Metal alkoxides electrochemical synthesis

The direct electrochemical synthesis of metal alkoxides by the anodic dissolution of metals into alcohols containing conducting electrolytes was initially demonstrated by Szilard in 1906 for the methoxides of copper and lead.19 More recently the method has received some attention particularly in the patent literature.29-25 The preparation of the ethoxides of silicon, titanium, germanium, zirconium and tantalum by electrolysis of ethanolic solutions of NH Cl has been patented, although the production of the ethoxides was found to cease after several hours.24,25... 

It should be mentioned that by changing the conditions of electrochemical synthesis — such as nature of the alcohol, the purity of the metal used as anode, the nature and concentration of the conductive additive, the voltage (usually 30 — 110 V DC is applied), the temperature, and even the construction of the cell — one can significantly effect the process of the anodic dissolution or even change its mechanism. As it has already been mentioned, the electrochemical dissolution of iron in alifatic alcohols gives insoluble iron (II) alkoxides [1005], while in 2-methoxyethanol a soluble iron (HI) complex is obtained [1514], Another example is provided by tantalum dissolution in isopropanol while high-purity metal is rapidly dissolved anodically [1639], the one containing impurities is passivated. Therefore, it is quite clear that each synthesis requires careful study, optimization of parameters of electrochemical synthesis, and isolation and purification of final products. 

Recently, Ce43 75+0(0Pr )i3( Pr0H) was obtained by irradiation of Ce(OPr )4 PrOH solutions in (-CH2OMe)2 and PrOH by the visible light [1803]. Partial reduction ofalkoxides occurs in the course of electrolysis of metal alkoxides (see above). Oxoalkoxoniobates NbO(OR)3 (R=Me, Et, Bu), the products of oxidation of the Nb(OR)4, were isolated in the course of electrolysis performed with Nb anode and Pt cathode [911]. Although the reduction products are highly unstable (they cannot be used for synthesis of oxides) their formation in electrochemical synthesis should be taken into consideration because their subsequent oxidation by 02 (or even its traces) leads to ox-oalkoxides. 

The metatheis of acetates with the alkali alkoxides (method 5) can be used for the preparation of the methoxides and ethoxides of all three elements and is the only reaction leading to Hg(OR)2 [1623]. The trans-esterification of Zn(OMe)2 (method 6), according to [1121], can be carried out only in the presence of LiOR (forming soluble bimetallic complexes). The direct electrochemical synthesis on the anodic oxidation of metals in alcohols has been described for Zn(OEt)2 and a series of cadmium derivatives (Cd(OR)2 — obtained in the presence of such donor ligands as Dipy, Phen, and Dmso [98]) (method 2). 

The most reproducible results have been obtained [32] in protic solvents such as z -BuOH and dimethyletanolamine with almost quantitative yields (Table 5.2, Example 11). A successful synthesis in protic media is in agreement with the opinion of the authors of Ref. 10 that a protic solvent is required for the electrosynthesis of PcH2. Sodium methylate as a source of alkoxide anions performs a nucleophilic attack at the cyano group of PN to form l-alkoxy-3-iminoisoindoline as an intermediate which is further reduced and cyclizated [19]. This mechanism is similar to the one described by Petit et al. [10] and Tomoda et al. [19], where metal alkoxides were used for the electrochemical preparation of Pc. 

Shreider, Turevskaya, E. P., Kozlova, N. and Turova, N. Y., Direct electrochemical synthesis of metal alkoxides, Inorg. Chim. Acta, 53, L73 (1981). 

The strategy of metal alkoxides synthesis is entirely related to the electronegativity of the element concerned. Some electropositive metals, such as alkali metals, alkaline earth metals, and lanthanides, react directly with alcohols. But some less electropositive metals such as magnesium and aluminum require a catalyst (I or HgCy for successful reaction with alcohols. Use of electrochemical synthesis by anodic dissolution of some metals or metalloids (Sc, Y, Ti, Zr, Nb, Ta, Fe, Co, Ni, Cu, Pb, Si, Ge, etc.) in dry alcohol performs a promising procedure because it does not produce any by-products except hydrogen gas. Another applicable method for the synthesis of some alkoxides (B, Si, Ti, Zr, Hf, Nb, Ta, Fe, etc.) is the reaction of their chlorides with alcohols which require a base such as... 

The interaction of metal with alcohol (on catalytic action of BeCl2, HgCl2 or I2) is used only for the synthesis of the ethoxide, which precipitates on reflux (method 1). In the case of MeOH or PrOH, the stable viscous liquid alkoxide halids are formed [1645, 1624, 1120]. The electrochemical approach (method 2) was applied only for the preparation of Be(OC2H4OMe)2 [1652],... 

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