Grignard reagents anodic oxidation

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 ... 

Electrolysis of Grignard reagents in ether produces the saturated and unsaturated hydrocarbons to be expected if the anode oxidizes a real or potential carbanion to the corresponding free radical. 

The electrochemical oxidation of l-MeO-7-alkyl-CHT in MeOH yields 2-alkyltropones, while the thermal rearrangement of 3-MeO-7-alkyl-CHT to l-MeO-4-alkyl-CHT followed by its anodic oxidation in MeOH affords 4-alkyltropones (equation 21). l-MeO-7-alkyl-CHT is prepared by the regioselective alkylation of 7,7-diMeO-CHT with a Grignard reagent and Cul, while 3-MeO-7-alkyl-CHT is also regioselectively prepared by alkylation of 7,7-diMeO-CHT with an alkyl lithium. 

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 behavior of BnMgBr (5d) is similar to that observed for compounds with higher alkyl groups, i.e. only the coupling product was detected and the earlier report on the additional formation of benzyl alcohol was not confirmed. On the other hand, reactions of Ar" radicals formed in the anodic oxidation of aryl Grignard reagents are different from those established for Aik, as is evident from the percent distribution of parent radicals in major products given in Table 7. 

Reactions of Ph" radicals (12h) formed at anodes yield styrene (30), biphenyl (31), p-terphenyl (32), insoluble hydrocarbon of high molecular weight and, in smaller amounts, benzene (33) as well as ethanol (19). 30 was the main product for substituted reagents 5q and 5r but for unsubstituted 5e only if the current efficiency, Teu was low. For higher Y i values 31 became the chief organic product. However, in contrast to aliphatic Grignard reagents, except methyl, the current efficiency was always much below 100%. In order to explain the above results the possibility of another route of anodic oxidation, different... 

Anodic oxidation of Grignard reagents (5) in the presence of styrene (30), butadiene (36) or vinyl ethyl ether (37) was investigated by Schafer and Kuntzel as an interesting (for preparative use) extension of other anodic reactions with olefins. The electrolysis was carried out at constant current density at Pt, Cu or graphite electrodes. It was found that the products obtained depend on the electrode material, as is seen from the data presented in Table 9. 

TABLE 10. Organoelemental compounds produced by electro-oxidation of Grignard reagents 5 on sacrificial anodes... 

Metal alkyls can be generated by anodic oxidation of a grignard reagent at a sacrificial metal anode. Tetraethyl lead is generated in this manner [16] ... 

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]. 

A better knowledge of electrochemical properties of the Grignard reagents would allow a more accurate prediction of their reactivities toward one and the same ketone, since electron transfer (ET) involves the transfer of one electron. It was hoped that it would be possible to relate reaction rates to the anodic oxidation potential of RMgX. 

Direct Si-C bonding to the H-terminated silicon surface is expected to lead to a modified surface that has a much better stability against oxidation than that of the H-terminated surface. It has been reported that the (111) Si surface can be completely methylated by anodization in a Grignard reagent. " ... 

Carbanions (those with the largest use are Grignard reagents, organolithians, or anions at the vicinal position to an unsaturated system) are easily oxidized. The anode material may be platinum, vitreous carbon, graphite, as well as titanium oxide. 

The rates of reaction for the different Grignard reagents were largely linearly correlated to the anodic oxidation potentials of the reagents. 5-Hcxenylmagnesium bromide produced I -addition product of which 509 was cycli/cd. 

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