Group 16 metal alkoxides

Transfer hydrogenation of ketones catalyzed by a transition-metal complex or a main group-metal alkoxide is a useful method to produce secondary alcohols. Pure organic compounds such as 2-propanol [2,281,282] and formic acid [283] are preferably used as hydrogen donors in place of hydrogen gas. This method is convenient for a small- or medium-scale reduction... 

The main group metal alkoxides are diamagnetic and do not show ESR spectra. The paramagnetic transition metal complexes studied, however, have yielded information concerning the delocahzation of the unpaired electrons. 

There are a number of reactions known involving metal alkoxides and organometallic compounds of main group elements (Li, Mg, Al, Sn, or Zn). The fundamental reaction generally involves elimination of a main group metal alkoxide. The composition of the reaction products is considerably influenced by (i) the nature and chemistry of the reactants as well as their stoichiometry, (ii) the reaction conditions, and (iii) the presence of other reactive substrates in the reaction medium. Some typical reactions are illustrated by Eqs (2.301)-(2.319). 

We have demonstrated (a) the diversity of mechanistic pathways that exists for the thermal decompositon of metal alkoxides, and (b) how the preferred pathway is a function of the nature of the alkoxide ligand. A perhaps surprising conclusion that emerges from the study is that neither M-OR nor MO-R bond homolysis plays a direct role in these decompositions. It should be noted, however, that we have only examined the alkoxides of oxophilic metal ions that are not easily reduced. It is conceivable that for the later transition metal ions that are both less oxophilic and more reducible, decompositon pathways that lead to the reduction of the metal ion may become more important. These include -hydrogen abstraction followed by reductive elimination (cf., eq. 6), as well as M-OR bond cleavage. Note that P-hy(6ogen abstraction has been demonstrated to be quite facile for the platinum-group metal alkoxides. 

Complex Formation Between Two Alkoxides (Method 2.1). The pronounced Lewis basicity of the alkoxide ligands of the alkaU and alkaline earth metal alkoxides explains their capacity to form heterometalhc conplexes in solution with the vast majority of high-valent transition or main group metal alkoxides, for example ... 

Metal alkoxides are compounds in which a metal is attached to one or more alkyl groups by an oxygen atom. Alkoxides are derived from alcohols by the replacement of the hydroxyl hydrogen by metal. 

Much work has been done on the structure of the metal alkoxides (49). The simple alkaU alkoxides have an ionic lattice and a layer stmcture, but alkaline earth alkoxides show more covalent character. The aluminum alkoxides have been thoroughly studied and there is no doubt as to their covalent nature the lower alkoxides are associated, even in solution and in the vapor phase. The degree of association depends on the bulkiness of the alkoxy group and can range from 2 to 4, eg, the freshly distilled isopropylate is trimeric (4) ... 

Another group of chelants that form stable chelates at high pH because of metal—alkoxide coordination are the sugar acids, such as gluconic acid [526-95-4] (1 )- Utility for this group is found in high alkalinity botde washes and other cleansers (19). 

Many factors influence the chemical behavior of an alkoxide, including leaving group, metal ion, solvent and temperature. Electrophile geometry can also promote one type of alkoxide behavior over another. 

To complete the synthesis of keto disaccharide 16, the cyclic carbonate must be cleaved and the C-4 hydroxyl group must be oxidized selectively. With respect to the former objective, it is well known that a metal alkoxide (e. g. sodium methoxide) can easily... 

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... 

Sol-gel chemistry offers a unique advantage in the creation of novel organic-inorganic hybrids. The sol-gel process begins with a solution of metal alkoxide precursors [M(0/f) ] and water, where M is a network-forming element, and R is typically an alkyl group. Hydrolysis and... 

A similar steric effect was observed in the reaction of benzyl carboxylate (44). When 44a-d were treated with Bu OK under solvent-free conditions at around 100 °C for 30 min, the corresponding condensation products 45a (75%), 45b (66%), 45c (64%), and 45d (84%) were obtained in the yields indicated [9] (Scheme 6). When the same reactions of 44a-d and Bu OH were carried out in toluene under reflux for 16 h, no condensation product was obtained and 44a-d were recovered unchanged. In solution reactions, exchange of the alkoxy group occurs among the substrate, reagent, and solvent. Therefore, the alkoxy groups of the ester, metal alkoxide, and alcohol used as a solvent should be identical. 

The most common sol-gel process employs metal alkoxides of network forming elements (M(0R) where M is Si, B, Ti, Al, etc. and R is often an alkyl group) as monomeric precursors. In alcohol/water solutions the alkoxide groups are removed stepwise by hydrolysis reactions, generally employing acid or base catalysts, and are... 

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