Alkaline earths as main group reagents in molecular catalysis

Abstract

The past decade has witnessed some remarkable advances in our appreciation of the structural and reaction chemistry of the heavier alkaline earth (Ae = Mg, Ca, Sr, Ba) elements. Derived from complexes of these metals in their immutable +2 oxidation state, a broad and widely applicable catalytic chemistry has also emerged, driven by considerations of cost and inherent low toxicity. The considerable adjustments incurred to ionic radius and resultant cation charge density also provide reactivity with significant mechanistic and kinetic variability as group 2 is descended. In an attempt to place these advances in the broader context of contemporary main group element chemistry, this review focusses on the developing state of the art in both multiple bond heterofunctionalisation and cross coupling catalysis. We review specific advances in alkene and alkyne hydroamination and hydrophosphination catalysis and related extensions of this reactivity that allow the synthesis of a wide variety of acyclic and heterocyclic small molecules. The use of heavier alkaline earth hydride derivatives as pre-catalysts and intermediates in multiple bond hydrogenation, hydrosilylation and hydroboration is also described along with the emergence of these and related reagents in a variety of dehydrocoupling processes that allow that facile catalytic construction of Si-C, Si-N and B-N bonds.

Imine hydrogenation with simple alkaline earth metal catalysts

“…The inability to lower the catalyst concentration is not due to catalyst decomposition by hydrolysis. Although organocalcium complexes are substantially more sensitive towards hydrolysis than most transition metal catalysts, often catalyst (or initiator) concentrations down to 0.1 mol% are feasible 5,19,43 . We attribute the need for higher catalyst concentrations to formation of larger aggregates (vide infra).…”

“…Increasing social pressure towards a sustainable society, however, dictates replacement of costly, and often harmful, precious metals by more abundant first-row transition metals or even biocompatible redox inactive main group metals [2][3][4][5][6] . The alkaline earth metal calcium does not possess partially filled d orbitals for substrate activation, but has recently shown catalytic activities in the hydrogenation of C= C double bonds with molecular H 2 (ref.…”

Diborane heterolysis: breaking and making B–B bonds at magnesium

“…42,43 We, thus, carried out the reaction of compound 6 with B 2 pin 2 . Although the reaction performed in d 8 -toluene required brief heating to ensure the formation of a homogeneous solution, inspection of the resultant 1 H and 13 C { 1 H} NMR spectra revealed the emergence of two new sets of β-diketiminate ligand environments, which had been formed in a strict 1 : 1 ratio by relative integration.…”

Kudos and Renaissance of s-Block Metal Chemistry

“…Future tasks related to the chemistry of these s-block organometallics include stability and reactivity [17], catalysis and stoichiometric conversions [18,21], Lewis acidity and nucleophilicity [24]. Quantum chemical calculations are valuable tools to elucidate the agostic and π-interactions between the s-block metal ions and multiple bonds [25,26].…”

“…Recent studies have elucidated the suitability of magnesium hydrides as storage materials for hydrogen, one of the future energy carriers. In the recent past, increasing efforts have been undertaken to develop a sophisticated chemistry of these elements [19][20][21]. The chemistry of methanediides [22] and the relevance of secondary interactions [23], discussed in this Special Issue, may be viewed as representative examples.…”