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 diethyl acetal precursors (5d, 5e) were isolated because trifluoromethyl-substituted phosphonium dimers did not form. Precursors 5d and 5e were incompatible with a base-catalyzed template approach, so a new acid-catalyzed template procedure was developed to generate the tetrafluoroborate salts (S,S)-trans-[Fe(CH3CN)2(PR2-CH2CHNCH(Ph)CH(Ph)NCHCH2–PR2)][BF4]2 (3d, 3e). Both 3d and 3e were converted to precatalysts 1d and 1e via CO addition reactions. Complexes 1b, 1d, and 1e were inactive for the ATH of acetophenone, while complexes 1a and 1c were active. Compound 1a showed very high activity, with a turnover frequency of 30 000 h–1 at 28 °C, and is currently the most active iron ATH catalyst. Compound 1c produced more enantiopure (R)-1-phenylethanol, with an ee of 90%, and is the most selective iron catalyst reported to date for the ATH of acetophenone. The activity of complexes 1a–e for ATH was compared to those of known complexes 1f (R = Ph), 1g (R = Et), 1h (R = i-Pr), and 1i (R = Cy), and the most active catalysts were defined by a narrow range of electronic (ν(CO)) as well as steric (Tolman cone angles) parameters.