Alkoxide-Based Methods

Different strategies in the solution are described in the literature for the preparation of CSD precursors, where the prominence of the type of chemicals transcends the former grouping of solution methods. In general, but especially in the case of complex oxide compositions, neither of them can be used alone to obtain the liquid precursor due to difficulties in finding available metal reagents of the same family (e.g., alkoxides, carboxylates, and p-diketonates) for all the cations involved in the system. Therefore, a combination of several approaches is used to stabilize all the metal reagents in a common and stable solution. Some of these methods are shown here. They are divided considering the type of precursor used to obtain a stable solution, more than the traditional classification of solution methods shown above. Seven major solution processes are explained next alkoxide-based methods, carboxylate- and p-diketone-based methods, Pechini methods, diol routes, amine-based methods, polymer-assisted methods, and aqueous solution-gel methods. 

The synthesis of solutions by sol-gel mainly uses metal alkoxides dissolved in alcohol solvents. These reagents react through hydrolysis and condensation reactions [13]  

In addition to using homometallic alkoxides as reagents in sol-gel chemistry, efforts are also directed to the design of heterometallic alkoxides that provide a high homogeneity and reactivity to the synthesized solution [14,15], This permits obtaining complex oxide thin films with better properties. 

In some cases, it is not possible to obtain a synthetic or commercial metal alkoxide, or these are extremely reactive toward hydrolysis. Then, modified alkoxides, as well as other metal reagents containing hgands different from —OR groups, can be used. Important research groups are mainly devoted to the design and synthesis of new metal compounds of interest for CSD [14,15]. However, the most used non-pure alkoxide metal compounds in CSD are carboxylates and P-diketonates. 

Metal carboxylates are salts of carboxylic acids (R—COOH), where R is an alkyl group. In complex oxide systems, carboxylates are usually used as a source of cations with a low valence number, whose metal alkoxides are not viable (e.g., Ba , Ca , and Na ). Carboxylates can be dissolved in their corresponding carboxylic acid. Short alkyl chain carboxylates have a strong polar nature, and thus they can be dissolved in polar solvents such as water or 

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In Section 9.2, intermolecular reactions of titanium—acetylene complexes with acetylenes, allenes, alkenes, and allylic compounds were discussed. This section describes the intramolecular coupling of bis-unsaturated compounds, including dienes, enynes, and diynes, as formulated in Eq. 9.49. As the titanium alkoxide is very inexpensive, the reactions in Eq. 9.49 represent one of the most economical methods for accomplishing the formation of metallacycles of this type [1,2]. Moreover, the titanium alkoxide based method enables several new synthetic transformations that are not viable by conventional metallocene-mediated methods. 

Hamada, T., Suzuki, D., Urabe, H., Sato, F. Titanium Alkoxide-Based Method for Stereoselective Synthesis of Functionalized Conjugated Dienes. J. Am. Chem. Soc. 1999, 121, 7342-7344. 

The sol-gel technology, based on various alkoxides, allows production of classical silica glasses, as well as multicomponent materials, merging silicates with titanates, borates and a variety of other oxides (Zn, La, Al, Li, B, K, etc.). The alkoxide gel method can be also used for production of certain non-silicate oxide glass-like materials (e.g., ZrCV, etc.)25. 

An alio-threonine analogue was prepared in this way. The Nebraska group has also explored the use of diallyl phosphonates driven by the need to develop mild deprotection methods [77]. Treatment of ketophosphonates with alkoxide base led to the formation of difluoroenolates and thus difluoromethylketones [78]. The lithiophosphate then acts as a synthetic equivalent for the difluoro-methyl anion synthon (Eq. 21). 

The pore liquid is removed during drying (Step 5). Drying of colloidal gels (Method I) is relatively easy because the pores arc large (100 nm). Alkoxide-based gels have very small pores (1-10 nm). and thus large... 

It is now common experimental practice to react ketones with lithium diisopropyl amide (LDA) in order to generate the enolate of the ketone. This methodology has largely replaced the older approach to enolates, which employed alkoxide bases to remove a proton alpha to the carbonyl group. Comparison of the equilibrium constants for these two acid-base reactions reveals why the LDA method is preferable. The use of the amide base leads to essentially complete conversion of die ketone to its enolate (Keq 1016). At equilibrium, there is virtually no... 

The method Is limited to activated halides which are stable to alkoxide bases and aqueous acids. According to the discoverer,the Krdhnke oxidation is inferior to the Sommelet reaction for benzyl halides, especially those bearing electron-withdrawing groups, such as nitro. Some representative examples are shown in Scheme (P and equations As can be seen, the Krdhnke oxidation is an... 

M. Kogure, T. Sato, T. Tanaka, S. Yasujima, T. Suzuki and H. Ohya, Preparation of antimony acid cation exchange membranes by using metal alkoxide-gel method, Bull. Chem. Soc. Jpn., 1991, 1991, 1618 H. Ohya, R. Paterson, T. Nomura, S. McFadzean, T. Suzuki and M. Kogure, Properties of new inorganic membranes prepared by metal alkoxide methods. Part I A new permselective cation exchange membrane based on Si/Ta oxides, J. Membr. Sci., 1995, 105, 103-112. 

This type of reaction leads to the formation of an inorganic polymer or a three-dimensional network formed of metal oxianions. The above-described process is called metal alkoxide-based sol-gel. The literature contains excellent reports providing in-depth analyses of this method [100,223]. The sol-gel process allows for very good chemical homogeneity and offers the possibility of obtaining metastable phases, including the amorphous phase. This process normally promotes the formation of amorphous metal oxides, which require thermal or hydrothermal treatment to promote crystallization. Several factors affect the sol-gel process, including the kind... 

State-of-the-art condensation chemistry has changed little since the turn of the 20th century when Dimroth first reported his novel method for 1,2,3-triazole synthesis. Still, replacement of the alkoxide base and alcohol solvent with several alternatives has extended the scope of the original reaction and has made possible a variety of interesting tandem processes vide infra). 

The development of the CrOs-pyridine and DMSO-based methods has decreased the number of instances in which older oxidation techniques are used. One such method, the Oppenauer oxidation is the reverse of the Meerwein-Pondorff-Verley reduction (Chapter 5). It involves heating the alcohol to be oxidized with an aluminum alkoxide in the presence of a carbonyl compound, which acts as the hydrogen acceptor. The reaction is an equilibrium process and proceeds through a cyclic transition state. 

Depending on the desired materials properties and structure, these approaches have been explored for sol-gel processing of alkoxide-based precursors and further developed in combination with many experimental parameters that can also be adjusted to control the sol composition and particle size, including alkoxide/ water ratio, concentration of the precursors, reaction temperature, reaction time, drying methods, choice and concentration of the catalyst, and the solvent used [42]. Besides the well-known hydrolytic sol-gel processes, even nonhydro-lytic approaches have been reported as a powerful method in the synthesis of mixed oxide materials and will be discussed in more detail subsequently [43]. Another processing option to tailor the material properties is the application of soft or hard templates [44]. In addition to the chemical parameters, drying and thermal treatments or firing processes determine the final architecture, chemical... 

Most of 3DOM metal oxides are fabricated by an alkoxide-based sol-gel process. The typical procedures include (i) preparation of a colloidal crystal template by ordering monodispersive microspheres (e.g., PS, PMMA or silica) into a face-centered close-packed array (ii) infiltration of a metal alkoxide solution into voids in the colloidal crystals, and in-situ solidification of the precimsor via a sol-gel process and (iii) template removal via a calcination or extraction pathway [99,217-219], Using the alkoxide-based sol-gel method, 3DOM oxides of Si, Ti, Zr, Al, Sb, W, Fe, and 3DOM mixed oxides of some of them can synthesized due to the moderate reactivity of their alkoxide preciuTsors [99], However, most of the other metal alkoxides exhibit high reactivity and their reactions are too quick to be... 

Weak acid (Section 1 16) An acid that is weaker than 1130" Weak base (Section 1 16) A base that is weaker than HO Williamson ether synthesis (Section 16 6) Method for the preparation of ethers involving an Sfj2 reaction between an alkoxide ion and a primary alkyl halide... 

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