Hydroxyl hydrogen abstraction

Specifically, hydroxyl radicals can oxidize organics by hydroxylation, hydrogen abstraction, electrophilic addition, and electron transfer, depending upon the nature of organic compounds. 

Hydroxyl radicals, OH, can undergo several types of reactions with chemical species in aqueous solution. The types of reactions that are likely to occur are hydroxylation, hydrogen abstraction, electron transfer, and radical-radical recombination. Hydroxylation reaction occurs readily with aromatic and unsaturated aliphatic compounds, which result in the formation of hydrox-ylated radicals ... 

Tamp-OH = 2,4,6-tris(dimethylaminomethyl)phenyl HMPA = hexamethylphosphortriamide OP[NMe2]3. Rf = CH(CH3)2. R f = CMe(CF3>2 teaHs -triethanolamine, teaH-n (n = 1-3) indicates the number of hydroxyl hydrogens abstracted. 

The base-catalysed rearrangement of ring-substituted benzoins in aqueous methanol is reported to be initiated by rate-determining a-hydrogen abstraction rather than a mechanism with initial hydroxyl hydrogen abstraction as in the general a-ketol rearrangement (Scheme 93). ... 

As a class of compounds, the two main toxicity concerns for nitriles are acute lethality and osteolathyrsm. A comprehensive review of the toxicity of nitriles, including detailed discussion of biochemical mechanisms of toxicity and stmcture-activity relationships, is available (12). Nitriles vary broadly in their abiUty to cause acute lethaUty and subde differences in stmcture can greatly affect toxic potency. The biochemical basis of their acute toxicity is related to their metaboHsm in the body. Following exposure and absorption, nitriles are metabolized by cytochrome p450 enzymes in the Hver. The metaboHsm involves initial hydrogen abstraction resulting in the formation of a carbon radical, followed by hydroxylation of the carbon radical. MetaboHsm at the carbon atom adjacent (alpha) to the cyano group would yield a cyanohydrin metaboHte, which decomposes readily in the body to produce cyanide. Hydroxylation at other carbon positions in the nitrile does not result in cyanide release. 

The hydroxyl hydrogen in phenol is particularly susceptible to abstraction by a free radical. 

Hydroxy radical and sulfate radical anion, though they may sometimes give rise to similar products, show quite different selectivity in their reactions with unsaturated substrates. In particular, the sulfate radical anion has a somewhat lower propensity for hydrogen abstraction than the hydroxyl radical. For example, the sulfate radical anion shows little tendency to abstract hydrogen from mcthacrylic acid.232... 

Typical non-enolising aldehydes are formaldehyde and benzaldehyde, which are oxidised by Co(III) Ce(IV) perchlorate and sulphate , and Mn(III) . The main kinetic features and the primary kinetic isotope effects are the same as for the analogous cyclohexanol oxidations (section 4.3.5) and it is highly probable that the same general mechanism operates. kif olko20 for Co(III) oxidation of formaldehyde is 1.81 (ref. 141), a value in agreement with the observed acid-retardation, i.e. not in accordance with abstraction of a hydroxylic hydrogen atom from H2C(OH)2-The V(V) perchlorate oxidations of formaldehyde and chloral hydrate display an unusual rate expression, viz. 

VALGiMiGLi L, BANKS J T, INGOLD K u and LuszTYK J (1995) Kiuetic solveut effects on hydroxylic hydrogen atom abstractions are independent of the nature of the abstrarting radical. Two extreme tests using vitamin E and phenol, J Am Chem Soc, 117, 9966-71. 

The major reactions carried out by hydroxyl and nitrate radicals may conveniently be represented for a primary alkane RH or a secondary alkane RjCH. In both, hydrogen abstraction is the initiating reaction. 

The a,( -unsaturated aldehyde 452 is generated from the unstable spiro-oxetane 451, and hydrogen abstraction from the aldehydic C-H bond by 3449 gave a triplet radical pair 453 and 454. Intersystem crossing and radical recombination followed by intramolecular nucleophilic attack of the hydroxyl group toward the ketene functionality furnish the diastereomeric products 54 and 55 (Scheme 102) <20000L2583>. 

To explore the mechanism of allylic hydroxylation, three probe substrates, 3,3,6,6-tetradeuterocyclohexene, methylene cyclohexane, and /l-pinenc, were studied (113). Each substrate yielded a mixture of two allylic alcohols formed as a consequence of either retention or rearrangement of the double bond. The observation of a significant deuterium isotope effect (4-5) in the oxidation of 3,3,6,6-tetradeuterocyclohexene together with the formation of a mixture of un-rearranged and rearranged allylic alcohols from all three substrates is most consistent with a hydrogen abstraction-oxygen rebound mechanism (Fig. 4.48). 

Reaction 13.22 is the net result of two steps, the first being the photochemical cleavage of the 0-0 bond in di-terf-butylperoxide (reaction 13.23), followed by the abstraction of the hydroxylic hydrogen in phenol by the ferAbutoxyl radical (reaction 13.24). 

From the above it is clear that DMPO can undergo the addition-oxidation mechanism with water as the nucleophile, provided a suitable oxidant is present. With a primary alcohol competing, the O-connected alkoxy spin adduct is formed in addition to HO-DMPO". On the other hand, with a hydroxyl radical source a competing alcohol will undergo hydrogen abstraction by HO" and form an a-hydroxyalkyl radical which forms a C-connected spin adduct. This criterion clearly can distinguish between the two mechanisms at least in model systems (for recent examples, see Reszka and Chignell, 1995 Janzen et al., 1995 Thomas et al., 1996). 

It should additionally be noted that a number of the paths of the schemes above have received some confirmation in a number of literature reports dealing with the photolysis and photo-oxidation of other polyesters [32-35], Because these reports investigated poly(butylene terephthalate) (PBT), poly(ethylene naphthalate) and poly(butylene naphthalate), however, they may not have direct application to understanding of the processes involved in PET and PECT and so have not been discussed in this present chapter. All do contain support for the formation of radicals leading to CO and C02 evolution, as well as the hydrogen abstraction at glycolic carbons to form hydroperoxides which then decompose to form alkoxy radicals and the hydroxyl radical. These species then were postulated to undergo further reaction consistent with what we have proposed above. 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

Scheme 60). Griesbeck et al. assume that in a non-polar solvent such as benzene the intramolecular electron transfer from the methionic sulfur group is much faster than the abstraction of hydrogen from the hydroxyl group of the unprotected amino acid. C-Hydrogen abstraction leads to 313, whereas previous lactonization of the zwitterionic biradical 311 yields 314. Since the cis-hydroxy acid is not detected it is conceivable that it cyclizes immediately to the lactone 314. Photolysis of the corresponding methyl ester under the same conditions attains improved yields (84% combined) of two diastereomeric tricyclic products in a ratio of 48 52. 

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