Transfer Hydrogenation of Flavanones and ortho‐Hydroxychalcones to 1,3‐Diarylpropanols Catalyzed by CNN Pincer Ruthenium Complexes

The transfer hydrogenation of flavanones and ortho‐hydroxychalcones catalyzed by ruthenium pincer complexes RuCl(CNNPh)(disphosphine) has allowed the synthesis of ortho‐hydroxy 1,3‐diarypropanols in 80–88 % yield, under mild reaction conditions and short reaction times (1 h) in 2‐propanol. The amount of the co‐catalyst NaOiPr has been found crucial for the selective reduction of flavanones to ortho‐hydroxy 1,3‐diarypropanols vs. flavan‐4‐ols. Preliminary results show that with pincer catalysts bearing (S,R)‐Josiphos, flavanone is reduced to the corresponding (S)‐alcohol in moderate conversion (36 %) and up to 92 % ee. Feeling the pinch: Efficient reduction of flavanones and 2’‐hydroxychalcones to 1,3‐diarylpropanols has been achieved with the pincer ruthenium catalysts RuCl(CNN)(diphosphine) with NaOiPr in 2‐propanol via transfer hydrogenation. The amount of base has been found to be a crucial parameter for the selectivity of this reactions. Preliminary results show that with chiral pincer catalysts, flavanone is reduced with 92 % of ee