Ruthenium-catalyzed reduction of racemic tricarbonyl(η6-aryl ketone)chromium complexes using transfer hydrogenation: A simple alternative to the resolution of planar chiral organometallics

Racemic planar chiral (η6-aryl ketone)Cr(CO)3 complexes (aryl ketone = 1-indanone, 1-tetralone, 4-chromanone and thiochroman-4-one) were prepared by refluxing the aryl ketone with Cr(CO)6 in a 10:1 mixture of dibutyl ether and THF. The reductions of the organometallic ketones by transfer hydrogenation in 2-propanol containing KOH and the catalyst precursor, generated from [RuCl2(η6-benzene)]2 and (−)-ephedrine, resulted in optically active syn-(R,S)-(η6-aryl alcohol)Cr(CO)3 and (R)-(η6-aryl ketone)Cr(CO)3 compounds in 31–97% ee. Reduction of racemic (η6-thiochroman-4-one)Cr(CO)3 with the catalyst precursor generated from (+)-norephedrine, instead of (−)-ephedrine, inverted the configuration of the products obtained. Syn-(S,R)-(η6-thiochroman-4-ol)Cr(CO)3 and (S)-(η6-thiochroman-4-one)-Cr(CO)3 were isolated in 49% and >95% ee, respectively. The free aryl ketones were reduced using the same conditions as their respective chromium complexes, giving aryl alcohols in high ee (>95%). Reactions of non-rigid acetophenone, propriophenone and their tricarbonylchromium complexes resulted in moderate to low ee.