J. Appl. Cryst.の発刊に際して

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC–main group element adducts, and might be...

NHCs in Main Group Chemistry.

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.

Alkaline earths as main group reagents in molecular catalysis.

Organoborane compounds present a class of versatile synthetic intermediate for myriad organic transformations. The direct addition of a B–H bond across unsaturated bond—namely, hydroboration—is a powerful tool for the preparation of organoborane derivatives. This review outlines recent advances in catalytic hydroboration of unsaturated organic compounds, specifically those involving C-X (X = N, O) bonds. We will discuss the chemical behavior of both transition metal catalysts and main group catalysts in hydroboration. Emphasis will also be placed on the reaction mechanism of these catalytic reactions. Furthermore, recent achievements in catalytic hydroboration of carbon dioxide CO2 will be highlighted.

Catalytic Hydroboration of Carbonyl Derivatives, Imines, and Carbon Dioxide

This Communication describes the hydrogenation of carbon dioxide to methanol via tandem catalysis with dimethylamine and a homogeneous ruthenium complex. Unlike previous examples with homogeneous catalysts, this CO2-to-CH3OH process proceeds under basic reaction conditions. The dimethylamine is proposed to play a dual role in this system. It reacts directly with CO2 to produce dimethylammonium dimethylcarbamate, and it also intercepts the intermediate formic acid to generate dimethylformamide. With the appropriate selection of catalyst and reaction conditions, >95% conversion of CO2 was achieved to form a mixture of CH3OH and dimethylformamide.

Tandem amine and ruthenium-catalyzed hydrogenation of CO2 to methanol.

A β-diketiminato n-butylmagnesium complex is presented as a selective precatalyst for the reductive hydroboration of organic nitriles with pinacolborane (HBpin). Stoichiometric reactivity studies indicate that catalytic turnover ensues through the generation of magnesium aldimido, aldimidoborate and borylamido intermediates, which are formed in a sequence of intramolecular nitrile insertion and inter- and intramolecular B–H metathesis events. Kinetic studies highlight variations in mechanism for the catalytic dihydroboration of alkyl nitriles, aryl nitriles bearing electron withdrawing (Ar(EWG)CN) and aryl nitriles bearing electron donating (Ar(EDG)CN) substitution patterns. Kinetic isotope effects (KIEs) for catalysis performed with DBpin indicate that B–H bond breaking and C–H bond forming reactions are involved in the rate determining processes during the dihydroboration of alkyl nitriles and Ar(EDG)CN substrates, which display divergent first and second order rate dependences on [HBpin] respectively. In contrast, the hydroboration of Ar(EWG)CN substrates provides no KIE and HBpin is not implicated in the rate determining process during catalysis. Irrespective of these differences, a common mechanism is proposed in which the rate determining steps are deduced to vary through the establishment of several pre-equilibria, the relative positions of which are determined by the respective stabilities of the dimeric and monomeric magnesium aldimide and magnesium aldimidoborate intermediates as a result of adjustments to the basicity of the nitrile substrate. More generally, these observations indicate that homogeneous processes performed under heavier alkaline earth catalysis are likely to demonstrate previously unappreciated mechanistic diversity.

/pdf/magnesium-catalysed-nitrile-hydroboration-1v28opio4e.pdf

Magnesium-catalysed nitrile hydroboration

Treatment of β-diketiminato Mg and Ca amidoborane compounds with B(C6F5)3 induces hydride elimination and formation of alkaline earth hydrido-tris(pentafluorophenyl)borate derivatives. Both species react with CO2 to provide formate complexes, one of which has been structurally characterised, and may be applied to the highly selective reductive hydroboration of CO2 with pinacolborane (HBpin) to provide the methanol equivalent, CH3OBpin.

/pdf/selective-reduction-of-co2-to-a-methanol-equivalent-by-b-pnn14ltvzu.pdf

Selective reduction of CO2 to a methanol equivalent by B(C6F5)3-activated alkaline earth catalysis

The potassium aluminyl complex K[Al(NONAr )] (NON=NONAr =[O(SiMe2 NAr)2 ]2- , Ar=2,6-iPr2 C6 H3 ) reacts with 1,3,5,7-cyclooctatetraene (COT) to give K[Al(NONAr )(COT)]. The COT-ligand is present in the asymmetric unit as a planar μ2 -η2 :η8 -bridge between Al and K, with additional K⋅⋅⋅π-aryl interactions to neighboring molecules that generate a helical chain. DFT calculations indicate significant aromatic character, consistent with reduction to [COT]2- . Addition of 18-crown-6 causes a rearrangement of the C8 -carbocycle to form the isomeric 9-aluminabicyclo[4.2.1]nona-2,4,7-triene anion.

Reduction vs. Addition: The Reaction of an Aluminyl Anion with 1,3,5,7-Cyclooctatetraene.

Reactions of β-diketiminato magnesium and calcium hydrides with 1 atm of CO result in a reductive coupling process to produce the corresponding derivatives of the cis-ethenediolate dianion. Computational (DFT) analysis of this process mediated by Ca, Sr, and Ba highlights a common mechanism and a facility for the reaction that is enhanced by increasing alkaline earth atomic weight. Reaction of CO with PhSiH3 in the presence of the magnesium or calcium hydrides results in catalytic reduction to methylsilane and methylene silyl ether products, respectively. These reactions are proposed to ensue via the interception of initially formed group 2 formyl intermediates, an inference which is confirmed by a DFT analysis of the magnesium-centered reaction. The computational results identify the rate-determining process, requiring traversal of a 33.9 kcal mol-1 barrier, as a Mg-H/C-O σ-bond metathesis reaction, associated with the ultimate cleavage of the C-O bond. The carbonylation reactivity is extended to a variety of magnesium and calcium amides. With primary amido complexes, which for calcium include a derivative of the parent [NH2]- anion, CO insertion is facile and ensues with subsequent nitrogen-to-carbon migration of hydrogen to yield a variety of dinuclear and, in one case, trinuclear formamidate species. The generation of initial carbenic carbamoyl intermediates is strongly implicated through the isolation of the CO insertion product of a magnesium N-methylanilide derivative. These observations are reinforced by a DFT analysis of the calcium-centered reaction with aniline, which confirms the exothermicity of the formamidate formation (ΔH = -67.7 kcal mol-1). Stoichiometric reduction of the resultant magnesium and calcium formamidates with pinacolborane results in the synthesis of the corresponding N-borylated methylamines. This takes place via a sequence of reactions initiated through the generation of amidatohydridoborate intermediates and a cascade of reactivity that is analogous to that previously reported for the deoxygenative hydroboration of organic isocyanates catalyzed by the same magnesium hydride precatalyst. Although a sequence of amine formylation and deoxygenation may be readily envisaged for the catalytic utilization of CO as a C1 source in the production of methylamines, our observations demonstrate that competitive amine-borane dehydrocoupling is too facile under the conditions of 1 atm of CO employed.

/pdf/alkaline-earth-centered-co-homologation-reduction-and-amine-2qbp9o7uxa.pdf

Alkaline Earth-Centered CO Homologation, Reduction, and Amine Carbonylation

Reaction between a β-diketiminato magnesium hydride and carbon monoxide results in the isolation of a dimeric cis-enediolate species through the reductive coupling of two CO molecules. Under catalytic conditions with PhSiH3, an observable magnesium formyl species may be intercepted for the mild reductive cleavage of the CO triple bond.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.201505851

Mathew D. Anker

Papers

Magnesium-catalysed nitrile hydroboration

Selective reduction of CO2 to a methanol equivalent by B(C6F5)3-activated alkaline earth catalysis

Reduction vs. Addition: The Reaction of an Aluminyl Anion with 1,3,5,7-Cyclooctatetraene.

Alkaline Earth-Centered CO Homologation, Reduction, and Amine Carbonylation

Alkaline-Earth-Promoted CO Homologation and Reductive Catalysis