Alkaline-Earth Metal Compounds

Table of Contents

1. Frontmatter

2. Heavy Alkaline-Earth Metal Organometallic and Metal Organic Chemistry: Synthetic Methods and Properties

   Ana Torvisco, Karin Ruhlandt-Senge

   Abstract

   Despite the growing utility of heavy alkaline-earth metal organometallics, their high reactivity resulted in limited accessibility. Over the last decade, synthetic strategies have been developed to overcome challenges, resulting in unprecedented growth of this chemistry. The synthetic accessibility is going hand in hand with exciting and ever expanding utilities of the compounds in applications as diverse as catalysis or materials chemistry. This article provides a comprehensive overview of the various synthetic methodologies along with considerations on their structure–property relationships.

   Graphical Abstract

   The careful examination and development of synthetic strategies towards organometallic and metal organic compounds of the heavy alkaline-earth metals have contributed much to the rapid growth of their chemistry. This article highlights the various synthetic methodologies towards the target compounds.

   Full size image

3. Heavier Group 2 Grignard Reagents of the Type Aryl-Ae(L) n -X (Post-Grignard Reagents)

   Matthias Westerhausen, Jens Langer, Sven Krieck, Reinald Fischer, Helmar Görls, Mathias Köhler

   Abstract

   Whereas hitherto no general procedure has been developed for the synthesis of alkylcalcium halides, arylcalcium halides were found to be easily accessible post-Grignard reagents. The large discrepancy between the inertness of metallic calcium and the organocalcium derivatives requires an activation of the metal prior to use. The arylcalcium compounds can be obtained in large yields from iodoarenes and in smaller yields from bromoarenes. Chloro- and fluoroarenes represent no suitable substrates for the direct synthesis with calcium. Diverse substituents and functional groups are tolerated in para and meta position, whereas in ortho position the tolerance of functional groups is reduced. Only the para-phenyl substituted arylcalcium derivatives are destabilized and instantaneously undergo degradation reactions. These arylcalcium complexes commonly crystallize with six-coordinate calcium centers in distorted octahedral environments. Bulky substituents allow the preparation of organometallics with smaller coordination numbers whereas the use of multidentate ethers with small bites (intraligand O···O distance) leads to compounds with seven- or eight-coordinate calcium atoms. The arylcalcium halides show spectroscopic properties and reactivities more similar to organolithium compounds than to classic Grignard reagents. Reduction of iodoarenes is also possible with strontium and barium, but these homologous reagents are more reactive than the calcium derivatives enhancing the tendency to cleave solvent molecules. The objective of this review includes the credo that arylcalcium halides and pseudohalides are valuable synthons in organometallic chemistry which can be at least as valuable as organolithium reagents due to their ease of preparation and manageability on the one hand and their high and tunable reactivity on the other.

   Graphical Abstract

   Full size image

4. Stable Molecular Magnesium(I) Dimers: A Fundamentally Appealing Yet Synthetically Versatile Compound Class

   Cameron Jones, Andreas Stasch

   Abstract

   The chemistry of the group 2 metals (alkaline-earth metals) is dominated by the +2 oxidation state. In 2007, we reported the first examples of magnesium(I) dimers which are stable at ambient temperature. Since that time a number of other examples have come forward and their fundamental and applied chemistry has rapidly emerged. All of these complexes feature “deformable” covalent Mg–Mg single bonds, and are kinetically stabilised towards disproportionation processes by the incorporation of sterically bulky, anionic or dianionic, chelating N-donor ligands. The high reactivity of magnesium(I) dimers has led to their use as specialist reagents in organic and organometallic/inorganic synthesis. They have proved especially useful as hydrocarbon soluble, stoichiometric, selective, and safe reducing agents in these areas. This chapter focuses on stable molecular compounds of the general type LMgMgL (L = anionic ligand), paying attention to their synthesis, structure, bonding, theoretical and spectroscopic analysis, as well as their further chemistry.

   Graphical Abstract

   Full size image

5. Modern Developments in Magnesium Reagent Chemistry for Synthesis

   Robert E. Mulvey, Stuart D. Robertson

   Abstract

   Last year (2012) commemorated the 100th anniversary of the award of the Nobel Prize to Victor Grignard for his development of Grignard reagents (at the simplest level expressed as “RMgX”, where R is an organic group and X is a halogen), one of the most widely utilised classes of synthetic reagent. A century on but only in the past decade or so has magnesium reagent chemistry entered a new and exciting phase, surpassing the limitations of what traditional Grignard reagents can do. Modern magnesium reagents have been designed to possess essentially comparable reactivity to their great rivals, organolithium reagents (traditional Grignard reagents are orders of magnitude less reactive than organolithium reagents), but at the same time to maintain the superior selectivity and broader functional group tolerance of their traditional ancestors. The key to the design of these new reagents is their multicomponent constitution with the organomagnesium engine powered by an activating alkali-metal additive, often a halide salt or another organometallic entity. This chapter outlines some of the most significant advances in this emerging field.

   Graphical Abstract

   Full size image

6. Alkaline-Earth Metal Complexes in Homogeneous Polymerization Catalysis

   Jean-François Carpentier, Yann Sarazin

   Abstract

   This chapter describes the use of complexes of the alkaline-earth metals (magnesium, calcium, strontium, and barium) in homogeneous polymerization catalysis. The latest developments in the polymerization of monomers containing C=C unsaturations (ethylene, styrene, dienes, acrylates) and in the ring-opening polymerization (ROP) of cyclic esters [lactides (LA), ε-caprolactone, trimethylene carbonate] are covered. The mechanisms of chain growth polymerization of olefins and immortal ROP of cyclic esters, which is of particular relevance to these metals, are presented. In view of the current interest devoted to the organometallic chemistry of the larger alkaline-earth metals, the implementation of well-defined catalyst systems based on calcium, strontium, and barium for the polymerizations of styrene and lactide is highlighted.

   Graphical Abstract

   Full size image

7. Homogeneous Catalysis with Organometallic Complexes of Group 2

   Mark R. Crimmin, Michael S. Hill

   Abstract

   This chapter provides details of the recent progress in heavier Group 2-catalyzed small molecule transformations mediated by well-defined heteroleptic and homoleptic complexes of the form LMX or MX₂, where L is a monoanionic ligand and X is a reactive σ-bonded substituent and M = Mg, Ca, Sr, and Ba. The intra- and intermolecular heterofunctionalization (hydroamination, hydrophosphination, hydrosilylation, hydroboration, hydrogenation, and hydroacetylation) of alkenes, alkynes, dienes, carbodiimides, isocyanates, pyridines, quinolines, and ketones is discussed, along with the dimerization of aldehydes, the trimerization of isocyanates, and the dehydrogenation of amine-boranes and the dehydrogenative coupling of amines with silanes. While studies in this field have focused largely on biocompatible and inexpensive catalysts of calcium and the heavier elements, the field has renewed interest in the chemistry of organomagnesium complexes.

   Graphical Abstract

   Full size image

8. Chiral Ca-, Sr-, and Ba-Catalyzed Asymmetric Direct-Type Aldol, Michael, Mannich, and Related Reactions

   Tetsu Tsubogo, Yasuhiro Yamashita, Shū Kobayashi

   Abstract

   Recent progress in asymmetric direct-type aldol, Michael, Mannich, and related reactions using chiral Ca, Sr, and Ba catalysts was summarized in this chapter. Ca, Sr, and Ba are very attractive, because they are abundant and ubiquitous elements found in nature, and form relatively safe and environmentally benign compounds compared with heavy transition metals. However, their use as catalysts in asymmetric synthesis has been limited compared with that of transition metal catalysts. Their strong Brønsted basicity and mild Lewis acidity are promising and attractive characteristics, and can influence their catalytic activity as well as their chiral modification capability in a positive manner. It was revealed that several catalytic asymmetric carbon–carbon bond-forming and related reactions proceeded smoothly in high enantioselectivites using the chiral Ca, Sr, and Ba catalysts.

   Graphical Abstract

   Full size image

9. Backmatter