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

The chiral-recognition processes of homoproline (hpro) and [Ir(pq)(2)( MeCN)(2)](PF6) (pq is 2-phenylquinoline; MeCN is acetonitrile) are investigated, in favor of formation of the thermodynamically stable diastereomers Delta-[Ir(pq)(2)(D-hpro)] and Delta-[Ir(pq)(2)(L-hpro)]. Moreover, the diastereoselective photoreactions of Delta-[Ir(pq)(2)(D-hpro)] and Delta-[Ir(pq)(2)(L-hpro)] are reported in the presence of O-2 at room temperature. Diastereomer Delta-[Ir(pq)(2)(L-hpro)] is dehydrogenatively oxidized into imino acid complex Delta-[Ir(pq)(2)(hpro-H-2(2))] (hpro(2)H(2) is 3,4,5,6-tetrahydropicalinate), while diastereomer Delta-[Ir(pq)(2)(D-hpro)] occurs by interligand C-N cross-coupling and dehydrogenative oxidation reactions, affording three products: Delta-[Ir(pq)(D-pqh)] [pqh is N-(2-phenylquinolin-8-yl)homoproline], Delta-[Ir(pq)(2)(hpro-H-2(2))], and Delta-[Ir(pq)2(D-hpro-H-2(6))] [hpro-H-2(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.

Automated summary

The chiral recognition processes and diastereoselective photoreactions of homoproline and [Ir(pq)(2)(MeCN)(2)](PF6) were investigated. Different diastereomers were formed, with one being dehydrogenatively oxidized into an imino acid complex, while the other underwent C-N cross-coupling and dehydrogenative oxidation reactions. The competitive reactions were controlled selectively through optimization of reaction parameters.