1.6.4 Rare Earth Metal Mediated Domino Reactions

AbstractRare earth metals, comprising 17 chemical elements in the periodic table, are relatively abundant in the Earth’s crust despite their name. In the series of lanthanides, a systematic contraction of the ionic radii is observed when going from lanthanum to lutetium (often referred to as the lanthanide contraction), but this variation is so smooth and limited, with ca. 1% contraction between two successive lanthanides, that it is possible to fine-tune the ionic radii, Lewis acidity, and Brønsted basicity of rare earth complexes. As a result of the large size of the lanthanide ions compared to other metal ions, lanthanide ions have high coordination numbers, varying from 6 to 12. Due to the strong oxophilicity of rare earth elements, their metal ions have a hard Lewis acidic nature. Most particularly, rare earth metal trifluoromethanesulfonates [M(OTf)3] have been regarded as new types of Lewis acids and are stable and active in the presence of many Lewis bases. Another important type of rare earth metal species, the rare earth metal alkoxides [M(OR)3], exhibit both Lewis acidity and Brønsted basicity. These collated characteristic features of rare earth based complexes, such as high coordination numbers, a hard Lewis acidic nature, high compatibility with various functional groups, ease of fine-tuning, and multifunctionality, have led to the development of a variety of domino reactions catalyzed largely by rare earth metal trifluoromethanesulfonates and alkoxides.