Anodic organometallic compounds

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. 

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. 

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. 

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

Electrochemical reactions serve as efficient and convenient methods for the synthesis of organoelemental compounds. There are four major methods for the formation of element (metal)-carbon bonds. The first method utilizes the anodic oxidation of organometallic compounds using reactive metal anodes. In the second method, the organic compounds are reduced using reactive metal cathodes. The third method involves the cathodic reduction of organic compounds in the presence of metal halides. The fourth one utilizes both the cathodic and the anodic processes. 

Oxidation of Organometallic Compounds Using Reactive Metal Anodes... 

The anodic oxidation of Grignard reagents using reactive metal anodes such as A1 [10,11], Zn [10,12], Mn [12], Cd [10,12], Bi [12], and B [13] is known to give the corresponding organometallic compounds [Eq. (2)]. This reaction has been used for the synthesis of tetraalkyllead on an industrial scale (NALCO process) [14,15]. 

Other examples have been provided by the anodically initiated isomerization of several decarbene metal carbonyl couples [197]. Electrochemical induction of chemical reactions can also be successfully used for conversion of alcoolates into ketones with simultaneous reduction of aromatic halides [198], the tetramerization of aziridines [199], or ligand substitution [200] in organometallic compounds. A useful review on this subject was published [201]. 

Metallocenes and their derivatives are another large class of organometallic compounds, such as the well-known ferrocene. Typically, they are chemically synthesized by reacting the metal ion with a cycloolefin activated by a very strong base. The electrochemical process works under milder conditions in an undivided cell. The chosen metal is oxidized at the anode, and at the cathode the olefin is reduced 53... 

Active electrocatalyst forming soluble species whose utilization is likely for the electroformation of organometallic compounds or low-chained polymers. One typical example is the industrial preparation of tetra-alkyl-lead through the electrooxidation of the Grignard reactant on bed-leached anodes [24,25],... 

A one-pot preparation of 4-iodomethyloxetan-2-one is reported (93SL899). Electrochemically generated organometallic compounds react with ketones in a Reformatsky reaction to give esters or lactones (93AG1218) and the formation of P-lactones is favoured when a sacrificial indium anode is used (94JOC3161). 

A variety of substituted radicals may be generated by the electro-oxidation of salts of 1,3-dicarbonyl compounds, aliphatic nitrocompounds, phenols, oximes, alkynes, and organometallic compounds. In all of these reactions, an anionic (or partially anionic) species is formed from which the intermediate radical is generated, e.g., in the anodic oxidation of diethylmalonate... 

Grignard reagents or lithium alkyls exhibit low oxidation potentials and provide an excellent source of both alkyl and aryl radicals. It appears that subsequent oxidation of intermediate species, as is observed in some Kolbe electrosyntheses, is often avoided in the anodic reactions of these organometallic compounds, but practical difficulties have tended to limit progress in this area. It is anticipated that improved design of electrochemical cells will alleviate these problems. " ... 

I have tried to show that the use of a metal as the sacrificial anode in electrochemical oxidation can form the basis of a very wide range of syntheses of inorganic and organometallic compounds. We believe that the method is indeed a choice one for a series of ligands derived from weak mono- and dibasic organic acids, and that while we have not investigated all metals ourselves, there seems every reason to assume that the range of syntheses could be extended to most areas of the Periodic Table. 

TABLE 6. Electrical Conductivity and Dissociation Schemes for Some Complexes Used in Synthesis of Organometallic Compounds in Anodic Processes... 

Organoboron complexes of the MR BR3 andMBR4 MAlR4 types merit attention from the safety standpoint. Numerous metal alkyls can be prepared by electrolysis of these compounds (Table 11). It is recommended that the dimethyl ether of diethylene glycol should be used as solvent. A small amount of water is added to increase the electrical conductivity of the solution [110]. Electrolysis of tetraalkylboron complexes in aqueous solution with a soluble anode has also been described [58]. In this last case alkyl derivatives of mercury, bismuth, and lead are obtained. Tin and antimony anodes become passivated, and the yields of the respective organometallic compounds are small. 

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