Zirconium alkoxide catalysts

Zirconium alkoxide catalysts were used for the aldol-Tishchenko reaction shown in Equation 18 [23]. In the reaction, diacetone alcohol (55) is converted to the corresponding enol by removal of acetone, and adds to an aldehyde. Enantioselective version of the reaction was also examined [24]. 

Zirconium alkoxides are used for cross-linking and hardening of isocyanate, epoxy, siUcon, urea, melamine, and terephthalate resins in the sol-gel process as catalysts in condensation and as water repellents. Zirconium alkoxides hydroly2e in moist air, but more slowly than titanium alkoxides. 

The vapor-phase esterification of ethanol has also been studied extensively (363,364), but it is not used commercially. The reaction can be catalyzed by siUca gel (365,366), thoria on siUca or alumina (367), zirconium dioxide (368), and by xerogels and aerogels (369). Above 300°C the dehydration of ethanol becomes appreciable. Ethyl acetate can also be produced from acetaldehyde by the Tischenko reaction (370—372) using an aluminum alkoxide catalyst and, with some difficulty, by the boron trifluoride-catalyzed direct esterification of ethylene with organic acids (373). 

Mn(II) > Mg(II).270 It should be underlined that titanium and zirconium alkoxides are efficient catalysts for both stages of reaction. Lanthanide compounds such as 2,2/-bipyridyl, acetylacetonate, and o-formyl phenolate complexes of Eu(III), La(III), Sm(III), Er(III), and Tb(III) appear to be even more efficient than titanium alkoxides, Ca or Mn acetates, Sb203, and their mixtures.273 Moreover, PET produced with lanthanides has been reported to exhibit better thermal and hydrolytic stability as compared to PET synthesized with the conventional Ca acetate -Sb203 catalytic system.273... 

Ziegler-Natta catalysis, 431, 449 Zinc diacetate catalysts, 71 Zirconium alkoxides, 68... 

The zirconium alkoxides Zr(OR)4 are reported as inactive for the epoxidation of olefins under the conditions recommended with the titanium analogs. When synthesized by reaction of Zr(CH2CMe3)4 with silica, followed by hydrolysis or calcination, a solid as active as the related Ti-based catalyst is obtained. The low selectivity for the formation of the epoxide is related to the fact that the same Zr centers catalyze both the formation and the decomposition of the epoxide.46... 

Oxidation of primary, secondary and benzylic alcohols with TBHP or CHP, mainly catalyzed by Mo and Zr derivatives, were performed by different authors. As an example, Ishii, Ogawa and coworkers reported the conversion of secondary alcohols such as 2-octanol to ketones mediated by catalyst 39 and TBHP. The oxidation of cyclic alcohols depended on steric factors. Zirconium alkoxides may act as catalysts in the conversion of different alcohol typologies with alkyl hydroperoxides . Secondary alcohols, if not severely hindered, are quantitatively converted to the corresponding ketones. The selectivity for equatorial alcohols is a general feature of the system, as confirmed by the oxidation of the sole cis isomer 103 of a mixture 103-bl04 (equation 68). Esters and acids could be the by-products in the oxidation of primary alcohols. 

Kobayashi and colleagues developed a catalytic enantioselective method for the allylation of imines 24 by substituted allylstannanes 25 with chiral zirconium catalysts 26 and 27 prepared from zirconium alkoxides and l,l -bi-2-naph-thol derivatives (Scheme 10) [19]. The allylation of aromatic imines 24 with 25 afforded the corresponding homoallylic amines 28 in good yields (71-85%) with high stereoselectivities (87-99% ee). 

Thermolysis of tin and lead alkoxozirconates leads to the formation of metals. The mass-spectral data indicate the presence ofbarium and aluminium derivatives in the gas phase, but no preparative data are accessible for them. The major application of zirconium and hafnium alkoxides lies now in the sol-gel technology of zirconate-titanate and solid solutions Zr02-Y203 (see Section 10.3), Except in the synthesis of oxide materials, the alkoxides of zirconium and hafnium are traditionally used in the polymer chemistry, where they are applied as the components in catalysts [1278, 1269] and as additives to polymers, improving their characteristics [825, 1403] and so on. Already in 1930s Meerwein has proposed the use of zirconium alkoxides for the reduction of aldehydes intoprimary alcohols (Meerwein-Schmidt reaction) [1420],... 

The first zirconium-catalyzed oxidation reaction was reported by Kaneda, in which a zirconium oxide complex or zirconium alkoxide as the catalyst and t-BuOOH as an oxidant were employed to oxidize primary and allylic alcohols into aldehydes in high yields without formation of carboxylie acids [31]. 

For the selective liquid phase alkylation of naphthalene with propene to form 2,6-diisopropylnaphthalene, mordenite again seems to be the most promising catalyst [42]. However, while this zeolite shows a high selectivity for 13-substitution of naphthalene, its activity is low compared to faujasites [43, 44]. Chemical vapor deposition, variation of the aluminum content and of the solvent are methods used to improve the catalytic behavior of mordenite catalysts in this reaction. Chemical vapor deposition of silicon or zirconium alkoxides were claimed to improve the I3,l3 -selectivity of zeolite Beta in this reaction [45]. A recurrent matter of dispute are the parameters influencing the yield ratio of 2,6- and 2,7-diisopropylnaphthalene. Both molecules are presumed to have the same molecular dimensions. With regard to the formation of 2,6-diisopropylnaphthalene, alkylation with isopropylbromide... 

Chmura et al. also prepared air and moisture resistant chiral imino phenoxide complexes of zirconium and titanium, 14 [16]. They envisioned to study the effect of supporting ligand chirality on the stereoselectivity of LA ROP reaction. But at the end, they did not gain acceptable evidence enable to support any relationship. They showed that all isolated polymers had similar and moderate heterotactic microstructure which implied simple chain end control mechanism and resulted to the selective racemic enchainment during the propagation process. First, they investigate polymerization in toluene at 80°C and ambient temperature in which titanium complexes were absolutely inactive and zirconium coxmterparts showed moderate activity after 2 and 24 hours, respectively. Then they checked out solvent free conditions at 130°C and received almost complete conversion after 30 minutes for both titanium and zirconium alkoxide complexes (Table 7.2, entry 33-36). In this condition, titanium coxmterpart, in contrast to zirconium, resulted to full atactic polymer. Their investigation also showed that zirconium complex retained its activity in moisture or with lactic acid impurity in crude monomer which is deleterious for most metal alkoxide catalysts. 

The effects of different synthesis parameters on the properties of zirconia aerogels prepared by sol-gel process have been investigated by many research groups. In fact, taking into account that the hydrolysis of zirconium alkoxides is very fast, it has been noted that the rate of the condensation process can be influenced by the concentration of an acid catalyst leading to zirconia aerogels with different textural properties. The amount of added water... 

Tetra-alkoxy titanates are the most commonly used catalysts for PBT pol)mierization [15, 20]. As opposed to the PET process, where a different catalyst is used in each of the two polymerization phases, in the PBT process, a single catalyst is typically used for both reaction steps. The most frequently used titanates are tet-rabutoxy titanate (Ti(OBu) ) [26, 27, 29, 30] and tetraisopropoxy titanate (Ti(OiPr) ) [31]. Ti(OBu), and Ti(OiPr) are very efficient catalysts which do not generate side effects, especially discoloration. Titanium and zirconium alkoxides are sometimes involved in the same preparation [23]. Other compounds have also been also proposed CHgCOONa [28] Ti(OBu), (95%) + CHjCOONa (5%) [29] Ti(OBu), (50%) + Sn(OBu), (50%) [26] Ti(OBu) + Ca(OAc)2 [32] Ti(OBu) + Mg(OAc)2 [30] TiOj-xH O [25], etc. These catalytic... 

Ziegler-Natta catalysts were prepared in a nitrogen atmosphere by syringe or inert atmosphere transfer of the titanium or zirconium alkoxide to toluene solvent followed by dropwise syringe addition of the triethylaluminum. The metal alkoxide concentration was 0.5 M and aluminum titanium or zirconium ratio was 4 1 unless stated otherwise. The C H., nC H., nC.Hg,... 

The ROP of lactide in the presence of metal compounds of tin, aluminum, zinc, titanium or zirconium as catalysts proceeds via the coordination-insertion mechanism. The initiators in this type of reaction are usually metal alkoxides. In the first step, temporary coordination of lactide through the carbonyl group with the metal in the initiator leads to increased nucleophilicity of the alkoxide and electrophilicity of the carbonyl group, thereby facilitating the insertion of the monomer into the metal O bond. This is the most investigated and applied method for the synthesis of PLA due to the mild reaction conditions, since the reaction proceeds via covalent species. High molecular weights of 200000 g mol are easily achievable with minimum side reactions and racemization. 

Dialkylaminoethyl acryhc esters are readily prepared by transesterification of the corresponding dialkylaminoethanol (102,103). Catalysts include strong acids and tetraalkyl titanates for higher alkyl esters and titanates, sodium phenoxides, magnesium alkoxides, and dialkyitin oxides, as well as titanium and zirconium chelates, for the preparation of functional esters. Because of loss of catalyst activity during the reaction, incremental or continuous additions may be required to maintain an adequate reaction rate. 

Transesterification of methyl methacrylate with the appropriate alcohol is often the preferred method of preparing higher alkyl and functional methacrylates. The reaction is driven to completion by the use of excess methyl methacrylate and by removal of the methyl methacrylate—methanol a2eotrope. A variety of catalysts have been used, including acids and bases and transition-metal compounds such as dialkjitin oxides (57), titanium(IV) alkoxides (58), and zirconium acetoacetate (59). The use of the transition-metal catalysts allows reaction under nearly neutral conditions and is therefore more tolerant of sensitive functionality in the ester alcohol moiety. In addition, transition-metal catalysts often exhibit higher selectivities than acidic catalysts, particularly with respect to by-product ether formation. 

Meerwein-Ponndorf-Verley-Oppenauer catalysts typically are aluminum alkox-ides or lanthanide alkoxides (see above). The application of catalysts based on metals such as ytterbium (see Table 20.7, entries 6 and 20) and zirconium [85, 86] has been reported. 

Scheme 20.28) [84, 118]. The first step in this procedure is reduction of the ketone, followed by the acetylation of the formed alkoxide. It may be noted that aluminum(III) isopropoxide and zirconium(IV) isopropoxide do not catalyze the acetylation. With these catalysts, the alcohol is obtained. 

Tetraalkyl titanates are the most commonly used catalysts for PBT polymerization [8], The varieties of titanates include tetraisopropyl titanate (TPT), tetrabutyl titanate (TBT) and tetra(2-ethylhexyl) titanate (TOT). Titanates effectively speed the reaction rate with few detrimental effects on the resin. Alkoxy zirconium and tin compounds, as well as other metal alkoxides, may also be used in PBT polymerization. 

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