TitaniumLewis Acids for Asymmetric Catalysis: Synthesis and Structural Characterization of Dichloro[diolato(2−)-κO,κO′]bis(solvent)titanium ([TiCl2(diolato)(solvent)2]) Complexes

Abstract

The complexes [TiCl2{(R,R)-TADDOLato}(DME)]⋅MeCN (3), and [TiCl2{(R,R)-1-Nph-TADDOLato}(MeCN)2]⋅CH2Cl2 (4b) (DME=1,2-dimethoxyethane; (R,R)-TADDOLato=(4R,5R)-2,2-dimethyl-α,α,α′,α′-tetraphenyl-1,3-dioxolane-4,5-dimethanolato(2−)-κO,κO′; (R,R)-1-Nph-TADDOLato=(4R,5R)-2,2-dimethyl-α,α,α′,α′-tetra(naphthalen-1-yl)-1,3-dioxolane-4,5-dimethanolato(2−)-κO,κO′) were prepared and isolated in high yield as stable crystalline materials (Scheme 1). They constitute ideally suited and easy-to-handle catalyst precursors for a large number of Ti-catalyzed asymmetric reactions, for which they have been previously generated in situ. The X-ray crystal structures of 3 and 4b show a distorted octahedral geometry around Ti with the chloro ligands in mutual trans positions (Figs. 5 and 6). The new chiral diols α-(1S,3R)-3-hydroxy-2,2,3-trimethylcyclopentyl]-α-phenylbenzenemethanol (13a), derived from camphoric acid (5), and (M)-6,6′-dimethyl-α,α,α′,α′-tetraphenyl[1,1′-biphenyl]-2,2′-dimethanol (15) were prepared (Schemes 3 and 4). These new ligands are able to form mononuclear complexes with the TiIVCl2 fragment. The corresponding complex 14 derived from 13a was characterized by X-ray as a mixed THF/MeCN adduct.