Aluminum complexes Schiff bases

Scheme 32 Organoalumlnum compounds used as substitute of aluminum porphyrins Schiff base (a) and calixarene aluminum complexes (b).

Ito K, Tsutsumi H, Setoyama M, Saito B, Katsuki T. Enantioselective hydrophosphonylation of aldehydes using an aluminum binaphthyl Schiff base complex as a catalyst. Syn-lett 2007 (12) 1960-1962. 

The major developments of catalytic enantioselective cycloaddition reactions of carbonyl compounds with conjugated dienes have been presented. A variety of chiral catalysts is available for the different types of carbonyl compound. For unactivated aldehydes chiral catalysts such as BINOL-aluminum(III), BINOL-tita-nium(IV), acyloxylborane(III), and tridentate Schiff base chromium(III) complexes can catalyze highly diastereo- and enantioselective cycloaddition reactions. The mechanism of these reactions can be a stepwise pathway via a Mukaiyama aldol intermediate or a concerted mechanism. For a-dicarbonyl compounds, which can coordinate to the chiral catalyst in a bidentate fashion, the chiral BOX-copper(II)... 

Five-coordinate aluminum alkyls are useful as oxirane-polymerization catalysts. Controlled polymerization of lactones102 and lactides103 has been achieved with Schiff base aluminum alkyl complexes. Ketiminate-based five-coordinate aluminum alkyl (OCMeCHCMeNAr)AlEt2 were found to be active catalyst for the ring-opening polymerization of -caprolactone.88 Salen aluminum alkyls have also been found to be active catalysts for the preparation of ethylene carbonate from sc C02 and ethylene oxide.1 4 Their catalytic activity is markedly enhanced in the presence of a Lewis base or a quaternary salt. 

Catalytic activity of the aluminum-Schiff base system is dramatically enhanced by adding a bulky Lewis acid (Table 2). Inoue et al. reported that a combination of 3 with 2c led to over 1000 times acceleration in the polymerization of PO at room temperature compared with the polymerization in the absence of 2c.The resulting polymers have narrow MWDs, molecular weights close to those estimated, assuming that every molecule of 3 forms one polymer chain. The same accelerating effect of 2c is also demonstrated in the polymerization of PO by using aluminum-phthalocyanine and aluminum-tetraazaannulene complexes, 4 and 5, which exhibit very low catalytic activities without 2c. 

The key point of the high-speed living polymerization is the steric suppression of an undesired reaction between the nucleophilic growing species and the Lewis acid, for which not only the steric bulk of the Lewis acid but also that of the porphyrin ligand is considered important. The benefit of using a Lewis acid holds even for the aluminum complexes with phthalocyanine (11), tetraazaannulene (12), and. Schiff bases (13-15). As initiators, these complexes exhibit much lower activity for the polymerization of PO than aluminum porphyrin 1 (X=C1). 

As reported by Spassky et al. [62], aluminum complexes of Schiff bases as initiators exhibit much lower activities than aluminum porphyrins for the ringopening polymerization of epoxides. In fact, the polymerization of PO (500 equiv) using a Schiff base complex (Salphen)AlCl (13) as initiator proceeded extremely slowly at room temperature to attain only 4% conversion in 8 d. Even at 80 °C, the polymerization was slow, and required 6 d for completion, affording a polymer with broad and bimodal MWD (Fig. 32A). 

In a number of nonenzymatic reactions catalyzed by pyridoxal, a metal ion complex is formed—a combination of a multivalent metal ion such as cupric oi aluminum ion with the Schiff base formed from the combination of an amino acid and pyridoxal (I). The electrostatic effect of the metal ion, as well as the electron sink of the pyridinium ion, facilitates the removal of an a -hydrogen atom to form the tautomeric Schiff base, II. Schiff base II is capable of a number of reactions characteristic of pyridoxal systems. Since the former asymmetric center of the amino acid has lost its asymmetry, donation of a proton to that center followed by hydrolytic cleavage of the system will result in racemic amino acid. On the other hand, donation of a proton to the benzylic carbon atom followed by hydrolytic cleavage of the system will result in a transamination reaction—that is, the amino acid will be converted to a keto acid and pyridoxal will be converted to pyridoxamine. Decarboxylation of the original amino acid can occur instead of the initial loss of a proton. In either case, a pair of electrons must be absorbed by the pyridoxal system, and in each case, the electrostatic effect of the metal ion facilitates this electron movement, as well as the subsequent hydrolytic cleavage (40, 43). 

A series of pyridoxal amino acid Schiff base complexes have been prepared in which Al is trigonally coordinated by N and two O atoms.415 These provide a model for the intermediate in the pyridoxal-catalyzed reactions of amino acids. Some Schiff base complexes produced in reactions of the bases with Al(OPri)3 have been assigned a structure with five-coordinate aluminum.416417... 

The assymetric Strecker reaction of diverse imines, including aldimines as well as ketoimines, with HCN or TMSCN provides a direct access to various unnatural and natural amino acids in high enantiomeric excesses, using soluble or resin-linked non-metal Schiff bases the corresponding chiral catalysts are obtained and optimized by parallel combinatorial library synthesis [93]. A rather general asymmetric Strecker-type synthesis of various imines and a, 9-unsaturated derivatives is catalyzed by chiral bifunctional Lewis acid-Lewis base aluminum-containing complexes [94]. When chiral (salen)Al(III) complexes are employed for the hydrocyanation of aromatic substituted imines, excellent yields and enatio-selectivities are obtained [94]. 

Several metallic species other than titanium have been reported. Kobayashi showed that the tin(II) complex (16, Fig. 4) modified by cinchonine, a natural alkaloid, catalyzed the reaction to give the chiral cyanohydrin of cyclohexanecar-baldehyde in 90% ee [54]. Corey applied a magnesium complex of chiral bisox-azoline (17, Fig. 4) to the asymmetric silylcyanation. High selectivity of up to 95% ee was observed in the reactions of aliphatic aldehydes compared with benzaldehyde (52% ee) [55]. An aluminum complex of a peptide containing the phenolic Schiff base (7) was shown by Mori and Inoue to be an efficient catalyst for the addition of TMSCN to aldehydes [56,57]. 

Polystyrenes have also been used to support chromophores useful in organic light-emitting diodes (OLEDs). Week and coworkers have attached tris(2-phenylpyridine) iridium complexes to aminomethylated polystyrene using a Schiff base reaction, 4 [21]. There was no major diminution of the desirable luminescence properties of the iridium complexes (high emission quantum yields of 0.23 and lifetimes of about a microsecond). Similar results have been reported for aluminum and boron 8-hydroxy quinoline complexes tethered to polystyrene using Schiff base condensation [22]. 

C, Pm = 0.80). In 2007, Nomura et al. reported the synthesis of Schiff base aluminum complex 4 [88] with flexible but bulky BuMe2Si substituents, which exhibited the highest isoselectivity in the ROP of rac-LA to form isotactic stereoblock PLA materials with a Pm value of 0.98 and a of 210°C. More recently, highly active yttrium phosphasalen initiators were reported for the stereocontrolled ROP of rac-lactide [89]. Changing the phosphasalen structure enables access to isoselectivities (Pm = 0.84) or hetero-selectivities (P, = 0.87) (Fig. 1). 

Although living anionic pol5unerization seemed to be the versatile method for the preparation of telechelic from oxiranes, with substituted oxiranes some side reactions leading to unsaturated species occur (reaction 93). An alternative initiating system based on aluminum complexes of Schiff bases was proposed for the preparation of o, (w-telechelics form differently substituted oxiranes (284-286). 

Vincens, V. Le Borgne, A. Spassky, N. Stereoelective ohgomeiization of methyloxirane with a chiral aluminum complex of a Schiff s base as initiator. Makmmol. Chem., Rapid Commun. 1989, 10, 623-628. 

Le Borgne, A. Vincens, V Jouglard, M. Spassky, N. Ring-opening oligomerization reactions using aluminum complexes of Schiff s bases as initiators. Makromol. Chem., Macromol. Symp. 1993, 73, 37 6. 

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