One-electron donors

Dyes, polymethine used for dyes having at least one electron donor and one electron acceptor group linked by methine groups or aza analogues aUopolar cyanine, dye bases, complex cyanine, hemicyanine, merocyanine, oxonol, streptocyanine, and styryl. Supersensitization has been reported for these types—18 cites for cyanines, 3 for merocyanine, and 6 for all other polymethine types. 

Organomagnesium halide reagents RMgX (RX = CjH,Br, i-PrBr, n-BuBr, PhBr) react in toluene-ether with (t7 -Cp)2MoH2 to form compounds II, in which each molecule contains four Mo—Mg bonds " . When II is dissolved in THF a red solution is formed, which after concentration yields orange crystals of III. Compound III is monomeric with a Mo—Mg bond of length 273.2 pm, consistent for Mo as a one-electron donor. 

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

Such a mechanism would have to involve the nitrosyl ligand acting in a non-innocent manner, changing from a three-electron donor to a one-electron donor in the intermediate complex. Such participation of the nitrosyl ligand has precedent in related systems (108). 

Another well-known transformation of carbonyl derivatives is their conversion to pinacols (1,2-diols) via an initial one-electron reduction with highly active metals (such as sodium, magnesium, aluminum, samarium iodide, cerium(III)/ I2, yttrium, low-valent titanium reagents (McMurry coupling), etc.), amines, and electron-rich olefins and aromatics as one-electron donors (D).43 Ketyl formation is rapidly followed by dimerization44 (equation 22). 

Shumate WJ, Mattem DL, Jaiswal A, Burgess J, Dixon DA, White TR, Honciuc A, Metzger RM (2006) Spectroscopic and rectification studies of three donor-sigma-acceptor compounds, consisting of a one-electron donor (pyrene or ferrocene), a one-electron acceptor (perylenebisimide), and a C19 swallowtail. J Phys Chem B110 11146-11159... 

In a similar fashion the bonding in H2 might be formally regarded as a complementary pair of one-electron donor-acceptor interactions, one in the ot (spin up ) and the other in the 3 (spin down ) spin set.8 In the long-range diradical or spin-polarized portion of the potential-energy curve, the electrons of ot and (3 spin are localized on opposite atoms (say, at on HA and 3 on HB), in accordance with the asymptotic dissociation into neutral atoms. However as R diminishes, the ot electron begins to delocalize into the vacant lsB(a) spin-orbital on HB, while (3 simultaneously delocalizes into Isa on HA, until the ot and (3 occupancies on each atom become equalized near R = 1.4 A, as shown in Fig. 3.3. These one-electron delocalizations are formally very similar to the two-electron ( dative ) delocalizations discussed in Chapter 2, and they culminate as before (cf. Fig. 2.9) in an ionic-covalent transition to a completely delocalized two-center spin distribution at... 

Scheme 12. Domino anionic-radical polycyclizations with one-electron-donor reagents...

Method (i) is a route commonly utilized in monometal nitrosyl complexes. The nitrosyl ligand may function as (formally) a three-electrop donor (NO+) with a linear bonding mode, or as (formally) a one-electron donor (NO ) with a bent (—120°) M-N-0 arrangement. Conversion of the M-NO system to a M-NO system has two effects. First, it increases the metal oxidation state by two second, it generates a vacant coordination site. The dinitrosyl cluster Os3(CO)8(NO)2, which has... 

In method (ii), the group X (e.g., a halogen) may function as a three-electron donor when present as a bridging group, but as a one-electron donor when combined to one metal center. The simple act of bridgeopening creates a vacant coordination site on one metal atom. Behavior of this sort is invoked to explain the stereospecific incorporation of I3CO in the complex Os3(CO)10Cl2. 

The monosubstituted adduct offers the ready synthesis of a whole range of monosubstituted adducts (see Scheme 6) it is often possible to isolate in these reactions intermediates that are not readily obtained by alternative methods. Thus, in the reaction with halogen acids to yield the bridged hydrido complexes HOs3(CO)10X, it is possible to identify the intermediate HOs3(CO)uX complex in which the halogen functions as a one-electron donor bonding to only one metal center (158). 

For the dibridged species, such as the halogen derivatives HOs2(CO)i0X (X = Cl, Br), we have used this type of behavior to explain the stereospecific incorporation of, 3CO into the molecules (160). The halogen group varies from a three-electron donor in bridged molecules to a one-electron donor in the terminally bonded species. 

A further innovation in this work is the recognition that monomers with more than one electron-donor site, e.g. an aromatic ring or an oxygen atom in addition to the double bond, can form two kinds of complex with the C+, only one of which can isomerise with incorporation of the monomer into the chain. Therefore there are then... 

There is sufficient information on CT complexes of this kind in Foster s book [6] for the Kus to be estimated. By extrapolating from 1,3,5-trinitrobenzene and 1,4-dinitrobenzene to PhN02 for any one electron donor (mesitylene or hexamethyl benzene), and from CC14 to solvents of D > ca 10, we find that at least for the three hydrocarbon monomers it is very unlikely that Kus > 10 2 1-mol 1. Therefore, since [Sv] = ca 10 mol-1 1 ([molar vol]"1), we find for m = 1 mold"1, that [MSv] <0.1 mold"1, which means that for these monomers >90% of the monomer is not bound in a CT complex, and the first objection is therefore not relevant. For the monomers containing both 7t- and n-donor groups, i.e., the VE, the Kus may well be greater, and therefore the formation of CT complexes may be important for these. 

The Fe-protein, whose molecular structure is shown in Figure 31,40,41 acts as a one-electron donor to the FeMo-protein. This electron-donating ability arises from the propensity of the Fe4S4 cluster to undergo a one-electron oxidation. 

The action of one-electron redox systems is readily understandable in the context of inner- and outer-sphere mechanisms, whereas two-electron redox systems require additional considerations. First, if a double one-electron transfer is possible from an organic substrate to the same metal ion, does it mean that the same molecule of an organic donor provides these two electrons, or do two molecules of the substrate act as one-electron donors ... 

Neutral organic molecules can also be one-electron donors. For example, tetracyano-quinodimethane gives rise to anion-radical on reduction with 10-vinylphenothiazine or N,N,N, N -tetramethyl-p-phenylenediamine. Sometimes, alkoxide or phenoxide anions hnd their applications as one-electron donors. There is a certain dependence between carbanion basicity and their ability to be one-electron donors (Bordwell and Clemens 1981). 

Typical one-electron donors, for example, sodium naphthalene, also entrained the reaction of p-nitrocumyl chloride with sodium nitrite (Kornblum et al. 1970). 

The cyclodimerization depicted in Scheme 7.19 is one of the many examples concerning cation-radicals in the synthesis and reactions of cyclobutanes. An authoritative review by Bauld (2005) considers the problem in detail. Dimerization is attained through the addition of an olefin cation-radical to an olefin in its neutral form one chain ends by a one-electron reduction of the cyclic dimer cation-radical. Unreacted phenylvinyl ether acts as a one-electron donor and the transformation continues. Up to 500 units fall per one cation-radical. The reaction has an order of 0.5 and 1.5 with respect to the initiator and monomer, respectively (Bauld et al. 1987). Such orders are usual for branched-chain reactions. In this case, cyclodimerization involves the following steps ... 

The donor types D2, D4, and D5 of Keilin and Nicholls (37) all reduce compound II to ferric enz5mie in a one-electron process without detectable intermediates. Donors of type D2, phenols and amines, also reduce compound I to compoimd II. Nitrite, the only member of category D4, reduces compoimd I in a two-electron step as described earlier. Donors of type D1 reduce compound I to compound II, but have no appreciable effect upon compound II itself Reactivity of the one-electron donors seems independent of heme pocket binding in the free enzyme. 

It is nearly 50 years since a c-type cytochrome was shown to catalyze peroxidase activity in crude extracts of Pseudomonas fluorescens (40). The enzyme responsible was first purified some 20 years later by Ell-folk and Soninen from the closely related P. aeruginosa and shown to be a diheme cytochrome c peroxidase (CCP) (41). These bacterial diheme CCPs are quite distinct from the superfamily of plant and yeast peroxidases (42) and are widely distributed among the Gram-negative bacteria (41, 43 6). Diheme CCPs are located in the periplasm (Fig. 2), where they catalyze the two-electron reduction of H2O2 to H2O by soluble one-electron donors such as cytochromes c and cupredoxins. 

Reduction of nitro compounds with one-electron donors... 133... 

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