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From ammonia to dioxzoline: How did these ligands revolutionize the history of chemistry?
In the long history of chemistry, the discovery and application of various ligands continue to drive the innovation of catalysts and synthesis technologies. Bis(oxazoline) ligands, often referred to as BOX ligands, are a particularly important class of chiral ligands. This type of ligand has gradually become the standard for use in the field of asymmetric catalysis due to its unique structure and literature records. In this article, we will explore the synthesis, catalytic applications of dioxazolin, and how they followed the trend of history and became a turning point in the history of chemistry.
Dioxazoline ligands contain two oxazoline rings, and their structures usually have C2 symmetry and can be derived in various forms.
Synthesis process
The synthesis method of dioxzoline is quite mature and generally involves cyclization reaction of 2-aminoalcohol with various suitable functional groups. However, for dioxzoline, the most convenient synthesis method is to use bifunctional starting materials, so that two rings can be synthesized at once. Common materials include diacids or dicyano compounds, so the majority of dioxzoline is derived from these raw materials. The success of BOX and PyBOX largely stems from their convenient synthesis methods. These materials, such as malononitrile and bispyridine acid, can be purchased at low prices.
Applications of asymmetric catalysis
Dioxzoline ligands are widely used, especially in asymmetric catalytic reactions. When bridged BOX ligands are used for catalysis, the stereochemistry results are consistent with a twisted planar tetrahedral intermediate. Taking the 4-position substituent of oxzoline as an example, it will block one of the antipodal surfaces of the substrate, leading to asymmetric selection. For example, it has amazing application effects in various reactions, such as Mannich reaction, ene reaction, etc.
Metal complexes containing dioxzoline ligands show excellent catalytic properties, especially in carbon-carbon bond formation reactions.
Carbon–carbon bond formation reaction
Dioxazoline has excellent performance in carbon-carbon bond formation reactions. The earliest application was the seromanid cyclization reaction, and later this application was extended to 1,3-dipole cycloaddition and Diels-Alder reaction. and many other forms. Studies have shown that dioxazoline can successfully achieve asymmetric products in these reactions, showing people's widespread reliance on dioxazoline.
In addition, the application of dioxzoline in hydrosilation, fluorination catalysis and Wacker-type cyclization has also been gradually discovered, demonstrating its diversity in the field of catalysis.
Development History
The history of dioxazoline can be traced back to 1984, when Brunner first demonstrated the potential of this type of ligand in asymmetric catalysis, although the efficiency at that time was only 4.9%. After several years of research, Brunner re-evaluated the oxzoline ligand and explored the application of chiral pyridinozoline, which significantly improved the asymmetric induction effect. With the efforts of Nishiyama and Masamune, the application of dioxazoline in various catalytic reactions has gradually matured, becoming a model of asymmetric catalysts.
To date, many dioxzolines with different structures have been synthesized, and these structures still mainly revolve around the classic BOX and PyBOX precursors. Although many new ligands are constantly being developed, the classic functions of BOX and PyBOX are still the mainstream in the chemical world. Where will future development lead?
