Zwitterionic Iridium Complexes with P,N‐Ligands as Catalysts for the Asymmetric Hydrogenation of Alkenes

Abstract

Several zwitterionic iridium complexes based on chiral P,N-ligands with imidazoline or oxazoline donors and anionic tetraarylborate or aryltrifluoroborate substituents have been synthesized. The corresponding cationic analogues have also been prepared, to evaluate the effect of the covalent linkage between the anion and the cationic metal complex in catalytic reactions. The respective pairs of structurally analogous precatalysts have been compared for their efficacies in the asymmetric hydrogenation of unfunctionalized olefins. In most cases, the anionic derivatization has virtually no influence on the asymmetric induction of the iridium complex. This is in accordance with X-ray structural studies, which have shown that the chiral environment of the cationic metal center is not affected by the anionic substituent. Depending on the nature of the counterion employed, the zwitterionic catalysts proved to be significantly more reactive than their cationic counterparts in nonpolar solvents.

Asymmetric Hydrogenation of Olefins Using Chiral Crabtree-type Catalysts: Scope and Limitations

“…85 This ligand showed low enantioselectivities in the Ir-hydrogenation of trans-α-methylstilbene S4 (ee's up to 48% ee). 51 However, the presence of the anionic derivatization has a negative influence on the asymmetric induction of the iridium complex. Table 1, entry 22) 61,87 and cyclic dienes (ee's up to >99% for the trans isomer), 88 but was only moderate in the reduction of Z-olefins (ee's up to 72%).…”

NeoPHOX – a structurally tunable ligand system for asymmetric catalysis

“…An attractive feature of this approach is that by proper choice of the alkylmetal reagent and the protecting group, the steric properties of the ligand can be optimized for a specific application. PHOX ligands of this type have been previously prepared from serine and successfully used in iridium-catalyzed hydrogenation [ 29 – 30 ].…”

Asymmetric Hydrogenation Using Rhodium Complexes Generated from Mixtures of Monodentate Neutral and Anionic Phosphorus Ligands

“…Synthesis of monodentate phosphite and phosphoramidite ligands : In connection with our work on zwitterionic iridium complexes with anionic N,P ligands,11 we developed fluorinated tetraarylborate 1 a ,12 which we thought would be a suitable building block for the synthesis of phosphites and phosphoramidites bearing a non‐coordinating anionic unit. Two secondary amines 3 a and 4 a were prepared from 1 a through bromide 2 a (Scheme ).…”