From limestone to catalysis: application of calcium compounds as homogeneous catalysts

Kinetic stabilisation of a molecular strontium hydride complex using an extremely bulky amidinate ligand

The first dimeric strontium hydride complex (see picture, Dip = 2,6-diisopropylphenyl, Ad = 1-adamantyl), and analogous magnesium hydride systems, have been kinetically stabilised by extremely bulky amidinate ligands, developed for this study.

Easily accessible lithium compound catalyzed mild and facile hydroboration and cyanosilylation of aldehydes and ketones

Simple and readily accessible lithium compounds are found to be efficient single site catalysts for cyanosilylation and hydroboration of a range of aldehydes and ketones under ambient conditions.

Cyclooctatetraenyl calcium and strontium amido complexes

The first examples of Ca and Sr amido complexes built on a cyclooctatetraenyl platform are reported.

Neutral and anionic zinc compounds supported by a bis(imino)phenyl NCN ligand

The new zinc complexes 2,6-(ArNCH)₂C₆H₃ZnBu (6) and 2,6-(ArNCH)₂C₆H₃ZnCl₂Li(THF)₃ (7), supported by a bis(imino)phenyl NCN pincer ligand, were prepared (Ar = 2,6-ⁱPr₂C₆H₃).

Mild, calcium catalysed Beckmann rearrangements

A mild calcium catalysed Beckmann rearrangement has been realised, which forgoes the more traditional harsh reactions conditions associated with the transformation.

Well-defined styryl and biphenyl calcium complexes from dilithio compounds and calcium iodide: synthesis, structure and reactivity toward nitrous oxide

Efficient synthesis and structure elucidation of carbon–calcium σ-bonded compounds are of remarkable interest and importance in organometallic chemistry of the heavier s-block metals.

Donor-stabilised molecular Mg/Al-bimetallic hydrides

Low-molecular organometallic hydride complexes of the type [(do)_xMg{(μ₂-H)(AlMe₃)}₂] form in ethereal solvents and display inherent reactivity of existing magnesium hydrido bonding.

The Unusual Role of Aromatic Solvent in Single-Site AluminumI Chemistry: Insights from Theory

The single-site activation of strong σ bonds (such as that of H-H, P-H, and N-H) remains a significant challenge in main-group chemistry, with only a few cases reported to date. In this regard, recent exciting experiments performed with aluminum^I complexes hold significance because they have been seen to activate a variety of strong σ bonds. Such chemistry is generally seen to occur in aromatic solvents. Current computational studies with DFT reveal the interesting reason for this: an explicit aromatic solvent molecule acts as a catalyst by converting the aluminum^I complex into aluminum^III during the process. Different cases of σ-bond activation by aluminum^I complexes have been investigated and the efficiency of H-X (X=H, NHtBu, PPh₂ ) bond activation in the presence of an explicit benzene solvent molecule is orders of magnitude higher than that in its absence. The current work therefore reveals the chemistry of aluminum^I complexes to be richer and more complex than that previously realized, and shows it to be dependent on metal-solvent cooperativity; the first known example of its kind in main-group chemistry.

Bora-amidinate as a cooperative ligand in group 2 metal catalysis

Syntheses and crystal structures of the monomeric bora-amidinate (bam) complexes ^DIPPNBN-Mg·(THF)₃ and ^DIPPNBN-Ca·(THF)₄ are presented; ^DIPPNBN = HB[N(2,6-iPr₂-C₆H₃)]₂. The simplicity of their ¹H NMR spectra in THF-d₈ suggest that their monomeric solid state structures are retained in solution. ^DIPPNBN-Mg·(THF)₃ in C₆D₆, however, is in equilibrium with a dimeric species. Calculations (B3PW91/6-311++G**) reveal a very high localized negative charge (NPA: -1.103) on the N atoms in ^DIPPNBN-Mg. The strongly basic properties of the bam ligand are in agreement with catalytic activity of these complexes in the intramolecular alkene hydroamination. A mechanism is proposed in which the bam ligand is non-innocent and cooperative, playing an active role in substrate deprotonation and product protonation.

Influence of 18-Crown-6 Ether Coordination on the Catalytic Activity of Potassium and Calcium Diarylphosphinites in Hydrophosphorylation Reactions

The addition of 18-crown-6 ether (1,4,7,10,13,16-hexaoxacyclooctadecane) to tetranuclear [(thf)K(OPAryl₂)]₄ and [(thf)₄Ca(OPAryl₂)₂] yields the corresponding mononuclear complexes [(18C6)K(OPAryl₂)] [Aryl = Ph (1a), Mes (1b)] and [(18C6)Ca(OPAryl₂)₂] [Aryl = Ph (2a), Mes (2b)]. The metathesis reaction of [(thf)K(OPAryl₂)]₄ with CaI₂ yields the calciate [K₂Ca(thf)_x{OPMes₂}₄]. The addition of dimesitylphosphane oxide and crystallization from a hexane solution gives [K₂Ca{OPMes₂}₄{Mes₂P(O)H}] (3). The complexes [(thf)K(OPMes₂)]₄, [(thf)₄Ca(OPMes₂)₂], 1b, 2b, and the calciate 3 are tested as catalysts in the hydrophosphorylation of isopropylisocyanate with dimesitylphosphane oxide, quantitatively yielding N-isopropyl(dimesitylphosphoryl)formamide. The potassium complexes are more efficient catalysts than the calcium congeners, and coordination of 18-crown-6 decelerates the catalytic conversion.

Group 2 metal (Mg, Ca, Sr) silylamides supported by a cyclen-derived macrocyclic polyamine

Heteroleptic bis(silyl)amides of magnesium and calcium [(L)M{N(SiMe₃)₂}] [M = Mg, Ca; LH = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane; (Me₃TACD)H] were previously synthesized from LH and [M{N(SiMe₃)₂}₂]. Strontium bis(silyl)amides [Sr{N(SiMe₃)₂}₂(thf)₂] and [Sr{N(SiHMe₂)₂}₂(thf)_2/3] reacted with LH to give different types of products, depending on the presence of the β-SiH function. While the former underwent protonolysis to give the amido-bridged dimer [(L)Sr{N(SiMe₃)₂}]₂ (1), the latter gave the adduct [(LH)Sr{N(SiHMe₂)₂}₂] (2) as a stable solid. 2 slowly underwent an intramolecular Si-H/H-N dehydrocoupling in solution to give [{(L)SiMe₂N(SiHMe₂)}Sr{N(SiHMe₂)₂}] (3) by liberating H₂. The results of transamination of 1 with HN(SiHMe₂)₂ depended on the relative stoichiometric ratio. A 1 : 1 mixture in n-pentane gave [{(L)SiMe₂N(SiHMe₂)}Sr{N(SiMe₃)₂}] (4) and H₂, while excess HN(SiHMe₂)₂ gave the adduct 2 under similar conditions. Compounds 2 and 3 exhibit Sr↼H-Si interactions according to X-ray crystallography, NMR, and IR spectroscopy. Lighter congeners of elusive [(L)Sr{N(SiHMe₂)₂}] were isolable for Mg (5) and Ca (6).

Hydrodeoxygenation of isocyanates: snapshots of a magnesium-mediated C[double bond, length as m-dash]O bond cleavage

Organic isocyanates are readily converted to methyl amine products through their hydroboration with HBpin in the presence of a β-diketiminato magnesium catalyst. Although borylated amide and N-,O-bis(boryl)hemiaminal species have been identified as intermediates during the reductive catalysis, the overall reduction and C-O activation is metal-mediated and proposed to occur through the further intermediacy of well-defined magnesium formamidato, formamidatoborate and magnesium boryloxide derivatives. Examples of all these species have been identified and fully characterised through stoichiometric reactivity studies and the stability of the borate species leads us to suggest that, under catalytic conditions, the onward progress of the deoxygenation reaction is crucially dependent on the further activation provided by the Lewis acidic HBpin substrate. These deductions have been explored and ratified through a DFT study.

Mg(OMe)2 promoted allylic isomerization of γ-hydroxy-α,β-alkenoic esters to synthesize γ-ketone esters

This work concerns the Mg(OMe)₂ promoted allylic isomerization of γ-hydroxy-α,β-alkenoic esters with TMEDA as an additive. The isomerization proceeded under mild conditions and afforded γ-keto esters in high yield (up to 96%) within 2 h. Both (Z)- and (E)-γ-hydroxy-α,β-alkenoic esters were tolerated under the reaction conditions. This transformation involves the in situ formation of a dienolate intermediate from the easily accessible γ-hydroxy-α,β-alkenoic ester. The in situ generated dienolate can react with benzaldehyde and undergo a practical, useful tandem allylic isomerization-Aldol reaction to afford more functionalized compounds.

Two Catalytic Methods of an Asymmetric Wittig [2,3]-Rearrangement

Two different approaches for asymmetric catalytic Wittig [2,3]-rearrangement were developed. Allyloxymalonate derivatives were converted into homoallyl alcohols via organocatalytic or Ca²⁺-catalyzed pathways in moderate to high enantioselectivities.

Organomagnesium amide catalyzed cross-dehydrocoupling of organosilanes with amines

Heteroleptic organomagnesium complex pre-catalyzed cross-dehydrocoupling of silanes with amines.

Tethered cationic alkaline earth – olefin complexes

The utilisation of a specifically tailored aminofluoroalcohol possessing both a methoxy and an olefin dangling side arms enables the preparation of the first examples of discrete calcium- and strontium-olefin cationic complexes.

Calcium stilbene complexes: structures and dual reactivity

Addition of a calcium hydride complex to diphenylacetylene gave a complex in which the stilbene dianion symmetrically bridges two Ca²⁺ions. With H₂, this complex reacts as a double Brønsted base. In reaction with I₂it provides two electrons.

Transition metal free catalytic hydroboration of aldehydes and aldimines by amidinato silane

Benz-amidinato dichlorosilane [PhC(NtBu)₂SiHCl₂] has been reported to catalyze hydroboration of aldehydes at room temperature and aldimines under slightly harsh conditions.

Enantioselective magnesium-catalyzed transformations

This review updates the major progress in the field of enantioselective transformations promoted by chiral magnesium catalysts, covering the literature since 2007, illustrating the power of these green catalysts to promote many types of reactions.

A cationic aluminium complex: an efficient mononuclear main-group catalyst for the cyanosilylation of carbonyl compounds

Main-group catalysis: A cationic aluminium complex supported by a semi-bulky aminotroponate ligand was synthesized. It functioned as an excellent catalyst of the cyanosilylation of carbonyl compounds using TMSCN.

Magnesium amide catalyzed selective hydroboration of esters

Cheaper, inexpensive alkaline-earth metal amides as efficient pre-catalysts for the selective hydroboration of a large number of esters.