Anion radical intermediates metals

The fact that both the thermal and the photochemical insertion reactions yield the same products via formation of charge-transfer complexes leads to the conclusion that the reactive ion-radical pair in equation (52) is the common intermediate for both activation processes. Such a conclusion is further verified by the direct observation of anion-radical intermediates from the thermal reaction of TCNE and DDQ with various metal hydrides.188... 

Substituent effects observed for this reaction are entirely consistent with those described for electrophilic substitution and addition —only reversed. That is, the reactivity of an arene in metal reductions is increased by electron-withdrawing groups and decreased by electron-donating groups. Substituents that can stabilize the anion-radical intermediate facilitate the reduction (see Exercise 22-35). 

It is known that group 14 metals facilitate the electron transfer from the neighboring jt-system by virtue of so called interaction [196]. The LUMO level is decreased by such interaction, which in turn favors the reduction of the system. The silyl group also stabilizes the anion radical intermediate formed by a one-electron reduction [197,198]. These effects can also be explained in terms of the orbital interaction with a low-lying a orbital of the sily group instead of the vacant d orbital. 

In a study of electron-transfer reactions. House has examined the stabilities of the anion radicals derived from a number of y-cyclopropyl-ap-unsaturated ketones. It would appear that for the conversion of substrates containing the unit depicted by (240) into (241) a rearrangement rate in excess of 10 s is necessary in order to detect the anion radical intermediate present in metal-ammonia reductions. A rate of ca. 10 s is likewise required in lithium dimethylcuprate reactions. ... 

Nitrosoarenes are readily formed by the oxidation of primary N-hydroxy arylamines and several mechanisms appear to be involved. These include 1) the metal-catalyzed oxidation/reduction to nitrosoarenes, azoxyarenes and arylamines (144) 2) the 02-dependent, metal-catalyzed oxidation to nitrosoarenes (145) 3) the 02-dependent, hemoglobin-mediated co-oxidation to nitrosoarenes and methe-moglobin (146) and 4) the 0 2-dependent conversion of N-hydroxy arylamines to nitrosoarenes, nitrosophenols and nitroarenes (147,148). Each of these processes can involve intermediate nitroxide radicals, superoxide anion radicals, hydrogen peroxide and hydroxyl radicals, all of which have been observed in model systems (149,151). Although these radicals are electrophilic and have been suggested to result in DNA damage (151,152), a causal relationship has not yet been established. Nitrosoarenes, on the other hand, are readily formed in in vitro metabolic incubations (2,153) and have been shown to react covalently with lipids (154), proteins (28,155) and GSH (17,156-159). Nitrosoarenes are also readily reduced to N-hydroxy arylamines by ascorbic acid (17,160) and by reduced pyridine nucleotides (9,161). 

This difference of regioselectivity in alkylation of CHT is explained by the difference of the electrophile which reacts with the first active intermediate formed from CHT. Thus, the first active intermediate formed by one-electron transfer to CHT is an anion radical species (A) in both the electrochemical and the Li-metal reduction. 

As indicated in Chapter 8, the production of alkanes, as by-products, frequently accompanies the two-phase metal carbonyl promoted carbonylation of haloalkanes. In the case of the cobalt carbonyl mediated reactions, it has been assumed that both the reductive dehalogenation reactions and the carbonylation reactions proceed via a common initial nucleophilic substitution reaction and that a base-catalysed anionic (or radical) cleavage of the metal-alkyl bond is in competition with the carbonylation step [l]. Although such a mechanism is not entirely satisfactory, there is no evidence for any other intermediate metal carbonyl species. 

Reductions of certain aromatic ketones with metal hydrides have been shown to involve radical intermediates formed by an electron-transfer mechanism (25). For example, the reaction of aluminum hydride with dimesityl ketone in THF produced a violet solution that gave an EPR spectrum indicative of the presence of a paramagnetic species. The paramagnetic species is an intermediate in the reduction of the ketone, and is believed to be a radical cation-radical anion pair (25). 

The carbon dioxide anion-radical was used for one-electron reductions of nitrobenzene diazo-nium cations, nitrobenzene itself, quinones, aliphatic nitro compounds, acetaldehyde, acetone and other carbonyl compounds, maleimide, riboflavin, and certain dyes (Morkovnik and Okhlobystin 1979). The double bonds in maleate and fumarate are reduced by CO2. The reduced products, on being protonated, give rise to succinate (Schutz and Meyerstein 2006). The carbon dioxide anion-radical reduces organic complexes of Co and Ru into appropriate complexes of the metals(II) (Morkovnik and Okhlobystin 1979). In particular, after the electron transfer from this anion radical to the pentammino-p-nitrobenzoato-cobalt(III) complex, the Co(III) complex with thep-nitrophenyl anion-radical fragment is initially formed. The intermediate complex transforms into the final Co(II) complex with the p-nitrobenzoate ligand. 

The authors proposed the following picture of the silylene anion-radical formation. Treatment of the starting material by the naphthalene anion-radical salt with lithium or sodium (the metals are denoted here as M) results in two-electron reduction of >Si=Si< bond with the formation of >SiM—MSi< intermediate. The existence of this intermediate was experimentally proven. The crown ether removes the alkali cation, leaving behind the >Si - Si< counterpart. This sharply increases electrostatic repulsion within the silicon-silicon bond and generates the driving force for its dissociation. In a control experiment, with the alkali cation inserted into the crown ether, >Si — Si< species does dissociate into two [>Si ] particles. 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

Reaction 11 involves hydrogen atom transfer as proposed by Halpern et al. (13) in the mechanism of formic acid oxidation by cobalt (III) in aqueous solutions. In this reaction one could consider that as peracetic acid approaches the coordination sphere of Co111 and transfers the hydrogen atom to the coordinated acetate, the Co111 atom is transformed into a Co11 complex of peracetoxy radical (or Co111 complex of peracetate anion). Complexes of free radicals with metal ions have been postulated by Kochi (16). The substitution rate in this complex could be intermediate between the rate of substitution of cobalt (III) and cobalt (II) complexes owing to the contribution of the resonance structures ... 

The difference between the last two reactions may be also considered in terms of the complete electron transfer in both cases. If the a-nitrostilbene anion radical and the metal-locomplex cation radical are formed as short-lived intermediates, then the dimerization of the former becomes doubtful. The dimerization under electrochemical conditions may be a result of the increased concentration of reactive anion radicals near the electrode. This concentration is simply much higher in the electrochemical reaction because all of the stuff is being formed at the electrode, so there is more dimerization. Such a difference between electrode and chemical reactions should be kept in mind too. 

Radical intermediates were proposed at an early stage in the history of the Cannizzaro reaction, and this possibility has been resurrected to account for the detection of some 20% of a-D-benzyl alcohol in the products of the Cannizzaro reaction of a-D-benzaldehyde in aqueous alkaline dioxan. This has been rationalized in terms of formation of the benzaldehyde radical anion, which abstracts a H-atom from the solvent (Chung, 1982). Epr spectroscopy of reacting solutions of p-Cl-,/ -NO 2 and / -CF3-benzaldehyde as well as benzaldehyde itself in THF HMPA (9 1) yielded spectra of the aldehyde radical anions identical to those produced by the action of metallic... 

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