TEMPO electrode

The method uses a simple electrode made of a thin film of sol-gel organosilica doped with nitroxyl radicals deposited on the surface of an indium tin oxide (ITO) electrode. Thus, whereas in water benzyl alcohol is rapidly oxidized to benzoic acid, the use of the hydrophobic sol-gel molecular electrode TEMPO DE affords benzaldehyde only (Figure 1.9), with an unprecedented purity, which is highly desirable for the fragrance and pharmaceutical industries where this aromatic aldehyde is employed in large amounts. 

As a result, a highly enantioselective oxidation of (22) was achieved by using a TEMPO-modified graphite felt electrode in the presence of (—)-sparteine, where the enantiopurity of the remaining (22) was >99% and the current efficiency for (23) was >90% (Scheme 8) [51]. However, this selectivity has been questioned [52]. 

A variety of phenol couplings have been described. Those reported before 1991 have been reviewed [66]. 2-Naphthol (27) was oxidized to l,l -binaphthol (28) in high current efficiency on a graphite felt electrode coated with a thin poly(acrylic acid) layer immobilizing 4-amino-2,2, 6,6-tetramethylpiperidinyl-l-oxy (4-amino-TEMPO) (Scheme 10) [67]. 

Partially protected carbohydrates can be selectively oxidized at the primary hydroxy group to uronic acids at the nickel hydroxide electrode. At the same electrode, in polyhydroxy steroids, a preferential oxidation of the sterically better accessible hydroxyl groups is achieved [142]. By way of the mediator, TEMPO, carbohydrates that are only protected at the anomeric hydroxyl group are selectively oxidized at the primary hydroxyl group (Fig. 27) [143-145]. 

Coupling A graphite felt electrode chemically modified with TEMPO led to the enantioselective electrocatalytic coupling of 2-naphthol, 2-methoxynaphth-alene and 10-hydroxyphenanthrene with high enantioselectivity (up to 98% ee) in the presence of (-)-sparteine as a chiral base [366]. 

Shorthand notations such as ET (electron transfer), HAT (hydrogen atom transfer), BDE (bond dissociation energy), NHE (normal hydrogen electrode), CV (cyclic voltammetry), LFP (laser flash photolysis), EPR (electron paramagnetic resonance) and KIE (kinetic isotope effect) will be used throughout the chapter. In addition, recurring chemical compounds such as TEMPO (2,2,6,6-tetramethylpiperidine-Ai-oxyl), HBT (1-hydroxyben-zotriazole), BTNO (benzotriazole-A-oxyl), HPI (iV-hydroxyphthalimide), PINO (phthal-imide-iV-oxyl), NHA (A-hydroxyacetanilide) and a few others will be referred to by means of the capital-letter acronym. 

E. M. Belgsir and H. J. Schafer, Selective oxidation of carbohydrates on Nafion -TEMPO-modified graphite felt electrodes, Electrochem. Commun., 3 (2001) 32-35. 

Osa and coworkers [405,466] developed a graphite felt electrode modified with 2,2,6,6-tetramethylpiperidin-l-yloxyl (TEMPO) and applied it to enantioselective, electro-catalytic oxidative coupling of naphthol, naphthyl ether, and phenanthrol in the presence of (—)-sparteine as a base. The enantioselectivity of the coupling products were as high as 98%. 

Recently, Osa and coworkers [502] have reported highly enantioselective electroca-talytic oxidation of racemic monoalcohols using a TEMPO-modified graphite felt electrode in the presence of (—)-sparteine. Optically almost pure i -isomeric alcohols remained unreacted. Highly enantioselective lactonization of racemic diols was also achieved by using the same TEMPO-modified electrode to give (S)-isometric lactones [503]. 

The molecular motion of redox couples within polymer-coated electrodes has recently been investigated by making use of both nitroxide spin probes and various cationic spin probes [94-97]. Spin probes, such as the nitroxide probe TEMPO (see Sect. 2.1.2) and its derivatives, have well-defined electrochemistry and their ESR spectra in viscous media exhibit effects due to incomplete rotational averaging of the g and hyperfine coupling constant tensors. Analysis of the spectra [98] allows deductions to be made concerning the molecular rotation. Such analysis has been performed for spin probes incorporated into various polymer films. 

The modification of electrodes with PVC membranes has found applicability in ion selective electrode work [99] (so-called "coated wire electrode ). The molecular motion of species within such electrodes has been investigated by Compton and Waller [100]. Using a range of derivatives of the nitroxide spin probe TEMPO, they were able to show how the rotational correlational time was dependent upon the molecular volume of the probe and, by use of variable-temperature apparatus, how this varied with temperature. The effect of various plasticizers upon the molecular motion within the PVC membrane was investigated, rotational correlational times being dependent upon the nature of the plasticizer and the loading level. The effect of loading level upon the correlation time was shown to correlate with data obtained by Compton Maxwell [101] for the response times of K+ ion selective electrodes based upon PVC modified electrodes. 

A few miscellaneous oxidations using oxoammonium salts generated from TEMPO or substituted TEMPO analogs have been reported in the literature. These include the electrochemical oxidation of thiols to disulfides by a TEMPO-modified felt electrode [66], the electrochemical oxidation of amines to imines or nitriles [67], the cleavage of benzyl ethers by a single electron transfer mechanism with an oxoammonium bromide salt [68], and the dibromination of propargyl acetates by catalytic oxoammonium tribromide generated from a nitroxide and bromine [69]. 

A third type of indirect electrochemical process involves processes in which the mediator is fixed at the electrode, such as 4-amino-2,2,6,6-tetramethylpiperidinyl-l-oxyl (TEMPO) modified graphite felt electrodes. 

Interestingly, Osa et al. reported an electrochemical synthesis of enantioenriched binaphthyl type dimers (up to 98% ee) via constant potential electrolysis of 2-naphthol, 2-methoxylnaphthalene, and 9-phenanthrol at 0.6 V vs. Ag/AgCl on a TEMPO-modified graphite felt electrode in the presence of a stoichiometric amount of (-)-sparteine (TEMPO = 2,2,6,6-tetramethylpiperidin-l-yloxyl). ... 

Tab. 1 Electrocatalytic oxidation of organic compounds on TEMPO-modified CF electrode...

Tab. 2 Enantioselective and stereoselective, electrocatalytic oxidations on TEMPO-modified GF electrode in the presence of (-)-sparteine...

Simple flow through micro-reactor cells for operation in two-electrode mode have been proposed based on low pressure [39, 40] and high pressure [41] designs. Recently Birkin and coworkers proposed a versatile design based on a Viton microchannel foil [42] (see Fig. 3) for acetoxylation and TEMPO-mediated oxidations [43]. 

Kashiwagi Y, Yanagisawa Y, Kurashima F, Anzai J-i, Osa T, Bobbitt JM (1996) Enantioselective electrocatalytic oxidation of racemic alcdiols mi a TEMPO-modified graphite felt electrode by use of chiral base. Chem Commun 24 2745-2746... 

Herranz, T., Garcia, S., Martinez-Huerta, M.V, Pena, MA., Fierro, J.L.G., Somodi, R, Borbath, 1., Majrik, K., Tempos, A. Rojas, S. ElectiooxidationofCO and methanol on well-characterized carbon supported PtjiSn electrodes. Effect of crystal structure./ni. J. Hydrogen Energy 37 (2012), pp. 7109-7118. 

---