Phenoxaphosphine-Based Diphosphine Ligands. Synthesis and Application in the Hydroformylation Reaction

Abstract

The synthesis of a new series of diphosphine ligands based on 2,7-di-tert-butyl-9,9-dimethylxanthene (1), p-tolyl ether (2), ferrocene (3), and benzene (4) backbones, containing one or two 2,8-dimethylphenoxaphosphine moieties, is reported. The ligands were employed in the rhodium-catalyzed hydroformylation of 1-octene. For all four ligand backbones, introduction of phenoxaphosphine moieties led to an increase in catalytic activity and a decrease in regioselectivity toward the linear aldehyde product. Xanthene-based ligands 1a-1c yielded highly active and regioselective hydroformylation catalysts; ligands containing p-tolyl ether and ferrocene backbones 2a-2c and 3a-3c provided less active and less regioselective catalysts. Catalysts containing benzene-derived ligands 4a and 4b showed a remarkable preference for the formation of the branched aldehyde product. The coordination behavior of ligands 1-4 under hydroformylation conditions was investigated using high-pressure NMR and IR spectroscopy, revealing the distinct steric and electronic properties of the diphenylphosphine and 2,8-dimethylphenoxaphosphine moieties in ligands 1-4. The phosphacyclic moieties proved to be less basic and less sterically demanding toward other ligands in metal complexes than the acyclic diphenylphosphine moieties. For ligands that contain rigid backbones, the lack of conformational freedom in these phosphacyclic moieties does lead to repulsive interactions between the substituents of the two phosphorus donor atoms, resulting in an increase in the bite angle of the ligand. The low catalytic activity of rhodium catalysts modified by benzene-based ligands 4a-4c was attributed to the quantitative formation of HRh(L) 2 under hydroformylation conditions.

Synthesis of Phospholane–Phosphoramidite Ligands and their Application in Asymmetric Catalysis

“…Unexpectedly, the signal of the phospholane P atom underwent a significant downfield shift and appears at δ =63.8 ppm, whereas the R 2 NPCl 2 signal lies at δ =83.1 ppm, a very unusual upfield shift compared to that of similar compounds ( δ ≈150–170 ppm) 5. 13 With a value of J P,P′ =356 Hz, the coupling constant is within the range reported for phosphine‐dichlorophosphine compounds 14. The condensation of PCl 2 compound 3 with ( R a )‐ and ( S a )‐(1,1′‐binaphthalene)‐2,2′‐diol in the presence of NEt 3 resulted in the clean formation of L3 and L4 , respectively.…”

Synthesis and Reactivity of Chiral, Wide‐Bite‐Angle, Hybrid Diphosphorus Ligands

“…Gratifyingly, formation of the unwanted cyclic phenoxaphosphine side-product during the second lithiation step, concomitant with P-Ph cleavage, [18] was completely suppressed by protecting phosphine 6 as its borane adduct, intermediate 6-(BH 3 ) ( 31 P δ = 18.5 ppm, broad). The two methyl groups in the 4,4Ј-position of the diphenyl ether backbone are required to avoid side-reactions during lithiation.…”

Synthesis of Mono- and Bis-Pyrazoles Bearing Flexible p-Tolyl Ether and Rigid Xanthene Backbones, and Their Potential as Ligands in the Pd-Catalysed Suzuki–Miyaura Cross-Coupling Reaction

“…Transition metal complexes of phosphorous (P), nitrogen (N), sulphur (S), and oxygen (O)-based ligands and their hybrids have provided chemists with a wide opportunity and a quasi-exhaustive tool for creating C-C bonds of significant interest [1,2]. The reactivity and the catalytic behaviour of these complexes largely depend on the nature of the coordinating atoms, their relative position within the molecular architecture, and the relative flexibility or rigidity of the ligand backbone, as they greatly influence steric and electronic properties of the resulting complex [3,4]. Therefore, the fine-tuning of these properties in order to synthesize ligands of particular interest has been an interesting strategy.…”