hydroxylamine to form the hydroxamic acid. 

The first synthesis of a 3,5-diarylisoxazole from aryl hydroxamic acid chlorides and sodium phenyl acetylides was that effected by Weygand and Bauer in 1927. Beginning in 1946, when Quilico and Speroni showed that acid chlorides of hydroxamic acids on treatment with alkalies readily yielded nitrile oxides,numerous isoxazole and especially A -isoxazoline derivatives have been prepared. 

As in 10-55 hydrazides and hydroxamic acids can be prepared from carboxylic esters, with hydrazine and hydroxylamine, respectively. Both hydrazine and hydroxylamine react more rapidly than ammonia or primary amines (the alpha effect, p. 445). Imidates, RC(=NH)OR, give amidines, RC(=NH)NH2. Lactones, when treated with ammonia or primary amines, give lactams. Lactams are also produced from y- and 5-amino esters in an internal example of this reaction. 

Deacylation of hydrophobic p-nitrophenyl alkanoates Hydroxamic acid and phenyl ester derivatives had alkyl or fluoroalkyl substituents. Rate effects depend on selectivity of binding of fluoro- and hydro-carbon derivatives Kunitake et ai, 1984... 

While the lone pair on the hydroxyl oxygen of hydroxamic acid 77 would be tightly bound resulting in a weak noH-cr N Qr interaction, the anion 78 would possess a high energy pair of electrons that would enhance the anomeric effect and drive the rearrangement. 

Figure 19.2 Effect of pH on malachite recovery using hydroxamic acid as collector.

Acidolytic treatment using DCM hexafluoroisopropanoI (1 1) for 2 h at ambient temperature afforded the hydroxamic acid in only 45% yield. However, it is worth noting that the tethered Fmoc-N(Pr)-<9-2-chIorotrityI polystyrene, on treatment with similar acidolytic cocktail effected quantitative release of Fmoc-N(Pr)-OH. 

In conclusion, we anticipate that A-Fmoc-aminooxy-2-chlorotrityl polystyrene will prove an indispensable reagent for the solid-phase synthesis of hydroxamic acids by multiple and combinatorial approaches. Not only is its production both efficient and cost effective, but release of the assembled hydroxamic acid derivative is readily accomplished using mild acidolytic reagents. 

The rate-enhancing effect of cationic detergents was analyzed by using Hill s equation. The observed exponent (n = 3 — 4) suggests that polymer-bound detergents facilitate the subsequent binding acceleratively hence the sigmoidshaped dissociation behavior of hydroxamic acid. 

Hydroxamic acids and oximes have relatively low p.fifa-values, and the nucleophilicity is enhanced due to the cr-effect. It is thus anticipated that micellization of these functions should produce highly nucleophilic systems. The typical nucleophilic rate constants for the reaction with PNPA are summarized in Table 3. It is seen that the rate constants in the micellar and Table 3... 

Nitrosobenzenes react with the carbonyl group of aldehydes to yield hydroxamic acids 73, according to reaction 20. Recently, the reactions between some X-substituted nitrosobenzenes (X = H, p-Me, p-C 1, m-Cl, p-Br) and formaldehyde were reported194 in order to investigate the mechanism of the hydroxamic acid formation. The mechanism reported in Scheme 9 involves a first equilibrium yielding the zwitterionic intermediate 74 which rearranges (by acid catalysis) into hydroxamic acid 75. The presence of a general acid catalysis, the substituent effect (p values of the Hammett equation equal —1.74),... 

As points of reference, we will take two well-established hydrogen-bond donor/ acceptors, H2O and NH3. Their computed gas-phase Vs,max and Vs,mm are in Table 5, along with the same data for all of the molecules that have been discussed hydroxylamine (5), dimethylhydroxylamine (6), acetoxime (7), acetohydroxamic acid (8), and the isomeric pairs of oximes examined in the last section. Finally, we included an additional hydroxamic acid, 11, to see the effects of the strongly electron-withdrawing cyano group. 

The hydrogen bonding in hydroxamic acids produces, in effect, five-membered rings (as in 1) and thus it should not be expected to be linear. For the hydroxamic acids in Table 2, the O—H---0 angles are between 118° and 122°. 

The structures of these molecules show the effects of intramolecular electrostatic interactions. Two examples are the lone pair—lone pair repulsion that is an important determinant of hydroxylamine and oxime conformations, and the intramolecular hydrogen bonding in hydroxamic acids that promotes the near-planarities of their —C(=0)—NO frameworks. 

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