A phosphite ligand refers to a phosphine ligand in which the atoms bonded to the P atom are all O atoms. Compared with traditional phosphine ligands, phosphite ligands have the advantages of simple synthesis, resistance to air oxidation, large structural plasticity, and the like, and they exhibit very good catalytic performance in various types of organic synthesis. Phosphite ligands can be classified into monodentate phosphite ligands and bidentate phosphite ligands. The monodentate phosphite ligands have strong structural rigidity, while the structure of bidentate phosphite ligands is highly controllable compared to monodentate phosphite ligands. In addition, the bidentate phosphite ligands are stable in air and tightly bound to the metal. Therefore, the bidentate phosphite ligands are more widely used when catalyzing organic reactions.
Phosphite ligands have the advantages of simple synthesis, easy availability of raw materials, and stable properties. phosphite is not sensitive to oxygen. At the same time, phosphites are weak ruthenium-electron donors and strong π-electron acceptors. As a ligand, phosphite has a catalytic effect on various types of reactions in organic synthesis such as asymmetric hydrogenation, asymmetric hydroxyformylation, asymmetric 1,4-addition, coupling reaction, etc..
- Asymmetric hydrogenation: Some phosphites and transition metal complexes have a relatively rigid structure, which helps to obtain effective chiral recognition and enhance the enantioselectivity of the product, thus showing very high potential in the asymmetric catalytic hydrogenation of dehydrogenated amino acids. Catalysts formed by phosphites and transition metals have been successfully used for the hydrogenation of ɑ-dehydroamino and β-dehydroamino acid derivatives, as well as the asymmetric ruthenium-catalyzed hydrogenation of itaconate and enamine.
- Asymmetric hydroxyformylation: The hydroformylation of an olefin with a complex formed by ruthenium generally exhibits a high catalytic activity and good regioselectivity. Therefore, catalysts formed by the combination of phosphite and ruthenium are widely used to catalyze the hydroformylation of olefins. For example, bisphosphite ligand ruthenium catalysts have been widely used for the production of aldehydes by low pressure processes. The optically active product, aldehyde, is an important intermediate in the fields of fine chemicals, medicines and pesticides.
- Asymmetric 1,4-addition reaction: The use of chiral phosphite ligand and copper-forming catalyst for the asymmetric conjugate addition of ɑ,β-unsaturated compounds contributes to the high activity and excellent enantioselectivity of the ɑ,β-unsaturated compounds, and also contributes to the construction of chiral centers and synthesis of functionalized molecules. It should be used to catalyze various biologically active compounds and their intermediates, such as musk ketone [(R)-Muscone], anti-mycobacterial agent Erogorgiaene, anti-cancer drug Clavularin B, cardiovascular disease drug PGE1, neurotoxin (-)-Pumiliotoxin C, and ibuprofen [(+)-Ibuprofen].
- Coupling reaction: Some phosphite ligands can be used to catalyze the coupling reaction. For example, bisphosphite contains two P atoms, which can well complex with Pd to form a stronger complex, which has better catalytic effect and can catalyze the direct coupling reaction of halogenated aromatics with amines (Buchwald- Hartwig).
Phosphite ligands can be classified into monodentate phosphite ligands and bidentate phosphite ligands.
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