Methyl radical transfer

The presence of Au(I) may likewise catalyze the reaction of Au(III) with methylcob(III)alamin Wood has included this couple under his Redox Switch category. Like PtCl l-, the reaction of AuC14 with CH3B12 is enhanced by the presence of Br" (46). It remains to be proven that the Au reaction does involve methyl radical transfer, presumably with subsequent oxidation of B12r [a cob(II)alamin compound] to the observed aquocobalamin. However, the available evidence for the formation of Au(II) intermediates is more extensive and convincing (200) than for Pt(III) intermediates (201). 

The activation energy of this isomerization (38) would be regarded to be the same as that for (12). The formation of i-butyraldehyde as an important product requires not only the loss of a carbon atom from the structure, but also an internal H atom transfer from an adjacent carbon atom. Zeelenberg [181] and Fish [27] proposed a (1,4) methyl radical transfer,... 

Metathetical reactions of methyl radicals transfer of hydrogen atoms... 

Homolytic cleavage of the Co-C bond of methylcobalamin leads to methyl-radical transfer. For this reaction to occur, the attacking species must be a free radical, and so the generation of such a radical intermediate is necessary either by the one equivalent oxidation of a metal ion in the reduced state of a redox couple (e.g., > SnIII), or by a one equivalent reduction of a... 

This methyl radical transfer onto the nickel-methanocorphin center would proceed from the methyl-coenzyme M and would be followed by protonation to form CH4. Protonation on nickel of a Ni-CHs complex indeed leads to methane through reductive elimination in the intermediate complex LnNi(H)(CH3). The structure of the enzyme is still unknown, however. 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

Another possible mechanism also involves a methyl radical (mechanism 3). In this case, electron transfer from one of the reduced clusters on CODH/ACS to methyl-Co(III) would form a methyl-Co(II) species that can disproportionate to form Co(I) and methyl-Ni(II). 

Investigation of direct conversion of methane to transportation fiiels has been an ongoing effort at PETC for over 10 years. One of our current areas of research is the conversion of methane to methanol, under mild conditions, using li t, water, and a semiconductor photocatalyst. Research in our laboratory is directed toward ad ting the chemistry developed for photolysis of water to that of methane conversion. The reaction sequence of interest uses visible light, a doped tungsten oxide photocatalyst and an electron transfer molecule to produce a hydroxyl i cal. Hydroxyl t cal can then react with a methane molecule to produce a methyl radical. In the preferred reaction pathway, the methyl radical then reacts with an additional wata- molecule to produce methanol and hydrogen. 

Perchlorotriphenyl methyl radicals are particularly persistent . Among the factors contributing to the exceptional persistency of this kind of radicals the steric shielding of the a-(tricovalent) carbon is predominant. Only hydrogen or electron can reach the carbon radical. Thus, when perchloro radicals are formed in a DMSO-alkaline hydroxide solution an electron transfer occurs, leading to the perchlorocarbanions. It is assumed that the donor is the DMSO carbanion. 

Studies have been carried out on the methylated complex [H3C-Niin(17)(H20)]2+, which is obtained from the reaction of methyl radicals (generated by pulse radiolysis) with [Ni(17)]2+. The volumes of activation are consistent with the coherent formation of Ni—C and Ni—OH2 bonds, as expected for the generation of a Ni111 complex from a square planar Ni11 precursor.152 The kinetics of reactions of [H3C-Niin(17)(H20)] + involving homolysis, 02 insertion and methyl transfer to Crn(aq) have been determined, and intermediates have been considered relevant as models for biological systems.153 Comparing different alkyl radicals, rate constants for the... 

A rapid scission of the Me—B bond (on the picosecond timescale) generates the methyl radical (CH ) and BMe3. The coupling of pyridine and methyl radicals within the solvent cage completes the methyl transfer (equation 47). 

Peroxynitrite reacts with heme proteins such as prostacycline synthase (PGI2), microperoxidase, and the heme thiolate protein P450 to form a ferryl nitrogen dioxide complex as an intermediate [120]. Peroxynitrite also reacts with acetaldehyde with the rate constant of 680 1 mol 1 s" 1 forming a hypothetical adduct, which is decomposed into acetate, formate, and methyl radicals [121]. The oxidation of NADH and NADPH by peroxynitrite most certainly occurs by free radical mechanism [122,123], Kirsch and de Groot [122] concluded that peroxynitrite oxidized NADH by a one-electron transfer mechanism to form NAD and superoxide ... 

Cyclohexadienylidenes, disubstituted at the 4-position are expected to be kinetically more stable than the parent carbene, however, the rearrangement to benzene derivatives is still very exothermic. The gas phase chemistry of 4,4-dimethyl-2,5-cyclohexadienylidene Is was investigated by Jones et al.100,101 The gas phase pyrolysis of the diazo compound 2s produces a mixture of p-xylene and toluene, and by crossover experiments it was demonstrated that the methyl group transfer occurs intermolecularly via free radicals. Thus, the pyrolysis of a mixture of the dimethyl and the diethyl derivative 2s and 2t... 

An unexpectedly encountered peak in the spectrum of reframidine (27) is due to the (M + 14)+ ion, formed by the transfer of a methyl radical from one molecular ion to another, followed by hydrogen expulsion (77). Furthermore, an (M - 16)+ peak has been spotted in the spectrum of amurensine (24), being attributable to the elimination of a methyl radical from the (M — 1)+ ion (77). The presence of a rather unusual (M — 17)+ peak in the spectrum of amuren-sinine (25) remains unexplained (71). 

Two situations are conducive to ipso attack. If polar effects come into play in stabilizing the transition state of the addition of the radical, then frequently ipso attack is encountered. This is clearly brought out in the different behaviour of adamantyl and methyl radicals towards the same substrate. It has been firmly established that while methyl and phenyl radicals are electroneutral, the bridgehead adamantyl radical behaves as a nucleophilic species (80ACR51). If this adamantyl radical is reacted with thiophene substrates made electron deficient by the presence of suitable substituents, then the transition state of the addition step may have the character of a charge-transfer complex the site at which the... 

Notice that whereas in Eq. 16-43 the methyl group is transferred as CH3+ by nucleophilic displacement on a carbon atom, the transfer to Hg2+ in Eq. 16-44 is that of a carbanion, CH3, with no valence change occurring in the cobalt. However, it is also possible that transfer occurs as a methyl radical.420 Methyl corrinoids are able to undergo this type of reaction nonenzymatically, and the ability to transfer a methyl anion is a property of methyl corrinoids not shared by other transmethy-... 

An alternative route for the synthesis of TV-methyl amino acids without racemization is shown in Scheme 8.[98 This method includes the use of TBPB in the presence of copper(I) octanoate. The proposed mechanism of this free radical reaction is given in Scheme 8. Electron transfer from copper(I) to TBPB affords the copper(II), benzoate, and tBuO radical 4, which undergoes (3-scission to acetone and methyl radical 5. In turn, electron transfer from the urethane to the copper(II) ion, followed by proton transfer, affords the corresponding urethane radical 6, which reacts with the methyl radical 5 to give the desired product in overall yields of 54% (Z derivative) or 57% (Boc derivative), respectively. 

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