The solution structures of chiral Ti4+ alkoxides

Abstract

1,4-Di-O-methyl-L-threitol and 1,2,5,6-tetra-O-methyl-L-mannitol were prepared by new, more convenient, and higher yielding routes. These and 1,2;5,6-di-O-isopropylidene-D-mannitol formed 2:2 complexes with Ti(OiPr)4, but did not readily form 1:2 complexes in the presence of excess Ti4+. 1H and 13C nuclear magnetic resonance spectra showed that the structures in solution of the 2:2 complexes were analogous to solid-state structures of tartaric acid derivatives, i.e., possessing bridging diolate oxygens. In contrast, R,R- and meso-diisopropyl tartrates, Ν,Ν′-dibenzyl tartramide, and Ν,Ν′-di(2-phenyl)ethyl tartramide behaved very differently. The tartramides formed mixtures of complexes that became dominated by 2:3 complexes in the presence of excess Ti(OiPr)4. Two tertiary tartramides failed to provide well-defined complexes. The R,R-tartrate formed a monocyclic (nonbridged) 2:2 complex, but an equilibrating mixture of 1:2 and 2:2 complexes with excess Ti4+. The equilibrium shifted upon cooling toward a 1:2 complex possessing a bridging diolate oxygen, as did the meso 1:2 complex. Other temperature-dependent phenomena were also studied. Keywords: titanium(IV), chiral alkoxides, solution structures, NMR, Sharpless catalysts.