CHEMISTRY: Pinned Propeller

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Science  12 Jan 2007:
Vol. 315, Issue 5809, pp. 162d-163d
DOI: 10.1126/science.315.5809.162d

Many coordination complexes have been prepared with threefold symmetry. However, exploiting chirality in such compounds tends to be challenging, in some cases because labile ligands scramble their orientation about the metal center, and in others because there is no feasible means of asymmetric induction in the synthesis, which therefore affords a racemic product mixture that must be laboriously resolved. Most chiral catalysts instead rely on a twofold symmetric motif.

Axe et al. have used an embedded ligand stereocenter to direct and enforce the threefold helical chirality of a tris(phenolate) titanium complex. Their tetradentate ligand consists of a central nitrogen atom bound through benzylic carbons to three alkyl-substituted phenol rings. One of these benzylic carbons also bears a methyl group in an enantiopure configuration. When the ligand reacts with a Ti(IV) precursor, the phenyl rings fan out in a propeller arrangement around the metal, and the methyl group induces a specific helical cant. Nuclear magnetic resonance spectroscopy confirmed exclusive formation of a single diastereomer in solution. The structure was likewise characterized in the solid state by x-ray crystallography. — JSY

Org. Lett. 10.1021/ol062655w (2006).

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