Ethene anion radical

The Three-electron Bond of the Ethene Anion Radical... 

The structure of the ethene anion radical remains unknown, but a simple HMO picture of this species (Scheme 64) reveals a three electron bond loosely analogous to those found in some radicals, such as the nitroxyl radicals. 

Scheme 64. The three electron bond of the ethene anion radical.

These assumptions suggest that the anion radical of ethene, a two-center ir-system, would be the best candidate for such a purpose. Not ethene, with an isolated ir-system, but 1,3-butadiene and its derivatives give observable anion radicals (Levy Myers 1964, 1966). 

The energy of the SOMO of an ethene derivative which is substituted with conjugating, electron withdrawing substituents is of course much lower than that of ethene itself. An extreme case of such an anion radical is that of tetracyanoethylene (TCNE), which has been isolated as the tetrabutylammonium salt. A reasonably direct, experimental examination of the SOMO distribution of this anion radical was possible through polarized single crystal neutron diffraction studies [108]. Interestingly, the pi SOMO was found not to be centered directly around the two alkene carbon nuclei, but rather to be bent back, away from the alkene C-C bond, as is theoretically expected for an MO which is anti-bonding between these two carbons (Scheme 65). 

The LUMOs of conjugated pi systems are typically much lower in energy than that of ethene, so that the reduction of these systems to the corresponding anion radicals is more facile. The electrochemical reduction of 1,3-butadiene in liquid ammonia solution at —78 °C, in fact, yields an anion radical which is stable enough to permit observation by ESR spectroscopy (Scheme 66) [109]. 

Although alkali metal/liquid ammonia reductions (Birch reductions) of simple alkenes is difficult, presumably as a result of the very high energy of an ethene type LUMO, the corresponding reduction of non-terminal alkynes to trawi -alkenes is an efficient and useful synthetic tool for accessing trans-alkenes [116]. The mechanism for this reaction (Scheme 69), involves the homogeneous reduction of the alkyne to the corresponding anion radical by the solvated electrons present in liquid ammonia solutions of alkali metals. 

Most technically important polymerizations of alkenes occur by chain mechanisms and may be classed as anion, cation, or radical reactions, depending upon the character of the chain-carrying species. In each case, the key steps involve successive additions to molecules of the alkene, the differences being in the number of electrons that are supplied by the attacking agent for formation of the new carbon-carbon bond. For simplicity, these steps will be illustrated by using ethene, even though it does not polymerize very easily by any of them ... 

Examples include control of molecular weight in step-growth polymerization, number-average degree of polymerization in step-growth polymerization of nonstoichio-metric monomer mixtures, free-radical and anionic polymerizations of styrene, and ethene oligomerization to linear 1-olefins in the Shell Higher Olefins Process. 

Chain-growth polymerization involves the sequential step-wise addition of monomer to a growing chain. Usually, the monomer is unsaturated, almost always a derivative of ethene, and most commonly vinylic, that is, a monosubstituted ethane, 1 particularly where the growing chain is a free radical. For such monomers, the polymerization process is classified by the way in which polymerization is initiated and thus the nature of the propagating chain, namely anionic, cationic, or free radical polymerization by coordination catalyst is generally considered separately as the nature of the growing chain-end may be less clear and coordination may bring about a substantial level of control not possible with other methods. ... 

In multielectron transfer processes, the reduction of CO2 can yield formic acid, carbon monoxide, formaldehyde, methanol, or methane that is, the primary electrochemical process supplies Ci compounds. These reactions can proceed at reasonable reduction potentials between —0.24 and —0.61 V (NHE) (Equations (6.12-6.16) the reduction potentials, E°, refer to pH 7 in aqueous solutions versus NHE), while the formation of the C02 radical anion is estimated to take place at —2.1 V.104 Reduction of CO (in the presence of H + ) supplies CH2" radicals that may yield methane directly or leads to higher hydrocarbons (e.g., ethene or ethane) by recombination.24,105 Efficient formation of ethene (together... 

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