Methyl radical intermediates

In addition to this pathway of metabolism and activation, methyl radical intermediates may also be involved in the toxicity and metabolism of 1,2-dimethylhydrazine catalysed by haemoglobin, peroxidases and cytochrome P450 (Augusto et al., 1985). 

Several studies have shown that the reactive binding species generated by 1,1-dimethylhydrazine metabolism may be free radical intermediates. Rat liver microsomes and rat hepatocytes are capable of metabolizing 1,1-dimethylhydrazine to form methyl radical intermediates (Albano et al. 1989 Tomasi et al. 1987). The formation of tliese radicals was inhibited by the addition of inhibitors of cytochrome P-450 (SKF 525A, metyrapone, and carbon monoxide) and inhibitors of the flavin-containing monooxygenase system (methimazole). The formation of free radicals could also be supported nonenzymatically by the presence of copper ion (Tomasi et al. 1987). These data indicate that at least two independent enzyme systems and one nonenzymatic pathway may be involved in the metabolism of... 

Lead tetraacetate (LTA) reacts with olefins via several mechanisms.23 The first one implies a methyl radical intermediate generated by the decomposition of LTA and results in the formation of a monoacetate after addition of the methyl group. Polar mechanisms give the gem- and pfc-diacetate. Sonochemical enhancement of the first process was predicted by Ando et ah, who indeed found an example of sonochemical switching when the reaction was performed on styrenes (Fig. 3). 24.25... 

Any time two radicals recombine, the chain stops. These are termination steps. Since the concentration of radicals is much less than that of either methane or Cl, termination steps are relatively rare. One of the termination steps is mechanistically important the reaction mixture always contains a small amount of ethane, and this only could have occurred if the reaction had proceeded by way of a methyl radical intermediate. 

The chemistry is clearly complex most pertinently, a methyl radical can be oxidized by flame species to formaldehyde either directly or via a methoxy radical intermediate (Eqns (7)—(9)). Subsequent hydrogen atom abstraction yields formyl radical (Eqn (10)) once collisionaUy stabilized, formyl radical can form CO (Eqn (11)) and, presuming complete combustion, it ultimately reacts to form CO2 (Eqn (13)). The methyl radical intermediate, alternatively, can combine with another methyl radical to form ethane (Eqn (12)), which can then yield ethyl radicals by chemically activated C—H bond fission or via H atom abstraction, opening up a wide variety of side reactions. (In unsaturated systems common to HC fuel combustion, another common initiation step involves addition of a reactive flame species to a double bond to yield a radical center.)... 

Like carbocations most free radicals are exceedingly reactive species—too reac tive to be isolated but capable of being formed as transient intermediates m chemical reactions Methyl radical as we shall see m the following section is an intermediate m the chlorination of methane... 

Methane oxidations occur only by intermediate and high temperature mechanisms and have been reported not to support cool flames (104,105). However, others have reported that cool flames do occur in methane oxidation, even at temperatures >400 ° C (93,94,106,107). Since methyl radicals caimot participate in reactions 23 or 24, some other mechanism must be operative to achieve the quenching observed in methane cool flames. It has been proposed that the interaction of formaldehyde and its products with radicals decreases their concentrations and inhibits the whole oxidation process (93). 

A typical oxidation is conducted at 700°C (113). Methyl radicals generated on the surface are effectively injected into the vapor space before further reaction occurs (114). Under these conditions, methyl radicals are not very reactive with oxygen and tend to dimerize. Ethane and its oxidation product ethylene can be produced in good efficiencies but maximum yield is limited to ca 20%. This limitation is imposed by the susceptibiUty of the intermediates to further oxidation (see Figs. 2 and 3). A conservative estimate of the lower limit of the oxidation rate constant ratio for ethane and ethylene with respect to methane is one, and the ratio for methanol may be at least 20 (115). 

Apart from the nuclear bromination observed (Section 2.15.13.1) in the attempted radical bromination of a side-chain methyl group leading to (396), which may or may not have involved radical intermediates, the only other reaction of interest in this section is a light-induced reduction of certain hydroxypyrido[3,4-f)]pyrazines or their 0x0 tautomers analogous to that well-known in the pteridine field (63JCS5156). Related one-electron reduction products of laser photolysis experiments with 1 -deazaflavins have been described (79MI21502). 

The oxidation of hydrocarbons involves the sequential formation of a number of similar reactions in which various intermediate radicals which are combinations of carbon, hydrogen and oxygen are formed. In the simplest case, the oxidation of medrane, the methyl radical CH3 plays an important part both in direct oxidation to CO(g) and in indirect oxidation duough the formation of higher hydrocarbons such as CaHe before CO is formed. The chain reactions include... 

There are very few homolytic reactions on triazolopyridines. A suggestion that the ring opening reactions of compound 1 involved free radical intermediates is not substantiated (98T9785). The involvement of radical intermediates in additions to ylides is discussed in Section IV.I. The reaction of radicals with compound 5 and its 1-substituted derivatives gives 4-substituted compounds such as 234 (96ZOK1085). A more detailed study of the reaction of the 1-methyl and 1-phenyl derivatives with r-butanol and ammonium persulfate produced 4-methyl substitution with a silver nitrate catalyst, and the side chain alcohol 235 without the catalyst (96ZOK1412). 

The wide variety of methods available for the synthesis of orga-noselenides,36 and the observation that the carbon-selenium bond can be easily cleaved homolytically to give a carbon-centered radical creates interesting possibilities in organic synthesis. For example, Burke and coworkers have shown that phenylselenolactone 86 (see Scheme 16), produced by phenylselenolactonization of y,S-unsaturated acid 85, can be converted to free radical intermediate 87 with triphenyltin hydride. In the presence of excess methyl acrylate, 87 is trapped stereoselectively, affording compound 88 in 70% yield 37 it is noteworthy that the intramolecular carbon-carbon bond forming event takes place on the less hindered convex face of bicyclic radical 87. 

Recall from Section 2.9 that most radicals are very reactive. Because one of the products is another radical, this reaction is a propagation step (a step in which one reactive radical intermediate produces another). In a second propagation step, the methyl radical may react with a chlorine molecule ... 

The positive charge should reside on a complex entity, and there is no ready means for assessing the products of the neutralization process. Although we know that neutralization must yield 3.8 intermediates/100 e.v., there is no chemical evidence for their contribution to the product distribution. This cannot be interpreted by neutralization yielding predominantly hydrogen atoms, ethyl radicals, or methyl radicals. One can quantitatively account for these intermediates on the basis of the distribution of primary species and second- and third-order ion-molecule reactions (36). 

With 6-alkenoic acids the intermediate radical partially cyclizes to a cyclopentyl-methyl radical in a 5-exo-trig cycHzation [139] (Eq. 6) [138 a, 140] (see also chap. 6). To prevent double bond migration with enoic acids the electrolyte has to be hindered to become alkaline by using a mercury cathode. Z-4-Enoic acids partially isomerize to -configurated products. Results from methyl and deuterium labelled carboxylic acids support an isomerization by way of a reversible ring closure to cyclopropyl-carbinyl radicals. The double bonds of Z-N-enoic acids with N > 5 fully retain their configuration [140]. 

After the first unsuccessful attempts to record a matrix IR spectrum of the methyl radical, reliable data were obtained by the use of the vacuum pyrolysis method. IR spectra of the radicals CH3 and CD3 frozen in neon matrices were measured among the products of dissociation of CH3I, (CH3)2Hg and CD3I (Snelson, 1970a). The spectra contained three absorptions at 3162 (1 3), 1396 V2) and 617 cm (I l) belonging to the radical CH3 and three bands 2381, 1026 and 463 cm assigned to the radical CD3. Normal coordinate analysis of these intermediates was performed and a valence force field calculated. In accordance with the calculations, methyl radical is a planar species having symmetry >31,. 

Methyl radicals have heoi detected in the gas i iase over a Sr/LajO, catalyst during the reaction of CH4 with NO, provided Oj is present in the system. In the absence of O2 the concentration of CHj- radicals decreases almost to the background level. The results indicate that the enhanced effect of Oj on the reduction of NO by CH4 may be due to surface-generated gas-phase CH,- radicals, but in the absence of O2 another reaction pathway may be dominant. Evidence has been found for the presence of CHjNO, a likely intermediate in the radical reaction, at temperatures up to 800 °C. 

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