Carboxymethyl radical

Homolytic substitution has been little studied, and work has been confined to the reaction of dibenzofuran with carboxymethyl radicals produced from acetyl peroxide or di-tcrt-butyl peroxide in boiling acetic acid or by pyrolysis of chloroacetylpolyglycolic acids. The method of analysis of the resultant mixture of 1- (55%), 4- (30%), and 3-dibenzofuranacetic acid (15%) was crude, but the results were in accord with simple HMO calculations. The amount of the 1-substituted product is perhaps surprising in view of the steric hindrance at this position. 

Although they suggested the formation of methyl radicals, they did not actually detect the methyl radicals in aqueous acetic acid. On the other hand, Kaise et al. have observed carboxymethyl radicals CH2COOH and methyl radicals CH3 by using a flow cell by ESR spectroscopy.29) We examined the dependence of radical concentrations on the flow rate and found that the amounts of these two intermediate radicals changed, reflecting the different reaction paths.39 31)... 

The stoichiometric oxidation of alkenes by Mn(OAc)3 in acetic acid at 120 °C affords 7-lactones n good yield, via the homolytic addition of carboxymethyl radical to the double bond (equations 104 and 205).504-506... 

In termination reactions, all mesomeric structures may contribute. Cases in point, where one would not immediately expect this to play a significant role are the a-carboxymethyl radicals (Wang et al. 2001). For some of the nucleobase radicals also more than one mesomeric structure maybe written (Chap. 10), and it is not unlikely that also here this aspect has to be taken into account. 

Wang W-F, Schuchmann MN, Bachler V, Schuchmann H-P, von Sonntag C (1996) The termination of CH2OH/CH2O radicals in aqueous solutions. J Phys Chem 100 15843-15847 Wang W-F, Schuchmann MN, Schuchmann H-P, von Sonntag C (2001) The importance of mesomery in the termination of a-carboxymethyl radicals from aqueous malonic and acetic acids. Chem Eur J 7 791-795... 

The proposed mechanism219 involves the addition of carboxymethyl radicals, formed by the oxidation of acetic acid by Mn(III), followed by oxidation of the resulting radical ... 

Kooyman et a/.232-234 showed that no direct reaction between the arene and Mn(IIl) occurred in the manganic acetate oxidation of benzene, chlorobenzene, and toluene in acetic acid. Products were satisfactorily explained by the participation of carboxymethyl radicals as intermediates analogous to those formed in the manganic acetate oxidation of olefins [see Section II.B.3.a]. Substituted... 

By contrast, oxidation of p-cymene by Mn(III) acetate246 afforded products derived from reactions of intermediate p-cymyl and carboxymethyl radicals, indicating that reaction did not proceed via electron transfer. 

An alternative route for the oxidation of carboxylic acids not involving decarboxylation has been demonstrated for the reaction of manganese-(III)219 221 232 234 and cerium(IV).238a-b Carboxymethyl radicals are formed in the reaction. 

C—H bond, in HO Ac the carboxymethyl radical. This type of reaction occurs by rate-determining enolization of the radical-producing species (de Klein, 1977 Southwick, 1970 van der Ploeg et al., 1968). 

Manganese(m) acetate oxidation (cf. Vol. 3, p. 34) of camphene gives (186) as a 95 5 mixture by carboxymethyl radical insertion no rearranged products were obtained, in contrast to /3-pinene which gave Wagner-Meerwein products only, and no free-radical insertion.279 The E- and Z-isomers of (187) probably result from a non-concerted biradical intermediate formed by benzyne addition to camphene.280 Benzyl-lithium adds to the aminocamphor (188) exclusively from the exo-side whereas only the competing enolization reaction occurs with more sterically hindered organometallics.281... 

Similar Sjj2 mechanism takes place when H atoms react with two other thioethers, each containing either carbonyl or carboxyl groups. The pulse radiolysis of acidic aqueous solutions of 2-(methylthio)-ethanoic acid and S-ethylthioacetone yielded carboxymethyl radicals, CHjCOOH, and acetylmethyl radicals, CH2-C(=0)-CH3, respectively. Support for this assignment was given by DFT calculations and radical scavenging experiments with trrt-butanol. DFT calculations indicated that the thermochemistry of the Sjj2 reactions should be feasible with AG between -29 and -33 kcal mol" in both cases. 

The oxidation of substituted aromatic hydrocarbons by Mn(OAc)3 in refluxing acetic acid proceeds by two competing mechanisms (a) a free radical addition of carboxymethyl radical to the nuclear ring, forming 2-arylacetic acids and (b) a benzylic hydrogen abstraction by Mn resulting in the formation of benzyl acetates. " In the presence of strong acids such as sulfuric, trichloro- or trifluoro-acetic acids, only benzylic oxidation products are readily formed at room temperature (equation 207). ... 

For the reaction of Mn " acetate with substituted toluenes in acetic acid (130 °C) under anaerobic conditions, an analogous correlation between log( iy i ) and ff exists (Heiba et al. [Ha] Figure 2). Here, however, the reaction is not the direct interaction of Mn" with the arene but the reaction of carboxymethyl radicals, generated from Mn " acetate, with the a-hydrogen atoms of the alkyl group on the arene (eqs. (22) and (23)). 

In the case of Mn , an analog of eq. (34) also occurs. In addition Mn can attack acetic acid by an enol mechanism to produce carboxymethyl radicals ... 

Manganese(III) acetate oxidation of cyclohexane gives the same products as the Co(OAc>3 oxidation. The selectivity of these two oxidants is, however, different. The former thermolyzes to give carboxymethyl radicals and the reaction products are formed by... 

The mechanism accounting for the production of these three compounds is a nonchain free-radical process involving carboxymethyl radicals ... 

When decomposition of the metal salt acetate occurs more easily than electron transfer, the aromatic substrate is attacked by methyl radicals and/or carboxymethyl radicals. Both Pb(IV) and Ce(IV) acetates lead to methylation and carboxymethylation (Heiba et al., 1968a Heiba and Dessau, 1971), while Mn(III) acetate leads only to the latter (Heiba et al., 1969a). Carboxymethylation produces not... 

Exposure of longifolene to manganic acetate in refluxing acetic acid-acetic anhydride furnished in poor yield (9%) the product (66), the stereochemistry of which remains unresolved. Under the same conditions, camphene gave in 30% yield y-lactone mixture (67), in which one isomer predominated 49). A carboxymethyl radical arising from the thermolysis of Mn (III) acetate is considered to be the reactive species in such reactions (50). 

Alkyl radicals can be obtained by abstraction of a hydrogen atom from an alkyl group by another radical. This method was utilized for the generation of benzyl radicals from toluene with tert-butoxy radical obtained on heating di-ier -butyl peroxide. Benzoyl and carboxymethyl radicals have also been obtained by this method. The reaction gives rise to a complex mixture of products and therefore is of rather limited use. 

Reaction with Allenes. Reaction of Mn(OAc)3-generated carboxymethyl radicals with allenes produce furan-2(5//)-ones (eq 22). At least one aryl group on the allene is necessary for the reaction to proceed. The other substituent has no effect on regios-electivity. In case of monosubstituted aUenes, the corresponding dimethyl-2-(2-oxoethylidene)malonates are obtained in fair yields (eq 23). 

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