Discrepancy between Proline and Homoproline in Chiral Recognition and Diastereomeric Photoreactivity with Iridium(III) Complexes

Abstract

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.

Remote control: stereoselective coordination of electron-deficient 2,2′-bipyridine ligands to Re(i) and Ir(iii) cores

“…Whilst both isomers might be expected to form, inspection of the solid-state structure shows the presence of only the Δ R,S isomer and, given that the yield of the initially isolated solid was 71%, the complex is clearly formed diastereoselectively (see later). Similar discrimination has been observed in related systems 9 but remains rare 10 and, in this case given the remote position of the asymmetric unit, somewhat unexpected.…”

Regioselective Dehydrogenation of the Secondary Amine Complexes into Imine Complexes under Visible-Light Irradiation

“…These are consistent with our previous observation in the chiral recognition of Δ-[Ir(pq) 2 (MeCN)) 2 ] + with pro and hpro. 17 We can not obtain the pure product of Δ-[Ir(pq) 2 (Lmpro)] diastereomer by the reaction of Δ-[Ir(pq) 2 (MeCN) 2 ]-(PF 6 ) and L-mpro under the reaction condition, because it is thermodynamically unstable. 19 Moreover, the reaction time was prolonged to 72 h to obtained hip complexes Λ-[Ir(pq) 2 (L-hiq)] and Δ-[Ir(pq) 2 (L-hiq)] due to the high steric hindrance of the hip ligand.…”

“…20 This was not observed in the five-membered heterocycle pro complex and the stable diastereomers Δ-[Ir(pq) 2 (L-mor)] and Δ-[Ir-(pq) 2 (L-hiq)], but also was found in Λ-[Ir(pq) 2 (L-hpro)] complex. 17,21 Optimization Photoreaction Conditions for the Regioselective Dehydrogenation. In the previous observation of the diastereoselective photoreaction in the presence of O 2 , we found that the Δ-[Ir(pq) 2 (L-hpro)] diastereomer was dehydrogenatively oxidized into imino acid complex Δ-[Ir(pq) 2 (hpro-2H α )], while the Δ-[Ir(pq) 2 (D-hpro)] diastereomer occurs interligand C−N cross-coupling and dehydro-genatively competitive reactions.…”

“…In the previous observation of the diastereoselective photoreaction in the presence of O 2 , we found that the Δ-[Ir(pq) 2 (L-hpro)] diastereomer was dehydrogenatively oxidized into imino acid complex Δ-[Ir(pq) 2 (hpro-2H α )], while the Δ-[Ir(pq) 2 (D-hpro)] diastereomer occurs interligand C−N cross-coupling and dehydro-genatively competitive reactions. 17 The imine regioisomer Δ-[Ir(pq) 2 (D-hpro-2H α ′)] (75%) is a main product at low temperature, indicating that the dehydrogenative oxidation reaction of the secondary amine is regioselective. This encourages us to further observe the dehydrogenative oxidation reaction of the secondary amine complexes.…”

Diastereoselective Photoreaction of Ir(III) Amine Complexes for Generation of New Multidentate Ligands in situ via a Postcoordinated Interligand‐Coupling Strategy

“…Further observations showed that the photoreaction of Ir(III) AA complexes was diastereoselective in the presence of O 2 at room temperature; the oxidative dehydrogenation into imino acid complex was observed in Ʌ ‐ D or Δ‐ L diastereomer, while two competitive pathways were found in Ʌ ‐ L or Δ‐ D diastereomer, affording an imino acid and an interligand C—N coupling product (Scheme 1b). [ 15‐16 ] In light of these findings, we are intrigued by what will happen in Ir(III) amine complexes, which has only been found in dehydrogenative oxidation into imine in the previous reports, [ 11 ] especially in diamine complexes which incorporate two or more stereogenic centres in a molecule, resulting in multiple diastereomers. Here, we demonstrate that the photoreactions of Ir(III)‐mempy complexes are diastereoselective in the presence of O 2 , namely, Ʌ ‐ R and Δ‐ S diastereomers are oxidized into the corresponding imine complexes, while Ʌ ‐ S and Δ‐ R diastereomers are interligand C—N cross‐coupling and dehydrogenative oxidation depending on reaction temperature.…”