Metal alkoxides reactions catalysed

The use of DMSO in recent studies has been largely upon the premise that the problem of base association would be avoided. Some doubt as to the validity of this assumption arises when the results of a conductometric study are considered (Exner and Steiner, 1974). Ion-pairing constants for lithium, sodium, potassium and cesium t-butoxide in DMSO have been evaluated as 108, 106, 270 and 200 M-1 respectively. Not only do these results suggest that there is base association in DMSO but they also imply that base-catalysed reactions involving alkali metal alkoxides in DMSO should be affected by the nature of the cation. If these conclusions are valid and if the possible involvement of the dimsyl anion in these reactions is also taken into consideration, then the choice of DMSO to remove the problem of base association can be a poor one, especially if the base is a lithium or sodium salt of a hindered alkoxide. It is far better to avoid association effects by the use of crown ethers (Bartsch et al., 1973, 1974, 1975). On the other hand, the use of lithium and potassium t-butoxide in DMSO solvent might aid in distinguishing reactivities of free ions and of ion pairs in certain processes. 

Transesterifications of aliphatic carbonate esters with glycols are catalysed by alkali metal alkoxides. No catalyst is needed for the transesterification of diaryl carbonates with aliphatic diols. Alkyl carbonate esters and p-xylylene glycol undergo transesterification reactions when certain titanium compounds are used as catalysts. The preparation of aromatic polycarbonates by transesterification is best... 

Alkali metal alkoxides catalyse the alcoholysis of esters, by a mechanism analogous to basic hydrolysis. Additionally, alkoxides catalyse the reaction of alcohols with carbon monoxide to give formate esters ... 

Another area that has expanded in recent years concerns the Industrial Applications of metal alkoxides. Besides the previously mentioned deposition of metal oxides in the microelectronic and ceramics industries there have also been major developments in the catalytic activity of early transition metal alkoxo compounds in several important homogeneous reactions. This has stimulated a growing interest in the mechanisms of reactions catalysed by metal alkoxides. 

Another type of reaction involving a metal alkoxide and a carbonyl compound was noticed as early as 1925 by Verley and Meerwein and Schmidt,who observed that alkoxides of magnesium, calcium, and particularly aluminium could catalyse reduction of aldehydes in the presence of excess ethyl alcohol as shown by Eq. (2.336) ... 

Reactions using picoline 1-oxides have been less successful than those using picolines. 2- and 4-Picoline 1-oxides did not react with aromatic aldehydes under various acidic and basic conditions, 199 but poor yields of stilbazole oxides were obtained using piperidine acetate in boiling tolu-ene . Under these conditions, the picolines performed no better than their oxides. 3-Methyl-4-nitropyridine 1-oxide reacted with benzaldehyde in presence of piperidine, circumstances under which 3-methyl-4-nitropyridine did not reacti 3 More successful have been reactions of the oxides catalysed by metal alkoxides or hydroxides (p. 383). 

In this reaction, the incoming alkoxide group is almost certainly co-ordinated to the metal centre in the key stages of the reaction. A particularly elegant demonstration of this is seen in the zinc(n) catalysed transesterification of 4-nitrophenyl picolinate with HO(CH2)2NH(CH2)2NH2 (Figs. 5-87 and 5-88). In the initial step of the reaction, an intermediate complex 5.38 may be isolated (Fig. 5-87). 

The replacement of the acid with an ester often allows the more facile purification of the starting material, so that stoichiometry is maintained to high extents of reaction. This facilitates a higher degree of polymerization. The reaction must be catalysed and the small molecule evolved is now an alcohol (Pilati, 1989). While p-toluenesulfonic acid has been employed, it is less effective than in the direct esterification, and metal catalysts, such as titanium alkoxides, are used instead, to avoid side reactions from the strong acid ... 

A major advantage of heterogeneous over homogeneously catalysed MPVO reactions is that the catalysts can easily be separated from the liquid reaction mixture. So far, several examples of heterogeneously catalysed MPVO reactions have been reported. The catalysts comprise (modified) metal oxides which exhibit either Lewis acid or basic properties. The reaction mechanisms involved have in common that the first step consists in the formation of an alkoxide-like species, while the reactions proceed via cyclic six-membered transition states, comparable to those in homogeneous systems. 

Catalyst initiator for reactions where only one compound is used, for example, where one part of the molecule initiates the reaction while another part catalyses it. These compounds consist of a metal centre with alkoxide ligands. 

A number of organometallic compounds have been applied to date as catalyst-initiators for the ROP. The metal will catalyse the reaction, while one of the alkoxide ligands will act as an initiator. 

The tungsten alkylidyne complex W(=CBu )(OBu )3, which, if monomeric, is only 12-electron, catalyses the metathesis of 3-heptyne at a spectacular rate. Note that the presence of strongly Ti-donating alkoxy groups destabilizes some of the metal d-orbitals (p. 1 51) so that the 18-electron rule is no longer expected to hold. In this case the reactions are too fast to obtain evidence for tungstenacy-clobutadiene intermediates. Another route to the catalysts, however, is provided by the remarkable reaction between the triply bonded tungsten alkoxides W IOR) and alkynes. 

---