Anodic organometallics

It had been shown in the preceding sections that the initial step in a number of cathodic and anodic reactions yields organic radicals, which then undergo further oxidation, reduction, or dimerization. In some cases reactions of another type are possible reaction of the radical with the electrode metal, yielding organometallic compounds which are then taken up by the solution. Such reactions can be used in the synthesis of these compounds. 

Anodic processes can also be used for tetraethyllead electrosynthesis. Here solutions of organometallic compounds are used the overall reaction is replacement of the metal in these compounds by another metal, lead. One such process uses a melt of the compound NaAl(C2H5)4, from which radicals QHj are produced anodically. The process is highly efficient, but it is not easy to isolate the TEL produced from the melt. More convenient is a commercial process involving the anodic oxidation of the Grignard reagent C2H5MgCl ... 

Synthesis with sacrificial electrodes is employed as a direct method in several other preparations of organometallic compounds and complexes. 3-Hydroxy-2-methyl-4-pyrone derivatives of Sn 1 (and of Zn, Cu, In and Cd as well) were prepared using the metal as an anode. The low oxidation state Sn(II) compound is obtained by direct electrolysis134. 

Figures 3 and 4, respectively, show thin-layer current-potential curves for polycrystalline Au and Ir in molar sulfuric acid before and after exposure to a 2 mM HQ solution. For smooth Au, no changes in the voltammetric curves are seen. In comparison, a prominent anodic oxidation peak is observed for Ir after pretreatment with HQ. These observations, which indicate that Ir is reactive towards HQ but Au is not, are consistent with what is known from the literature on homogeneous organometallic chemistry (21) Ir and Pt complexes are reactive towards a variety of organic compounds, whereas Au is inert.

The reactions appear to be similar to organometallic synthesis, where the reduction is performed by the metal instead of electricity. However, these reactions have been shown to be essentially different from the corresponding organometallic reactions. This method has valuable advantages. As the anode reaction is controlled, an undivided cell can be used, the reaction occurs in one-step, the conditions are quite simple, and so on. Sibille and Perichon et al. have found that the sacrificial zinc anode is quite effective for trifluoromethylation of aldehydes to form trifluoromethylated alcohols in almost quantitative yields (Eq. 6) [19]. The reaction proceeds via the reduction of Zinc(II) salts, followed by a chemical reaction between the reduced metal, CF3Br, and aldehyde. 

Organometallic compounds can be generated at the electrode in three ways. An alkyl halide is reduced at an active cathode, for example, Pb, Sn, which reacts with the intermediate radical [163, 164]. A Grignard reagent or an at-complex is oxidized at an active anode and the intermediate radical reacts with the anode... 

The success of these reactions was intriguing because, under normal reaction conditions, group 14 organometallic compounds serve as carbanion synthons. The anodic oxidation reaction successfully reversed this polarity thereby expanding the overall synthetic utihty of group 14 organometallic intermediates. 

In the mid-1960s, Dessy and coworkers [12, 13] provided an extensive survey of the anodic and cathodic reactions of transition metal organometallic species, including binary (homoleptic) carbonyls, and this provided a stimulus for many later detailed studies. Whereas the electrochemistry of heteroleptic transition metal carbonyls is covered elsewhere in this volume, that of the binary carbonyls, which is covered here, provides paradigms for the electrochemistry of their substituted counterparts. A key aspect is the generation of reactive 17-electron or 19-electron intermediates that can play key roles in the electrocatalytic processes and electron-transfer catalysis of CO substitution by other ligands. 

The electrooxidation of Grignard reagents (5) on reactive metal anodes produces the corresponding organometallic compounds, or more generally organoelemental compounds. 

The electrochemical reduction of bromotrifluoromethane (CF3Br) in DMF in a cell fitted with a sacrificial zinc anode and a stainless steel or nickel cathode is a typical case where the organometallic compound formation can be realized according to two different processes, and where a transient species having a different reactivity compared to usual organometallics can be produced. 

These results indicate that zinc ions formed by oxidation of the anode do not play a part or only have side effects in the direct electroreductive carbon—carbon bond formation carried out with a zinc anode and a nickel catalyst. In these reactions, a nickel organometallic is involved. 

Coupling reactions (a) using organometallic compounds, and (b) at the anode 477... 

Anodic 1 e-oxidation of organic and inorganic anions or organometallics is a... 

A great variety of substituted radicals for dimerization can be generated by anodic oxidation of anionic species r5"Me5+, e.g., sodium salts of 1,3-dicarbonyl compounds, aliphatic nitro compounds, phenols, oximes, alkynes, thio-lates or organometallics (Eq. (157) ). 

---