Radical mechanism, addition organometallic compounds

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. 

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). 

This class of reaction is manifested especially by (typically five-coordinated) low-spin d cobalt(II) complexes, such as [Co(II)(CN)5]3-, [Co(II)(DMGH)iB] (DMGHi = dimethylglyoxime, B = axial ligand such as pyridine, triphenylphosphine, etc.) and cob(Il)alamin (vitamin Bn ). Important examples include the oxidative addition of organic halides leading to the formation of organometallic compounds by free-radical mechanisms ... 

In a direct comparison of the palladium-catalyzed and the hexabutyldistannane-mediated ene-halogcnocyclization of unsaturated a-iodo carbonyl compounds, identical mixtures of regio- and stereoisomers were obtained311. Thus, it was suggested that both reactions proceed via a radical mechanism, and that palladium does not initiate an organometallic cycle via oxidative addition, alkene insertion and reductive elimination. 

Organometallic compounds with a 17-electron configuration are often labile toward associative ligand exchange. Radical chain mechanisms are well established for phosphine substitution on metal carbonyl hydrides (Scheme 23), the 17-electron chain carrier being in most cases non hydridic. This mechanism, however, was also shown to operate for OsH2(CO)4 via the 17-electron hydride complex OsH(CO)4 [137]. Thus, phosphine addition to the radical prevails over the dimerization, which indeed occurs in the absence of phosphine [33] (section 6.5.7), and over other possible decomposition pathways. The second step of the chain propagation process in Scheme 23, for this osmium system, is another example of atom transfer to a hydride radical (section 6.5.6). 

Due to the unpaired electrons, paramagnetic compounds often react as radicals. In the section on oxidative addition below, we ll see that radical mechanisms can be involved. In fact, many organometallic mechanisms involve radicals, and having a knowledge of which orbitals these radicals reside in is informative. The identity of the singly occupied orbitals is predicted by an examination of the d orbital splitting. 

During CCT, there are occasions where organometallic compounds form a Co—C bond that is stable at room temperatures. Investigation of the mechanism of this reaction with deuterated substrates proved that hydrogen transfer from cobalt porphyrin to a double bond (Scheme 8.3) occurred stereospecifically through cis-addition. Cis-addition supposes a concerted reaction mechanism. Stereospecific cis-addition hardly could be observed if reaction of LCo-H addition to a double bond proceeds by a three-center mechanism that requires formation of intermediate radical from the olefin ... 

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