Electroreduction indirect

Indirect electrosynthesis of reactive formyl transition metal compounds involves an initial step of reduction of metal carbonyls to radicals followed by transfer of a hydrogen atom from trialkyltin hydrides190. Electroreduction of metal carbonyls yields products of dimerization and loss of CO from the radical anion. Electroreduction in the presence of R3SnH yields the formylmetalcarbonyls ... 

Asymmetric electroreduction of ketones to the corresponding chiral alcohols has recently been reported. Typical examples are the reduction of ketones bearing chiral auxiliaries [68, 69], and the indirect reduction of ketones with alcohol dehydrogenase (ADH), as a mediator (Scheme 32) [70]. 

Mediators employed in electroreduction media play an important role in indirect electroorganic synthesis. Surveyed here are the reactions in which metal complexes are used as electroreductive catalysts. 

Indirect electroreductions use metal complexes, metal ions, and metal organic compounds as mediators. Their characteristic features are well documented both in monographs [8, 11, 13, 15, 17] and reviews [249-252]. This chapter reviews the potential of such mediators for electroreduction, which involve recyclable metal and nonmetal complexes, but not enzymatic systems. 

Indirect electroreduction with Ni, Co, and Fe complexes has been well studied, and will be discussed first. Furthermore, the synthetic use of palladium, rhodium, and ruthenium complexes as mediators in the electroreduction of organic compounds are finding increasing apphcations. Metallic complexes derived from chromium, manganese, molybdenum, tungsten, and rhenium have also been used as mediators for special conversions. Recently, tin, zinc, and mercury... 

The Co(III) complexes with the 1,4,8,11-tetraazacyclotetradecane (cyclam) ligand are found to behave as redox mediators for the indirect electroreduction of meso-1,2-dibromo-l,2-diphenylethane (261) leading to tr ns-stilbene (262) in a DMF-Bu4NCl04-(Pt) system (Scheme 98) [400]. 

Titanium ions can also he used as redox catalysts for the indirect cathodic reduction of nitro compounds (417). The electroreduction is carried out in an H20-H2S04/Ti(S04)2-(Pb/Cu) system at 45 80°C under 5 20Am . Nitrobenzene, dinitrobenzene, nitrotoluene, 2,4-dinitrotoluene, 2-nitro-m-xylene, nitro-phenol, 2,4-dinitrophenol, nitrophenetole, o-nitroanisole, 4-nitrochlorotoluene, ni-trobenzenesulfonic acid, and 4,4 -dinitro-stilbene-2,2 -disulfonic acid can all be reduced by this procedure to the corresponding amino compounds (418) in good yields (Scheme 146) [513-516]. Tin... 

Indirect electroreduction of allyl alcohols leading to the corresponding unsaturated hydrocarbons is attained using a mercury electrode in a strongly acidic medium containing an iodide salt [554]. The reaction involves transformation of the alcohol into the iodide, the reaction of the iodide with mercury, the protonation of the... 

Table 8. Reduction potentials of organic mediators for indirect electroreductions...

In several examples the reductive halide-hydrogen exchange has been studied on a preparative scale. For example, the indirect electroreduction of 2-chloropyridine in DMF using anthracene as mediator gives pyridine in 83-86 % yield 2 . For the dehalogenation of 1-chlorohexane (80% yield), naphthalene is applied as redox catalyst. Similarly, 6-chloro-hexene yields 1-hexene (60%) and methylcyclopentane (25%), which is the product of the radical cyclization . The indirect electrochemical reduction of p- and y-bromocarboxylic esters forms coupling and elimination products besides the dehalogenated products... 

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