The molecular and crystal structure of Shibasaki’s (R)-ALB chiral catalyst

Abstract

Shibasaki’s heterobimetallic compound [LiAl{()-BINOL}] and its enantiomer, often referred to as ()- and ()-ALB, respectively, are used as catalysts in asymmetric synthesis (BINOLH = 1,1′-bi(2-naphthol)). Crystalline ()- and ()-ALB are conveniently prepared by reacting LiAlH with 2 equiv. of ()- or ()-BINOLH, respectively, in THF and evaporating the solvent under vacuum. However, their molecular and crystal structures have never been reported in detail. We herein show that the as-crystallized ()-ALB material is a single-phase THF solvate containing [Al{()-BINOL}] anions, [Li(thf)] fragments bonded to one of the binolate O atoms, and a lattice THF molecule. This ion-paired structure was authenticated by single-crystal X-ray diffraction and then used as a starting point for the Rietveld refinement of the powder diffraction pattern. Crystallographic and NMR characterization data converge in indicating that the three THF molecules bonded to the Li ion withstand vacuum treatment. On NMR timescale, a single structure is detected in THF- solution, whereas CD solutions give more complex, multicomponent spectra that hint to aggregation effects.

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