The chiral-recognition processes of homoproline (hpro) and [Ir-(pq) 2 (MeCN) 2 ](PF 6 ) (pq is 2-phenylquinoline; MeCN is acetonitrile) are investigated, in favor of formation of the thermodynamically stable diastereomers Λ-[Ir(pq) 2 (D-hpro)] and Δ-[Ir(pq) 2 (L-hpro)]. Moreover, the diastereoselective photoreactions of Δ-[Ir(pq) 2 (D-hpro)] and Δ-[Ir(pq) 2 (L-hpro)] are reported in the presence of O 2 at room temperature. Diastereomer Δ-[Ir(pq) 2 (L-hpro)] is dehydrogenatively oxidized into imino acid complex Δ-[Ir(pq) 2 (hpro-2H 2 )] (hpro-2H 2 is 3,4,5,6-tetrahydropicalinate), while diastereomer Δ-[Ir(pq) 2 (D-hpro)] occurs by interligand C−N cross-coupling and dehydrogenative oxidation reactions, affording three products: Δ-[Ir(pq)(D-pqh)] [pqh is N-(2-phenylquinolin-8-yl)homoproline], Δ-[Ir(pq) 2 (hpro-2H 2 )], and Δ-[Ir(pq) 2 (D-hpro-2H 6 )] [hpro-2H 6 is 2,3,4,5-tetrahydropicalinate]. The C−N cross-coupling and dehydrogenative oxidation reactions are competitive, and the dehydrogenative oxidation reactions are regioselective. By optimization of the photoreaction parameters such as the diastereomeric substrate, solvent, and temperature as well as base, each possible competitive product is selectively controlled. In addition, density functional theory calculations are performed to elucidate the distinctly chiral recognition between proline and hpro with an iridium(III) complex.

A diastereoselective photooxidation of α-amino acid (AA) complexes into imino acid complexes using a chiral iridium(III) complex as a photosensitizer and stereo-controller under visible light irradiation and oxygen was developed. It was found that the oxidative rate of Δ-[Ir(pq) 2 (L-AA)] (pq is 2-phenylquinoline) diastereomer is significantly higher than that of the corresponding Δ-[Ir(pq) 2 (D-AA)] diastereomer, providing a new protocol for kinetic resolution of AAs via a nonenzymatic pathway. Moreover, the thermodynamic controlled strategy offered a complemental method for the diastereoselective hydrogenation of imine bonds using NaBH 4 as a reductant under the chiral Ir(III) complex as a stereocontroller. The combination of diastereoselective photooxidation and reduction processes results in a new protocol for deracemization of α-amino acids under mild conditions. Mechanism study strongly indicates that singlet oxygen is a key participant in the reaction and the α-C−H bond cleavage of AAs is the rate-determining step.

The postcoordinated interligand-coupling strategy provides a useful and complementary protocol for synthesizing polydentate ligands. Herein, diastereoselective photoreactions of Λ-[Ir­(pq)2(d-AA)] (Λ-d ) and Λ-[Ir­(pq)2(l-AA)] (Λ-l , where pq is 2-phenylquinoline and AA is an amino acid) are reported in the presence of O2 under mild conditions. Diastereomer Λ-d is dehydrogenatively oxidized into an imino acid complex, while diastereomer Λ-l mainly occurs via interligand C–N cross-dehydrogenative coupling between quinoline at the C8 position and AA ligands at room temperature, affording Λ-[Ir­(pq)­(l-pq-AA)]. Furthermore, the photoreaction of diastereomer Λ-l is temperature-dependent. Mechanistic experiments reveal the ligand–radical intermediates may be involved in the reaction. Density functional theory calculations were used to eluciate the origin of diastereoselectivity and temperature dependence. This will provide a new protocol for the amination of quinoline at the C8 position via the postcoordinated interligand C–N cross-coupling strategy under mild conditions.

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