Electron transfer variants

It is evident from the exceptions noted that the mechanism proposed above does not fully capture the pathways open to the Patemo-Biichi reaction. A great deal of effort has been devoted to deconvoluting all of the possible variants of the reaction. Reactions via singlet state carbonyls, charge-transfer paths, pre-singlet exciplexes, and full electron transfer paths have all been proposed. Unfortunately, their influence on product... 

In this last interpretation, B could promote the electron transfer through a superexchange mechanism involving the (P B H) charge-transfer state. A variant of this model involves an internal charge-transfer state of the Bchl dimer [172]. 

Steady state kinetics and protein-protein binding measurements have also been reported for the interaction of these mutant cytochromes with bovine heart cytochrome c oxidase [120]. The binding of cytochrome c variants to the oxidase occurred with increasing values of Kj in the order He (3 x 10 Mol L ) < Leu = Gly < wild-type < Tyr < Ser (3 x 10 molL ). Steady-state kinetic analysis indicated that the rate of electron transfer with cytochrome c oxidase increased in the order Ser < He < Gly < Leu < Tyr < wild-type, an order notably different from that observed for a related analysis of the oxidation of these mutants by cytochrome c peroxidase [85]. This difference in order of mutant turnover by the oxidase and peroxidase may arise from differences in the mode of interaction of the cytochrome with these two enzymes. 

As part of a subsequent study concerning primarily second-site revertant yeast iso-l-cytochrome c variants, Hazzard et al. evaluated the effect of converting Lys-72 to an aspartyl residue by site-directed mutagenesis on the electron transfer kinetics of the cytochrome c-cytochrome c peroxidase complex [136]. Lys-72 was of interest for this purpose, because it is involved in the hypothetical model for the complex formed by these two proteins that was proposed by Poulos and Kraut on the basis of molecular graphics docking [106]. In these... 

Probably the most familiar radical reactions leading to 1,2-D systems are the so called acyloin condensation and the different variants of the "pinacol condensation". Both types of condensation involve an electron-transfer from a metal atom to a carbonyl compound (whether an ester or an aldehyde or a ketone) to give a radical anion which either dimerises directly, if the concentration of the species is very high, or more generally it reacts with the starting neutral carbonyl compound and then a second electron is transferred from the metal to the radical dimer species (for an alternative mechanism of the acyloin condensation, see Bloomfield, 1975 [29]). 

Swartz and Stenzel (1984) proposed an approach to widen the applicability of the cathode initiation of the nucleophilic substitution, by using a catalyst to facilitate one-electron transfer. Thus, in the presence of PhCN, the cathode-initiated reaction between PhBr and Bu4NSPh leads to diphe-nydisulfide in such a manner that the yield increases from 10 to 70%. Benzonitrile captures an electron and diffuses into the pool where it meets bromobenzene. The latter is converted into the anion-radical. The next reaction consists of the generation of the phenyl radical, with the elimination of the bromide ion. Since generation of the phenyl radical takes place far from the electrode, this radical is attacked with the anion of thiophenol faster than it is reduced to the phenyl anion. As a result, instead of debromination, substitution develops in its chain variant. In other words, the problem is to choose a catalyst such that it would be reduced more easily than a substrate. Of course, the catalyst anion-radical should not decay spontaneously in a solution. 

Romanian scientists compared one-electron transfer reactions from triphenylmethyl or 2-methyl benzoyl chloride to nitrobenzene in thermal (210°C) conditions and on ultrasonic stimulation at 50°C (lancu et al. 1992, Vinatoru et al. 1994, Chivu et al. 2006). In the first step, the chloride cation-radical and the nitrobenzene anion-radicals are formed. In the thermal and acoustic variants, the reactions lead to the same set of products with one important exception The thermal reaction results in the formation of HCl, whereas ultrasonic stimulation results in CI2 evolution. At present, it is difficult to elucidate the mechanisms behind these two reactions. As an important conclusion, the sonochemical process goes through the inner-sphere electron transfer. The outer-sphere electron transfer mechanism is operative in the thermally induced process. 

As emphasized by this latter study, it has been relatively straightforward to identify myoglobin variants that are five-coordinate in both oxidation states, but it has been far more difficult to identify variants or derivatives that are six-coordinate in both oxidation states. Myoglobins with this characteristic would have the potential to provide considerable insight into the role of various types of axial ligand in regulating the electron transfer reactivity of cytochromes and other types... 

These include 14-crown-4 ethers (30a-c), their aza analogs and other crown compounds. The phenoxide can provide a convenient counterion for the hthium cation. Otiier variants may involve fluorescence and photoinduced electron transfer. See text in Section III.A.3. 

An interesting variant of a geometric isomerization was observed for the 7,7-dimethylbicyclo[4.1.0]hept-2-ene system. The electron transfer reaction of the highly strained rrans-fused isomer (30) with 1-cyanonaphthalene rapidly converted it to the cw-fused system (31) [224], The observed rearrangement requires inversion at one of the tertiary cyclopropane carbons. This can be accomplished either by removal of a hydrogen (proton) or by cleavage of a cyclopropane or an allylic bond. 

One of the favourite generic arrangement is the molecular triad, consisting of a photoactive centre (PC), an electron donor (D) and an electron acceptor (A). In systems such as D-PC-A, the charge separated state D+-PC-A is obtained in two consecutive-electron transfer processes after excitation of PC. Of course, several variants exist, depending on the electron transfer properties of PC and its excited state, PC, as well as on the precise arrangement of the various components (PC-Ai-A2 or D2-Di-PC, in particular, if PC is an electron donor or an electron acceptor, respectively). 

Fig. 10.27. Ionic mechanism ("variant A") and radical mechanisms ("variants Ba or B2") for the addition of a Grignard reagent to an aldehyde. SET = single electron transfer.

A further variant is the oxidation of olefins by Mn(III) acetate in the presence of halide ions. Thus, oxidation of cyclohexene by Mn(III) acetate in acetic acid at 70°C is slow, but addition of potassium bromide leads to a rapid reaction. Cyclohexenyl acetate was formed in 83% yield.223 In contrast to what would be expected for an electron transfer mechanism, norbomene (ionization potential 9.0 eV) was unreactive at 70°C, whereas cyclohexene (ionization potential 9.1 eV) and bicyclo[3,2,l] oct-2-ene reacted rapidly. The low reactivity of norbomene can be explained, if oxidation involves attack at the allylic position... 

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