Chiral complexes of titanium containing a linked amido-cyclopentadienyl ligand: synthesis, structure, and asymmetric imine hydrogenation catalysis

Abstract

A series of mono- and disubstituted derivatives (−)-( S)-Ti(η 5:η 1-C 5Me 4SiMe 2NCHMePh)(X)Cl (X=CH 2SiMe 3, BH 4) and (−)-( S)-Ti(η 5:η 1-C 5Me 4SiMe 2NCHMePh)X 2 (X=OSO 2CF 3, O iPr, Me, CH 2Ph) was prepared from (−)-( S)-Ti(η 5:η 1-C 5Me 4SiMe 2NCHMePh)Cl 2 without significant racemization at the stereogenic center. The monosubstituted complexes are formed as mixtures of diastereomers. One diastereomeric monoalkyl ( S Ti, S C)-Ti(η 5:η 1-C 5Me 4SiMe 2NCHMePh)(CH 2SiMe 3)Cl was characterized by X-ray single crystal structure analysis. When the (−)-( S)-NCHMePh group is attached to planar chiral ring moieties 3- t BuC 5H 3, C 9H 6, and C 9H 5(SiMe 3)-3 and coordinated at the titanium center, diastereomeric mixtures are formed. A series of titanium complexes Ti(η 5:η 1-C 5R 4SiMe 2NR')Cl 2 (R=H, Me; R′=CHMeC 10H 7, CHMeCMe 3, CHPhCMe 3, CHMeC 6H 11, (1 S)-pinanyl-3, (1 R)-bornyl-2) containing an enantiomerically pure linked amido-cyclopentadienyl ligands were synthesized and characterized by 1H- and 13C-NMR spectroscopy, mass spectrometry, and elemental analysis. The crystal structure of a three-legged piano-stool molecule was determined for (+)-(1 S)-Ti(η 5:η 1-C 5Me 4SiMe 2NCH 2pinanyl-3)Cl 2 by a single-crystal X-ray diffraction study. Upon activation with n-butyllithium a selection of these dichloro complexes catalyzed the hydrogenation of acetophenone N-benzylimine with good conversions for R=H, but with low enantioselectivities.