CHAPTER 4: Asymmetric Catalysis with Chiral Magnesium and Calcium Systems

In this chapter we describe the recent developments in the context of asymmetric catalysis by chiral magnesium (Mg) and calcium (Ca) species. Up to 1995, both metals were not very popular in the prominent field of asymmetric catalysis. However, critically important features – exploitable in (asymmetric) catalysis – of both Mg and Ca include: (i) a low electronegativity; (ii) a stable +II oxidation state, meaning that two covalent bonds with anions may be formed and applied in a variety of bond activations; (iii) the ability to offer various coordination sites due to their larger ionic radii. Furthermore, these group 2 metals show mild but significant Lewis acidity in acid–base interactions. Taken together, these characteristics render both Mg and Ca very promising metal components of highly functionalised acid–base catalyst systems. In this account, we describe the development of chiral alkaline-earth metal catalysts and their use in asymmetric carbon–carbon bond formations as well as the one-pot formation of several C–C and C–X bonds.