Radicals metal atom reactions

This chapter presents developments in the activation and functionalization of carbon-hydrogen bonds that have been discovered since 1993. Major breakthroughs in hydrocarbon activation appeared in the early 1980s, and in the following decade, an explosion of discoveries was seen in new examples of metal complexes that could activate C-H bonds. Mechanisms for cleavage included oxidative addition, electrophilic cleavage, radical H-abstraction, and metal atom reactions, and several texts are available that summarize the first decade of this work. " ... 

In the synthesis of molecules without functional groups the application of the usual polar synthetic reactions may be cumbersome, since the final elimination of hetero atoms can be difficult. Two solutions for this problem have been given in the previous sections, namely alkylation with nucleophilic carbanions and alkenylation with ylides. Another direct approach is to combine radical synthons in a non-polar reaction. Carbon radicals are. however, inherently short-lived and tend to undergo complex secondary reactions. Escheirmoser s principle (p. 34f) again provides a way out. If one connects both carbon atoms via a metal atom which (i) forms and stabilizes the carbon radicals and (ii) can be easily eliminated, the intermolecular reaction is made intramolecular, and good yields may be obtained. 

The valency of the metal ion changes in every step so that a single atom of heavy metal (Me) may produce many free radicals. Metal chelating compounds, such as citric, tartaric or phosphoric acids, ascorbic acid, phytin or phosphatidic acids, combine with metals to form non-reactive compounds so that the oxidation reactions are inhibited and natural food antioxidants are saved. 

The site of reaction on an unsaturated organometallic molecule is not restricted to the most probable position of the metallic atom or cation or to a position corresponding to any one resonance structure of the anion. This has been discussed in a previous section with reference to the special case of reaction with a proton. Although the multiple reactivity is particularly noticeable in the case of derivatives of carbonyl compounds, it is not entirely lacking even in the case of the derivatives of unsaturated hydrocarbons. Triphenylmethyl sodium reacts with triphenylsilyl chloride to give not only the substance related to hexaphenylethane but also a substance related to Chichi-babin s hydrocarbon.401 It will be recalled that both the triphenyl-carbonium ion and triphenylmethyl radical did the same sort of thing. 

In this study, the metal centred radicals were formed indirectly by the photolysis of (Me3CO)2 yielding Me3CO radicals which then abstracted a hydrogen atom from the (n-Bu)3MH (M = Ge or Sn). The resulting metal based radical reacts with the carbonyl compound by adding to the oxygen atom (reaction 3). 

This conclusion falls in line with the fact that the anion radical could neither be detected after collision of the parent halide with alkali metal atoms in the gas phase (Compton et ai, 1978) nor upon y-irradiation in apolar or weakly polar solid matrixes at 77 K by esr spectroscopy (Symons, 1981). However, these observations are not absolute proofs that the anion radicals do not exist they might exist and be too short lived to be detectable. On the other hand, the reaction medium and the driving force conditions are quite different from those in the electrochemical experiments, which rendered necessary an independent investigation of the problem in the latter. 

Probably the most familiar radical reactions leading to 1,2-D systems are the so called acyloin condensation and the different variants of the "pinacol condensation". Both types of condensation involve an electron-transfer from a metal atom to a carbonyl compound (whether an ester or an aldehyde or a ketone) to give a radical anion which either dimerises directly, if the concentration of the species is very high, or more generally it reacts with the starting neutral carbonyl compound and then a second electron is transferred from the metal to the radical dimer species (for an alternative mechanism of the acyloin condensation, see Bloomfield, 1975 [29]). 

Nitrogen oxides. Nitric oxide (NO) itself, has been shown to be a poor nitrosating agent (28), probably because it is unable to abstract an amino-H atom to generate the dialkyl-amino radical, which might then combine with further NO. However, the presence of even a small amount of air results in complete conversion, presumably via oxidation of NO to NO2. Nitrosation by NO is catalyzed by metal salts, such as Znl2, CuCl, and CUSO4. The metal catalyzed reaction is inhibited in acid or aqueous media (29). 

Adenosylcobalamin (coenzyme 812) carries a covalently bound adenosyl residue at the metal atom. This is a coenzyme of various isomerases, which catalyze rearrangements following a radical mechanism. The radical arises here through homolytic cleavage of the bond between the metal and the adenosyl group. The most important reaction of this type in animal metabolism is the rearrangement of methylmalonyl-CoAto form succinyl-CoA, which completes the breakdown of odd-numbered fatty acids and of the branched amino acids valine and isoleucine (see pp. 166 and 414). 

Free radicals are atoms or groups of atoms possessing an odd (unpaired) electron. Radical recombination occurs when active flame propagating species (O , H and OH) recombine (heterogeneously) on particle surfaces or (homogeneously) as a result of gas phase reactions catalysed by alkali metal atoms in the flame, e.g. 

Thermolysis on insulating wool. Kaowool or other refractory wools are valuable for reduced radiative heat losses from a hot crucible. However, their use causes some increase in the extent of pyrolysis of substrate vapor by the crucible assembly and this, in rare instances, may spoil a metal atom synthesis. Only one example is known at present. The reaction of palladium atoms with benzyl chloride gives very low yields of t73-benzylpalladium chloride when the palladium is evaporated from an alumina crucible insulated with Kaowool, but a 30-50% yield with an uninsulated crucible. It has been established that this is due to enhanced formation of product-destroying radicals on the hot Kaowool. 

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