Stereoelectronic Factors in Iron Catalysis: Synthesis and Characterization of Aryl-Substituted Iron(II) Carbonyl P–N–N–P Complexes and Their Use in the Asymmetric Transfer Hydrogenation of Ketones

A series of five (S,S)-trans-[Fe(CO)(Br)(PR2-CH2CH═NCH(Ph)CH(Ph)N═CHCH2-PR2)][X] compounds (1a–c, X = BPh4; 1d,e, X = BF4) were synthesized and tested for the asymmetric transfer hydrogenation (ATH) of acetophenone. Three of the complexes had methyl-substituted aryl groups (a, R = para-CH3C6H4; b, R = ortho-CH3C6H4; c, R = 3,5-(CH3)2C6H3), and two had trifluoromethyl-substituted aryl groups (d, R = para-CF3C6H4; e, R = 3,5-(CF3)2C6H3). Using both known and new phosphonium dimers, [cyclo-(PR2CH2CH(OH)−)2][Br]2 (2a–c), in a one-pot template reaction, the corresponding (S,S)-trans-[Fe(CH3CN)2(PR2-CH2CH═NCH(Ph)CH(Ph)N═CHCH2-PR2)][BPh4]2 complexes (3a–c) were generated and then converted to precatalysts 1a–c via CO addition reactions. While investigating compounds 1a–c, an alternative route for synthesizing phosphonium dimers was developed that allowed the facile introduction of tetrafluoroborate counterions. Compounds 1d and 1e could not be synthesized using previously developed methods; phosphinoacetaldehyde...