The Atropo-Enantioselective Reduction of Configurationally Unstable Biaryl Hydroxy Aldehydes - A Novel Approach to Axially Chiral Biaryls

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Abstract
The oxazaborolidine-assisted atropo-enantioselective catecholborane reduction of configurationally unstable biaryl hydroxy aldehydes to axially chiral biaryl alcohols by (dynamic) kinetic resolution is achieved in enantiomeric ratios (er) of up to 92:8. Using the same chiral auxiliary, the M- or, optionally, the P-configurated atropisomer can be obtained in good er's - just by variation of the relative quantity of the achiral reductant. This hints at the existence of two competing reaction pathways with opposite asymmetric inductions. The enantiomeric ratios observed strongly depend on the relative steric demand of the substituents ortho to the biaryl axis.

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