Reductive electron transfer

Because the breadth of chemical behavior can be bewildering in its complexity, chemists search for general ways to organize chemical reactivity patterns. Two familiar patterns are Br< )nsted acid-base (proton transfer) and oxidation-reduction (electron transfer) reactions. A related pattern of reactivity can be viewed as the donation of a pair of electrons to form a new bond. One example is the reaction between gaseous ammonia and trimethyl boron, in which the ammonia molecule uses its nonbonding pair of electrons to form a bond between nitrogen and boron ... 

Whatever the reason may be behind the strict necessity to deprotonate the flavin donor, the reduced and deprotonated flavin was established in these model studies to be an efficient electron donor, able to reduce nucle-obases and oxetanes. In the model compounds 1 and 2 the pyrimidine dimer translates the electron transfer step into a rapidly detectable chemical cycloreversion reaction [47, 48], Incorporation of a flavin and of a cyclobutane pyrimidine dimer into DNA double strands was consequently performed in order to analyse the reductive electron transfer properties of DNA. 

SCHEME 1 Schematic illustration of the biological process of 02 dismutation into 02 and H202 catalyzed by Cu, Zn-SOD via a cyclic oxidation-reduction electron transfer mechanism. (Reprinted from [98], with permission from Elsevier.)... 

H.-A. Wagenknecht, Reductive electron transfer and excess electron migration in DNA. Angew. Chem., Int. Ed. 42, 2454-2460 (2003). 

The redox potential diagram in eq. 1 illustrates that the effect of optical excitation is to create an excited state which has enhanced properties both as an oxidant and reductant, compared to the ground state. The results of a number of experiments have illustrated that it is possible for the excited state to undergo either oxidative or reductive electron transfer quenching (2). An example of oxidative electron transfer quenching is shown in eq. 2 where the oxidant is the alkyl pyridinium ion, paraquat (3). 

R acts as an electron acceptor and is therefore reduced (reductive electron transfer Figure 6.19) ... 

Figure 6.19 Molecular orbital representation of reductive electron transfer...

The oxidative and reductive properties of molecules can be enhanced in the excited state. Oxidative and reductive electron transfer processes according to the following reactions ... 

Anion-radicals are the first products of reductive electron transfer. Mntnal repulsion of the primary and snbseqnent excess electrons forms the basis of the impediment of polyelectron reduction at the initial step. However, an orbital, which already has one electron, can be populated by another electron if a proper redncer is chosen. This reducer must be able to overcome Coulomb repulsion. As a resnlt, more or less stable dianions are formed. Like anion-radicals, dianions can reversibly give excessive electrons back. If skeleton rearrangement is absent, the initial uncharged molecules are regenerated. 

The majority of the enzyme-catalyzed reactions discussed so far are oxidative ones. However, reductive electron transfer reactions take place as well. Diaphorase, xanteneoxidase, and other enzymes as well as intestinal flora, aquatic, and skin bacteria—all of them can act as electron donors. Another source of an electron is the superoxide ion. It arises after detoxification of xenobiotics, which are involved in the metabolic chain. Under the neutralizing influence of redox proteins, xenobiotics yield anion-radicals. Oxygen, which is inhaled with air, strips unpaired electrons from these anion-radicals and gives the superoxide ions (Mason and Chignell 1982). 

Azzena and colleagues have shown the [l,2]-Wittig-type rearrangement of aromatic acetals 55 induced by reductive electron transfer with lithium to give 56 (equation 30) , which was originally discovered by Schlenk and Bergmann in 1928 (with sodium). ... 

Apart from the all-carbon backbone, poly(vinyl ester)s also exhibit a unique 1,3-diol structure (see Fig. 1). This structure is a common motif in many natural materials, e.g. carbohydrates. A number of oxidative or reductive electron transfer processes catalysed by natural redox systems are imaginable for this motif. The 1,3-diol structure is unique for a synthetic polymer and cannot be found in any other synthetic polymer class of significance. This explains the unusual biodegradation properties discussed below. 

The collision between reacting atoms or molecules is an essential prerequisite for a chemical reaction to occur. If the same reaction is carried out electrochemically, however, the molecules of the reactants never meet. In the electrochemical process, the reactants collide with the electronically conductive electrodes rather than directly with each other. The overall electrochemical Redox reaction is effectively split into two half-cell reactions, an oxidation (electron transfer out of the anode) and a reduction (electron transfer into the cathode). 

Methyl viologen (/V, /V - d i m e t h I -4,4 - b i p r i d i n i u m dication, MV2+ ) can function as an electron acceptor.34 When MV2+ is linked to electron donor, photoinduced electron transfer would occur. For example, within molecule 24 the 3MLCT excited state of [Ru(bpy)3]2+ is quenched by MV2+ through oxidative electron transfer process. The excited state of [Ru(bpy)3]2 + can also be quenched by MV" + and MV°. The transient absorption spectroscopic investigations show that the quenching of the excited state of [Ru(bpy)3]2+ by MV + and MV° is due to the reductive electron transfer process. Thus, the direction of the photoinduced electron transfer within molecule 24 is dependent on the redox state of MV2 +, which can be switched by redox reactions induced chemically or electrochemically. This demonstrates the potential of molecule 24 as a redox switchable photodiode.35... 

In photochemical reductions, electron transfer from the reducing agent (which is usually the solvent) to bonded fluorine is the rate-determining step ... 

Charge reversal in the electron transfer can be observed if donor sensitizers are employed. For example, photosolvolysis of cyclohexene oxide (135), may proceed through the epoxide radical anion. Analogous fragmentation from stilbene oxide and extrusion of SC>2 from benzylsulfone has been reported when amine sensitizers are employed (136). In fact, reductive electron transfer to cyclic sulfites or carbamates, eq. 49 (137),... 

In principle, an organic molecule can accept as many electron pairs as it has low-lying vacant orbitals. In the same way, high-lying occupied orbitals can release not a single, but several electrons. Such multielectron processes can result in the formation of poly(ion radicals). As seen later, the main topic of interest in poly(ion radicals) consists in their spin multiplicity. Therefore, it is meaningless to divide the material in the anion and cation radical parts. Let us therefore discuss first the reductive electron transfer. 

Quinones are cyclic conjugated diketones. They are colored compounds used as dyes. They also play important roles in reversible biological oxidation-reduction (electron-transfer) reactions. 

Reductive Electron Transfer and Excess Electron Transport in DNA... 

V potential corresponds to E[dU/dU(H) ], Pz is a stronger electron donor than Py since the reduction potential of Pz in the excited state, E(Pz+ /Pz ), is -2.0 V [52]. In order to use Py and Pz as electron donors in DNA, we synthesized Py-dU and Pz-dU as described in the previous sections. By this synthetic approach, we are able to photoinitiate exclusively a reductive electron transfer since the intramolecular electron transfer in the Py-dU and the Pz-dU moieties can be regarded... 

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