Alcohol exchange

A1koxid.es of other alcohols are formed by alcohol exchange. The general stabihty 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 ... 

Solid esters are easily crystallisable materials. It is important to note that esters of alcohols must be recrystallised either from non-hydroxylic solvents (e.g. toluene) or from the alcohol from which the ester is derived. Thus methyl esters should be crystallised from methanol or methanol/toluene, but not from ethanol, n-butanol or other alcohols, in order to avoid alcohol exchange and contamination of the ester with a second ester. Useful solvents for crystallisation are the corresponding alcohols or aqueous alcohols, toluene, toluene/petroleum ether, and chloroform (ethanol-free)/toluene. Esters of carboxylic acid derived from phenols... 

For many alcohols, exchange of the hydroxyl group with solvent competes with... 

The reactivity of hydrido(ethoxo) complex 4 was examined (Scheme 6-15) [8]. Metatheses similar to those postulated for alcohol exchange (Eq. 6.5) occurred between HCl, LiCl, phenyl acetate or primary amines and yielded complexes 94. The reaction of 4 with cyclic anhydrides proceeded similarly to give iridium-assisted ring opening products 95. Heterocumulenes afforded the inserhon products 96 into the Ir-O bond. 

Alcohols (ROH) are often employed as the solvent medium for aikoxides compounds. In this situation, the alcohol also fills the role of reactant, in addition to its roles as solvent and common medium. Alcohol exchange reactions are well known in many of the more common systems that have been investigated. In simple terms, the reaction may be shown as follows ... 

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. 

The alcohol exchange reaction was shown above in equation (2). The reactive alkoxy group (OR) is replaced by an alkoxy group that has less hydrolysis sensitivity (OR ). A representative example here is the use of reagents such as zirconium n-propoxide and titanium /-propoxide, both of which possess polar bonds, for the production of lead zirconate titanate films. Commonly in these processes, R OH is 2-methoxyethanol (CH3OCH2CH2OH), which is generally present as a bidentate ligand.35... 

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.

Catalyzed Pot Stability. A sample of the above BA/HHA/25X ABDA copolymer In butanol/toluene was catalyzed with IX Nacure (S/S). The viscosity was observed qualitatively and an Initial aliquot was drawn down as a 2 mil film on glass and cured (150 C/3 min or room temperature for 35-45 d). After 24 h at ambient temperature one equivalent of methanol based on ABDA was detected by GLPC In the solution. After 5 d at ambient and 24 h at 40 C, two equivalents of HeOH were present. Indicating complete alcohol exchange. The solution polymer showed no thickening. Films were also drawn down with these samples and evaluated (Table VI). 

The limited number of compounds in this class have mostly been made by alcohol exchange of a better leaving group at anomeric hydroxamic esters. 

Reinitiation by RO Na+ would usually be slower, since ROH is relatively acidic. This results in a decreased polymerization rate and a broadening of the polymer molecular weight. The rate of polymerization will be relatively unaffected during most of the polymerization for the case in which ROH is less acidic than the polymeric alcohol. Exchange will occur in the later stages of reaction with a broadening of the MWD. 

Only when the rate of alcohol exchange is lowered by cooling is the nonequivalence of fluorine atoms evident in the NMR spectrum. These exchange effects are observed because at room temperature the chemical shift between fluorine atoms in the two environments is comparable to the exchange rate. Chemical shiftsf and coupling constants of nuclei in molecules are comparable to the rates of many chemical processes. Consequently this possible complication must always be kept in mind. Exchange phe-... 

As mentioned above, a variety of complexes of type (10) can easily be prepared by alcohol exchange and, in this way, compounds with R = Pr", Bun, Pen , Oct" have been isolated and characterized.221 However, when secondary alcohols have been used for exchange with (10), mixed alkoxy species were obtained. NMR showed that the bridging ethoxide ligands had not been replaced, a result that was confirmed by the crystal structure determination of the mixed ethoxide-isopropoxide and ethoxide-sec-pentoxide derivatives. 

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... 

Perfluoro tertiary alcohols are known, e.g. (CF3)3COH,139,140 (C6F5)3COH141 and (C2F5)3COH,142 and these will react by alcohol exchange to give metal tertiary perfluoroalkoxides, but these do not appear to have been systematically studied. 

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... 

Methoxy(tosyloxy)iodo]benzene is obtained from [hydroxy(tosyloxy)iodo]-benzene and trimethyl orthoformate. This iodane has been used for the preparation of the (-)-menthyloxy analog by simple alcohol exchange upon mixing in dichloromethane equimolar quantities of it with (-)-menthol and concentrating under reduced pressure (Scheme 9) [44]. 

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... 

A monomeric Ce(IV)-siloxide was prepared by an alcohol exchange reaction (Eq. 6). The 6-coordination in Ce(OSiPh3)4(DME) is considered as a (4 4- 2) environment with the four siloxy oxygen atoms occupying the sites of a distorted tetrahedron [53],... 

Mechanistic studies on the Dess-Martin oxidation have shown that the alcohol exchanges with one of the acetate groups in the periodinane to create a new iodinane which then undergoes a C—H bond cleavage in the manner shown in Scheme 8.10. 

---