Erbium is a lanthanide element and a rare earth element. Lanthanide salts are of great importance as lewis acids in green organic chemistry because of their stability and low to moderate toxicity, especially the erbium(III) salt, which is one of the most active lanthanoid(III) derivatives. Erbium(III) salts possess the most important characteristics of green catalysts: they are cheap, easily available, allow reactions to proceed under mild conditions, and can be reused several times without time-consuming purification protocols or significant loss of activity.
The Er(III) salts can be used as homogeneous catalysts and heterogeneous catalysts. However, Er(III) salts are currently mainly used as homogeneous catalysts, and there are few reports on them as heterogeneous catalysts.
Er(III) triflate: One of the main characteristics of Erbium (III) is the ability to coordinate oxygen atom. Therefore, Er(III) triflate [Er(OTf)3] is often used as a catalyst for the conversion of epoxide. For example, the epoxide can be successfully converted directly to 1,2-diacetate by mixing the epoxide and acetic anhydride together with the catalyst erbium (III) triflate.
Er(III) chloride: The lanthanide salts play a role as lewis acids in organic chemistry, and erbium (III) chloride has a very important position in green chemistry. Various 4,5-diaminocyclopent-2-enone can be effectively prepared by using a very small amount of erbium (III) chloride hexahydrate as catalyst, Erbium (III) chloride hexahydrate is widely present in many natural and synthetic biologically active compounds, and it even can reduce the need for strict drying of experimental conditions. The reaction diagram is shown in Figure 1.
Figure 1 Erbium(III) chloride in the synthesis of trans-4,5-diaminocyclopent-2-enones
There are basically two different strategies for the conversion from homogeneous erbium catalysts to heterogeneous erbium catalysts (as shown in Figure 2), which include metals in metal-organic frameworks (MOFs) or on large surface area inorganic carriers.
Figure 2． Strategies to convert Er(III)-based homogeneous catalysts into heterogeneous catalysts
Many multifunctional catalysts, especially bifunctional catalysts, are designed and applied to asymmetric catalysis to combine a metal ion (acting as lewis acid moiety) with a lewis or brønsted base in the form of an organic functional group. Just as norephedrine was first tested by Tiseni and Peters as a chiral ligand for Er(III), the tertiary amino group was inserted into an aliphatic β-amino alcohol. The greatest advantage of this catalyst is that it can be recovered and reactivated via acid/base wash, and it can be reused without any significant loss of activity.