Abstract
The asymmetric reduction of ketones and imines by transfer of hydrogen from isopropanol as the solvent catalyzed by metal complexes is a very useful method for preparing valuable enantioenriched alcohols and amines. Described here is the development of three generations of progressively more active iron catalysts for this transformation. Key features of this process of discovery involved the realization that one carbonyl ligand was needed (as in hydrogenases), the synthesis of modular ligands templated by iron, the elucidation of the mechanisms of catalyst activation and action, as well as the rational synthesis of precursors that lead directly and easily to the species in the catalytic cycle. The discovery that iron, an abundant element that is essential to life, can form catalysts of these hydrogenation reactions is a contribution to green chemistry.
Highlights
Green chemistry Homogeneous catalyst development has traditionally focused on the more active second- and third-row transition metals in combination with simple ligands.[1]
Replacement of one acetonitrile ligand with CO, gave precursors (6a and 7a) to extremely active asymmetric transfer hydrogenation (ATH) catalysts.[46]. This second generation of catalysts showed marked improvements over the original 6,5,6 systems in both activity (TOF of 28 000 h−1 at 28 °C) and selectivity (up to 82% enantioselectivity for acetophenone reduction to (S)-1-phenylethanol) when the (R,R)-catalyst 7a derived from the diamine (R,R)-dpen was used.[46]
The reduced tridentate ligand was mixed with half an equivalent of phosphonium dimer (1a, 1e or 1g) as well as base to produce tetradentate bis(acetonitrile)(P–N–NH–P)iron(II) compounds.[63]. These bis(acetonitrile) species were found to be inactive for ATH, and as such a ligand exchange was performed to generate the corresponding trans carbonyl-chloride complexes 28–30 (Fig. 23)
Summary
Green chemistry Homogeneous catalyst development has traditionally focused on the more active second- and third-row transition metals in combination with simple ligands.[1]. The reduced tridentate ligand was mixed with half an equivalent of phosphonium dimer (1a, 1e or 1g) as well as base to produce tetradentate bis(acetonitrile)(P–N–NH–P)iron(II) compounds.[63] These bis(acetonitrile) species were found to be inactive for ATH, and as such a ligand exchange was performed to generate the corresponding trans carbonyl-chloride complexes 28–30 (Fig. 23) This synthesis provides highly active precatalysts in acceptable yields.[63] The single crystal X-ray diffraction structure of precatalyst 29 confirmed the presence of the amine and imine functionalities based on bond lengths, and indicated that the amino proton and chloro ligand were located on opposite sides of the plane defined by the PNNHP ligand.[63] Remarkably, 31P NMR indicated that only one diastereomer of the precatalyst was formed. It is important to note that this synthesis tolerated a large degree of variability, namely changes in the
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