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Asymmetric Reduction of Ketones with Catecholborane Using 2,6-BODOL Complexes of Titanium(IV) as Catalysts
Authors:Ian Sarvary  Fredrik Almqvist  Torbjörn Frejd
Institution:1. Organic Chemistry Department of Chemistry, Lund University P.O. Box 124, 221 00 Lund (Sweden) Fax: (+46) 46-222-41-19;2. Department of Organic Chemistry Umeå University, 901 87 Umeå (Sweden)
Abstract:Reductions performed with TiIV complexes of ligands based on bicyclo2.2.2]octane diols 5 and 6 are effective catalysts in the reduction of prochiral ketones to optically active alcohols, with catecholborane as the reducing agent. Methyl ketones are favored and enantiomeric excesses (ee) of ≤98 % have been achieved with acetophenone as the substrate. Several other substrates were tested, among them 2-octanone, which gave 2-octanol in 87 % ee. Further details of the method were examined, for example, temperature, solvent composition, amount of molecular sieves (4 Å), and catecholborane quality, as well as the sensitivity of the ligands towards acids. NMR spectroscopic methods were used to gain some insight into the complexes formed between the ligands and Ti(OiPr)4]. A dimeric structure is proposed for the pre-catalyst.
Keywords:asymmetric catalysis  diol ligands  ketones  reduction  structure elucidation  titanates
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