Electron transfer processes, ascorbate

The kinetic models for these reactions postulate fast complex-formation equilibria between the HA- form of ascorbic acid and the catalysts. The noted difference in the rate laws was rationalized by considering that some of the coordination sites remain unoccupied in the [Ru(HA)C12] complex. Thus, 02 can form a p-peroxo bridge between two monomer complexes [C12(HA)Ru-0-0-Ru(HA)C12]. The rate determining step is probably the decomposition of this species in an overall four-electron transfer process into A and H202. Again, this model does not postulate any change in the formal oxidation state of the catalyst during the reaction. 

It is important that nitroxides can oxidize ascorbate anions and thiols, by one-electron transfer processes. Indeed, when ascorbate is added to a nitroxide solution the ESR spectrum of the nitroxide is largely replaced by that of ascorbate (Liu et al., 1988b). 

In basic solutions ascorbate is apparently oxidized preferentially by the electron transfer process, which goes to completion in less than 2 fts after termination of the electron pulse (see Structure I). In nitrous-oxide-saturated acid solutions (pH 3.0-4.5), A and two other species which were shown to be OH-radical adducts were observed (37), thus confirming earlier observations (18,19,23, 25). The ascorbate radical anion was identified by its doublet of triplets spectrum that maintains its line position from pH 13 to 1. One OH-radical adduct (IV) shows a doublet, the lines of which start to shift below pH 3.0 it has a pK near 2.0, a decay period of about 100 fxs, and probably does not lead to formation of A". The other OH-radical adduct (II) is formed by addition of the OH radical to the C2 position its ESR parameters are = 24.4 0.0002 G and g == 2.0031 0.0002. Time growth studies suggest that this radical adduct converts to the ascorbate anion radical (III) with r 15 fxs, and accounts for 50% of the A signal intensity 40 fxS after termination of the electron pulse. The formation of the three radicals can be summarized as shown in Scheme 1. 

Miscellaneous Electron Transfer Processes. The electron transfer from ascorbate/ascorbic acid to a number of organic free radicals has been studied by the pulse radiolysis technique. The corresponding rate constants are summarized in Table IV. 

The stoichiometric redox reactions of ascorbic acid with oxidizing metal ions and metal chelates, of the type illustrated in Scheme 1, are also involved in the mechanisms of oxidation of ascorbic acid by various oxidants since they function as very efficient catalysts for such reactions. Further details concerning electron transfer processes in the metal chelates of ascorbic acid will be presented in the following discussion of the role of simple metal ascorbate chelates and of mixed ligand ascorbate chelates in the oxidation of ascorbic acid by molecular oxygen. 

Finally it should be pointed out that there is an alternate mechanism for the two successive electron transfer processes indicated by the sequence 14-> 15 -> 16- 13 a. It is quite possible that the ascorbate radical anion dissociates from the mixed ligand complex 15, prior to reoxidation of the Cu(I) ion, and recombines with another Cu(II) chelate prior to the final electron transfer step indicated by 16 17 +... 

The photocycloaddition of L-ascorbic acid derivatives (e.g., 93) with 4-chlorobenzaldehyde (94) and benzyl methyl ketone led to preferential attack on the less hindered a-face of the enone with approximately 2 1 regioselectivity (33% de for 96) (see Scheme 22) [147]. When the substrate was changed to benzophenone, the regioselectivity was reversed, even though the facial selectivity remained the same (35% de). This was proposed to be the result of a mechanistic switchover, from a 1,4 diradical process for benzophenone to a photoinduced electron transfer process for the other substrates. 

A theoretical study of the oxidation of triose reductone (27) has been completed, and the results compared with the oxidation of ascorbic acid. Oxidation of the acid by nitroxlde radical (28) has been examined kinetically by e.s.r. spectroscopy. An intermediate was observed and a 2-step, one-electron transfer process 56... 

In the presence of a biological unit that can react as a base (such as water or a nitrogen atom from a different molecule), the hydrogen atom from the C=NH unit in 69 is transferring electrons to carbon (forming a CN unit, a nitrile), and this electron transfer process leads to an elimination reaction that forms ethylene, and Fe(II),with transfer of the OH unit to a suitable acid. The products are ethylene and cyanoformate (which then decomposes to HCN + CO2). Ascorbic acid is utilized in this transformation. This sequence is one example in which an elimination process plays a key role in a biosynthetic transformation. 

Kinetics of the oxidation of ascorbic acid by S20g in the pH range 3.4-4.6, in the absence of metal ion catalysis, gave the empirical rate law -d[S20g ]/df = k([H2A] -I- [HA ])[S20g ] which is unaffected by the addition of a radical trap suggesting parallel two-electron transfer processes from H2A and HA to dehydroascorbic acid. The role of transition-metal-ion catalysis on these and other reactions of S20 has been studied. 

Electron transfer within the [M(HA)(02)](re 1)+ complex was envisioned as a two-stage process in which first, a 2p electron of the ascorbate oxygen is transferred to a t2g non-bonding or an eg antibonding orbital of the metal ion. The subsequent step is the transfer of an electron to the 7i 2p or 7ij2p orbital of the oxygen molecule. 

Detailed kinetics of some of these and other processes have been studied, e.g. oxidation by [Os(bipy)3]3+ of J , SCN",167 OH" 122 reaction of [Os(LL)3]3+ with OH- to give 02 (LL = bipy, phen, 5-Mephen, 4,4-Me2bipy),168 oxidation of ascorbic acid by [Os(LL)3]3+ (LL = bipy, 4,4 -Me2bipy) 169 and the photoinduced electron transfer reactions of acidopentaamminecobalt(III) complexes with [Os(bipy)3]2+.170 The outer-sphere oxidation of ascorbic acid by [Os(bipy)3]2+ incorporated in a coating on a graphite electrode has recently been reported.170... 

The first reaction describes the excitation of uranyl ions. The excited sensitizer can lose the energy A by a non-radiative process (12b), by emission (12c) or by energy transfer in monomer excitation to the triplet state (12d). Radicals are formed by reaction (12e). The detailed mechanism of step (12e) is so far unknown. Electron transfer probably occurs, with radical cation and radical anion formation these can recombine by their oppositely charged ends. The products retain their radical character. Step (12g) corresponds to propagation and step (12f) to inactivation of the excited monomer by collision with another molecule. The photosensitized initiation and polymerization of methacrylamide [69] probably proceeds according to scheme (12). Ascorbic acid and /7-carotene act as sensitizers of isoprene photoinitiation in aqueous media [70], and diacetyl (2, 3-butenedione) as sensitizer of viny-lidene chloride photopolymerization in a homogeneous medium (N--methylpyrrolidone was used as solvent) [71]. 

The propensity of 02 to remove protons from substrates accounts for its reactivity with acidic reductants and their overall oxidation. Thus, combination of 02 - with protic substrates (a-tocopherol, hydroquinone, 3,5-di-t-butylcatechol, L(- -)-ascorbic acid) yields products that are consistent with an apparent one-electron oxidation of the substrate and the production of HOOH. However, the results of electrochemical studies provide clear evidence that these substrates are not oxidized in aprotic media by direct one-electron transfer to 02. The primary step involves abstraction of a proton from the substrate by 02 to give substrate anion and the disproportionation products of HOO- (HOOH and O2). In turn, the substrate anion is oxidized by O2 in a multistep process to yield oxidation products and HOOH. Thus, by continuously purging the O2 that results from the... 

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