The substance containing yttrium with catalytic function is referred to as a yttrium catalyst. Yttrium is a malleable gray-black metal and one of the rare earth metals, which can react with hot water and is easily soluble in dilute acid. The content of yttrium in the earth's crust is very small and usually found in silicon germanium, black rare earth ore and xenotime. The main valence of yttrium is +3, and the valence of yttrium in most yttrium catalysts is +3.
The use of yttrium catalysts is not very extensive, but it exhibits good catalytic performance in some reactions (such as polymerization, Michael addition reaction, Mannich reaction), and has the advantages of environmental protection, so it still plays a role in the field of organic catalysis.
- Mannich reaction: The Mannich reaction, also known as amine methylation, is an organic chemical reaction in which a compound containing an active hydrogen (usually a carbonyl compound) is condensed with formaldehyde and a secondary amine or ammonia to form a β-amino (carbonyl) compound. Some rare earth metal compounds have a good catalytic effect on the Mannich reaction and thus can often be used as a catalyst for the Mannich reaction. For example, Y(OTf)3 can catalyze the novel Mannich reaction of N-alkoxycarbonylpyrroles, formaldehyde and primary amine hydrochlorides.
Figure 1. Yttrium catalyst catalyzed Mannich reaction
- Michael addition reaction: The Michael addition reaction is an important reaction for synthetic drugs, natural products and fine intermediates. In particular, Michael addition of α,β-unsaturated aldehydes, ketones and heterocyclic compounds is one of the most effective methods for extending the carbon chain and introducing reactive groups. Moreover, the Michael addition reaction product of α,β-unsaturated aldehyde, ketone and heterocyclic compound has important biological activity and photoelectric activity. Michael addition reactions often use catalysts such as Lewis acid and protonic acid. The trifluoromethylsulfonate-based rare earth compound Y(OTf)3 has a very slow hydrolysis rate and is stable in water, which can effectively overcome the shortcomings of traditional Lewis acid (AlCl3, TiCl4 and SnCl4) in-water instability and can be reused. The activity does not decrease. In addition, Y(OTf)3 catalyst has the advantages of low toxicity and environmental friendliness compared with conventional catalysts such as Lewis acid and protonic acid, and thus can be used as an excellent catalyst for Michael addition reaction. For example, under the catalysis of Y(OTf)3, a Michael addition reaction can be carried out on hydrazine and cyclohexenone to synthesize 3-(3'fluorenyl) cyclohexenone in high yield.
Figure 2. Yttrium catalyst catalyzed Michael addition reaction
- Ferrier Rearrangement reaction: Owing to their versatile chemical transformations, alkyl and aryl 2,3-unsaturated glycosides are important chiral intermediates in the synthesis of biologically active molecules and new functional materials. A variety of reagents have been used to promote this reaction, including Bronsted acids, Lewis acids, as well as other reagents such as oxidants. Due to their special properties and high catalytic activity, rare earth metal salts as catalyst have recently gained more and more applications. Y(OTf)3 is a highly efficient catalyst for Ferrier Rearrangement reaction, which can promotes the transformation of various glucals to the O- and S-2,3-unsaturated glucosides. For example, Y(OTf)3 can catalyze Ferrier Rearrangement of 3,4,6-tri-O-acetyl-D-glucal with nucleophiles, including ethanol, cyclohexanol, phenol and so on. The yttrium catalyst catalyzed Ferrier Rearrangement reaction generally has the advantages of short reaction time, simple post treatment, high yield and good selectivity.
Figure 3. Yttrium catalyst catalyzed Ferrier Rearrangement reaction
- Su, Yu-Chia. (2019), "Air-stable di-nuclear yttrium complexes as versatile catalysts for lactide polymerization and copolymerization of epoxides with carbon dioxide or phthalic anhydride." Polymer 167, 21-30.
- Chuanxin Zhang, Jingchao Dong. (2001), "Y(OTf)3-catalyzed novel Mannich reaction of N-alkoxycarbonylpyrroles, formaldehyde and primary amine hydrochlorides." Tetrahedron Letters 42, 461–463.
- Peiran Chen; Shan Li. (2014), "Y(OTf)3 as a highly effificient catalyst in Ferrier Rearrangement for the synthesis of O- and S-2,3-unsaturated glycopyranosides." Tetrahedron Letters 55, 5813–5816.
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