Alkoxides alcohol exchange

Zirconium tetrachloride is instantly hydrolyzed in water to zirconium oxide dichloride octahydrate [13520-92-8]. Zirconium tetrachloride exchanges chlorine for 0x0 bonds in the reaction with hydroxylic ligands, forming alkoxides from alcohols (see Alkoxides, METAl). Zirconium tetrachloride combines with many Lewis bases such as dimethyl sulfoxide, phosphoms oxychloride and amines including ammonia, ethers, and ketones. The zirconium organometalLic compounds ate all derived from zirconium tetrachloride. 

Alkoxides can be formed also by reaction of zirconium dialkylarnines with alcohols, and alkoxides can be exchanged also be transesterification reactions. 

Esters react with alcohols in either aeidic or basic solution to exchange alkoxy groups (ester interchange) by mechanisms which parallel hydrolysis. The alcohol or alkoxide acts as the nucleophile ... 

Metal alkoxides undergo alkoxide exchange with alcoholic compounds such as alcohols, hydro-xamic acids, and alkyl hydroperoxides. Alkyl hydroperoxides themselves do not epoxidize olefins. However, hydroperoxides coordinated to a metal ion are activated by coordination of the distal oxygen (O2) and undergo epoxidation (Scheme 1). When the olefin is an allylic alcohol, both hydroperoxide and olefin are coordinated to the metal ion and the epoxidation occurs swiftly in an intramolecular manner.22 Thus, the epoxidation of an allylic alcohol proceeds selectively in the presence of an isolated olefin.23,24 In this metal-mediated epoxidation of allylic alcohols, some alkoxide(s) (—OR) do not participate in the epoxidation. Therefore, if such bystander alkoxide(s) are replaced with optically active ones, the epoxidation is expected to be enantioselective. Indeed, Yamada et al.25 and Sharp less et al.26 independently reported the epoxidation of allylic alcohols using Mo02(acac)2 modified with V-methyl-ephedrine and VO (acac)2 modified with an optically active hydroxamic acid as the catalyst, respectively, albeit with modest enantioselectivity. 

To address the hydrolysis sensitivity issue for materials with highly polar bonds, one of two strategies is typically employed (1) alcohol exchange or (2) chelation (reaction) of the starting alkoxide with a ligand that is less susceptible to attack by water. Chelation processes are discussed in detail in the next section. 

Scheme 12.26 Proposed mechanism for the MPV reduction catalyzed by MCM-41-supported Nd-alkoxide complexes (1 and 8A-D) (i) and (iv) alcohol exchange reactions (ii) adduct formation and (iii) acetone elimination.

U(OEt)5 is easily prepared by oxidation of U(OEt)4 by bromine in ethanol, followed by addition of the calculated quantity of sodium ethoxide, a reaction which yields181 only Np(OEt)4Br in the case of Np(OEt)4. U(OEt)s is commonly used as the starting material for the preparation of other uranium(V) alkoxides by alcohol exchange reactions. A comprehensive review of metal alkoxides includes a useful discussion of the uranium(V) compounds.182... 

Hence a new alkoxide or phenoxide can be obtained if the ester produced is more volatile and is fractionated out of the mixture. This method has proved very useful for the preparation of tertiary alkoxides as it appears to be much less prone to steric factors than alcohol exchange. Thus the r-butoxides of the Group IV metals Ti, Zr and Hf are readily synthesized from their isopropoxides (equation 24).90... 

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... 

Since aluminum alkoxides function effectively as catalysts for alcohol exchange in esters,38 89 reduction of keto esters by aluminum isopropoxide frequently yields the isopropyl ester of the hydroxy acid. Thus the reduction of the cyclopropane derivative XLIV gave the isopropyl ester of the reduced acid (XLV).40 Similarly the substituted methyl /3-keto ester (XLYI)41 gave upon reduction the isopropyl esters of the stereoisomeric reduced acids (XLVII). 

Because of the catalytic effect of aluminum alkoxides on alcohol exchange in esters, reduction of esters of hydroxy ketones usually is accompanied by cleavage of the ester group and the product is a glycol. Ether and acetal linkages are not affected. Thus, the reduction of the acetate (LII) gave a 94% yield of the glycol (LIII).42... 

Unlike the monomeric U(OR)6, U(OR)s are usually associated U(OPti)5 is dimeric with two alkoxide bridges giving six coordination. U(OEt)5 is the easiest compound to synthesize and can be converted into others by alcohol exchange (Figure 11.10). 

Alkoxides of other alcohols are formed by alcohol exchange. The general stability of the alcohols in. exchange is primary > secondary > tertiary, although the reaction can be driven in the opposite direction by removal of the more volatile alcohol ... 

The isopropoxide is crystalline, subliming in a vacuum at 170°, but other alkoxides, prepared from the isopropyl compound by alcohol exchange, are non-volatile and presumably polymerized by Ce—O(R)—Ce bridges. 

Finally, alcohol exchange reactions are a means of obtaining more appropriate precursor, starting from commercial or easily accessible metal alkoxides (see Sec. II.C). 

The nature of the solvent can likewise play a critical role in the synthesis. The solvents are not inert with respect to the reaction sequence. It is important to choose solvents for the complete solubility of alkoxides or salts. Solvents play a direct role through the process of alcohol exchange. 

Glycols are highly reactive towards metal alkoxides to form homoleptic and mixed alkoxo-glycolate derivatives. This alcohol exchange reaction (see Section 7.10.3.1.3) is driven by the formation of chelated or bridged structures with both (dianionic) OCR2CR2O—... 

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