Radicals hydroxymethyl

Simple alkyl radicals such as methyl are considered to be nonnucleophilic. Methyl radicals are somewhat more reactive toward alkenes bearing electron-withdrawing substituents than towards those with electron-releasing substituents. However, much of this effect can be attributed to the stabilizing effect that these substiments have on the product radical. There is a strong correlation of reaction rate with the overall exothermicity of the reaction. Hydroxymethyl and 2-hydroxy-2-propyl radicals show nucleophilic character. The hydroxymethyl radical shows a slightly enhanced reactivity toward acrylonitrile and acrolein, but a sharply decreased reactivity toward ethyl vinyl ether. Table 12.9 gives some of the reactivity data. 

Minisci reactions have also been applied to these compounds. formation by exposure to w-CPBA and O-methylation with Meerwein s reagent converted 54 into 55. Nucleophilic attack of the hydroxymethyl radical, generated with ammonium sulfate, provides an alternate route to 2-hydroxymethyl pyridines 56. 

Fig. 26. Schematic representation of the decay mechanisms of the hydroxymethyl radical. The 3s Rydberg state and the ground state have a conical intersection leading to the ground state H2CO + H products. (From Hoffman et al.170)...

Organometallic radicals are important intermediates in biological and catalytic reactions. The structure and formation mechanism of radicals trapped in y-irradiated molecular sieves exposed to methanol and ethylene have been studied by EPR spectroscopy. It was found that Ag CH2OH+ radical with one-electron bond between Ag and C is formed by the attack of -CH2OH hydroxymethyl radical on Ag+ cation. 

Based on hyperfine splittings used for the simulation of the 109Ag 13CH2OH+ spectrum it was calculated that the unpaired electron in the silver hydroxymethyl radical is localised to the extent of 29% on Ag and 46% on C and H nuclei. By comparison these spin... 

Figure 1. EPR A/CH3OH zeolite A(A ) and atoms at different sites B, CH2OH hydroxymethyl radicals E, Ag CH2OH silver hydroxymethyl radical cation...

At the UB3LYP/6-31G(d) and CBS-RAD levels of theory, barriers for the limited number of hydroxymethyl radical additions exhibit larger MADs from experiment (8.5 and 7.3 kJ/mol) than those observed for the methyl radical additions (Tables 6.15 and 6.16). In contrast, barriers obtained at the UB3LYP/6-311+G(3df,2p) and G3(MP2)-RAD levels... 

Table 6.17 Comparison of experimental barriers at 0 K (kJ/mol) for the addition of hydroxymethyl radical C OH to alkenes CH2=CXY with those calculated with wavefunction-based electronic structure methods.8...

Calculated barriers for a selection of cyanomethyl radical additions are presented in Tables 6.19 and 6.20. The CBS-RAD method performs particularly well (MAD of 1.3 kJ/mol) for the selected cyanomethyl radical additions. However, the other levels of theory show somewhat larger mean absolute deviations (5.9 - 14.8 kJ/mol). With the exception of CBS-RAD (MD of -0.9 kJ/mol), all levels give higher barriers than those observed experimentally (MD of +5.9 to +14.8 kJ/mol). The correlation with experiment (R2 = 0.76 - 0.85) is somewhat poorer than that for the methyl and hydroxymethyl radical additions (Tables 6.15 - 6.18). 

Recent studies conducted by the same group revealed that the radical cation of toluene generated by photoinduced electron transfer can be deprotonated in a protic cosolvent and thus efficient trapping by electrophilic alkenes is feasible, yielding benzylation products. Secondary hydrogen abstraction by the benzyl radical from methanol generates hydroxymethyl radicals, which can also be used for preparative hydroxymethylation of alkenes (Scheme 18) [24],... 

Grotheer, H.-H., G. Riekert, D. Walter, and Th. Just, Non-Arrhenius Behavior of the Reaction of Hydroxymethyl Radicals with Molecular Oxygen, J. Phys. Chem., 92, 4028-4030 (1988). 

The conformational barriers in acyclic radicals are smaller than those in closed-shell acycles, with the barrier to rotation in the ethyl radical on the order of tenths of a kilocalorie per mole. The barriers increase for heteroatom-substituted radicals, such as the hydroxymethyl radical, which has a rotational barrier of 5 kcal/mol. Radicals that are conjugated with a n system, such as allyl, benzyl, and radicals adjacent to a carbonyl group, have barriers to rotation on the order of 10 kcal/mol. Such barriers can lead to rotational rate constants that are smaller than the rate constants of competing radical reactions, as was demonstrated with a-amide radicals, and this type of effect permits acyclic stereocontrol in some cases. "... 

As aldehydes are not detected in aqueous sodium hydroxide, they must be rapidly further oxidized. Possibly they form diolates, which should be strongly adsorbed at the electrode and have been identified as intermediates in the anodic oxidation of aldehydes On the other hand it is questioned wether aldehydes are intermediates at all in the further oxidation of the a-hydroxymethyl-radical to the carboxylic add ). 

The methoxy radical may also be converted to the hydroxymethyl radical (CH2OH) by reaction with methanol,... 

The two isomers CH3O and CH2OH have quite different reaction characteristics. The hydroxymethyl radical mainly reacts with oxygen to form the second desired end product, formaldehyde ... 

This third primary process produced the hydroxymethyl radical which was needed to explain the third low energy peak in the translational energy distribution curve. From the areas associated with each of the peaks in the translational distribution curves they were able to determine that the relative quantum yields for each of these primary process were 9,4,1, and respectively, for reactions 19, 20, and 21. 

These redox chain reactions, which cycle iron(II) and iron(III), have advantages over methods that use stoichiometric quantities of oxidants because the hydroxymethyl radical is also a good reductant and, at high oxidant concentrations, it may be oxidized more rapidly than it adds to (72). The disadvantage of this type of reaction is that the initial radical is generated by a relatively non-selective hydrogen atom abstraction reaction. To be efficient, the H-donor must be used in large excess it is often a cosolvent. Nonetheless, this is a very practical method to prepare hydroxyalkylated and acylated heteroaromatic and related derivatives. 

Schuchmann H-P, von Sonntag C (1981) Photolysis at 185 nm of dimethyl ether in aqueous solution Involvement of the hydroxymethyl radical. J Photochem 16 289-295 Schuchmann H-P, von Sonntag C (1984) Methylperoxyl radicals a study ofthey-radiolysis of methane in oxygenated aqueous solutions. Z Naturforsch 39b 217—221 Schuchmann MN, von Sonntag C (1977) Radiation chemistry of carbohydrates. Part 14. Hydroxyl radical-induced oxidation of D-glucose in oxygenated aqueous solution. J Chem Soc Perkin Trans 2 1958-1963... 

Although a-hydroxyalkyl radicals such as the hydroxymethyl radical are oxidized without an adduct being noticed [reaction (14) k= 1.6 x 108 dm3 mol1 s 1], such a complex becomes apparent in the case of P-hydroxyalkyl radicals [reactions (15) and (16) k15 = 3 x 107 dm3 mol1 s k16 = 330 s Freiberg and Meyer-stein 1980], whereby the epoxide is formed (Soylemez and von Sonntag 1980). 

The reduction of disulfides is also only given by the hydroxymethyl radical anion [reaction (18)] while the hydroxymethyl radical itself is practically unreac-tive (Akhlaq et al. 1989). 

The 2-hydroxy-2-propyl radical anion is a stronger reductant than the hydroxymethyl radical anion. Thus, a number of reactions are readily given by the for-... 

It is important to note that the rate of reaction of alkyl radicals with thiols does not simply correlate with the exothermicity of the reaction, i.e., with the BDE of the C-H bond to be formed. For example, the tertiary 2-hydroxypropyl radical reacts more readily with thiols than the primary hydroxymethyl radical, and this reacts even faster than the methyl radical (Table 6.4). The reason for this surprising behavior has been discussed in terms of the charge and... 

Alkyl radicals are nucleophilic radicals (cf. Walbiner et al. 1995 Wu and Fischer 1995 Wu et al. 1995 Heberger and Lopata 1998), and the preferred position of addition at a polarized C-C double bond is reversed compared to that of the electrophilic OH. Thus, in the case of Ura, the hydroxymethyl radical adds preferentially to the C(6)-position k 104dm3 mol1 s1 (Schuchmann et al. 1986 Chap. 10.5)]. 

Schuchmann H-P, von Sonntag C (1988) The oxidation of methanol and 2-propanol by potassium peroxodisulphate in aqueous solution free-radical chain mechanisms elucidated by radiation-chemical techniques. Radiat Phys Chem 32 149-156 Schuchmann H-P, Wagner R, von Sonntag C (1986) The reactions of the hydroxymethyl radical with 1,3-dimethyluracil and 1,3-dimethylthymine. Int J Radiat Biol 50 1051-1068 Schuchmann MN, von Sonntag C (1982) Determination of the rate constants of the reactions C02 + OH" -> HC03 and barbituric acid —> barbiturate anion + H+ using the pulse radiolysis technique. Z Naturforsch 37b 1184-1186... 

Wu JQ, Fischer H (1995) Absolute rate constants for the addition of hydroxymethyl radicals to al-kenes in methanol solution. Int J Chem Kinet 27 167-179... 

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