Aluminum alkoxides complex

Based on the catalytic activity of aluminum alkoxides in the Meerwein-Ponndorf-Verley-Oppenauer reaction, Berkessel et al. envisioned that aluminum complexes can act as alcohol racemization catalysts [32]. Aluminum alkoxide complexes generated from a 1 1 mixture of AlMes and a bidentate ligand such as binol or 2,2 -biphenol were effective catalysts for alcohol racemization. At room temperature, 10mol% of the aluminum catalyst racemized 1-phenylethanol completely within 3h in the presence of 0.5 equiv. of acetophenone. The aluminum catalysts were... 

In the past, several aluminum-alkyl, halide, and alkoxide complexes supported by multidentate ligands were examined for their catalytic lactide polymerization activities. To this end, monomeric aluminum complexes 148a, b (Fig. 21) were synthesized in our laboratory for producing polyesters with thiolate end groups [137]. These complexes initiated polymerizations under reflux condition in toluene and xylene forming PLAs with narrow molecular weight distributions (PDIs 1.15-1.25). 

Recently, systems have been developed where the aluminum alkoxide is covalently bonded to solid porous silica [73]. This system takes advantage of the exchange reaction between the alkoxide and the hydroxy-terminated free molecules to produce a catalytic process, i.e., to produce a larger number of polymer chains than aluminum complexes present. The initiator/catalyst used can easily be recovered by filtration and recycled. In addition, the polymers obtained are free from metal residues. 

The mechanistic complexities of stereoselectivity is further evidenced by a recent report by Maudoux et a/. who describe a chiral aluminum salen catalyst that generates highly isotactic PLA from rac-lactide (Pm-0.90). In this example, the kinetics indicated a dominant chain-end control mechanism, which contrasts to other chiral aluminum salen catalysts where enantiomorphic site control is thought to predomi-nate. ° All the previously mentioned chiral aluminum salen alkoxide systems require multiple days at elevated temperatures to polymerize -200 equiv. of lactide. The low activity of chiral aluminum salen systems towards lactide polymerization is a major drawback of these systems. 

The aluminum-chelated alkoxide can also be used unhydrolyzed, in a mixture with prehydrolyzed TEOS [51]. This mixture is concentrated under reduced pressure and the fibers are dry [49] [51] or wet spun [48]. Mullite fibers were prepared from a single phase precursor [52], whereby the alumina source was a viscous solution of aluminum formoacetate. After concentration, the dope was dry spun, yielding fibers which contained orthorhombic mullite after firing at 1250°C. Mullite fibers can also be prepared from another single phase liquid precursor, a mixture of an alkoxysilane, such as tetramethoxysilane or ethyl silicate, and an aluminum chlorohydrate polyol complex, such as the 1,2- dihydroxypropane complex, Al2(0H)5CI-H20-CH3CH(0H)CH20H[46]. 

The proligands aluminum complex 91 was readily synthesized as mentioned in the Scheme 6.18 [69]. The bis-alkoxide Al species 91 is active... 

Although aluminum alkoxide- and aluminoxane-based catalysts have shown promise for the isospecific polymerization of epoxides, the poorly defined nature of these species has significantly hampered their use in such polymerizations. Some examples of discrete aluminum complexes have been reported, however. 

Ooi and Maruoka found that aluminum alkoxide (83) prepared from aluminum complex (82) and tertiary propargyl alcohol reacts with highly electrophilic aldehydes, such as chloral and pentafluorobenzaldehyde, to give alkynyl transfer products in good yield (Scheme 6.65) [84]. Bisphenol structure of ligands on aluminum center is the most important feature to obtain the alkynylated products, and thus, in reactions with (84) or (85) instead of (83) dramatical decrease in the yield of alkynyl product was observed. Same group also reported that cyanide transfer reaction with acetone cyanohydrin as a cyanide donor catalyzed by complex (82) [84]. 

Double Alkoxides. Complex double alkoxides are formed when a solution of an alkaU or alkaline earth metal alkoxide is added to a solution of an alkoxide of aluminum, titanium, or tirconium and a series of such compounds have been prepared (44). 

The lithium aluminum hydride-aluminum chloride reduction of ketones is closely related mechanistically to the Meerwein-Ponndorf-Verley reduction in that the initially formed alkoxide complex is allowed to equilibrate between isomers in the... 

A series of bis(phenoxide) aluminum alkoxides have also been reported as lactone ROP initiators. Complexes (264)-(266) all initiate the well-controlled ROP of CL, NVL.806,807 and L-LA.808 Block copolymers have been prepared by sequential monomer addition, and resumption experiments (addition of a second aliquot of monomer to a living chain) support a living mechanism. The polymerizations are characterized by narrow polydispersities (1.20) and molecular weights close to calculated values. However, other researchers using closely related (267) have reported Mw/Mn values of 1.50 and proposed that an equilibrium between dimeric and monomeric initiator molecules was responsible for an efficiency of 0.36.809 In addition, the polymerization of LA using (268) only achieved a conversion of 15% after 5 days at 80 °C (Mn = 21,070, Mn calc 2,010, Mw/Mn = 1.46).810... 

As described previously for aluminum, in order to obviate the kinetic complexity arising from aggregation, several groups have examined potentially less complicated single-site monoalkoxides. For example, complexes (299)-(301) are active for the ROP of CL, 6-VL and even /3-PL at 0 °c.888 Polydispersities are low (< 1.10) even up to 90% conversion and Mn increases linearly with conversion for CL, although initiator efficiencies are typically 50-60%. Lanthanocene alkyls, such as (302) and (303), and hydrides, (304), exhibit almost identical reactivity for the polymerization of CL and <5-VL to the alkoxides (299)-(301) (although no activity for /3-PL was observed). 

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... 

Aluminum(III) complexes are amongst the most common Lewis acids. In particular, aluminum halide species (e.g., A1C13, AlBr3) are commercially available and are widely used for various reactions. Other types of Lewis acid such as aluminum alkoxides, alkylaluminum halides, and trialkylaluminum species are also used for many kinds of Lewis-acid-mediated reactions. 

Dauben et al. (15) applied the Aratani catalyst to intramolecular cyclopropanation reactions. Diazoketoesters were poor substrates for this catalyst, conferring little asymmetric induction to the product, Eq. 10. Better results were found using diazo ketones (34). The product cyclopropane was formed in selectivities as high as 77% ee (35a, n = 1). A reversal in the absolute sense of induction was noted upon cyclopropanation of the homologous substrate 34b (n = 2) using this catalyst, Eq. 11. Dauben notes that the reaction does not proceed at low temperature, as expected for a Cu(II) precatalyst, but that thermal activation of the catalyst results in lower selectivities (44% ee, 80°C, PhH, 35a, n = 1). Complex ent-11 may be activated at ambient temperature by reduction with 0.25 equiv (to catalyst) DIBAL-H, affording the optimized selectivities in this reaction. The active species in these reactions is presumably the aluminum alkoxide (33). Dauben cautions that this catalyst slowly decomposes under these conditions. 

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