An oxazole ligand refers to a ligand containing one or more oxazole rings in the molecule while an oxazoline ligand refers to a ligand containing one or more oxazoline rings in the molecule and a thiazole ligand refers to a ligand containing one or more thiazole rings in the molecule. The oxazole ligand and the oxazoline ligand are five-membered heterocyclic compounds containing both nitrogen and oxygen atoms, and the thiazole ligand is a five-membered heterocyclic compound containing both nitrogen and sulfur atoms. These three ligands share some common features. First, nitrogen atoms can form complexes with many metal centers because of their electron donating properties. Therefore, oxazole ligands, oxazoline ligands and thiazole ligands are widely used. Secondly, the oxazole ligand and the oxazoline ligand are usually prepared from a chiral amino alcohol as a raw material, and the amino alcohol can be obtained by simply adding ammonia to a ubiquitous amino acid which is ubiquitous in nature. Therefore, they are featured by wide source of raw materials, simple preparation and economy. Finally, the chiral groups of oxazolyl ligands, oxazoline ligands and thiazole ligands are close to the metal coordination center, so direct and efficient chiral induction can be achieved in the metal catalysis process.
Figure 1. Oxazoline ligand
Oxazole ligands, oxazoline ligands and thiazole ligands have wide applications in organic synthesis and material chemistry due to their wide range of raw materials, simple preparation process and excellent structure.
- Material chemistry: Thiazole ligands can be combined with transition metals (such as ruthenium, platinum, rhodium, iridium, etc.) to form a wide variety of phosphorescent materials. Among them, due to the relatively large atomic number of the lanthanum element, the thiazole ligand and the complex molecule can generate an extremely strong spin-orbit coupling, benefiting to the emission of phosphorescence. In addition, the thiazole ligand and the ruthenium metal complex have high thermal stability and adjustable luminescent color, so that the luminescent device can be prepared by solution processing or vacuum evaporation. Light-emitting devices convert electrical energy into light energy through electroluminescence, which has great application prospects in many fields, such as display devices, lighting devices, and chemical and biological sensors. The complex formed by the thiazole ligand and the metal is also a vital chemical intermediate and a fine chemical raw material, and plays an important role in food additives, perfumes, dyes and the like.
- Organic Synthesis: Nitrogen-containing ligands are necessary in organic synthesis, and among the nitrogen-containing ligands, oxazolines are a prominent class. Oxazoline ligands have been widely used in many types of organic synthesis reactions due to their wide range of raw materials, convenient synthesis, structural diversity and asymmetric induction performance, especially in asymmetric synthesis reactions. The currently common reactions catalyzed by oxazoline ligands are asymmetric allyl alkylation, asymmetric cyclopropanation, asymmetric hydrosilation of carbonyl, asymmetric Wacker-type cyclization, asymmetric hydrogenation reaction, asymmetric oxidation cross-coupling reaction, Heck reaction, asymmetric addition reaction, asymmetric aerobic alkoxylation reaction, and the like. Among all the oxazoline ligands, the oxazoline ligands exhibit high catalytic activity and enantioselectivity, and become the most widely used and most useful oxazoline ligands. Thiazole ligands also play a role in the field of organic synthesis, and the reactions involved include asymmetric Friedel-Crafts alkylation, asymmetric alkylation, asymmetric Henry reaction, and the like.
- Biology and medicine: The complex formed by the thiazole ligand and the transition metal has stronger lipid solubility and cell penetrating property, which makes the antibacterial spectrum more extensive, is less prone to drug resistance, and has better medical value. In the biological field, thiazole complexes can be applied to plant growth regulators, inhibitors, herbicides, etc., and are one of the most widely used heterocyclic drugs in nature.
The oxazole ligand can be classified into a chiral oxazole ligand and an achiral oxazole ligand depending on whether the molecular structure has a chirality. Further, the number of oxazole rings contained in the structure may be classified into a monooxazolyl ligand, a bisoxazolyl ligand and a trioxazole ligand ligand.
According to the different coordination heteroatoms in the oxazoline ligands, the oxazoline ligands can be classified into N-type oxazoline ligands, O-type oxazoline ligands, P-type oxazoline ligands, S-type oxazoline ligands, and the like.
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