Salen-Based Systems

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... 

Very few methods developed recently are applicable to the cyanation of not only electron-rich but also electron-deficient aromatic, as well as aliphatic aldehydes. Titanium(IV)-derived complex 1 (Fig. 1), reported by Uang, catalyzes the hydro cyanation of aromatic, a, 3-unsaturated, and aliphatic aldehydes with high enantioselectivity (>88% ee for all substrates reported) [15]. Similarly, Ti(IV)-catalyst 2 developed by Choi has proved to be highly enantioselec-tive for the cyanation of various classes of aldehydes (>90% ee for most substrates) [16]. Belokon and co-workers reported two chiral salen-based systems (salen)VO catalyst 3a and [(salen)TiO]2 3b, both of which provided moderate levels of enantioselection when applied to the asymmetric cyanation of diverse... 

Currently, the most efficient catalysts in terms of activity and enantioselectivity are binaphthol- and salen-based systems. The former is exemplified by a combination of BINOL and magnesium iodide, which catalyzes the conversion of prochiral ketones into enantiomerically enriched lactones with up to 65% ee [346]. When BINOL derivatives are combined with organoaluminum reagents, enantioselectivities of up to... 

Zhu and coworkers have applied another Ga-based system to the desymmetriza-tion of meso epoxides using acetylide anions as nucleophiles. A complex generated from Mc3Ga and a novel salen provides modest selectivities and yields using phenylacetyhde as the nucleophile ... 

Besides mono-Cp systems,958,959,973-989 other half-sandwich complexes with different aromatic ligands were investigated.969-971,982,983,985,990-993 Other studies were concerned with polymerization conditions, including catalyst heterogenization.990,994-1006 Other catalysts such as mono-276 and bis-benzamidinate1007 complexes, bis(phenolato) Ti complexes, 8 and salen-based Ti complexes,1009 are also able to polymerize styrene to a... 

Immobilization of a sulfonated chiral manganese-salen catalyst on a fimctio-nalized Merrifield resin yielded a remarkably active epoxidation catalyst [42]. Its activity and enantioselectivity was examined by epoxidation of 6-cyanochromene, indene, styrene, 4-methylstyrene, and trans-stilbene using m-CPBA/NMO and quantitative yields were obtained in less than 5 min. Enantioselectivities were between 33% (4-methylstyrene) and 96% ee (6-cyanochromene). The same complex was also supported on silica and a layered double hydroxide (LDEI) and the catalytic performances of the systems were compared. Recycling experiments were carried out and the silica-based system showed metal leaching combined with a significant decrease in yield and ee. The layered double hydroxide- and resin-catalysts exhibited a slight decrease in activity and constant ee values in five consecutive reactions. 

Since the Co(II) to Co(III) complexes were redox active, an electrochemical method of analysis seemed viable for the quantification of the two species in the reaction. The specific electrochemical technique developed to monitor the activation reaction allowed the simultaneous quantitative measurement of (salen)Co(II) and (salen)Co(III) species in the medium. The principle of the method is based on the electro-oxidation of both species on a platinum-rotating electrode linearly polarized with respect to a standard electrode [7]. The electrochemical reactions operative with this cyclic voltammetry technique involve the single electron oxidation of each species and occur at the revolving surface of the electrode. With this salen ligand system, the Co(II) to Co(III) transformation was determined as being fully reversible, while the Co(III) to Co(IV) reaction was irreversible. 

The use of salen-based aluminum catalysts for asymmetric hydrophosphonylation reactions has been reported however, the selectivity tended to be low [199, 200]. Katsuki modified the structure of the salen ligand and developed a catalyst system for... 

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