Metal alkoxides addition reactions

Figure 27-1 shows the types of distribution of inorganic substances. Table 27-1 summarizes the results ofits distribution for different metal alkoxide/alcohol reaction systems. The moisture-conditioned specimens refer to wood with only bound water below the fiber saturation point therefore, water is distributed only within the cell walls. Water-saturated specimens have, however, free water in the cell cavities in addition to the bound water within the ceU walls. In this way, the use of these specimens with differently distributed water in the wood cells can make it possible to prepare wood composites with inorganic substances distributed differently (Saica, 1992, 1993a Ogiso, 1993). 

The process of inversion (racemization) without exchange is called isoinversion. This involves an ion-pair mechanism. Indeed, in the presence of a crown ether, to separate the ions, the percentage of racemate increases (307). In fact, the stereochemical course of many metal-alkoxide catalyzed reactions in nonpolar solvents can be drastically modified by addition of catalytic amounts of crown ethers to the medium. For this reason, ion pairs, in low dielectric media, play a remarkable role as intermediates in reactions in which the negative ion is a carbanion. For example. Cram studied the rate of exchange (ke) and racemization kj as a function of the substituents present with deuterium-labeled 9-methylfluorene (309). The most interesting case was kjk < 0.5, racemization without exchange. This happens for X = di-... 

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. 

Most successful approaches involving addition reactions in the presence of chiral additives utilize organolithium, organomagnesium and the recently introduced organotitanium reagents, which are known to coordinate with amines, ethers, metal amides and alkoxides. 

In addition to amines, alkali metal alkoxides, hydroxides, and metals themselves can be used in borylation reactions [Eq. (69)] (90IZV886). 

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). 

Metal-metal bonds in molybdenum and tungsten alkoxides provide a ready source of electrons for oxidative-addition reactions and addition reactions involving ir-acidic ligands. 

Fig. 5.18 Schematic and TEM image of reaction scheme to prepare metal nanoparticles encapsulated within metal oxide coating on oxidized MWCNTs. Metal NPs are added to developing metal alkoxide sol followed by addition of oxidized MWCNTs and water for hydrolysis. Adapted with permission from [228], (2012) American Chemical Society.

Metal-Oxygen Compounds. Trialkyltin alkoxides are remarkable for the variety of addition reactions they undergo with carbonyl and thiocarbonyl compounds. Bloodworth and Davies have reported reactions of tri-w-butyltin alkoxides with isocyanates, carbon dioxide, sulfur dioxide, isothiocyanates, carbon bisulfide, chloral, and ketene. The reactions observed were as follows ... 

Metal-Nitrogen Compounds. Very little work has been done on addition reactions of metal-nitrogen compounds. The trimethyltin dimethylamide apparently does undergo reactions analogous to those of the trialkyltin alkoxides just discussed. For example, the following reactions were observed with carbon dioxide, carbon disulfide, and phenylisocyanate (57) ... 

Various metal alkoxides are ideal starting materials for the preparation of metal (hydrous) oxides by the described aerosol techniques, because many of these compounds are in the liquid state at room temperature, easily vaporized, and exceedingly reactive with water vapor. Additional advantage is the purity of the resulting powders, because the only products of the chemical reactions are the metal (hydrous) oxide and alcohol. The particles are, therefore, free of impurities, such as various ions, normally present in solids prepared from different salts. 

Hanawalt and Richey observed that additions of alkali-metal alkoxides to diaUtylmag-nesium led in some reactions to behavior resembling that of trialkylmagnesates. This includes enhanced reactivities in addition to pyridine leading to 4- or 2-aIkyl-substituted... 

On the other hand, approaches to the use of catalytic amounts of chiral ligands have been developed. Thus, the use of a sub-stoichiometric amount (50 mol%) of DBNE (1) affords A,A-diphenylphosphinylamine with 85% in 69% yield121a. Similarly, 25 mol% of chiral aziridinyl alcohol 56 (R = - ) affords (V,(V-diphenylphosphinylamine with 65% in 60% yield123. In the enantioselective addition reaction of diethylzinc to a nitrone, 20 mol% of the metal alkoxide of diisopropyl tartrate 62 catalyzed the formation of a... 

The generation of dichlorocarbene for addition to olefins has been realized by the use of chloroform and alkali metal alk-oxides4 6 (preferably potassium feri-butoxide), sodium trichloro-acetate,6 butyllithium and bromotrichloromethane,7 and the reaction of an ester of trichloracetic acid with an alkali metal alkoxide.2,8 The latter method, which is here illustrated by the preparation of 2-oxa-7,7-dichloronorcarane, generally gives higher yields of adducts. 

In organic solvents the acidity functions H corresponding to hydrogen dissociation from neutral indicator acids were reported for solutions of alkali metal alkoxides in various alcohols (2), using nitroanilines (21), aminobenzenecarboxylic acids (22), or indols (23) as indicators. For addition reactions of methoxide and ethoxide ions to neutral indicator acids, acidity functions J (also denoted as Hr) based on use of nitrobenzenes (21) and a-cyanostilbenes (18) as indicators in methanol and dimethylsulfoxide-methanol and -ethanol mixtures were reported. Recently (24) the acidity function J- (denoted as Jm) was derived for methoxide ion solutions in methanol using substituted benzaldehydes as indicators. These scales involve arbitrary choice of water as the solvent for determination of the dissociation constant of the anchoring acid. 

U(OEt)5 is easily prepared by oxidation of U(OEt)4 by bromine in ethanol, followed by addition of the calculated quantity of sodium ethoxide, a reaction which yields181 only Np(OEt)4Br in the case of Np(OEt)4. U(OEt)s is commonly used as the starting material for the preparation of other uranium(V) alkoxides by alcohol exchange reactions. A comprehensive review of metal alkoxides includes a useful discussion of the uranium(V) compounds.182... 

They proposed a polymerization scheme closely related to other well-known chemical reactions of metal alkoxide with carbonyl compounds (20). In Scheme 2, complex [A] is converted to [B] by hydride ion transfer from the alkoxyl group to the carbon atom of aldehyde (Meerwein-Ponndorf reduction). Addition of one molecule of monomer to the growing chain requires transfer of the alkoxide anion to the carbonyl group to form a new alkoxide [C]. Repetition of these two consecutive processes, i.e., coordination of aldehyde and transfer of the alkoxide anion, constitutes the chain propagation step. 

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