Metal alkoxides transesterification

Metal alkoxides cataly2e the Tishchenko condensation of aldehydes (62), the transesterification of carboxyhc esters, the Meerwein-Poimdorf reaction (63), and other enolization and condensation reactions. 

The mechanism for cyclic formation via depolymerization is the same type of transesterification which occurs on polymerization, as outlined in Scheme 3.3. Metal alkoxides such as tetraalkyl titanates or dibutyl tin alkoxides have proven... 

Ruthenium(III) catalyses the oxidative decarboxylation of butanoic and 2-methylpropanoic acid in aqueous sulfuric acid. ° Studies of alkaline earth (Ba, Sr) metal alkoxides in amide ethanolysis and of alkali metal alkoxide clusters as highly effective transesterification catalysts were covered earlier. Kinetic studies of the ethanolysis of 5-nitroquinol-8-yl benzoate (228) in the presence of lithium, sodium, or potassium ethoxide revealed that the highest catalytic activity is observed with Na+.iio... 

The use of supported metal complexes in transesterification reactions of TGs is not new. An earlier patent claimed that supported metals in a hydroxylated solid could effectively catalyze transesterification. The catalyst preparation used an inert hydrocarbon solvent to attach transition metal alkoxide species to the support surface. The reaction, however, was carried out in the presence of water. The author claimed that water was essential in preparing materials with good catalytic activity. Among the metals employed, titanium catalysts showed the best activity. However, it was not clear from the preparation method if reproducibility could be easily achieved, an important requirement if such catalysts were to be commercially exploited. 

Metal alkoxides such as V-, Nb- and Ta-(V) alkoxides, as well as oxides in various oxidation states-in particular Nb(II, III, IV, and V), V(III, IV, and V) and Ti (IV)-oxides-have been studied in transesterification reactions involving EC and... 

Alkali metal alkoxides such as KOH, NaOH, and CH3ONa are the most effective catalysts in alkali-catalyst transesterification. When using KOH, NaOH, and CH3ONa alkali-catalyst for FAME conversion, the active catalytic species were the methoxide anion (CH 0 ), formed by the reaction between methanol and hydroxide ions of KOH and NaOH. In addition, the methoxide anion was formed by dissolution of sodium methoxide. Sodium methoxide causes the formation of several byproducts, mainly sodium salts, that have to be treated as waste and additionally require high-quality oil (16). However, KOH has an advantage because it can be converted into KOH by reaction with phosphoric acid, which can serve as a fertilizer. Since KOH is more economical than sodium methoxide, it is the preferred choice for large-scale FAME production process. 

Parameters that influence the number of transesterifications are temperature, reaction time, and type and concentration of catalyst or initiator [44]. Depending on the metal used, the initiator is more or less active towards side-reactions such as transesterification reactions [44,45]. The relative reactivity of different metal alkoxide initiators towards chains already formed has been reported to be Bu2Sn(OR)2>Bu3SnOR>Ti(OR)4>Zn(OR)2>Al(OR)3 [44]. 

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... 

Metal alkoxides (homo as well as hetero) undergo facile and reversible alcoholysis and transesterification reactions that can be represented by the following equations ... 

From Alcoholysis and Transesterification. Metal alkoxides of higher, unsaturated, or branched alcohols are difficult to prepare directly and are usually made from lower metal alkoxides by means of alcoholysis ... 

The principle found for zinc(II) was applied to copperdi) complex models by Young et al. (25). The hydroxyl function of copper complex 27a deprotonates with a pK value of 8.8 to yield 27b, which cleaves phosphodiester bis(2,4-dinitrophenyl) phosphate (BDP ) by transesterification to produce 28 (A(BDP ) = 7.2 x 10 M" sec at 25°C see Scheme 5). The analogous complex with a hydroxyethyl pendent cleaves the diester predominantly by hydrolysis, which suggests that the reactive species is not Cu -alkoxide, but Cu —OH . The rate ife(BDP ) of 9.5 X 10" M sec is about two orders of magnitude smaller than the phosphoryl-transfer reaction. This copper model study shows that metal-alkoxide species may be more effective nucleophiles, as has been seen with zincdD-model complex 24. Thus, future models may be designed that are composed of a metal-alkoxide function and a proximate metal-hydroxide function. 

They arrived at the conclusion that the substitutions proceeds via two competing pathways direct substitution with inversion of configuration (path A) or a two-step process involving first the deprotonation of the TZ-phosphinate followed by substitution and hydrolysis, also with overall inversion of configuration (B). Path B requires two equivalents of carbanionic reagent, whereas A requires only one. They also studied the factors that affect the stereoselectivity of these reactions. They found that anionic intermediates 47 and 48 are configurationally stable but metal alkoxides cause epimerisation of the phosphorus atom on 46 by transesterification. 

The scope of organic synthesis, essentially, is the synthesis of new molecules from existing molecules. The addition reactions could be realized as a very important category in which two or more different molecules reacted with each other to form a new compound. Transesterification and cyclization reactions also have been used for the synthesis of new compounds, sometimes. There are excessively other reactions in this area but we only consider some of them which could be catalyzed or co-catalyzed with metal alkoxides. There are also many routs for the classification of these reactions but the focus of this chapter is on metal alkoxides so we use particular types of metal for this purpose and divide metals to four categories of s-block (alkali and alkaline earth), p-block, d-block (transition), and f-block (actinides and lanthanides) metals. 

The d-block or transition metal alkoxides make the third category. Titanium alkoxides (methoxide, ethoxide, propoxide, isopropoxide, butoxide and isobutoxide) have been able to catalyze transesterification reaction of fatty acids (reaction 7.13) [64]. The results of Nawaratna et aV.s researches showed that the catalytic behavior of titanium alkoxides in transesterification of fatty acids depended on their steric effects meaningfully and lower steric hindrance increased the yield and selectivity of the produced ester [64]. 

The actinides and lanthanides or f-block metal alkoxides are the last group. According to the report of Nawaratna and co-workers, yttrium isopro-poxide has been catalyzed transesterification reaction of fatty acids (reaction 7.13) [78]. Hatano et al. have used this strategy in the presence of lanthanum alkoxides for transestrification of carboxylic esters and preparation of new esters [79]. 

The metal alkoxides affect substantially these reactions. Thus Ti(OPr )4 and OV(OPr )3, cocondensates act as catalysts in transesterification of tetramethoxy- and tetraethoxysilanes (TMOS and TEOS, respectively) during sol-gel nanocomposite preparation. ... 

Metal alkoxides undergo transesterification with carboxylic esters, and this affords a method of conversion from one alkoxide to anotha-. The reaction is reversible and can be written ... 

Additives that coordinate to Al perturb the polymerization kinetics in toluene. For instance, THF and cyclohexanone compete e-CL for coordination to aluminum (18), which decreases the propagation rate and may even inhibit polymerization. In contrast, kinetics is faster upon addition of a Lewis base, such as 4-picoline (19), because the coordination of this Lewis base onto the Al atom polarizes the metal-alkoxide bond and facilitates the monomer insertion. Although the reactivity of the active sites is increased, the extent of transesterification reactions is reduced, more likely for steric reasons. 

In addition to the above, alcoholysis or transesterification reactions of metal alkoxides themselves have been widely used for obtaining the targeted homo- and heteroleptic alkoxide derivatives of the same metal. Since the 1960s, the replacement reactions of metal dialkylamides with alcohols has provided a highly convenient and versatile route (Section 2.9) for the synthesis of homoleptic alkoxides of a number of metals, particularly in their lower valency states. 

As discussed already (Section 2.8), metal alkoxides undergo transesterification reactions with organic as well as silyl esters as illustrated by Eqs (2.201), (2.202), and (2.203) ... 

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