Electrochemical asymmetric oxidation

Very low asymmetric induction (e.e. 0.3-2.5%) was noted when unsymmetrical sulphides were electrochemically oxidized on an anode modified by treatment with (— )camphoric anhydride or (S)-phenylalanine methyl ester299. Much better results were obtained with the poly(L-valine) coated platinum electrodes300. For example, t-butyl phenyl sulphide was converted to the corresponding sulphoxide with e.e. as high as 93%, when electrode coated with polypyrrole and poly(L-valine) was used. 

In electrochemical oxidation of l-hydroxy-3-imidazoline-3-oxides containing one to four H atoms at a-C, one observes in ESR-spectra not only triplet splitting of the nucleus 14N of the nitroxyl group (a v 15-16 G) but also splitting of the neighboring protons (a// 18-20 G), with multiplets corresponding to their number (from doublet to quintet) (101). Unlike spatially hindered hydroxylamines which show reversibility in electrochemical oxidation, hydroxylamines with H at a-C are oxidized irreversibly. Oxidation of hydroxylamines with nitroxyl radical proceeds easily and with quantitative yields (102). In the oxidation of asymmetric polylluorinated hydroxylamines with Mn02, isomeric polyfluorinated nitrones have been obtained (103). 

Low asymmetric induction (e.e. 0.3-2.5%) was found (56) to occur when unsymmetrical sulfides were electrochemically oxidized on an anode modified by treatment with (->camphoric anhydride or (5)-phenylalanine methyl ester. 

A new development is that electrochemical oxidation of ferrocyanide to ferricyanide can be coupled with AD to give a very efficient electrocatalytic process [37]. Under these conditions, the amount of potassium ferricyanide needed for the reaction becomes catalytic and Eqs. 6D.6 and 7 can be added following Eq. 6D.4. Summation of Eq. 6D.1-6D.4, 6D.6, and 6D.7 gives 6D.8, showing that only water in addition to electricity is needed for the conversion of olefins to asymmetric diols and that hydrogen gas, released at the cathode, is the only byproduct of this process. In practice, sodium ferrocyanide is used in the reaction and the amount of this reagent used in comparison with the potassium ferricyanide method mentioned above has been reduced from 3.0 equiv. to 0.15 equiv. (relative to an equivalent of olefin). 

Matsumura and co-workers reported a memory effect of chirality in the electrochemical oxidation of 95 to give 96, although the enantioselectivity was modest (Scheme 3.25). The reaction is assumed to proceed via carbenium ion intermediate Q.46 The mechanism for asymmetric induction is not clear. A possible mechanism involves chiral acid (95)-mediated deracemization of racemic 96 produced by the electrochemical oxidation of 95. However, this suggestion may be eliminated based on the finding that treatment of racemic 96 with 95 in methanol containing 5% formic acid did not produce optically active 96. 

Minato D, Arimoto H, Nagasue Y, Demizu Y, Onomura O (2008) Asymmetric electrochemical oxidation of 1,2-diols, aminoalcohols, and aminoaldehydes in the presence of chiral copper catalyst. Tetrahedron 64 6675-6683... 

The asymmetry of the plots is related first to the asymmetry of reactants and prodncts. In the anodic oxidation the reactant is Cr(OH2)g and the product is Cr(OH2)6 +, hence and/p =f . On the other hand, in the cathodic reduction the reactant is Cr(OH2)6 + and the product is Cr(OH2)g, henceand/p = Xed- Th potential-energy curves of reactant and product are asymmetric for each reaction, and this asymmetry is reversed between electrochemical oxidation and reduction. With the data available in Appendix El, this contribution to the asymmetry of the cathodic reduction of the Cr(OH2)5 /Cr(OH2V+ couple system can be shown to be given by =/ox = 3.75x10 kJ mol A" and /p=, = 2.35xl03klmol- A-2. 

The thylakoid membrane is asymmetrically organized, or sided, like the mitochondrial membrane. It also shares the property of being a barrier to the passive diffusion of H ions. Photosynthetic electron transport thus establishes an electrochemical gradient, or proton-motive force, across the thylakoid membrane with the interior, or lumen, side accumulating H ions relative to the stroma of the chloroplast. Like oxidative phosphorylation, the mechanism of photophosphorylation is chemiosmotic. 

Mitchell s chemiosmotic theory postulates that the energy from oxidation of components in the respiratory chain is coupled to the translocation of hydrogen ions (protons, H+) from the inside to the outside of the inner mitochondrial membrane. The electrochemical potential difference resulting from the asymmetric dis-... 

In an another analogous set of reactions, Matsumura and coworkers have shown that electrochemically derived A -acyliminium ions can serve as electrophiles in asymmetric alkylation reactions (Scheme 25) [57]. In this case, the methoxycarbamate of a series of cyclic amines was oxidized in order to generate the AZ-a-methoxy carbamates. 

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