Metal halides reactions with alkoxides

The late transition and main group metals follow the anodic oxidation pathway analogous to that in aqueous solutions. The minimal oxidation potentials in these cases can in fact be very low (up to max. 3.0 V), whUe higher ones are readily applied to accelerate the process. The anodic reaction consists of dissolution of metal ions in the form of anionic halide complexes, which are later transformed into insoluble alkoxides by reaction with alkoxide anions generated at the cathode, for example (Lehmkuhl, 1975) ... 

Both reactants m the Williamson ether synthesis usually originate m alcohol pre cursors Sodium and potassium alkoxides are prepared by reaction of an alcohol with the appropriate metal and alkyl halides are most commonly made from alcohols by reaction with a hydrogen halide (Section 4 7) thionyl chloride (Section 4 13) or phosphorus tri bromide (Section 4 13) Alternatively alkyl p toluenesulfonates may be used m place of alkyl halides alkyl p toluenesulfonates are also prepared from alcohols as their imme diate precursors (Section 8 14)... 

From Metal Halides. The reaction of metal chlorides with alcohols can give metal chloride alkoxides (67—69), eg... 

Alkyl halides undergo Sn2 reactions with a variety of nucleophiles, e.g. metal hydroxides (NaOH or KOH), metal alkoxides (NaOR or KOR) or metal cyanides (NaCN or KCN), to produce alcohols, ethers or nitriles, respectively. They react with metal amides (NaNH2) or NH3, 1° amines and 2° amines to give 1°, 2° or 3° amines, respectively. Alkyl halides react with metal acetylides (R C=CNa), metal azides (NaN3) and metal carboxylate (R C02Na) to produce internal alkynes, azides and esters, respectively. Most of these transformations are limited to primary alkyl halides (see Section 5.5.2). Higher alkyl halides tend to react via elimination. 

We have already learnt that alkyl halides react with alcohols and metal hydroxide (NaOH or KOH) to give ethers and alcohols, respectively. Depending on the alkyl halides and the reaction conditions, both S l and Sn2 reactions can occur. Alkyl halides undergo a variety of transformation through Sn2 reactions with a wide range of nucleophiles (alkoxides, cyanides, acetylides, alkynides, amides and carboxylates) to produce other functional groups. 

The addition of a base, typically ammonia, to mixtures of transition metal halides and alcohols allows the synthesis of homoleptic alkoxides and phenoxides for a wide range of metals. Anhydrous ammonia was first used in the preparation of titanium alkoxides where the reaction is forced to completion by the precipitation of ammonium chloride.41 Although useful for the synthesis of simple alkoxides and phenoxides of Si, Ge, Ti, Zr, Hf, V, Nb, Ta and Fe, as well as a number of lanthanides,42-47 the method fails to produce pure /-butoxides of a number of metals.58 Presumably, secondary reactions between HC1 and Bu OH take place. However, mixing MC14(M = Ti, Zr) with the Bu OH in the presence of pyridine followed by addition of ammonia proves successful, giving excellent yields of the M(OBul)4 complexes.59... 

An in-depth study of the industrially important hydrolysis of titanium alkoxides has been carried out by Bradley.234,235 A number of intermediate complexes were isolated and characterized. The alcohol exchange reaction has been discussed previously. The addition of hydrohalous acids to alkoxides is clearly related to the reverse reaction, the addition of alcohols to metal halides. In general, the products of these two reactions will be the same (equation 59). Hence, complete substitution will occur to give metal halides that are known to form only alcoholates with alcohols (equations 60 and 61),31,236... 

An important development of this method has recently been carried out by Mehrotra and Veith. The reaction with the excess of halide was found to provide bimetallic alkoxide halides. Their interaction with the alkoxometallate of the third metal provided access to trimetallic alkoxides ... 

The most usual synthetic routes to the derivatives of platinum group metals are the exchange reactions of the complexes containing halide ligands with alkali metal alkoxides (method 5), alcoholysis of the same kind derivatives (usually by phenols, method 4), alcoholysis of hydroxide complexes (method 3), and redox reactions — reduction of chlorides or 0s04 in alcohol media (method 7) (Table 12.25). 

Two reactants are needed to form the ether an allylic halide and an alkoxide ion. Alkoxide ions are commonly formed by the reaction of an alcohol with sodium metal ... 

Phosphines and their derivatives are known to be very useful ligands toward transition metals and a variety of complexes with phosphine as a ligand have been prepared for all kinds of transition metals. If one of the substituents on a coordinating tertiary phosphorus compound is abstracted as an anion, it would form a cationic phosphenium complex. Actually this strategy has been widely used, and halide, hydride, and alkoxide have been abstracted as an anion by an appropriate Lewis acid. An alternative method to prepare cationic phosphenium complexes is a direct reaction of a phosphenium cation with a transition metal complex having appropriate... 

It is convenient to note here that a common reaction of alcohols with transition metal halides in the presence of phosphine is the decarbonylation and sometimes dehydrogenation to give metal carbonyls. These reactions proceed via alkoxide intermediates which then undergo 8-H transfer ... 

Two principle strategies have been employed for the synthesis of siloxide-containing molecular precursors. The first involves a silanolysis, or condensation, reaction of the Si - OH groups with a metal amido, alkyl, halide, or alkoxide complex. The second method involves salt metathesis reactions of an alkali metal siloxide with a metal halide. Much of our work has been focused on formation of tris(tert-butoxy)siloxide derivatives of the early transition metals and main group elements. The largely imexplored regions of the periodic table include the lanthanides and later transition metals. 

Alkoxide and aryloxide ligands are excellent ligands for the actinides. As a result, these ligands have been studied extensively in the coordination chemistry and reactivity of tri-, tetra-, penta-, and hexavalent actinides. The alkoxides and aryloxides can be synthesized by a variety of routes the two most popular routes include direct reaction of actinide halides with alkali metal salts of the alkoxide or aryloxide of interest and protonolysis of actinide amides by alcohols. 

The reaction of metal halides with an alkoxide or aryloxide (most commonly an alkali metal alkoxide), can resnlt in metathetic exchange to give the required complex (equation 6). Such reactions can also result in the formation of heterometallic species (such as NaZr2(OR)4) this can be a problem when homometallic aUcoxides are desired, but it is an important route to heterometallic alkoxides and aryloxides. The nature of the product can be influenced by the alkali metal, the alkoxide ligand, and the relative amounts of the reactantsd ... 

Carboxylate complexes are often synthesized by refluxing the acid with the metal salts (such as carbonate, sulfate, oxide, etc.), or by reaction of the sodium or silver salt of the acid with the metal halide. Insertion of CO2 into a-bonded organotransition metal species has also been used to generate carboxylate complexes, as has exchange reactions with metal alkoxides (see Section 3.3). 

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