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| - Catalysis of the Michael Addition Reaction by Late TransitionMetal Complexes of BINOL-Derived Salens
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| Abstract
| - Salen metal complexes incorporating two chiral BINOL moieties have been synthesized andcharacterized 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 andnaphthoxides that are independent from the Lewis acid center. These characteristics allowindependent alteration of the Lewis acidic and Brønsted basic sites. These unique complexes havebeen shown to catalyze the Michael reaction of dibenzyl malonate and cyclohexenone with goodselectivity (up to 90% ee) and moderate yield (up to 79% yield). These catalysts are also effectivein the Michael reaction between other enones and malonates. Kinetic data show that the reactionis first order in the Ni·Cs-BINOL-salen catalyst. Further experiments probed the reactivity of theindividual Lewis acid and Brønsted base components of the catalyst and established that bothmoieties are essential for asymmetric catalysis. All told, the data support a bifunctional activationpathway in which the apical Ni site of the Ni·Cs-BINOL-salen activates the enone and thenaphthoxide base activates the malonate.
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