Electron transfer reduction

Fukuzumi, S., Ohkubo, K., Wenbo, E., Ou, Z., Shao, J., Kadish, K.M., Hutchison, J.A., Ghiggino, K.P., Sintic, P.J. and Crossley, M.J. (2003) Metal-centered photoinduced electron transfer reduction of a gold(III) porphyrin cation linked with a zinc porphyrin to produce a long-lived charge-separated state in nonpolar solvents. Journal of the American Chemical Society, 125, 14984-14985. 

The benzyl group can serve as a hydroxy-protecting group if acidic conditions for ether cleavage cannot be tolerated. The benzyl C—O bond is cleaved by catalytic hydrogenolysis,176 or by electron-transfer reduction using sodium in liquid ammonia or... 

Fig. 5 Marcus plot of electron transfer reductions of alkyl halides, as contrasted with archetypal SN2 substitution processes (Finkelstein reactions, circled see Eberson,...

The electron-transfer reduction of a large number of aromatic molecules involving an aryl carbon-heteroatom a-bond produces a frangible anion radical which decomposes to the corresponding aryl radical and an anion containing the heteroatom. The most widely investigated compounds in this... 

In the numerous cases where k jk = 0, the overall reaction amounts to a substitution of the halogen in the halide by the aromatic anion radical followed by the electron-transfer reduction of the ensuing radical by another aromatic anion radical, i.e. the sum of steps (92) and (94) might rather occur as in Scheme 6. 

At this point, it can be concluded that the direct and indirect electrochemical approach of the reaction in the case of aryl halides has provided a quantitative kinetic demonstration of the mechanism and the establishment of the nature of the side-reactions (termination steps in the chain process). In poor H-atom donor solvents, the latter involve electron-transfer reduction of the aryl radical. 

A large number of these values are close to the diffusion limit. This is not actually very surprising since the coupling of the aryl radical with the nucleophile has to compete with quite rapid side-reactions, if only its electron-transfer reduction, for the substitution to be effective. When taking place homogeneously, the latter reaction itself at the diffusion limit and the parameter that governs the competition is Anu[Nu ]/Ad[RX]. This is the reason why a discussion of structure-reactivity relationships is necessarily restricted to a rather narrow experimental basis. 

Electron transfer reduction of pyridines in both acid and alkaline solution generates the protonated radical-anion. This rapidly accepts a further electron and a proton to give a mixture of dihydropyridines. Enamine structures in these dihydro-pyridines can tautomerise to the imine, which is more readily reduced than the original pyridine molecule. Further reaction of the 1,4-dihydropyridine leads to piperidine while reduction of the t, 2-dihydropyridine leads to a tetrahydropyridine in which the alkene group cannot tautomerise to the imine and which is not therefore reduced to the piperidine stage. The reaction sequence is illustrated for 2,6-dimethyl-pyridine 18 which yields the thermodynamically favoured cis-2,6-dimethylpiperidine in which the two alkyl substituents occupy equatorial conformations. 

Reduction of a Stable System Resulting in Reduction of a Ligand. Ligands, as a group, are far better electron donors than acceptors. Thus, electron transfer reductions of ligands should be rather unlikely, and in fact, no such case has hitherto been reported. 

For amine protection, sulfonamides such as 9 offer several advantages over urethanes 5 or amides 7. In particular, secondary amines protected as the urethane or the amide exist as mixtures of rotational isomers, confusing NMR characterization and making crystallization more difficult. The limitation has been that sulfonamides have been difficult to remove. Masanobu Uchiyama of the University of Tokyo reports (J. Am. Chem. Soc. 2004,126, 8755) the development of transition metal ate complexes that catalyze electron-transfer reduction. While the sulfonamide 10 is inert to Mg in THF, inclusion of a catalytic amount of the ate complex 11 led to 12 in quantitative yield. 

Alkali metals in liquid ammonia represent the most important class of the so-called dissolving-metal reductions of aromatics. First described in 1937, it is a highly efficient and convenient process to convert aromatic hydrocarbons to partially reduced derivatives.201 The recognition and extensive development of this electron-transfer reduction came from A. J. Birch,202,203 and the reaction bears his name. 

The nickel(IV) complex of ligand (4) has been used to oxidize thiourea and alkyl derivatives to the respective disulfides, NH2C(S)NHR—> NH=C(NHR)—S—S— C(NHR)=NH. Autocatalysis is observed around pH 4.5, whilst at pH > 6, a faster Ni(IV) >Ni III) reduction step is followed by slower Nifffl) —> Ni(II) reduction. In the intervening pH region (ca 5.5), behaviour indicative of a single step two-electron-transfer reduction of the Ni(TV) is observed.43... 

Potassium hydroxide when merely dissolved in methanol is not effective in the electron-transfer reduction of 9-diazofluorene and fluoren-9-ylides. Addition of DMSO to the system makes a drastic change, with the highest efficiency in pure DMSO (Handoo Kaul 1992 Handoo et al. 1983). 

This interesting methodology broadens the preceding electron transfer reduction of acetates and pivaloates, and seems very efficient for synthesizing protected 2 -deoxy uridine such as 85 in high yields (85 %) (Scheme 44). 

To utilize the strong reducing power of the 3(da po) excited states of the platinum and iridium dimers, the nonproductive back electron transfer reactions need to be inhibited. An effective way to accomplish this is to use acceptors that are thermally unstable after the initial electron transfer. Reduction of alkyl halides has been shown to lead to short-lived radical anions RXT, which rapidly decompose to give R- and X (k... 

Cathodic reduction of disubstituted alkynes is possible but proceeds with relatively low current efficiency. Reduction of 4-octyne under similar conditions as used for 1-hexyne yields only 50% 4-octene after transfer of 8 F mol-1. It is interesting to note that the ratio trans/cis for the cathodically obtained 4-octene was 3/l. Other methods29 involving electron-transfer reductions yield the corresponding trans isomers almost exclusively. 

From the limited data available, it seems that terminal alkynes can be efficiently reduced to the corresponding alkenes at mercury cathodes in (C4H9)4N+ electrolyte solutions. The cathodic reduction can be carried out in an organic-aqueous medium in which base related complications, associated with other electron-transfer reductions, can be avoided. Efficient reduction of alkenes has not proven possible. In competition, both benzenoid and alkyne functionalities are reduced. Selectivity can be improved by controlling the water content of the medium so that a terminal alkyne can be converted to an alkene in the presence of a benzenoid aromatic functionality. 

A new radical chain group transfer reaction which does not involve tin reagents has been reported. The reaction proceeds by a photosensitized electron transfer reductive activation of PhSeSiR.3 using 1,5-dimethoxynaphthalene as the sensitizer [95ACIE2669]. In contrast to the tellurium transfer described above, the selenium transfer reaction gave higher diastereoselectivity (4 1 vs 2 1). 

A parabolic driving force dependence of logket is also observed for electron-transfer reduction of fullerenes in PhCN, as shown in Fig. 13.10 [18, 28-32]. 

The photocurrent generation in the present system is initiated by photoinduced charge separation from the porphyrin excited singlet state (1H2P /H2P+ = -0.7 V vs. NHE) [78] in the dendrimer to C60 (C60/Cf>0 = -0.2 V vs. NHE) [78] in the porphyrin dendrimer-C60 complex rather than direct electron injection to conduction band of Sn02 (0 V vs. NHE) system [91] The reduced C60 injects electrons into the Sn02 nanocrystallites, whereas the oxidized porphyrin (H2P/H2P+ = 1.2 V vs. NHE) [78] undergoes electron-transfer reduction with iodide (I3 /I = 0.5 V vs. NHE) [78] in the electrolyte system [91]. 

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