Lithium Binaphtholates: Chiral Chains and Clusters

Abstract

Binaphthol (H 2 binol) is chiral by virtue of restricted rotation about the C À C bond linking the two naphthyl units, and the absolute configuration of (R)binaphthol (1) was first established unequivocally by X-ray diffraction in 1968. [1] The flexibility of the dihedral angle between the two naphthyl rings of this C 2symmetric molecule allows coordination to a wide range of metal centers (examples are known from 458 to 1108), and H 2 binol has been used as a chiral auxiliary in many metal-based enantioselective catalysts and reagents. Examples include the Yb(OTf) 3 -catalyzed asymmetric Diels ± Alder reaction, [2] the TiCl 4 -derived catalyst for the asymmetric ene reaction, [3] and the chiral modification of LiAlH 4 for use in asymmetric reductions. [4] Multi-binaphthol species have attracted recent interest; particularly successful examples are Mikamis Ti ± binaphtholate catalysts [5] and the heterometallic alkali metal/ lanthanide binaphtholates synthesised by Shibasaki et al. [6] Examples of catalytically active multi-binaphthol Ti clusters have also been reported. [7,8] In addition to these applications in asymmetric synthesis, binaphthol has also been of interest as a component of macrocycles for chiral recognition, [9,10] and as a component of helical polymers with novel properties such as the emission of polarized light. [11] In view of this widespread interest in binaphthol-containing species, it is surprising that no structural studies of the lithiated derivatives have been reported (inclusion compounds of rac-H 2 binol with NaOH and KOH have been structurally characterized [12] ). Lithiated H 2 binol has been used in the preparation of many enantioselective catalysts and has also been investigated as a chiral auxiliary in its own right. [13] Our interest in lithium binaphtholates arose from our use of LiHbinol in the synthesis of anhydrous analogues of Shibasakis heterometallic catalysts, [14] and herein we report on investigations of mono-and dilithiated derivatives of optically pure and racemic H 2 binol. The previously overlooked structures of these compounds show unexpected diversity. (5) were prepared by reaction of H 2 binol with stoichiometric quantities of nBuLi in THF at 0 8C. All of these lithiated derivatives were precipitated as microcrystalline solids from THF solutions by the addition of petroleum ether, and we were able to grow X-ray-quality single crystals of 2, 3, and 4 by the slow diffusion of petroleum ether into the THF solutions at room temperature. Despite numerous attempts and using a range of precipitating solvents, good quality single crystals of 5 could not be obtained from THF solutions. Lithiated H 2 binol compounds [Li(rac-Hbinol)] (2), [Li{(R)-Hbinol}] (3), [Li 2 (rac-binol)] (4), and [Li 2 {(R)-binol}]The monolithiated species 2 and 3 both crystallize as helical polymeric chains with Hbinol À units linked by [Li(thf) 2 ] bridges, as shown for 2 in Figures 1 and 2. The chain structure of 3 is very similar, but less symmetrical, containing two inequivalent Li atom...