Alkoxides groups

When L is a group capable of chelating a counterion such as alkoxide groups... 

Enols and alkoxides give chelates with elimination of alcohol. For example, in the reaction of the enol form of acetylacetone [123-54-6] all four alkoxide groups attached to zirconium can be replaced, but only two of the four attached to titanium (Fig. 3). Acetoacetic esters react similarly. 

Hydrolysis of TEOS in various solvents is such that for a particular system increases directiy with the concentration of H" or H O" in acidic media and with the concentration of OH in basic media. The dominant factor in controlling the hydrolysis rate is pH (21). However, the nature of the acid plays an important role, so that a small addition of HCl induces a 1500-fold increase in whereas acetic acid has Httie effect. Hydrolysis is also temperature-dependent. The reaction rate increases 10-fold when the temperature is varied from 20 to 45°C. Nmr experiments show that varies in different solvents as foUows acetonitrile > methanol > dimethylformamide > dioxane > formamide, where the k in acetonitrile is about 20 times larger than the k in formamide. The nature of the alkoxy groups on the siHcon atom also influences the rate constant. The longer and the bulkier the alkoxide group, the lower the (3). 

The advantages of titanium complexes over other metallic complexes is high selectivity, which can be readily adjusted by proper selection of ligands. Moreover, they are relative iaert to redox processes. The most common synthesis of chiral titanium complexes iavolves displacement of chloride or alkoxide groups on titanium with a chiral ligand, L ... 

Nucleophilic participation is important only for esters of alcohols that have pK <13. Specifically, phenyl and trifluoroethyl esters show nucleophilic catalysis, but methyl and 2-chloroethyl esters do not. This result reflects the fete of the tetrahedral intermediate that results fi om nucleophilic participation. For relatively acidic alcohols, the alkoxide group can be eliminated, leading to hydrolysis via nucleophilic catalysis ... 

Silicon alkoxide groups, 455 Silicon-containing polymers, 450-460 Silicon-methoxy bonds, hydrolysis of,... 

Initiation resulting from insertion of the monomer into the Al—Cl bond is followed by propagation involving insertion between the porphinato-aluminum and the alkoxide group of the growing polymer, coupled with P-scission of the C—O bond of the oxirane monomer (demonstrated by nmr results) it yields a polyether terminated by a CH2C1 end-group. 

The first example deals with poly[bis(trifluoroethoxy)phosphazene],PTFEP, film, whose surface is intrinsically hydrophobic but can be converted to hydrophilic by the action of NaOH in dioxane at 80 °C in the presence of tetrabutyl-ammonium bromide [514,515], or by metathetical exchange of trifluoroethoxy moieties with variable types of alkoxides groups [482,483,515] (vide supra). 

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... 

Poly(L-lactide), R=alkoxide group as initiator fragment... 

Compound (6) has the structure shown in Fig. 4(d). It formally has 14-electron configurations about each metal atom and all fourteen electrons are involved in the bonds represented by lines in the structural drawing. Either the U2-CO group makes no appreciable demand for tt electron density or there may be an indirect feeding of electrons from the lone-pair orbitals of the alkoxide groups to y2-C0. Alkoxide groups are known to be 7r-electron donors. 

The alkoxide group R is chosen so that the heavier alkali metal alkoxide MOR is soluble in ether or hydrocarbon solvents and so the by-product LiOR may easily be separated from the desired heavier alkali metal phosphide/arsenide (which is often insoluble in weakly or noncoordinating solvents such as ethers and hydrocarbons). 

Complex (J )-140 serves as a chiral Lewis acid and coordinates to the aldehyde at the less hindered /J-face of 141. i e-side cyanation of (J )-141 and the subsequent cleavage of the alkoxide group give the product 142. Because at this stage the catalyst turnover is blocked, the reaction cannot be carried out in a catalytic manner. 

Stereoselective cyclization controlled by a substituent remote from the reaction center is often difficult to achieve. However, 1-mediated cyclization of the substrates illustrated in Eq. 9.54 proceeds in a highly stereoselective manner when the hydroxy group is converted to a magnesium alkoxide prior to cyclization [99,100]. The effect of the alkoxide group is much more favorable than that of the corresponding TBS ether. 

Dimerisation and ester coordination restricts the number of sites available for alkoxides to two only, while maintaining a comparatively Lewis-acidic titanium centre, as needed for the reaction. In the dimer, the methine protons, alkoxide groups, and ester groups are inequivalent, but they show a rapid exchange on the H NMR timescale at room temperature, as the AG for the process is only 64 kl.mol. This process is much faster than the catalytic reaction, but due to the C2-symmetry of the tartaric esters the resulting structures of the dimers are the same. 

In ester hydrolysis, rate-limiting formation of the tetrahedral intermediate usually apphes (Sec. 6.3.1) since the alkoxide group is easily expelled. In contrast, amide hydrolysis at neutral pH involves rate-limiting breakdown of the tetrtihedral intermediate, because RNH is a poor leaving group. The catalytic effect of metal ions on amide hydrolysis has been ascribed to accelerated breakdown of the tetrahedral intermediate. 

In the sol-gel process, ceramic polymer precursors are formed in solution at ambient temperature shaped by casting, film formation, or fiber drawing and then consolidated to furnish dense glasses or polycrystalline ceramics. The most common sol-gel procedures involve alkoxides of silicon, boron, titanium, and aluminum. In alcohol water solution, the alkoxide groups are removed stepwise by hydrolysis under acidic or basic catalysis and... 

Treatment of the pyrrolidone mixture 5 with 2.1 equivalents of lithium diisopropylamide at — 78 °C for 1 hour, and then — 25 °C for 1 hour, yields the dianionic enolate. Alkylation at -117 °C or - 78 °C then provides a 50-80% yield of the (3S)-alkylated 3,4-tran.v-product as a 85 15 mixture of the 5-epimers19,20. No trace of the 3,4-a.s-product could be detected by NMR. The electrophile attacks from the side opposite to the alkoxide group. Evidently, in this case, the stereogenic center in the 5-position has no influence on the stereoselectivity. 

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