Olefin ligand refers to a type of ligand having a carbon-carbon double bond in its structure. Olefins have a long history as ligands and are widely found in various metal complexes. From the beginning of the simple olefin ligand to the appearance of olefin heteroligands, the types of olefin ligands are becoming increasingly abundant, and most of them are chiral olefin ligands. Olefin ligands exhibit high catalytic activity and stereoselectivity in asymmetric reactions and are therefore mainly used in the field of organic synthesis and the synthesis of a variety of compounds of high optical purity for potential pharmaceutical synthesis and biological activity.
Like other ligands, olefin ligands are not used alone, but rather, they can be combined with metal atoms to form olefin-metal complexes that play a role in asymmetric catalysis. Olefin ligands, which have been developed in recent years, are also widely used in various reactions catalyzed by metals, such as asymmetric hydrogenation, conjugate addition, imine/carbonyl compound addition, allyl substitution, intramolecular hydroformylation, chemical coupling, etc., and achieved very good results in some reactions, efficiently constructed a series of structurally important chiral compounds.
- Asymmetric addition reaction: olefin ligands and transition metals form olefin-metal complexes, which are widely used in asymmetric addition reactions, including asymmetric 1,2-addition, asymmetric 1,4-addition, and the like. The asymmetric 1,4-addition reaction should be carried out by using a metal-chiral olefin complex as a catalyst, and the addition of boric acid to alkenyl or ketene can be carried out under the conditions of potassium hydroxide being a base, dioxane and water as a solvent. Asymmetric 1,4-addition reaction is carried out to give an optically pure product. Various olefin ligands and metal complexes can be applied to asymmetric addition, and chiral thioolefin ligands exhibit some unique advantages in ruthenium-catalyzed asymmetric catalysis.
- Asymmetric hydrogenation reaction: Olefin ligands can be used not only for catalysis, but also for asymmetric hydrogenation reactions. For example, some chiral phosphorus-ene ligands are used for the hydrogenation of imines and enamines with good enantioselectivity.
- Asymmetric substitution reaction: The olefin ligand can also be substituted, but the application is less. It has been reported that the olefin ligand and the metal palladium complex catalyze the asymmetric allyl substitution reaction.
- Asymmetric coupling reaction: Olefin ligands have applications in some coupling reactions, but more in asymmetric Suzuki-Miyaura coupling reactions. Asymmetric Suzuki-Miyaura coupling reaction is very challenging. It has been reported that a phosphonene ligand having a spiroene core can achieve high-efficiency coupling of 1-bromonaphthyl phosphate with ortho-substituted phenylboronic acid under mild conditions, and a series of biaryl compounds having an axial chirality are obtained.
Depending on the type of atom in the structure, olefin ligands can be classified into simple olefin ligands and mixed olefin ligands.
- Simple olefin ligand: A simple olefin ligand refers to an olefin ligand containing only two atoms of C and H in the structure. For example, 1,4 cyclohexadiene ligand, 1,5-cyclooctadiene ligand, and the like.
- Mixed olefin ligand: A mixed olefin ligand refers to an olefin ligand having a structure in which other atoms (for example, a P atom, an S atom, or the like) are contained in addition to C and H atoms. The most common mixed olefin ligands are chiral phosphine olefin ligands and chiral thioolefin ligands.
Figure 1. Common olefin ligands
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