Heteroatom-based bidentate ligands are ubiquitous in the field of asymmetric catalysis. However, only a few ligands that coordinate to a transition metal through a carbonyl oxygen atom have been reported. Tomioka and co-workers have extensively studied the chiral amidophosphine L A 1 (see Scheme 1 for structure) in combination with rhodium or copper in the asymmetric Michael addition of a variety of organometallic reagents to a,b-unsaturated carbonyl compounds with moderate to excellent enantioselectivities. [1][2][3][4] In the coordination to rhodium(I), this P,O ligand was shown to behave as a hemilabile ligand, in which the amide carbonyl group is weakly bound to the transition metal.[1] More recently Reek et al. have published on a rhodium-based phosphine urea P,O-ligand system which gives moderate enantioselectivity in asymmetric hydrogenation. [5,6] Iridium complexes with chiral N,P ligands have emerged as highly efficient catalysts for the asymmetric hydrogenation of olefins. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] In contrast to rhodium and ruthenium diphosphine catalysts, they do not require a coordinating group near the C = C bond and, therefore, can be applied to a wide range of functionalized and unfunctionalized olefins. However, there are still important substrate classes that give unsatisfactory results with known catalysts. Therefore, the search for new ligands that could fill these methodological gaps continues.In a broad automated screening of various metal/ligand combinations, an iridium complex formed in situ from the ligand L A 1 and [Ir(cod)] 2 BAr F (cod = 1,5-cyclooctadiene; BAr F = tetrakis[bis-3,5-(trifluoromethyl)phenyl]borate) gave promising enantioselectivities of up to 68 % ee in the hydrogenation of (E)-1,2-diphenylprop-1-ene. This result was unexpected considering the assumed lability of the Ir À O bond. The ready accessibility and the modular nature of the ligand L A 1 prompted us to synthesize a diverse library of proline-derived P,O ligands by systematic variation of the substituents on the P and N atoms to evaluate their potential in the iridium-catalyzed asymmetric hydrogenation (Scheme 1).As described by Tomioka and co-workers, a linear approach was used to synthesize these P,O ligands with introduction of the phosphine moiety prior to the substituents at the N atom (Scheme 1). [4a-d] Starting from (S)-tert-butyl-2-(bromomethyl)pyrrolidine-1-carboxylate (1) precursors 2 were obtained by nucleophilic substitution with various metallated phosphines.[25] After removal of the N-Boc protecting group, the free amine was reacted with acetyl chlorides, isocyanates, or carbamoyl chlorides, allowing access to a variety of P,O ligands. The introduction of the N substituents generally proceeded in high yield for both, amides (43-98 %) and ureas (61-93 %). Oxygen-sensitive compounds such as di-tert-butyl-or dicyclohexylphosphines were protected as borane adducts to avoid oxidation. Scheme 1. Synthesis of the l-proline-based P,O ligands. 1a) KPPh 2 , THF for ...