Alkyl radicals, reduction potentials

Tlte reduction potential for an alkyl or benzyl radical, relative to that of the carbon-halogen bond from which it is derived, is important in determining the isolated products. Products are derived either by radical or by carbanion chemistry. The half-wave potential for the second polarographic wave of alkyl halides is connected with reduction of the radical. Sophisticated methods have been devised for deducing radical reduction potentials in cases where (his second wave is not seen. Values are collected in Table 4.4. 

Chemiluminescence also occurs during electrolysis of mixtures of DPACI2 99 and rubrene or perylene In the case of rubrene the chemiluminescence matches the fluorescence of the latter at the reduction potential of rubrene radical anion formation ( — 1.4 V) at —1.9 V, the reduction potential of DPA radical anion, a mixed emission is observed consisting of rubrene and DPA fluorescence. Similar results were obtained with the dibromide 100 and DPA and/or rubrene. An energy-transfer mechanism from excited DPA to rubrene could not be detected under the reaction conditions (see also 154>). There seems to be no explanation yet as to why, in mixtures of halides like DPACI2 and aromatic hydrocarbons, electrogenerated chemiluminescence always stems from that hydrocarbon which is most easily reduced. A great number of aryl and alkyl halides is reported to exhibit this type of rather efficient chemiluminescence 155>. 

For a particular iron(III) oxidant, the rate constant (log kpe) for electron transfer is strongly correlated with the ionization potential Ip of the various alkylmetal donors in Figure 4 (left) (6). The same correlation extends to the oxidation of alkyl radicals, as shown in Figure 4 (right) (2). [The cause of the bend (curvature) in the correlation is described in a subsequent section.] Similarly, for a particular alkylmetal donor, the rate constant (log kpe) for electron transfer in eq 1 varies linearly with the standard reduction potentials E° of the series of iron(III) complexes FeL33+, with L = substituted phenanthroline ligands (6). 

Indoles can be also be converted into their radical cations by the use of C1C>2 as the oxidant produced by pulse radiolysis. From the reactivity of the resultant cation it was possible to establish the one-electron reduction potential of the indole in question. Typical results from this are illustrated in Table 234. As can be seen, the one-electron reduction potential is influenced by alkyl substitution. 

Reduction of alkyl and benzyl halides proceeds in two one-electron addition steps. The first detectable product is the alkyl or benzyl radical and this is reduced further to the carbanion. Some alkyl iodides show two polarographic waves corresponding to the two steps. Alkyl bromides show only one two-electron wave and alkyl chlorides are not reducible in the available potential window. Benzyl halides also show only one wave and benzyl chlorides are reducible in the available potential range. Reduction potentials measured in dimethylformamide are collected in... 

In the ESR spectra of some related trimethylsilyl benzoquinone derivatives, as with the ketyls, the spin density in the quinone ring increases for trimethylsilyl substitution and decreases for alkyl substitution, consistent with the electron-accepting ability of the trimethylsilyl group. This ability is also manifested in the reduction potentials of the compounds (65). The ESR data for the trimethylsily ketyls and for the trimethylsilyl benzoquinone anion radicals are summarized in Table IX. 

The reductions of halogenated organic compounds (RX) involve the cleavage of carbon-halogen bonds [62]. Depending on the solvent, supporting electrolyte, electrode material and potential, it is possible to electrogenerate either alkyl radicals (R ) or carbanions (R ), which then can lead to the fonnation of dimers (R-R), alkanes (RH) and olefins [R(-H)] ... 

Evidence of both types of potential intermediate in reduction by Sml2, the alkyl radical and the ketyl radical 27, has been provided by radical cyclisation reactions. Mechanism 4, which involves an Sjj2 substitution, has been eliminated because optically active halides are completely racemised. The rate of addition of alkyl radicals to ketones is very slow (<102 dm3 mol-1 s-1) the resulting alkoxy radicals (26) are very reactive and could not... 

Scheffold and colleagues subsequently reported that alkyl iodides 249 add to xfl-unsaturated esters 248 catalyzed by 23 mol% hydroxycobalamine 247 (X=OH) and mediated by light to homolyze 253B [302]. The generation of the active Co(I) species 253A proceeded by electrochemical reduction at a potential of —1.2 V vs SCE. The transformation required a large excess of 248 to promote the addition of the initially generated alkyl radical. The reaction was applied to the total synthesis of the California red scale pheromone. The product... 

Electrochemical reduction of the salts (4) provides radicals (18) which dimerize or undergo further reduction to anions (32) or dianions (80MI43100). The reduction potentials are not much affected by substituents. Reduction with zinc in aprotic conditions gives bi(l,2-dithiol-3-yls) (59), and 3-chloro-l,2-dithiolylium salts (35a X = Cl) are converted into bi(l,2-dithiol-3-ylidenes) (20) (75TL3473). Divalent chromium converts the 3,5-dimethyl-l,2-dithiolylium cation into a dithioacetylacetonate ligand (72AJC2547). The reaction of 3,5-diamino-l,2-dithiolylium salts (8) or alkyl derivatives with thiols provides dithiomalonamides (60) by electron transfer (63ACS163). 

Ether solutions based on TAA salts are not reduced on noble metal electrodes. The major cathodic reaction of these solutions involves the cation reduction to trialkyl amine, alkane, and alkene (which are the stable disproportion products of the alkyl radical formed by the electron transfer to the cation) [3], Electrolysis of ethers such as THF or DME containing TBAP, formed in the catholyte tributyl amine, butane and butene, were unambiguously identified by NMR and GCMS analysis [3], In the presence of water (several hundred ppm and more), the electrolysis products were found to be tributyl amine and butene (butane was not detected) [3], The potential of this reduction reaction is higher than that of the dry solution, and it is clear that the initial electroactive species in this case is the... 

Reactions of PhCH2Cl and m-02NC6H4CH2Cl with 9 and 10 failed to produce the a-alkylated ketones under similar reaction conditions. The failure of a chain reaction with PhCH2Cl reflects the increased reduction potential of the alkyl halide while, in the case of the latter, the radical anion formed by the facile reduction does not readily undergo the fragmentation step required to continue the chain process. 

Such polycyclic aromatic hydrocarbons as anthracene or heteroaromatics as acridine, phenazine and 2,4,5-triphenyl oxazole act as Jt-donors for the Jt-acceptors AN and alkyl methacrylates [50-53]. Again, the interaction of the donor excited states with vinyl monomers leads to exciplex formation. But, the rate constants (k ) of these quenching processess are low compared to other quenching reactions (see Table 1). The assumed electron transfer character is supported by the influence of the donor reduction potential on the k value (see Table 1), and the detection of the monomer cation radicals with the anthracene-MMA system. Then, the ion radicals initiate the polymerization, the detailed mechanism of which is unsolved,... 

For systems that are powerful excited-state reductants, photoreduction of alkyl halides is observed (6.16). This reaction was initially interpreted to be an outer-sphere electron transfer to form the radical anion, which rapidly decomposes to yield R- and X . Subsequent thermal reactions yield the observed products, an SrnI mechanism (Figure 3a). While such a mechanism, SrnI, appears plausible for a metal complex with E°(M2 /3M2 ) < -1.5 V (SSCE), it seems unlikely for complexes with E°(M2 /3M2 ) > -1.0 V (SSCE). Reduction potentials for alkyl halides of interest are generally more negative than -1.5 V (SSCE) (1/7). Alkyl halide photoreduction is observed for binudear d complexes whose excited-state reduction potentials are more positive than -1.0 V (SSCE) in CH3CN. 

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