From alkoxides

Specialty Aluminas. Process control (qv) teclmiques permit production of calcined specialty aluminas ha nng controlled median particle sizes differentiated by about 0.5 ]lm. Tliis broad selection enables closer shrinkage control of high tech ceramic parts. Production of pure 99.99% -AI2O2 powder from alkoxide precursors (see Alkoxides, metal), apparently in spherical form, offers the potential of satisfying the most advanced appUcations for calcined aluminas requiring tolerances of 0.1% shrinkage. 

The usual products from alkoxides and acids are dialkoxytitanium diacylates (130,131). The third acyl group, but not the fourth, can often be iatroduced by a2eotropiag the lower alcohol with ben2ene (132). Using acetic anhydride, the same hexaacetoxydititanoxane is prepared from the chloride forms. 

A number of reaction variables or parameters have been examined. Catalyst solutions should not be prepared and stored since the resting catalyst is not stable to long term storage. However, the catalyst solution must be aged prior to the addition of allylic alcohol or TBHP. Diethyl tartrate and diisopropyl tartrate are the ligands of choice for most allylic alcohols. TBHP and cumene hydroperoxide are the most commonly used terminal oxidant and are both extremely effective. Methylene chloride is the solvent of choice and Ti(i-OPr)4 is the titanium precatalyst of choice. Titanium (IV) t-butoxide is recommended for those reactions in which the product epoxide is particularly sensitive to ring opening from alkoxide nucleophiles. ... 

Whether the formation of alkene 3 proceeds directly from alkoxide 4 or via a penta-coordinated silicon-species 6, is not rigorously known. In certain cases—e.g. for /3-hydroxydisilanes (R = SiMes) that were investigated by Hrudlik et al —the experimental findings suggest that formation of the carbon-carbon bond is synchronous to formation of the silicon-oxygen bond ... 

The intermediate in the second mechanism is identical with that postulated by Skell and Starer (1959) in the formation of carbonium ions from alkoxide and carbenes. 

The first stage of Kolbe s synthesis is analogous to the well-known aliphatic synthesis of alkyl carbonates from alkoxides and carbon dioxide ... 

Finally, /i-hydrogen transfer is the key step in the Meerwein-Pondorf-Verley (MPV) reduction of ketones by alcohols, catalyzed by aluminium alkoxides and many other catalysts. In that case, competition is not an issue, since polymerization is usually not thermodynamically favourable. The accepted mechanism for this reaction is direct transfer of the hydride from alkoxide to ketone. 

In principle, one could write two mechanisms for the MPV reduction a stepwise mechanism involving a discrete hydride intermediate, and a direct hydride transfer from alkoxide to ketone. These alternatives are similar to the two mechanisms usually assumed for transition-metal-catalyzed transfer... 

Acetylide (RC=CNa) and alkynide (RC=CMgX and RC=CLi) are good nucleophiles. They react with carhonyl group to from alkoxide, which under acidic work-up gives alcohol. The addition of acelylides and alkynides produces similar alcohols to organometallic reagents. 

Essentially the same behavior was reported earlier for Ti02 powders prepared from alkoxide.38 This may be accounted for by the change in the number of electrons from 4 (02) to 2 (acetone). 

Two routes of catalyst decomposition are also possible from alkoxide 29, fragmentation to form an epoxide or O-alkylation and subsequent fragmentation to an enol ether. Both of these tertiary amines can then be IV-alkylated to form new chiral, non-racemic quat salts. The quaternary ammonium catalyst can also be dequatemarized by nucleophiles to a tertiary amine, which can then undergo subsequent reactions [9c, 1 li,26b,87]. 

Apart from aluminium, many other metals were tested in Meerwein-Ponndorf-Verley reductions and Oppenauer oxidations during the early years of research on hydride transfer from alkoxides.26 A consensus was... 

The vertical electron affinity (EA) of acetone is given as —1.51 eV by Jordan and Burrow386. Lifshitz, Wu and Tiernan387 determine—among other compounds—the excitation function and rate constants of the slow proton transfer reactions between acclone-Ih, acetone-Dg and other ketones. The acetone enolate anion has been produced in a CO2 laser induced alkane elimination from alkoxide anions by Brauman and collaborators388-390. These show, e.g. that the methane elimination from t-butoxide anion is a stepwise process ... 

The earliest examples of such reactions are the hydride transfer from HCO to formaldehyde (Karpas and Klein, 1975), from DNOT as deuteride to (CH3)3B (Murphy and Beauchamp, 1976), from the conjugate base of 1,4-cyclohexadiene to benzaldehyde (DePuy et al., 1978b) and from alkoxide ions to singlet oxygen (Schmitt el al., 1979). 

One of the reactions involving hydride transfer, which has synthetic importance in solution chemistry, is the Meerwein-Ponndorf-Verley reduction of carbonyl compounds by hydride transfer from alkoxide ions. Similarly, it has been found possible to reduce formaldehyde, benzaldehyde, 2,2-dimethylpropanal and 1-adamantylcarboxaldehyde with methoxide ions in the gas phase (Ingemann el al., 1982b). The reaction trajectory of the hydride transfer from the methoxide ion to formaldehyde has also been studied by ab initio calculations (Sheldon et al., 1984b). 

Although collision induced dissociation (CID) is a well-known method for investigating the structures of cations in the gas phase (McLafferty, 1983), it has been applied much less to anions (Bowie, 1986). Actually, in some cases CID has been used to study the fragmentation mechanisms of anions, such as the elimination of molecular hydrogen from alkoxide ions (Hayes el al., 1984) or the primary fragmentation routes of ester enolate ions (Froelicher et al., 1985). 

Figure 4. Inhomogeneity of silica-aluminas prepared by various methods. A series of 17 commercial samples of silica-aluminas from seven different producers was submitted to microanalysis. All of them showed considerable fluctuations of composition at the scale of several tens of nanometers to several micrometers. These samples were prepared by coprecipitation or by the sol-gel method. It is not known whether some of these samples were prepared from alkoxides. Smaller but significant fluctuations at the micrometer scale were also observed for two laboratory samples prepared from alkoxides. The samples were dispersed in water with an ultrasonic vibrator. A drop of the resulting suspension was deposited on a thin carbon film supported on a standard copper grid. After drying, the samples were observed and analyzed by transmission electron microscopy (TEM) on a JEOL-JEM 100C TEMSCAN equiped with a KEVEX energy dispersive spectrometer for electron probe microanalysis (EPM A). The accelerating potential used was 100 kV.

The empty coordination sites on A1 and Zn are reversibly filled by electron pairs from alkoxide groups with intra- or intermolecular associate formation. The degree of association depends on the solvent, the type of metal M +, and the OR group in [(R0)4Al202M +] in benzene, n varies from 1 to 8. We assume that the monomers are inserted, with simultaneous ring opening, into the... 

Alkoxy- or aryloxypyridazines and the corresponding diethers are usually made from a halo- or dihalopyridazine and an equivalent amount of sodium alkoxide or phenoxide. As by-products 6-alkoxy-3(2 )-pyridazinones may be formed (particularly if aqueous bases are used) or other products may result from alkoxide exchange. A detailed examination of the reaction between 3,6-dichloropyridazine and various alkoxides revealed that the crude products, i.e., 3-alkoxy-6-chloropyridazines, are always contaminated with the starting material and the 3,6-bisalkoxy derivative. Lower temperatures and prolonged heating favor the preparation of 3-alkoxy-6-chloro-pyridazines and similar optimum reaction conditions for the synthesis of 3,6-dialkoxy- and phenoxypyridazines are reported. 

Alcohols (as well as amines, sulfides and many hydrocarbons) may act as overall hydride donors towards excited nonbonding-rr " states of carbonyl compounds and heterocycles. The synthetic as well as the photophysical aspects of these processes have been discussed extensively. These reactions will not be dealt with further here. Singlet oxygen also accepts hydride from alkoxides in the gas phase the mechanisms of such reactions have received considerable study. ... 

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