P-Stereogenic PN(H)P Iron(II) Catalysts for the Asymmetric Hydrogenation of Ketones: The Importance of Non-Covalent Interactions in Rational Ligand Design by Computation

The P-stereogenic PN(H) P pincer ligands (R(Me) PCH2CH2)(2)NH (R= Cy, (S, S)-1a; R= tBu, (S, S)1b; R= Ph, (R, R)-1c) and their iron(II) derivatives [FeBr2(CO)(PN(H) P)] (2a-2c) and [FeHBr(CO)(PN(H) P)] (3a-3c) were developed by DFT-driven ligand design. In a preliminary study, the P(Cy) Me-based pincer (S, S)-1a and its Fe(II) complex 3a were prepared, tested in the asymmetric transfer hydrogenation of acetophenone, and studied by Density Functional Theory (DFT). Based on the good agreement between the experimental and calculated enantioselectivity, rational design of the pincer by DFT was attempted, which suggested high enantioselectivity for the tert-butyl and phenyl analogues 3b and 3c. Therefore, a new synthetic protocol was developed for (R, R)-1c using Buono's (S)-(1-(OH) Et) P(Me) Ph center dot BH3 as P-stereogenic synthon. Against the DFT prediction, 3c gave 1-phenylethanol with 44% ee, which was reproduced by increasing the level of theory from DFT to post-Hartree-Fock Moller-Plesset (MP2). This result can be explained by the overestimation of the enantiodeterming CH/pi interaction by DFT, which reiterates the need for accurate energies in the assessment of small energy differences such as in asymmetric catalysis.