Metal isopropoxides

Although the precise mechanism has not yet been clarified, a possible mechanism is shown in Scheme 5.2. First, the iridium alkoxide 3 is produced from 1 and an alcohol, this step being stimulated by a base (K2CO3). A ]3-hydride elimination of 3 then yields a carbonyl product and the iridium hydride 4. The insertion of acetone into the iridium-hydride bond in 4, giving metal isopropoxide 5, is followed by exchange of the alkoxy moiety to regenerate 3. 

Treatment of scandium, yttrium and the lanthanides with refluxing isopropyl alcohol in the presence of HgCl2 (10—3—10 4 equivalents) leads to the metal isopropoxide, which can then be crystallized from the excess isopropyl alcohol (equation 3).17,18... 

In the presence of a metal isopropoxide (63) undergoes either intermolecular reduction to form (66), or reacts intramolecularly to form (65). The latter appears to be the product expected from transition state (64), derived from a boat conformation (Scheme 6). The intramolecular reaction is much faster than its intermolecular competitor with Ba or K" " as cation but when Al is the cation, i.e. classical MPV conditions, then only intermolecular reduction occurs." ... 

Cationic complexes of the type [M(diene)Li2]+ (M = Rh, Ir) are also active catalyst precursors for transfer hydrogenation using 2-propanol as a donor, but refluxing conditions as well as basic media are needed. Under these conditions, in the presence of potassium hydroxide, metal-isopropoxide species are formed facilitating the generation of the metal hydride catalyst (2), according to Equation 29 ... 

The differences in solubility of metal isopropoxides and their alcoholates have led to the crystallization of pure isopropoxide isopropanolates of some tetravalent elements. For example, zirconium tetraisopropoxide is a viscous supercooled liquid which dissolves in excess of isopropyl alcohol and crystallizes out in the form of pure white crystals which were characterized as Zr(OPr )4.Pr OH. Similarly tin tetraisopropoxide and cerium tetraisopropoxide have been obtained as crystalline alcoholates, Sn(OPr )4.Pr OH ° and Ce(OPP)4.Pr OH which were used as the starting materials for the synthesis of a number of new alkoxides. 

These reactions can be carried out quantitatively in the case of metal isopropoxides (or ethoxides) by the azeotropic fractionation of the liberated alcohol with a suitable solvent (e.g. benzene), yielding mixed alkoxides or alkoxide-ligand (carboxylate, fi-diketonate, aminoalkoxide) derivatives. Many of these mixed ligand derivatives can be recrystallized and appear to be stable to heat (volatilizing without decomposition under reduced pressure). 

All the above reactions have been found to be quite facile and have been mostly carried out with metal isopropoxides and also with ethoxides in some cases. The 1 1 molar reaction of the alcoholate, Zr(OPr )4.I OH with CH3COCI showed that the zirconium isopropoxide moiety is more reactive than the coordinated isopropyl alcohol molecule. 

The formation of the final prodnct (A) (characterized by careful analysis of metal, isopropoxide, and tertiary bntoxide contents separately) was explained on the basis that the replacement of fonr terminal isopropoxide groups on two zirconium atoms and a bridging OBn gronp between them stericaUy hindered further coordination with another tertiary bntyl alcohol molecnle. In view of the repeated confirmation of the plausible structure(s) in a large nnmber of such cases (Section 1), a brief account of the revealing parameters will be included particularly in cases for which X-ray structural data are not yet available. 

J. V. Bell, J. Heisler, H. Tannenbaum, and J. Goldenson, Infrared Spectra of Metal Isopropoxides, Anal. Chem. 25, 1720, 1953. 

Scheme 2.1 Schematic of nonhydrolytic hydroxylation reactions, (a) Reaction of metal halides with alcohols coordination of the alcohol to the metal center, followed by elimination of an alkyl halide RX. (b) Guerbet-type reaction of a metal isopropoxide with benzyl...

After extensive optimization of the reaction conditions (e.g., metal isopropoxides, chiral ligands, and additives), the best results were obtained with Ba(Of-Pr)2 (2.5 mol%), chiral ligand (F2-FujiCAPO, 2.5mol%), and CsF(2.5mol%) affording the products (19 and 20) in 91% yield... 

Catalytic amounts of mercuric chloride are usually employed in this preparation. Aluminum isopropoxide is a useful Meerwein-Potmdorf-Verley reducing agent in certain ester-exchange reactions and is a precursor for aluminum glycinate, a buffering agent (see Alkoxides, metal). 

Hydrolysis of metal-organic solutions Example. Ba(OC3H7)2 + Ti(OC5Hu)4 + H2O — BaTiOs (Barium isopropoxide and Titanium tertiary amyloxide are refluxed in isopropanol and then hydrolyzed with de-ionized water to produce a sol-gel. ... 

The milder metal hydnde reagents are also used in stereoselective reductions Inclusion complexes of amine-borane reagent with cyclodexnins reduce ketones to opucally active alcohols, sometimes in modest enantiomeric excess [59] (equation 48). Diisobutylaluminum hydride modified by zmc bromide-MMA. A -tetra-methylethylenediamme (TMEDA) reduces a,a-difluoro-[i-hydroxy ketones to give predominantly erythro-2,2-difluoro-l,3-diols [60] (equation 49). The three isomers are formed on reduction with aluminum isopropoxide... 

In contrast to the situation with copper-based catalysis, most studies on ruthenium-based catalysts have made use of preformed metal complexes. The first reports of ruthenium-mediated polymerization by Sawamoto and coworkers appeared in I995.26 In the early work, the square pyramidal ruthenium (II) halide 146 was used in combination with a cocatalyst (usually aluminum isopropoxide). 

Evans, D.A. Nelson, S.G. Gagne, M.R. Mud, A.R. J. Am. Chem. Soc., 1993,115,9800. It has been that shown in some cases reduction with metal alkoxides, including aluminum isopropoxide, involves free-radical intermediates (SET mechanism) Screttas, C.G. Cazianis, C.T. Tetrahedron, 1978, 34, 933 Nasipuri, D. Gupta, M.D. Baneijee, S. Tetrahedron Lett., 1984, 25, 5551 Ashby, E.C. Argyropoulos, J.N. Tetrahedron Lett.,... 

Two kinds of solution were prepared in advance. Solution A was a water solution containing an Si source, which was obtained by hydrolyzing metal alkoxide (tetraethylorthosilicate, TEOS) with a dilute tetrapropylammoniumhydroxide (TPA-OH)/water solution at room temperature. The molar ratio of Si to the template was 3. In peparation of ZSM-S zeolite nanoerystals, aluminium isopropoxide as an A1 source and sodium chloride were added into solution A. Solution B was an oi mic solution containing surfectant Nonionie surfactants, poljraxyethylene (15) cxslylether (C-15), polyoxyethylene (15) nonylphenylether (NP-15), and polyoxyethylene (15) oleylether (O-15), and ionic surfoctnnts, sodium bis(2-ethylhexyl) sulfosucdnate (AOT) and... 

A gel of diesel or crude oil can be produced using a phosphate diester or an aluminum compound with phosphate diester [740]. The metal phosphate diester may be prepared by reacting a triester with phosphorous pentoxide to produce a polyphosphate, which is then reacted with an alcohol (usually hexanol) to produce a phosphate diester [870]. The latter diester is then added to the organic liquid along with a nonaqueous source of aluminum, such as aluminum isopropoxide (aluminum-triisopropylate) in diesel oil, to produce the metal phosphate diester. The conditions in the previous reaction steps are controlled to provide a gel with good viscosity versus temperature and time characteristics. All the reagents are substantially free of water and will not affect the pH. 

Addition of potassium ferf-butoxide or of sodium isopropoxide to the solvent led to ignition of the latter. This was attributed to presence of free metal in the alkoxides, but a more likely explanation seems to be that of direct interaction between the powerful bases and the sulfoxide. 

Aluminium isopropoxide, Heavy metal salts Ward, D. S., unpublished information, 1974... 

Interaction of anhydrous hydrazine and titanium isopropoxide is explosive at 130° C in absence of solvent. Evaporation of solvent ether from the reaction product of tetrakis(dimethylamino)titanium and anhydrous hydrazine caused an explosion, attributed to formation and ignition of dimethylamine. /V-Metal derivatives may also have been formed. 

The scope of metal-mediated asymmetric epoxidation of allylic alcohols was remarkably enhanced by a new titanium system introduced by Katsuki and Sharpless epoxidation of allylic alcohols using a titanium(IV) isopropoxide, dialkyl tartrate (DAT), and TBHP (TBHP = tert-butyl-hydroperoxide) proceeds with high enantioselectivity and good chemical yield, regardless of... 

The most common catalysts for the Meerwein-Ponndorf-Verley reduction and Oppenauer oxidation are Alm and Lnm isopropoxides, often in combination with 2-propanol as hydride donor and solvent. These alkoxide ligands are readily exchanged under formation of 2-propanol and the metal complexes of the substrate (Scheme 20.5). Therefore, the catalytic species is in fact a mixture of metal alkoxides. 

As an example, Table 20.2 lists the rate of the racemization of 61 via an MPVO procedure utilizing the catalyst neodymium(III) isopropoxide (62) as a function of the solvent. In this case, an equimolar amount of acetone was applied as the oxidant. The best results were obtained with hydrocarbons such as hexane (entry 7) and heptane (entry 8) as solvents, while the reaction rates in dioxane (entry 2) and acetonitrile (entry 1) were much lower due to inactivation of the catalyst by coordination of the solvent to the metallic center (Table 20.2) [84]. 

Lanthanide(III) isopropoxides show higher activities in MPV reductions than Al(OiPr)3, enabling their use in truly catalytic quantities (see Table 20.7 compare entry 2 with entries 3 to 6). Aluminum-catalyzed MPVO reactions can be enhanced by the use of TFA as additive (Table 20.7, entry 11) [87, 88], by utilizing bidentate ligands (Table 20.7, entry 14) [89] or by using binuclear catalysts (Table 20.7, entries 15 and 16) [8, 9]. With bidentate ligands, the aluminum catalyst does not form large clusters as it does in aluminum(III) isopropoxide. This increase in availability per aluminum ion increases the catalytic activity. Lanthanide-catalyzed reactions have been improved by the in-situ preparation of the catalyst the metal is treated with iodide in 2-propanol as the solvent (Table 20.7, entries 17-20) [90]. Lanthanide triflates have also been reported to possess excellent catalytic properties [91]. 

The catalysts are best prepared in situ by mixing a half-equivalent of the di-chloro-metal aromatic dimer with an equivalent of the ligand in a suitable solvent such as acetonitrile, dichloromethane or isopropanol. A base is used to remove the hydrochloric acid formed (Fig. 35.3). If 1 equiv. of base is used, the inactive pre-catalyst is prepared, and further addition of base activates the catalyst to the 16-electron species. In the IPA system the base is conveniently aqueous sodium hydroxide or sodium isopropoxide in isopropanol, whereas in the TEAF system, triethylamine activates the catalyst. In practice, since the amount of catalyst is tiny, any residual acid in the solvent can neutralize the added base, so a small excess is often used. To prevent the active 16-electron species sitting around, the catalyst is often activated in the presence of the hydrogen donor. The amount of catalyst required for a transformation depends on the desired reaction rate. Typically, it is desirable to achieve complete conversion of the substrate within several hours, and to this extent the catalyst is often used at 0.1 mol.% (with SCR 1000 1). Some substrate-catalyst combinations are less active, requiring more catalyst (e.g., up to 1 mol.% SCR 100 1), in other reactions catalyst TONs of 10000 (SCR 10000 1) have been realized. 

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