Mechanism, radical organometallic compounds

This section provides the rationale for the selection of the chiral cyclopropyl and vinyl systems in the investigation of the mechanism of organometallic compound formation. The absolute configurations and optical purities of these systems have been established. The ability of these systems to perform as effective probes in such mechanistic studies resides in the knowledge of the stereochemistry of their radical and carbanion intermediates. Do the radical and carbanion intermediates retain or lose their configurations ... 

The introduction of additional alkyl groups mostly involves the formation of a bond between a carbanion and a carbon attached to a suitable leaving group. S,.,2-reactions prevail, although radical mechanisms are also possible, especially if organometallic compounds are involved. Since many carbanions and radicals are easily oxidized by oxygen, working under inert gas is advised, until it has been shown for each specific reaction that air has no harmful effect on yields. 

It might be mentioned that matters are much simpler for organometallic compounds with less-polar bonds. Thus Et2Hg and EtHgCl are both definite compounds, the former is a liquid and the latter is a solid. Organocalcium reagents are also known, and they are formed from alkyl halides via a single electron transfer (SET) mechanism with free-radical intermediates. "... 

It is unlikely that a single mechanism suffices to cover all conversions of organometallic compounds to alkyl halides. In a number of cases the reaction has been shown to involve inversion of configuration (see p. 762), indicating an Se2 (back) mechanism, while in other cases retention of configuration has been shown, implicating an Se2 (front) or SeI mechanism. In still other cases, complete loss of configuration as well as other evidence have demonstrated the presence of a free-radical mechanism. ... 

The removal of an electron from a carbanion oxidizes it to a free radical and sometimes, in the presence of oxygen, to a peroxide. Organometallic compounds give many radical-like reactions of course, and a possible oxidation mechanism for such compounds is a preliminary dissociation into radicals followed by oxidation of the radicals and the metal. 

The section about organometallic compounds of Ge, Sn and Pb was divided into neutral closed-shell molecules, which comprise the largest part of the chapter, cations and anions and finally radicals. The section about closed-shell molecules is further divided into papers which report about structures and properties of molecules with multiple and those with single bonds of Ge, Sn and Pb, and work that concentrates on the elucidation of reaction mechanisms. We included all studies which report about compounds of the heavier elements Ge to Pb even when the main focus of the work was on the lighter atoms C and Si or other elements. Thus, some work which is discussed here will also be found in the chapter which focuses on Si compounds8. 

To begin, let s consider the anionic polymerization of styrene. For an initiator, we will choose an organometallic compound an organic compound bonded to a metal atom) such as butyllithium, C4H9 Li+. Although the details differ, you should recognize the overall similarity of the mechanism for this anionic polymerization to that for the free radical polymerization of ethylene, above (initiation, propagation, and termination). 

In the drying of THF or ether (Figure 14.45), the sequence ketone —> ketyl —> hydroxylated radical —> hydroxylated organometallic compound —> alkoxide is of course not intended to convert all the ketone into product. The reaction depicted in Figure 14.46 features the same sequence of steps as Figure 14.45 (and there is thus no need to discuss their mechanism again). In the reaction of Figure 14.46, however, the reaction is intended to run to completion until all of the ketone has been consumed. The reason for this is that it is the purpose of this reaction to reduce the ketone to the alcohol. The... 

The majority of main group organometallic compounds for which SET mechanisms were postulated involves Grignard reagents [13,69-75] in their reactions with carbonyl compounds. Even the formation of these important laboratory reagents RMgHal (which does not fall within the scope of this article) is now believed to involve electron transfer mechanisms and radical formation at the surface of metallic Mg [76,77]. In contrast to Grignard compounds with their... 

An aryl halide such as chlorobenzene is relatively unreactive towards nucleophilic substitution. The S l and Sj. 2 pathways involve mechanisms that are not open to aryl halides. The greater s character of the sp bond makes it more difficult to cleave the bond to generate a carbo-cation. However, these restrictions do not apply to radical or carbanion chemistry. Hence, aryl halides undergo radical coupling reactions and metal insertion reactions, leading to organometallic compounds. 

Organometallic compounds of less active metals and metalloids (e.g., silicon," antimony, and bismuth, are quite inert to water. Organomercury compounds (RHgX or R2Hg) can be reduced to RH by H2, NaBITj, or other reducing agents." The reduction with NaBH4 takes place by a free-radical mechanism." Alkyl-Si... 

Apparently, the reactivity of organometallic compounds in the addition of olefins to Mt—C bonds is determined by the capability of these compounds to coordinate olefins. The formation of intermediate n-complexes ensures further insertion of olefin by a concerted mechanism with a low activation energy. Thus, a high reactivity of active centers, containing a transition metal, comparable to the reactivity of the radical active centers, is achieved. The activation energy of the propagation in olefin polymerization on catalysts containing transition metals (2-6 kcal/mol) does not exceed its value for the radical polymerization (Table 10). 

The mechanism of the initiation of the radical polymerisation of polar monomers in systems comprising organometallic compounds Homogeneous anionic polymerisation of unsaturated monomers Metallopolymers... 

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