Asymmetric reduction of ketones with catecholborane using 2,6-BODOL complexes of titanium(IV) as catalysts

Reductions performed with Ti-IV complexes of ligands based on bicyclo[2.2.2 ]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 less th an or equal to 98% have been achieved with acetophenone as the substrate. S everal other substrates were tested, among them 2-octanone, which gave 2-oc tanol in 87% ee. Further details of the method were examined, for example, temperature, solvent composition, amount of molecular sieves (4 Angstrom), and catecholborano quality, as well as the sensitivity of the ligands towar ds acids. NMR spectroscopic methods were used to gain some insight into the complexes formed between the ligands and [Ti(OiPr)(4)]. A dimeric structur e is proposed for the pre-catalyst.