Direct hydrogen abstraction mechanism

During the mechanistic studies of estrogen biosynthesis, selective oxidation of androstene-3,17,19-trione (n) to the corresponding carboxylic acid was found to proceed by iron porphyrin complexes (Scheme 14A) [253]. On the basis of substituent effect on the benzaldehyde oxidation and kinetic isotope effect, direct hydrogen abstraction mechanism has been proposed [254]. The relative reactivity of aldehydes and alkenes is as follows cyclooctene, styrene > aldehyde, terminal alkene > a, 3-unsaturated ketone. 

The value of CM has been determined by a number of groups as 6x 10 5 (Table 6.14.1. " However, the mechanism of transfer has not been firmly established. A mechanism involving direct hydrogen abstraction seems unlikely given the high strength of vinylic and aromatic C-M bonds. The observed value of Cy is only slightly lower than Ctr for ethylbenzene ( 7x 10"5). w... 

Mechanism II. Amines may be able to stabilize free radicals formed during the reaction between the fuel and oxygen. This could be done either by forming an adduct between a fuel radical, R , and the amine, followed by reaction to form stable products (28), or by direct hydrogen abstraction (25, 28, 29) to form a radical, T, from the inhibitor, which does not take any part in the fuel-oxygen chain reaction ... 

A direct cleavage of the ester to radicals similar to that observed in the vapor state94 and an internal hydrogen abstraction followed by radical formation98 have been proposed to explain the reaction in benzene. The internal hydrogen abstraction mechanism appears to be the more reasonable. Direct cleavage would yield ethoxy radicals, but no products derived from them are observed. The low quantum yield of ester disappearance observed for ethyl... 

The oxidation of hydrazine derivatives with diethyl azodicarboxylate is of particular interest because it involves direct hydrogen abstraction. The oxidation of keto hydrazones with lead tetraacetate leads to azoacetates, presumably by a free radical mechanism. 

Carbonyl halides containing a hydrogen atom are likely to undergo reaction in the troposphere with hydroxyl radicals, which dominate the day-time chemistry. Reaction 37 of carbonyl halides CHXO (where X = F or Cl) may occur by two mechanisms (i) direct hydrogen abstraction or (ii) radical addition to carbonyl. 

Extensive studies of the sensitizer dependence and the solvent dependence of the polarization patterns led to the identihcation of two parallel pathways of that deprotonation. One is a proton transfer within the spin-correlated radical pairs, with the radical anion A acting as the base. The other is a deprotonation of free radicals, in which case the proton is taken up by surplus starting amine DH. Furthermore, evidence was obtained from these experiments that even in those situations where the polarization pattern suggests a direct hydrogen abstraction according to Equation 9.6 these reactions proceed as two-step processes, electron transfer (Eq. 9.7) followed by deprotonation of the radical cation by either of the described two routes. The whole mechanism is summarized by Chart 9.3 for triethylamine as the substrate. Best suited for an analysis is the product V. 

As a photoinitiator for the polymerization of bulk methyl methacrylate, benzil was found to be considerably less efficient than benzoin or benzoin methyl ether (20), e,g. at photoinitiator concentrations of 10 M, the polymerization rate observed using benzoin was eight times that observed using benzil, despite the fact that the latter was found to absorb three times as much of the incident radiation (Fig. 1). The photo-initiating efficiency of benzil was improved by a factor of three on addition to the methyl methacrylate of 10% v/v tetrahydrofuran, whereas the same additive had no appreciable effect on rates of benzoin- and benzoin methyl ether-photoinitiated polymerizations direct evidence that photoinitiation by benzil proceeds by a hydrogen abstraction mechanism rather than by fragmentation. 

Applying M /halogenid catalysts, two different mechanisms are responsible for the start of the oxidation reactions [14, 19] the reaction of the substrates to radicals and the direct hydrogen abstraction by radicals X formed from XCo "(OAc)2. 

Owing to their relatively low ionization energies (IE) of ca 8.0-8.5 eV, phenols are also good electron donor solutes. Recent experimental studies of phenols in non-protic solvents showed that ionized solvent molecules react with phenol to yield not only phenol radical cations by electron transfer, but also phenoxy radicals by hydrogen transfer. An obvious question is whether, under these conditions, the latter radicals were formed from ionized phenols rather than by direct hydrogen abstraction, because proton transfer reactions could be facilitated upon ionization. This also raises a question about the influence of solvent properties, both by specific and non-specific interactions, on the mechanism and kinetics of deprotonation processes ... 

This latter product may arise via direct hydrogen abstraction or via a single-electron oxidation of the radical to the cation followed by elimination of a proton. Results of CYP17A catalyzed oxidation of substrates bearing deuterium labels at the Cl6, Cl7 and the methyl group a to the ketone are in agreement with this mechanism . The 17-0-acetyl-testosterone is probably best explained as... 

Kinetic isotope effects on the hydroxylation of a various types of hydrocarbons and oxidative demethylation of anisole derivatives both by cytochrome P-450 and by model systems were examined, and large kinetic isotope effects in the range between 4 and 12 were observed [46,191-196]. Direct hydrogen abstract followed by rapid oxygen rebound mechanism has been proposed for the hydroxylation reactions by P-450 and their model systems. Retention of configuration in many hydroxylations mediated by P-450 indicates an extremely rapid rebound step. 

Different mechanisms have been proposed for the photoreduction of azoalkanes by a number of hydrogen donors (1,4-cylohexadiene, alcohols, stannanes, and silanes) and amines. While direct hydrogen abstraction operates for the former group of donors, amines react with the triplet azoalkanes by charge transfer.The mechanistic details of photoreduction by the direct hydrogen donors depend on the donor structure however, intervention of the hydrazinyl radical 13 (Scheme 2) is a common feature of these photoreduction processes. 

For MDI based polyurethanes we have provided evidence for formation of a diphenylmethyl radical by direct excitation (248 nm) of the carbamate moiety as well as hydrogen abstraction by a tert-butoxy radical which is produced by excitation (351 nm) of tert-butyl peroxide. The diphenylmethyl radical readily reacts with oxygen. A proposed mechanism which accounts for the production (direct or indirect) and subsequent reaction with oxygen of the diphenylmethyl radical is shown in Scheme IV. The hydrogen peroxide product depicted in Scheme IV has been previously identified by FT-IR (7) we have simply provided a plausible mechanism for its formation. 

Waals radii of two atoms (2.90 A). Nevertheless, each reaction site is far apart the hydrogen abstraction from the benzyl group proceeds and the product was obtained in optically active form. In this case, another mechanism besides the direct intramolecular hydrogen abstraction from the benzyl group may be involved, like intermolecular hydrogen abstraction or hydrogen shift. 

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