Chain initiation electron-transfer

SRNl substitution include ketone enolates,183 ester enolates,184 amide enolates,185 2,4-pentanedione dianion,186 pentadienyl and indenyl carbanions,187 phenolates,188 diethyl phosphite anion,189 phosphides,190 and thiolates.191 The reactions are frequently initiated by light, which promotes the initiating electron transfer. As for other radical chain processes, the reaction is sensitive to substances that can intercept the propagation intermediates. 

Triethylamine as the electron donor was also used by Mattay and co-workers in tandem fragmentation cyclization reactions of a-cyclopropylketones. The initial electron transfer on the ketone moiety is followed by the fast cyclopropyl-carbinyl-homoallyl rearrangement, yielding a distonic radical anion. With an appropriate unsaturated side chain within the molecule both annealated and spi-rocyclic ring systems are accessable in moderate yields (Scheme 41) [62]. 

Oxidations initiated by thermally induced electron transfer in an oxygen-CT complex represent the thermal analog of the Frei photo-oxidation and are properly classified as hybrid type IlAOi-type IIaRH oxidations (Fig, 2), Such reactions require either zeolites with high electrostatic fields or substrates with low oxidation potentials. In addition, elevated temperatures are known to promote the thermally initiated electron-transfer step, although the possible intrusion of a classical free-radical initiation chain oxidation at higher temperatures must be considered. 

Many carboxylic acids lose carbon dioxide on either direct or sensitized irradiation, and in some cases (4.10 the evidence points to the operation of an initial electron-transfer mechanism rather than primary a-deavage. Cleavage occurs readily with acyl halides, and this can [ead to overall decarbonylation (4.11). Aldehydes also cleave readily, since the (0=)C—H bond is more prone to homolysis than the (0= C-C bond. This offers a convenient method for replacing the aldehydic hydrogen by deuterium in aromatic aldehydes (4.12. and a similar initial reaction step accounts for the production of chain-Iengtheped amides when formamide is irradiated in the presence of a terminal alkene (4.13). 

Entry no. 2 of Table 16 introduces the most remarkable aspect of Co(III) chemistry, namely its ability to oxidize non-activated C—H bonds (for a recent study, see Jones and Mellor, 1977) and we immediately see that Marcus theory here completely rules out the possibility of an initial electron-transfer step. This is predicted to be ca. 1016 times slower than what is actually observed. On the other hand, the theory correctly predicts the rate constant for oxidation of naphthalene under the same conditions, and the postulated direct abstraction of a 7t-electron is thus feasible (Cooper and Waters, 1967). The Co(III) trifluoroacetate study of entry no. 3, including only substrates without an alkyl side-chain, however, cannot be fitted to any physically realistic set of E°, A parameters. With E° = 1.83 V (value in 1.0 M HC104) A ought to be ca. 40 kcal mol-1 and the slope of the log k/AG0 regression line ca. —0.6 the A value is then in reasonable agreement with the estimated one but not the slope. With E° = 3.2 V (clearly not a physically very realistic standard potential) a A value of ca. 80 kcal mol-1 is required. This seems to be far too large for such a system. 

In related model complex studies, Isied and coworkers, have examined photo-induced (or pulse-radiolytically initiated) electron-transfer processes in which a polypyridine-ruthenium(II) complex is linked by means of a 4-carboxylato,4 -methyl,2,2 -bipyridine ligand and a polyproline chain to a [Co(NH3)5] + or [(-NH-py)Ru (NH3)5] acceptor. Chains composed of from zero to six cis-prolines have been examined. The apparent distance dependence of the electron-transfer rate constant, corrected for variations in the solvent reorganizational energy, seems to exhibit two types of distance dependence, 0.7-1A for short chains and /3 a0.3 A for long chains. A very detailed theoretical analysis of electron transfer in the complexes with four proline linkers has indicated that the electronic coupling is sensitive to conformational variations within the proline chain. ... 

This topic has been reviewed by Ingledew (55). The major components of the respiratory chain for T. ferrooxidans are a cytochrome oxidase of the Ci type, cytochromes c, and the blue copper protein rusticyanin. Initial electron transfer from Fe(II) to a cellular component takes place at the outer surface of the plasma membrane in the periplasmic space. The rate of electron transfer from Fe(II) to rusticyanin is too slow for rusticyanin to serve as the initial electron acceptor. Several proposals have been made for the primary site of iron oxidation. Ingledew (56) has suggested that the Fe(II) is oxidized by Fe(III) boimd to the cell wall the electron then moves rapidly through the polynuclear Fe(III) complex to rusticyanin or an alternative electron acceptor. Other proposals for the initial electron acceptor include a three-iron-sulfur cluster present in a membrane-bound Fe(II) oxidoreductase (39, 88), a 63,000 molecular weight Fe(II)-oxidizing enzyme isolated from T. ferrooxidans (40), and an acid-stable cytochrome c present in crude extracts of T. ferrooxidans (14). 

Reaction of osmium tetra-/>-tolylporphyrin dimer [Os(TTP)]2 with excess hexamethylsilacyclopropane at room temperature gave a metalloporphyrin dimethylsilylene complex (Equation (17)) <9lOM3977>. The mechanism for this reaction was thought to involve an initial electron transfer followed by a radical chain process. 

In the light-induced initiation step of the chain mechanism 2-nitropropyl anion transfers an electron to 1 yielding the corresponding radical anion 3. Bromide is expelled to give cyclo-proyl radical 5 via allylic radical 4. Reaction with 2-nitropropyl anion yields the radical anion 6 that propagates the chain by electron transfer to 1 with formation of product 2. ... 

These reactions also appear to be chain reactions that proceed through the mechanism. Dimethyl sulfoxide is a particularly favorable solvent for this reaction, probably because its conjugate base acts as an efficient chain initiator by transferring an electron to the nitroalkane. 

Therefore, in anoxic medium and (for example biogenic) in situ hydrogen production, this is an important pathway to initiate reduction chains by electron transfer processes. The hydrogen atom and hydrated electron are interconvertible. Reaction... 

The complexes [Fe (r -C5R5)(r -C6R 6)], R and R = H or Me, can be used in catalytic amount to initiate electron-transfer-chain (ETC) reactions shown in Chap. 5 (sometimes called electrocatalytic). Depending on the redox potential... 

Kornblum has reported on the alkylation of some tertiary nitro-com-pounds with nitro-paraffin salts (Scheme 155) a radical-anion chain mechanism is proposed, with initial electron transfer from the nitro-paraffin anion to the substrate. 

Reactions involving the peroxodisulfate ion are usually slow at ca 20°C. The peroxodisulfate ion decomposes into free radicals, which are initiators for numerous chain reactions. These radicals act either thermally or by electron transfer with transition-metal ions or reducing agents (79). 

The addition followed a radical chain mechanism initiated by photoinitiated electron transfer from the tertiary amine to the excited aromatic ketone and occurred with complete facial selectivity on the furanone ring (99TL3169). The yields increased and best results were obtained with sensitizers (4-methoxyacetophenone,... 

The ceric ion also is also known to trap carbon-centered radicals (initiator-derived species, propagating chains) by single electron transfer (Scheme 3.60). 

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