New chiral lithium aluminum hydrides based on biphenyl-2,2′-bisfenchol (BIFOL): structural analyses and enantioselective reductions of aryl alkyl ketones

A series of new chiral lithium aluminum hydrides based on BIFOL (biphenyl-2,2′-bisfenchol) and various alkyl alcohols (i.e., methanol, n-butanol, tert-butanol yielding BIFAl-H's) was synthesized and characterized by single crystal X-ray analyses. These investigations point to alkoxide redistribution for BIFAl-H-(O-tBu) (biphenyl-2,2′-bisfenchol aluminum hydride) species. The new BIFAl-H reagents are suitable to reduce aryl alkyl ketones with up to 62% ee. Computational transition structure analyses help to explain the experimentally observed enantioselectivities.