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ABSTRACT: Salen metal complexes incorporating two chiral BINOL moieties have been synthesized and characterized by X-ray crystallography. The X-ray structures show that this new class of Ni-BINOL-salen catalysts contains an unoccupied apical site for potential coordination of an electrophile and naphthoxides that are independent from the Lewis acid center. These characteristics allow independent alteration of the Lewis acidic and Brønsted basic sites. These unique complexes have been shown to catalyze the Michael reaction of dibenzyl malonate and cyclohexenone with good selectivity (up to 90% ee) and moderate yield (up to 79% yield). These catalysts are also effective in the Michael reaction between other enones and malonates. Kinetic data show that the reaction is first order in the Ni*Cs-BINOL-salen catalyst. Further experiments probed the reactivity of the individual Lewis acid and Brønsted base components of the catalyst and established that both moieties are essential for asymmetric catalysis. All told, the data support a bifunctional activation pathway in which the apical Ni site of the Ni*Cs-BINOL-salen activates the enone and the naphthoxide base activates the malonate.
The Journal of Organic Chemistry 04/2003; 68(5):1973-81. · 4.45 Impact Factor
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ABSTRACT: Metal salen complexes containing intramolecular bases have been designed for use as bifunctional catalysts. A ligand containing a quinoline base and the salen core has been synthesized for this purpose. Ruthenium, chromium, titanium, and zinc complexes of the salen have been formed and characterized. A crystal structure of a μ-oxo−titanium dimer incorporating the bifunctional ligand shows a typical salen coordination pattern and illustrates that the nitrogens of the quinoline groups participate in neither intra- nor intermolecular coordination. In addition, the quinoline groups orient in an appropriate manner to act as bases toward substrates coordinated at the apical positions of these complexes. As an indication that the quinoline bases can alter the activity of salen metal complexes, the addition of dialkylzincs to aldehydes was shown to be accelerated by the quinoline salen ligand relative to a salen lacking the quinoline groups.
01/2003;
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ABSTRACT: [formula: see text] The first catalytic, enantioselective addition of organoznic reagents to alpha-ketoesters is described. Modular bifunctional salen catalysts that contain Lewis acid and Lewis base activating groups accelerate the carbonyl addition to a much greater extent than the competing carbonyl reduction. alpha-Hydroxyesters containing new quaternary stereogenic centers are obtained in high yield and moderate enantiomeric excess. Enrichment to 98% ee can be effected by recrystallization of the corresponding alpha-hydroxy acid.
Organic Letters 11/2002; 4(22):3781-4. · 5.86 Impact Factor
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ABSTRACT: Lewis acid-Lewis base salen complexes have been identified as highly efficient catalysts for the addition of dialkylzincs to alpha-ketoesters. In contrast to aldehydes or ketones, the reaction between diethylzinc and alpha-ketoesters is significant in the absence of catalyst. In the presence of catalyst, the reaction rate is increased over 100-fold relative to the background. Furthermore, the reduction product, which is a major coproduct with other catalysts, is not observed with these bifunctional salens. As a result, high yields of the addition products can be obtained (57-99%). Both the Lewis acid and Lewis base portions of the catalyst are critical to the reactivity and selectivity. The two separate portions of the catalyst have been shown to function in a cooperative manner.
Journal of the American Chemical Society 11/2002; 124(43):12668-9. · 9.91 Impact Factor
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ABSTRACT: Salen metal complexes incorporating two chiral BINOL moieties have been synthesized. From a comparison of the parent salen and the BINOL−salen X-ray crystal structures, the effect of the BINOL portions has been examined. In particular, the effects of intramolecular hydrogen bonding on the salen core structure have been determined. The bis(naphthoxide) complexes arising from deprotonation of the BINOL−salen metal complexes have been found to catalyze the asymmetric addition of benzyl malonate to cyclohexenone in up to 90% ee.
02/2002;
