Calcium alkoxides

Alkaline earth metal alkoxides decompose to carbonates, olefins, hydrogen, and methane calcium alkoxides give ketones (65). For aluminum alkoxides, thermal stability decreases as follows primary > secondary > tertiary the respective decomposition temperatures are ca 320°C, 250°C, and 140°C. Decomposition products are ethers, alcohols, and olefins. 

In the second procedure, calcium nitrate was replaced by calcium alkoxide (60). Calcium and sificon alkoxides have very different rates of hydrolysis. To avoid the production of inhomogeneities, a slow and controlled hydrolysis of a mixture of sificon, calcium, and phosphorous alkoxide was performed. The resulting materials were highly homogenous, and monolithic pieces could be produced. The bioactivity of the gel-derived materials is equivalent or greater than melt-derived glasses. 

Calcium alkoxides do not promote the rearrangement and instead afford only ring... 

Calcium, tetrakis(methanol)dinitrato-structure, 1, 89 Calcium alkoxides synthesis, 2, 336... 

Several aluminum biphenolate complexes have been investigated as initiators for the ROP of PO.810,935 Unlike the TPP and salen-based systems, a cis coordination site is realistically accessible and in theory an alternative cis-migratory mechanism to the backside attack pathway might operate. However, NMR analyses on the resultant PPO show that stereochemical inversion still occurs when the biphenolate initiators are used (Scheme 22). It has also been confirmed that the same process occurs with the Union Carbide calcium alkoxide-amide initiator for both PO and CHO.810... 

The synthesis of precursors A and B (see Figure 12.10) has been described by Michalczyk et al. [102]. These compounds can be synthesised from platinum-catalysed hydrosilylation reactions, that is addition reactions between Si—H and C=C groups in the presence of a catalyst. Once the pure precursors are obtained, BSG can be synthesised by incorporation of calcium alkoxide during polycondensation of the precursors. 

Fig. 12.11 Transparent star gel monoliths prepared with calcium alkoxide (right) and without calcium (left).

Calcium alkoxides synthesis, 336 Calcium complexes acetylacetone, 372 amides, 164 malonic acid, 444... 

Shibasaki et al. also developed a barium complex (BaB-M, 14, Scheme 5) for the aldol reaction of acetophenone (la), making use of the strongly basic characteristic of barium alkoxide. The catalyst was prepared from Ba(0-z-Pr)2 and BINOL monomethyl ether, and the products were obtained in excellent yield with up to 70% ee (Scheme 6) [8], Shibasaki et al. attempted to incorporate a strong Bronsted base into the catalyst and developed a lanthanide heterobime-tallic catalyst (15) possessing lithium alkoxide moieties, which promoted the aldol reaction with up to 74% ee (Scheme 6) [9]. Noyori and Shibasaki et al. reported a calcium alkoxide catalyst (16) that was prepared from Ca[N(SiMe3)2]2,... 

Despite the fact that the metal ions enable to control the polymerisation, they also pose an important downside as the formed polymers will always contain metal residues. These metal ions might have detrimental health effects. Aluminium derivatives have already been linked to Alzheimer s disease, although this hypothesis is controversial [50], while tin octoate, despite being FDA approved, has already been shown to be cytotoxic. As tin levels often exceed 1000 ppm, the FDA has set a limit of 20 ppm for polyesters used for biomedical applications [51]. To circumvent this problem, attention has already been paid to less toxic catalysts including magnesium and calcium alkoxides. Conversely, attempts are being made to remove the catalyst residues from the polymer. 

Figure 2. In situ generation of a calcium alkoxide from bis(tetrahydrofuran)calcium, bis[bis(trimethylsilyl)amide] and an alcohol, for the ring Opening polymerization of cyclic esters...

A numbers of calcium alkoxides have been synthesized to date [7], Because their derivatives possess groups such as MeO , EtO , and PrO , they often form insoluble, large aggregates that have a tendency to continuously equilibrate in solution [8], Because of this, the synthesis of calcium alkoxides of low nuclearity is cumbersome and usually requires the use of sterically large chelating ligands [9] or Lewis acids to prevent aggregation. 

In the same time period of Price s work, Hill, Bailey and Fitzpatrick of Union Carbide Corporation developed some improved catalysts for polymerizing ethylene oxide to high polymer(l ). These new catalysts included improvements on the very early systems of Staudinger, i.e., strontium, calciiom, and zinc oxides and carbonates, as well as some new, even better, systems based on calcium alkoxides(ii) and amides(j . Bailey used the latter systems in 1958 to make water-soluble ethylene oxide-unsaturated epoxide copolymers(17). Vulcanizates of these copolymers were very water sensitive and thus not very useful in the conventional elastomer area. 

The precatalyst [ (ArNCMe)2CH Ca p-H](THF)]2 has been shown to react with ketones to yield the corresponding dimeric p-diketiminato calcium alkoxide products derived from insertion of the ketone into the Ca-H bond. These latter species have been shown to be catalyticaUy competent for the hydrosilylation of ketones. Stoichiometric preparations included reaction of [ (ArNCMe)2CH Ca p-H](THF)]2 with benzophenone, acetophenone, cyclohexanone, 1,3-diphenylacetone, and 2-adamantone to yield the corresponding alkoxides in 9-73 % following crystallization from hydrocarbon solutions. Quenching the reaction mixtures with MeaSiCl showed, in addition to the expected reduction products, moderate to significant... 

While calcium isopropoxide, Ca(0 Pr)2 has been employed successfully in the preparation of chiral catalysts, it was also found that calcium amide (e.g., Ca (HMDS)2) worked well in these reactions as a catalyst. Calcium amides have a stronger Brpnsted basicity and a higher solubility in many solvents compared with calcium alkoxides. Interestingly, it was reported that chiral calcium amide catalysts prepared from Ca(HMDS)2 could be employed successfully in [3+2] cycloaddition reactions (Scheme 4) [48]. 

In the past, many variations of the sol-gel process have been developed and used to produce powders with different Ca/P ratios, by altering not only the quantity and composition of precursors but also the processing variables. As the synthesis of HA requires a calcium to phosphorus molar ratio of 1.67 1 in the final product, a number of caldum/phosphorus precursor combinations have been used in the preparation of HA powders. For example, calcium nitrate or different calcium alkoxides and 2-ethyl-hexyl phosphate, triethyl phosphate or orthophos-phoric acid, have been used as the calcium and phosphoras precursors, respectively. The major limitation for appUcation of the sol-gel process was shown to be the very poor solubility of the calcium alkoxides in organic solvents, and the poor reactivity of the phosphorous compounds. The effective control of stoichiometry due to the volatility of the phosphorous compounds used also represented a challenge. 

Poologasundarampillai, G., Yu, B., Tsigkou, O., Wang, D., Romer, F., Bhakhri, V. et al (2014) Poly(gamma-glutamic acid)/silica hybrids with calcium incorporated in the silica network by use of a calcium alkoxide precursor. Chem. Eur. /., 20, 8149-8160. 

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