Isopropoxide ligand

A great deal of kinetie information on the AE reaetion has been obtained. The rate of reaetion is first order in allylie aleohol, Ti(0-iPr)2(tartrate), and TBHP. In addition, the rate is inversely-square dependent on isopropoxide. This refleets the required replaeement of two isopropoxide ligands on Ti(0-iPr)2(tartrate) with TBHP and the allylie aleohol. The rate-determining step is oxygen transfer from the peroxide to the olefin. 

The selectivity of the hydrogen transfer is excellent When employing a catalyst with deuterium at the a-positions of the isopropoxide ligands (17), complete retention of the deuterium was observed. A computational study using the density functional theory comparing the six-membered transition state (as in Scheme 20.3, the direct transfer mechanism) with the hydride mechanism (Scheme 20.3, the hydride mechanism) supported the experimental results obtained [36]. A similar mechanism has been proposed for the MPV alkynylations [37] and cyanations [38]. 

Whereas a mononuclear alkoxide complex of aluminium which incorporates azaindole ligands has been noted [116], so azaindoles have been reported to bridge Al-centres in oxides [23, 41, 42]. This is also true of an oxide-alkoxide which incorporates the bis(trifluoromethyl)isopropoxide ligand [42]. In the case of dinuclear aggregate [[(F3C)2C(H)]0 2Al[/r-0C(H)(CF3)2]-... 

The hydridic nature of early transition metal hydrides is normally inferred chemically by their ability to reduce acetone to isopropoxide ligands.111... 

Zirconia (Zr02), an unusually tough oxide ceramic, has been used to make very sharp table knives. Write a balanced equation for the hydrolysis of zirconium iso-propoxide in the sol-gel method for making zirconia powders. The isopropoxide ligand is the anion of isopropyl alcohol, HOCH(CH3)2. 

The primary nucleophilic culprit is the isopropoxide ligand of the Ti(0-i-Pr)4. The use of Ti(0-r-Bu)4 in place of Ti(0- -Pr)4 has been prescribed as a means to reduce this problem (the r-butoxide being a poorer nucleophile) [18]. Fortunately a better solution now exists in the form of the catalytic version of the reaction that uses only 5-10 mol % of Ti-tartrate complex and greatly reduces the amount of epoxide ring opening. Some comparisons of results from reactions run under the two sets of conditions are possible from the epoxidations summarized in Table 6A.3 [2,4,18,33-38]. 

The neodymium atoms are placed in three different environments as depicted in Fig. 37. However, all neodymium atoms are 7-coordinate adopting a distorted mono capped prismatic geometry. Most interesting structural feature are the presence of terminal ethyl and isopropoxide ligands at neodymium. More simple molecular arrangments were found in ligand related complexes Ln[(jt-... 

Transition-metal-catalysed epoxidations work only on allylic alcohols, so there is one limitation to the method, but otherwise there are few restrictions on what can be epoxidized enantioselectively. When this reaction was discovered in 1981 it was by far the best asymmetric reaction known. Because of its importance, a lot of work went into discovering exactly how the reaction worked, and the scheme below shows what is believed to be the active complex, formed from two titanium atoms bridged by two tartrate ligands (shown in gold). Each titanium atom retains two of its isopropoxide ligands, and is coordinated to one of the carbonyl groups of the tartrate ligand. The reaction works best if the titanium and tartrate are left to stir for a while so that these dimers can form cleanly. 

When the oxidizing agent (f-BuOOH, shown in green) is added to the mixture, it displaces one of the remaining isopropoxide ligands and one of the tartrate carbonyl groups. 

Now, for this oxidizing complex to react with an allylic alcohol, the alcohol must become coordinated to the titanium too, displacing a further isopropoxide ligand. Because of the shape of the complex the reactive oxygen atom of the bound hydroperoxide has to be delivered to the lower face of the alkene (as drawn), and the epoxide is formed in high enantiomeric excess. 

A controlled gas-solid phase reaction of titanium(IV) isopropoxide with silica surface pretreated at 600°C was achieved at between 110 and 180°C in an atomic layer deposition (ALD) reactor. Element determinations and DRIFTS measurements suggested the formation of two isopropoxide ligand containing complex on silica with a titanium density of -1.25 atoms/nm upport- At reaction temperatures exceeding 180°C the growth was no longer determined by the surface. The reaction at 160°C was followed by a calcination treatment at 500°C and the consecutive precursor - oxygen cycles were repeated from one to seven times. 

For this oxidizing complex to react with an allylic alcohol, the alcohol must become coordinated to the titanium too, displacing a further isopropoxide ligand. Because of the shape... 

One of the chemoselective and mild reactions for the reduction of aldehydes and ketones to primary and secondary alcohols, respectively, is the Meerwein-Ponndorf-Verley (MPV) reduction. The lifeblood reagent in this reaction is aluminum isopropoxide in isopropyl alcohol. In MPV reaction mechanism, after coordination of carbonyl oxygen to the aluminum center, the critical step is the hydride transfer from the a-position of the isopropoxide ligand to the carbonyl carbon atom through a six-mem-bered ring transition state, 37. Then in the next step, an aluminum adduct is formed by the coordination of reduced carbonyl and oxidized alcohol (supplied from the reaction solvent) to aluminum atom. The last step is the exchange of produced alcohol with solvent and detachment of oxidized alcohol which is drastically slow. This requires nearly stoichiometric quantities of aluminum alkoxide as catalyst to prevent reverse Oppenauer oxidation reaction and also to increase the time of reaction to reach complete conversion. Therefore, accelerating this reaction with the use of similar catalysts is always the subject of interest for some researchers. 

This bimetallic complex was characterized by elemental analysis, infrared spectrum and mass spectrum. The infrared indicates that the eight-membered ring is present as [8]annulene. It was suggested from the mass spectrum and iH-NMR studies that this complex exists with the isopropoxide ligands bridging between the cerium and aluminum ions (Figure 11). The bimetallic [8]annulene complex has also been made as its acetonitrile adduct. 7... 

We note that the bulky isopropoxide ligands yield weU-defined 4-coordinate Ti(OR)4 compounds, while methoxy, ethoxy, and n-butoxy compounds, as well as TiCU, are oligomeric in solution. Hydrolysis is followed by condensation reactions to yield Ti02- Synthesis at low hydrolysis ratio (=moles ofwater/moles of metal alkoxide) yields spherical aggregates of anatase Ti02 nanocrystaUites, that are relatively monodisperse. At high hydrolysis ratios. 

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