Metal alkoxides reactions between

Double alkoxides of zirconium with alkah metals of the type MZr2(OR) have been obtained by reaction of alkah metal alkoxides with zirconium alkoxides (220). Although these usually are monomeric derivatives, the reaction between zirconium tetra-/-butoxide [1071 -76-7] and sodium /-butoxide was found (221) to form dimeric NaZr(OC(CH2)2) ]2. 

From the preceding discussion, it is easily understood that direct polyesterifications between dicarboxylic acids and aliphatic diols (Scheme 2.8, R3 = H) and polymerizations involving aliphatic or aromatic esters, acids, and alcohols (Scheme 2.8, R3 = alkyl group, and Scheme 2.9, R3 = H) are rather slow at room temperature. These reactions must be carried out in the melt at high temperature in the presence of catalysts, usually metal salts, metal oxides, or metal alkoxides. Vacuum is generally applied during the last steps of the reaction in order to eliminate the last traces of reaction by-product (water or low-molar-mass alcohol, diol, or carboxylic acid such as acetic acid) and to shift the reaction toward the... 

Having established structural and electronic analogies between metal oxides and alkoxides of molybdenum and tungsten, the key remaining feature to be examined is the reactivity patterns of the metal-alkoxides. Metal-metal bonds provide both a source and a returning place for electrons in oxidative-addition and reductive elimination reactions. Stepwise transformations of M-M bond order, from 3 to 4 (37,38), 3 to 2 and 1 (39) have now been documented. The alkoxides M2(0R)6 (MiM) are coordinatively unsaturated, as is evident from their facile reversible reactions with donor ligands, eq. 1, and are readily oxidized in addition reactions of the type shown in equations 2 (39) and 3 (39). 

Most often in these processes, compounds such as acetic acid, acetylacetone (acac, 2,4-pentanedione), or amine compounds are employed, since these compounds readily react with alkoxides.4M877 A typical reaction involves the formation of new chemical species that (hopefully) possess physical and chemical characteristics that are more attractive in terms of solution stability and film formation behavior. An example reaction between a metal alkoxide and acetic acid is shown below ... 

LA represents Lewis acid in the catalyst, and M represents Bren sled base. In Scheme 8-49, Bronsted base functionality in the hetero-bimetalic chiral catalyst I can deprotonate a ketone to produce the corresponding enolate II, while at the same time the Lewis acid functionality activates an aldehyde to give intermediate III. Intramolecular aldol reaction then proceeds in a chelation-controlled manner to give //-keto metal alkoxide IV. Proton exchange between the metal alkoxide moiety and an aromatic hydroxy proton or an a-proton of a ketone leads to the production of an optically active aldol product and the regeneration of the catalyst I, thus finishing the catalytic cycle. 

In the first structurally characterized complexes of type A the metal-phosphorus triple bonds are kinetically stabilized by bulky substituents at the amido ligands. Therefore, these compounds reveal exclusively end-on reactivity via the phosphorus lone pair. This reactivity pattern seems also valid for the solution stable alkoxide derivative [(C/0)3Mo=P], for which the reaction potential is under investigation [13]. In contrast, due to their lesser degree of kinetic stabilization by bulky substituents the short-lived alkoxide containing complexes [(R 0)3W=Pj (R =t-Bu (3c), Ph (3d)), generated by the metathesis reaction between the alkoxide-dimer and the phosphaalkyne (cf. Eq. 8), show additionally a high side-on reactivity towards the phos-phaalkynes of the reaction mixture. Thus, there occurs a formal cycloaddition reaction with the phosphaalkynes, and a subsequent 1,3-OR shift yields the formation of four-membered diphospha-metallo-cyclobutane derivatives 6(Eq. 8) [15,31, 37]. 

The first metallasilsesquioxanes containing metal-metal triple bonds have been reported by Chisholm and Feher et al The reaction between two equivalents of 3 and Mo2(OtBu)6 in benzene or toluene at room temperature (Scheme 54) led to rapid replacement of all six alkoxide ligands and formation of orange, crystalline... 

Alkylation of enolate is an important synthetic method.27 The alkylation of relatively acidic compounds such as /i-dikctoncs, /i-ketoesters, and esters of malonic acid can be carried out in alcohols as solvents using metal alkoxides as bases. The presence of two electron-withdrawing substituents facilitates formation of the enolate resulting from removal of a proton from the carbon situated between them. Alkylation then occurs by an Sn2 process. Some examples of alkylation reactions involving relatively acidic carbon acids are shown in Scheme 1.5. These reactions are all mechanistically similar in that a... 

Tin(II) 2-ethylhexanoate is an important industrial initiator for cyclic ester polymerization [Duda and Penczek, 2001 Kricheldorf et al., 2001 Storey and Sherman, 2002]. Metal car-boxylates are useful initiators only in the presence of alcohols. The polymerization rate is very slow in the absence of alcohol, less than 1% of the rate in the presence of alcohol. The actual initiator is the metal alkoxide formed by the reaction between metal carboxylate and alcohol. 

Carbamates have been prepared by heating ethyl carbamate with a higherboiling alcohol in the presence or absence of catalysts [31-33], Aluminum iso-propoxide has been reported [34] to be an excellent catalyst for the interchange reaction between ethyl carbamate and benzyl alcohol. The interchange reaction is also effective for /V-alkyl carbamates as well as unsubstituted carbamates [35]. This catalyst is effective in preparing mono- and dicarbamates in excellent yields from primary and secondary alcohols and diols. Other effective catalysts are dibutyltin dilaurate [36], dibutyltin oxide [37], sulfuric acid or p-toluenesulfonic acid [31], and sodium metal (reacts with alcohols to give the alkoxide catalyst) [33]. 

Although phenyl-sodium and -potassium may be made from reactions between phenyl-lithium and the heavier metal alkoxides, the structures of the donor-free solid compounds are apparently still unknown. The dimeric adducts [NaPh.PMDTA]2 and [NaC6H4Ph.TMEDA]2 have been characterized1 and the structure of a solvent-free arylsodium [Na(C6H3Mes2-2,6)]2, 32 (Fig. 17), has been described.63 The sodium interacts with the ipso... 

In summary, the reaction between an alkali metal alkoxide and a poly-hydroxy compound in hot alcoholic media produces an alcoholate and, possibly, a small proportion of alkoxide adduct however, the conditions governing the ratio of alcoholate to adduct have not yet been well defined. Reactions with alkali metal hydroxides and cyanides produce mixtures (of alcoholate and adduct) that consist mainly of alcoholate. Occurrence of reactions between alkaline-earth metal hydroxides and polyhydroxy compounds in anhydrous alcoholic media has not been reported. 

Bismuth is an important element in many of the new high-temperature, oxide superconductors and in a variety of heterogeneous mixed oxide catalysts. Some of the methods employed in the preparation of these materials, namely sol-gel and chemical vapor deposition processes, require bismuth alkoxides as precursors and a number of papers on these compounds have recently been published.1 One synthetic route to bismuth alkoxides, which avoids the more commonly used trihalide starting materials and the often troublesome separation of alkali metal halides, involves the reaction between a bismuth amide and an alcohol according to the following equation ... 

The addition of a base, typically ammonia, to mixtures of transition metal halides and alcohols allows the synthesis of homoleptic alkoxides and phenoxides for a wide range of metals. Anhydrous ammonia was first used in the preparation of titanium alkoxides where the reaction is forced to completion by the precipitation of ammonium chloride.41 Although useful for the synthesis of simple alkoxides and phenoxides of Si, Ge, Ti, Zr, Hf, V, Nb, Ta and Fe, as well as a number of lanthanides,42-47 the method fails to produce pure /-butoxides of a number of metals.58 Presumably, secondary reactions between HC1 and Bu OH take place. However, mixing MC14(M = Ti, Zr) with the Bu OH in the presence of pyridine followed by addition of ammonia proves successful, giving excellent yields of the M(OBul)4 complexes.59... 

A rare example of the formation of an alkoxide ligand in a metal cluster compound is seen in the reaction between methyl fluorosulfonate and the triiron carbonyl clusters anion [Fe3(CO)9(/i3-MeCO)], which gives Fe3(CO)9(/x3-CMe)(/i3-OMe) by C—O bond cleavage.135,136 The C—O bond cleavage provides a possible model for a step in Fischer-Tropsch chemistry.121... 

Ionic fully fluorinated alkoxides of the heavier alkali metals may be prepared by the reaction between the anhydrous metal fluoride and various perfluorinated carbonyl compounds in donor solvents (equations 40 and 41).137... 

In any of the fluorinated alkoxides made this way there must always be a fluorine attached to the a-carbon. There is in fact an equilibrium between the metal perfluoroalkoxide and the metal fluoride and carbonyl fluoride compound. Thus these Group IA metal alkoxides cannot be used in metathe-tic reactions to prepare other metal alkoxides. For example, R3SiCl and NaOCF(CF3)2 react to yield RjSiF, NaCl and (CF3)2CO rather than R3SiOCF(CF3)2.138... 

Since this brief summary covers most of our present knowledge of the stereospecific polymerizations, it must be very evident that any mechanism now proposed must be based largely on intuition and cannot be detailed. Most authors agree that propagation probably occurs via an anionic reaction between a metal alkoxide and an epoxide,... 

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