Electron transfer catalysts

Reductive ring opening of epoxides in radical reactions in presence of titanocenes as electron transfer catalysts 98SL801. 

Triazenes can also be used to investigate the Sandmeyer mechanism proper, as shown in these and other papers from the same group (Ku and Barrio, 1981 Satya-murthy and Barrio, 1983). However, the investigations demonstrate that their modifications allow bromo-de-diazoniations to be performed without an electron transfer catalyst. It is interesting that Galli predicted the borderline character of bromo-de-diazoniation as early as 1981. 

In another example of a radical process at the pyrrole C-2 position, it has been reported that reductive radical cycloaddition of l-(2-iodoethyl)pyrrole and activated olefins, or l-(oj-iodo-alkyl)pyrroles 34 lead to cycloalkano[a]pyrroles 35 via electroreduction of the iodides using a nickel(II) complex as an electron transfer catalyst <96CPB2020>. Thus, it appears the radical chemistry of pyrroles portends to be a fertile area of research in the immediate or near future. 

One of the most prominent characteristics of Fe(+2) is its ability to undergo oxidation leading to Fe(+3). This was used by Uchiyama et al. when they reported on Fe(+2)-ate complexes as potent electron transfer catalysts [7, 8]. These ferrates are accessible from FeCl2 and 3 equiv. of MeLi. The Fe(+2/+3) oxidation potential of [Me3Fe(+2)]Li 19 in THF is —2.50 V, thus being in between those of Sml2 (—2.33 V) and Mg (—3.05 V). With these alkyliron-ate complexes it was possible to realize a reductive desulfonylation of various A -sulfonylated amines 20 with different basicity. By using Mg metal to restore the active Fe(+2) species 19 a catalytic reductive desulfonylation process was achieved (Scheme 4). 

Schwarzenbach et al. (1990) have shown that reductive transformations of a series of monosubstituted nitrobenzenes and nitrophenols in aqueous solutions containing reduced sulfur species occur readily in presence of small concentrations of an iron prophyrin as an electron transfer catalyst. 

Substantial research efforts should be dedicated also to the development of low cost multiple-electron transfer catalysts for oxygen production. Electrochemical losses related to O2 evolution are a considerable part of the overall inefficiencies. 

Low cost multiple electron transfer catalysts for oxygen production... 

An effective and mild electrocatalytic procedure for the deprotection of the 1,3-dithiane group of (68), giving the ketone (67), has been developed by using a small amount of tris(/ -tolyl)amine as a homogeneous electron-transfer catalyst (Scheme 26) [86]. The scope and limitations are discussed in detail [87]. The method can be applied also for oxidative removal of the 4-methoxybenzyl thioether protecting group from poly-cystinyl peptides [88]. 

The generation of an alkyllithium mediated by an arene-catalyzed PhS/Li exchange has been applied to thioethers Ifp- and 77 using naphthalene and DTBB, respectively, as the electron-transfer catalyst and used in the preparation of polyols. 

Collections of fundamental and thermodynamic data can be found in an earlier review [158] and in standard resources [13, 14]. However, due to the reactivity of iodine there are many less common or more reactive forms of iodine that have been less well characterized. For example, a blue 12 cation, a brown I3+, or a green I5+ cation are formed in concentrated sulfuric acid and 1+ is stabilized in donor environments such as pyridine [159]. So-called hypervalent iodine reagents have been developed as a versatile oxidation tool in organic synthesis and often iodine derivatives are employed as electron transfer catalysts. Some fundamental thermodynamic data and typical applications of iodine are summarized in Scheme 5. 

Dialkyl-4,4-bipyridinium halides (viologens) are useful electron-transfer catalysts. 

In studies of analogs of the redox cofactor pyrroloquinoline quinone (PQQ), synthetic efforts have focused initially on isosteric, isomeric structures that reflect on important mechanisms of electron-transfer catalysis mediated by PQQ. These studies provide insight into the choice of PQQ as an electron-transfer catalyst in nature, and bear directly on pharmaceutical applications of this vitamin-like nutritional factor. 

There has been some exploration of the mechanism of reduction of d transition metal complexes by M2+(aq) (M = Eu, Yb, Sm). Both inner- and outer-sphere mechanisms are believed to operate. Thus the ready reduction of [Co(en)3]3+ by Eu2+(aq) is necessarily outer-sphere. 2 However, the strong rate dependence on the nature of X when [Co(NH3)5X]2+ or [Cr(H20)5X]2+ (X = F, Cl, Br or I) are reduced by Eu2+(aq) possibly suggests an inner-sphere mechanism.653 The more vigorous reducing agent Yb2+ reacts with [Co(NH3)6]3+ and [Co(en)3]3+ by an outer-sphere route but with [Cr(H20)5X]2+ (X = halide) by the inner-sphere mechanism.654 Outer-sphere redox reactions are catalyzed by electron-transfer catalysts such as derivatives of isonicotinic acid, one of the most efficient of which is iV-phenyl-methylisonicotinate, as the free radical intermediate does not suffer attenuation through disproportionation. Using this catalyst, the outer-sphere reaction between Eu2+(aq) and [Co(py)(NH3)5]3+ proceeds as in reactions (18) and (19). Values found were ki = 5.8 x KFM-1 s 1 and k kx = 16.655... 

It would appear certain that the most important need in LCEC is the development of improved electrode materials. It may be possible in the near future to design an electrode that will give superior performance for certain classes of compounds. Modifying electrode surfaces by covalent attachment of various ligands or electron-transfer catalysts (including enzymes) can provide the key to better amperometric devices for all sorts of analytical purposes. Research in the area of chemically modified electrodes (CMEs) has been reviewed (see Chap. 13) [6,11]. Those interested in improving the performance of electrochemical detectors would do well to study these developments in detail. 

Other dyes which participate efficiently as electron transfer catalysts include the xanthenes (217,218), thianthrenes (219), phenothiazines (220), and anthraquinonesulfonates (221). 

While the exploration of the implications of irradiated semiconductor surfaces for organic chemistry have only recently been attempted, there now exists a growing body of experiments illustrative of their power (283). A typical example of the contrasting oxidative reactivity observed on irradiated semiconductor surfaces can be seen in the different product distributions obtained by oxidation of 1,1-diphenylethylene photo-electrochemically on Ti02, electrochemically on Pt (an inert electrode material), and with thermal single electron transfer catalysts in homogeneous solution, eq. 89 (284) ... 

For example, hydrogen can be produced from a sacrificial electron donor, a chromophore, an electron transfer catalyst and a redox catalyst. The role of the sacrificial electron donor is to... 

---