Highly Enantioselective Transfer Hydrogenation of Polar Double Bonds by Macrocyclic Iron(II)/(NH)2P2 Catalysts

We describe herein a new protocol for the synthesis of 2,2′-((1S,1′S)-ethane-1,2-diylbis­(phenylphosphanediyl))­dibenzaldehyde ((SP,SP)-5), which is the key intermediate in the synthesis of macrocyclic iron­(II)/(NH)2P2 catalysts for the highly enantioselective transfer hydrogenation of polar double bonds. The dialdehyde (SP,SP)-5 was obtained as a single diastereoisomer and enantiomer from an optically pure H-phosphinate in 33% yield over five steps. It was further converted to afford multigram quantities of the macrocyclic iron­(II)/(NH)2P2 complexes, which were tested in the asymmetric transfer hydrogenation of aryl alkyl ketones and imines in 2-propanol on a 100 mmol scale. Ten substrates, including challenging ones such as tert-butyl phenyl ketone and industrially relevant molecules such as 3,5-bis­(trifluoromethyl)­acetophenone, were reduced in high yield (89.0–99.7%), excellent enantioselectivity (95.8–99.4% ee), and with low catalyst loadings (S/C up to 10 000/1).