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

Reactivity of Strontium Hydride Supported by the Superbulky Hydrotris(pyrazolyl)borate Ligand

Hydrogenolysis of [(TpAd,iPr)Sr{CH(SiMe3)2}] (1) (TpAd,iPr = hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate) in hexane solution under 20 atm of H2 allowed for the isolation of strontium hydride [(TpAd,iPr)Sr(μ-H)]2 (2) in good yield. Complex 2 exhibits the dimeric nature in solid state, featuring two different bond modes between the Sr center and TpAd,iPr ligand. Treatment of complex 2 with PhC(H)═NtBu or PhCH2Bpin (Bpin = pinacolateborane) afforded the strontium amide complex [(TpAd,iPr)Sr{N(CH2Ph)(tBu)}] (4) and hydroborate complex [(TpAd,iPr)Sr{μ-HBpin(CH2Ph)}] (5), respectively. Reactions of complex 2 with 2-picoline, 2-phenylquinoline, or 2-phenylpyridine led to the formation of strontium 2-pyridylmethylene/2-picoline complex [(TpAd,iPr)Sr(2-CH2-Py)(2-picoline)] (6), reductively coupling diphenyl-biquinolide complex [{(TpAd,iPr)Sr}2(2,2'-Ph2-4,4'-dihydro-4,4'-biquinolide)] (7), and diphenyl-bipyridyl radical complex [(TpAd,iPr)Sr(6,6'-Ph2-2,2'-bipyridyl)] (8), separately. All of the complexes have been well characterized, including NMR spectrum and single-crystal X-ray analysis.

A series of neutral alkaline earth metal hydride complexes supported by a bulky, unsymmetrical β-diketiminate ligand, [{(Dip/TCHPNacnac)M(μ-H)}2] (M = Mg, Ca, Sr or Ba)

A bulky, unsymmetrical β-diketiminate ligand, [HC{MeCN(Dip)}{MeCN(TCHP)}]- (Dip/TCHPNacnac; Dip = 2,6-diisopropylphenyl, TCHP = 2,4,6-tricyclohexylphenyl), has been utilised in the preparation of a series of magnesium alkyl and calcium, strontium and barium amide complexes. Reaction of these with PhSiH3 afforded the first complete series of β-diketiminato heavier group 2 metal hydride complexes, [{(Dip/TCHPNacnac)M(μ-H)}2] (M = Mg, Ca, Sr or Ba). The unsymmetrical nature of the β-diketiminate ligand seemingly promotes stabilising interactions of ligand Dip groups with the metal centres in the Ca, Sr and Ba hydride complexes.

Transition Metal Mimetic π-Activation by Cationic Bismuth(III) Catalysts for Allylic C-H Functionalization of Olefins Using C═O and C═N Electrophiles

The discovery and utilization of main-group element catalysts that behave similarly to transition metal (TM) complexes have become increasingly active areas of investigation in recent years. Here, we report a series of Lewis acidic bismuth(III) complexes that allow for the catalytic allylic C(sp3)-H functionalization of olefins via an organometallic complexation-assisted deprotonation mechanism to generate products containing new C-C bonds. This heretofore unexplored mode of main-group reactivity was applied to the regioselective functionalization of 1,4-dienes and allylbenzene substrates. Experimental and computational mechanistic studies support the key steps of the proposed catalytic cycle, including the intermediacy of elusive Bi-olefin complexes and allylbismuth species.

Calcium(II)-Mediated Three-Component Selenylation of gem-Difluoroalkenes: Access to α,α-Difluoroalkyl-β-selenides

A calcium-mediated three-component selenylation of gem-difluoroalkenes using alcohols as nucleophiles and N-(phenylseleno)phthalimide as the selenylation agent has been developed for the efficient synthesis of various α,α-difluoroalkyl-β-selenides. This selenylation reaction exhibits broad substrate and functional group tolerance, along with high levels of chemo- and regioselectivity. Additionally, the synthetic utility of the developed transformation in the late-stage functionalization of drug molecules was demonstrated.

Fresh Impetus in the Chemistry of Calcium Peroxides

Redox-inactive metal ions are essential in modulating the reactivity of various oxygen-containing metal complexes and metalloenzymes, including photosystem II (PSII). The heart of this unique membrane-protein complex comprises the Mn4CaO5 cluster, in which the Ca2+ ion acts as a critical cofactor in the splitting of water in PSII. However, there is still a lack of studies involving Ca-based reactive oxygen species (ROS) systems, and the exact nature of the interaction between the Ca2+ center and ROS in PSII still generates intense debate. Here, harnessing a novel Ca-TEMPO complex supported by the β-diketiminate ligand to control the activation of O2, we report the isolation and structural characterization of hitherto elusive Ca peroxides, a homometallic Ca hydroperoxide and a heterometallic Ca/K peroxide. Our studies indicate that the presence of K+ cations is a key factor controlling the outcome of the oxygenation reaction of the model Ca-TEMPO complex. Combining experimental observations with computational investigations, we also propose a mechanistic rationalization for the reaction outcomes. The designed approach demonstrates metal-TEMPO complexes as a versatile platform for O2 activation and advances the understanding of Ca/ROS systems.

Ytterbium(II) Complex-Catalyzed Selective Single and Double Hydrophosphination of 1,3-Enynes

1,3-Enynes with conjugated alkene and alkyne moieties are attractive building blocks in synthetic chemistry. However, neither 4,1-hydrophosphination nor dihydrophosphination of 1,3-enynes has been reported. In this paper, the divalent ytterbium and calcium amide complexes supported by silaimine-functionalized cyclopentadienyl ligands (C5Me4-Si(L)=NR) were developed, which successfully catalyzed the efficient single and double hydrophosphination of 1,3-enynes with diarylphosphines. The hydrophosphination reactions selectively produced homoallenyl phosphines and (E)-propenylene diphosphines, respectively. This work demonstrated the potential of hemilabile silaimine-Cp ligands in the supporting the efficient and selective rare- and alkaline-earth catalysts.

Appropriation of group II metals: synthesis and characterisation of the first alkaline earth metal supported transition metal carbonite complexes

Nickel carbonite complexes supported by alkaline earth metals have been accessed via salt-metathesis of the corresponding alkali metal precursors. The new complexes undergo Schlenk-like exchange reactions in solution which have been investigated by NMR spectroscopy. Also their reactivity towards epoxides and carbon monoxide was studied.

Calcium-Ligand Cooperation Promoted Activation of N2O, Amine, and H2 as well as Catalytic Hydrogenation of Imines, Quinoline, and Alkenes

Bond activation and catalysis using s-block metals are of great significance. Herein, a series of calcium pincer complexes with deprotonated side arms have been prepared using pyridine-based PNP and PNN ligands. The complexes were characterized by NMR and X-ray crystal diffraction. Utilizing the obtained calcium complexes, unprecedented N2O activation by metal-ligand cooperation (MLC) involving dearomatization-aromatization of the pyridine ligand was achieved, generating aromatized calcium diazotate complexes as products. Additionally, the dearomatized calcium complexes were able to activate the N-H bond as well as reversibly activate H2, offering an opportunity for the catalytic hydrogenation of various unsaturated molecules. DFT calculations were applied to analyze the electronic structures of the synthesized complexes and explore possible reaction mechanisms. This study is an important complement to the area of MLC and main-group metal chemistry.

Alpha-metalated N,N-dimethylbenzylamine rare-earth metal complexes and their catalytic applications

This perspective summarizes our group's extensive research in the realm of organometallic lanthanide complexes, while also placing the catalytic reactions supported by these species within the context of known lanthanide catalysis worldwide, with a specific focus on phosphorus-based catalytic reactions such as intermolecular hydrophosphination and hydrophosphinylation. α-Metalated N,N-dimethylbenzylamine ligands have been utilized to generate homoleptic lanthanide complexes, which have subsequently proven to be highly active lanthanum-based catalysts. The main goal of our research program has been to enhance the catalytic efficiency of lanthanum-based complexes, which began with initial successes in the stoichiometric synthesis of organometallic lanthanide complexes and utilization of these species in catalytic hydrophosphination reactions. Not only have these species supported traditional lanthanide catalysis, such as the hydrophosphination of heterocumulenes like carbodiimides, isocyanates, and isothiocyanates, but they have also been effective for a plethora of catalytic reactions tested thus far, including the hydrophosphinylation and hydrophosphorylation of nitriles, hydrophosphination and hydrophosphinylation of alkynes and alkenes, and the heterodehydrocoupling of silanes and amines. Each of these catalytic transformations is meritorious in its own right, offering new synthetic routes to generate organic scaffolds with enhanced functionality while concurrently minimizing both waste generation and energy consumption. Objectives: We aim for the research summary presented herein to inspire and encourage other researchers to investigate f-element based stoichiometric and catalytic reactions. Our efforts in this field began with the recognition that potassium salts of benzyldimethylamine preferred deprotonation at the α-position, rather than the ortho-position, and we wondered if this regiochemistry would be retained in the formation of lanthanide complexes. The pursuit of this simple idea led first to a series of structurally fascinating homoleptic organometallic lanthanide complexes with surprisingly good stability. Fundamental studies of the protonolysis chemistry of these complexes ultimately revealed highly versatile lanthanide-based precatalysts that have propelled a catalytic investigation spanning more than a decade. We anticipate that this summative perspective will animate the synthetic as well as biological communities to consider La(DMBA)3-based catalytic methods in the synthesis of functionalized organic scaffolds as an atom-economic, convenient, and efficient methodology. Ultimately, we envision our work making a positive impact on the advancement of novel chemical transformations and contributing to progress in various fields of science and technology.

Fluorenyl-tethered N-heterocyclic carbene (NHC): an exclusive C-donor ligand for heteroleptic calcium and strontium chemistry

Exclusive C-donating ligands are rarely used with kinetically labile heavier alkaline earths (Ca, Sr, Ba). We report herein the aptitude of a combination of NHC with fluorenyl connected by a flexible -(CH2)2- linker as a ligand support for heteroleptic Ca- and Sr-N(SiMe3)2 and iodides. The Ca-N(SiMe3)2 complex even catalyzes the intramolecular hydroamination of aminoalkenes to showcase the effectiveness of this ligand framework.

Ca(II) and Yb(II) complexes featuring M(C≡C)4 structural motif: enforced proximity or genuine η2 -bonding?

Bis(carbazolide) complexes M[3,6-tBu2 -1,8-(RC≡C)2 Carb]2 (THF)n (R=SiMe3 , n=0, M=Ca, Yb; R=Ph, n=1, M=Ca, Yb; n=0, M=Yb) were synthesized through transamination reaction of M[N(SiMe3 )2 ]2 (THF)2 with two molar equivalents of carbazoles. The complexes feature M(η2 -C≡C)4 structural motif composed of M(II) ions encapsulated by four acetylene fragments due to atypical for alkaline- and rare-earth metals η2 -interactions with triple C≡C bond. This interaction is evidenced experimentally by X-ray diffraction, Raman spectroscopy in the solid state and by NMR-spectroscopy in the solution. According to QTAIM analysis there are 4 bond critical points (3;-1) between the metal atom and each of the triple bonds, which are connected by a strongly curved, almost T-shaped bond pathway.

Zinc hydride catalyzed hydroboration of esters

The conjugated bis-guanidinate (CBG)-supported zinc hydride {LZnH}2; L = {(ArHN)(ArN)-CN-C(NAr)(NHAr); Ar = 2,6-Et2-C6H3} (I) is utilized as a catalyst for the hydroboration of esters with pinacolborane (HBpin) under mild reaction conditions. Various aryl and alkyl substrates containing electron-donating, withdrawing, and cyclic groups of esters are effectively converted into alkoxy boronate esters as products upon hydroboration. Furthermore, stoichiometric experiments have been performed to understand the plausible reaction mechanism for the hydroboration of esters. Additionally, complex (I) was used for the hydroboration of carbonate, carboxylic acid, and anhydride substrates to showcase the broad substrate scope.

Borataalkenes, boraalkenes, and the η2-B,C coordination mode in coordination chemistry and catalysis

Borataalkenes and boraalkenes are the boron-containing isoelectronic analogues of alkenes and vinyl cations respectively. Compared with alkenes, the borataalkene and boraalkene ligand motifs in transition metal coordination chemistry are relatively underexplored. In this review, the synthesis of borataalkene and boraalkene complexes and other transition metal complexes featuring the η2-B,C coordination mode is described. The diversity of coordination modes and geometry in these complexes, and the spectroscopic and structural evidence supporting their assignments is disclosed as well as computational analysis of bonding. The applications of the borataalkene ligand motif in synthetic organic homogeneous catalysis, especially those involving geminal bis(pinacolatoboronates) and 1,4-azaborines, are discussed.

Alkaline earth metals: homometallic bonding

The study of alkaline earth metal complexes is undergoing a renaissance. Stable molecular species featuring Mg-Mg bonds were reported in 2007 and their reactivity has since been intensively investigated. Motivated by this work, efforts have also been devoted to the synthesis of complexes featuring Be-Be and Ca-Ca bonds. These collective endeavours have revealed that the chemistry of the group 2 metals is richer and more complex than had previously been appreciated. Here, a discussion of the nature of homometallic alkaline earth bonding is presented, recent synthetic advances are described, and future directions are considered.

Calcium(II)-Mediated Three-Component Selenofunctionalization of Alkenes under Mild Conditions

A mild and general protocol involving amnio- and oxyselenation of diverse alkenes for the efficient synthesis of organo-Se compounds is achieved via an environmentally benign calcium-catalyzed three-component reaction. This selenofunctionalization reaction exhibits excellent substrate/functional group tolerance and high levels of chemo- and regioselectivity. Its utility was exemplified in the late-stage functionalization and even aggregation-induced emission luminogen labeling of organo-Se compounds.

Accelerating σ-Bond Metathesis at Sn(II) Centers

Molecular main-group hydride catalysts are attractive as cheap and Earth-abundant alternatives to transition-metal analogues. In the case of the latter, specific steric and electronic tuning of the metal center through ligand choice has enabled the iterative and rational development of superior catalysts. Analogously, a deeper understanding of electronic structure-activity relationships for molecular main-group hydrides should facilitate the development of superior main-group hydride catalysts. Herein, we report a modular Sn-Ni bimetallic system in which we systematically vary the ancillary ligand on Ni, which, in turn, tunes the Sn center. This tuning is probed using Sn L1 XAS as a measure of electron density at the Sn center. We demonstrate that increased electron density at Sn centers accelerates the rate of σ-bond metathesis, and we employ this understanding to develop a highly active Sn-based catalyst for the hydroboration of CO2 using pinacolborane. Additionally, we demonstrate that engineering London dispersion interactions within the secondary coordination sphere of Sn allows for further rate acceleration. These results show that the electronics of main-group catalysts can be controlled without the competing effects of geometry perturbations and that this manifests in substantial reactivity differences.

Variable Ca-Caryl Hapticity and its Consequences in Arylcalcium Dimers

The dimeric β-diketiminato calcium hydride, [(Dipp BDI)CaH]2 (Dipp BDI = HC{(Me)CN-2,6-i-Pr2 C6 H3 }2 ), reacts with ortho-, meta- or para-tolyl mercuric compounds to afford hydridoarylcalcium compounds, [(Dipp BDI)2 Ca2 (μ-H)(μ-o-,m-,p-tolyl)], in which dimer propagation occurs either via μ2 -η1 -η1 or μ2 -η1 -η6 bridging between the calcium centers. In each case, the orientation and hapticity of the aryl units is dependent upon the position of the methyl substituent. While wholly organometallic meta- and para-tolyl dimers, [(Dipp BDI)Ca(m-tolyl)]2 and [(Dipp BDI)Ca(p-tolyl)]2 , can be prepared and are stable, the ortho-tolyl isomer is prone to isomerization to a calcium benzyl analog. Computational analysis of this latter process with density functional theory (DFT) highlights an unusual mechanism invoking the generation of an intermediate dicalcium species in which the group 2 centers are bridged by a toluene dianion formed by the formal attachment of the original hydride anion to the initially generated ortho-tolyl substituent. Use of a more sterically encumbered aryl substituent, {3,5-t-Bu2 C6 H3 }, facilitates the selective formation of [(Dipp BDI)Ca(μ-H)(μ-3,5-t-Bu2 C6 H3 )Ca(Dipp BDI)], which can be converted into the unsymmetrically-substituted σ-aryl calcium complexes, [(Dipp BDI)Ca(μ-Ph)(μ-3,5-t-Bu2 C6 H3 )Ca(Dipp BDI)] and [(Dipp BDI)Ca(μ-p-tolyl)(μ-3,5-t-Bu2 C6 H3 )Ca(Dipp BDI)] by reaction with the appropriate mercuric diaryl. Conversion of [(Dipp BDI)Ca(H)(Ph)Ca(Dipp BDI)] to afford [{{(Dipp BDI)Ca}2 (μ2 -Cl)}2 (C6 H5 -C6 H5 )], comprising a biphenyl dianion, is also reported. Although this latter transformation is serendipitous, AIM analysis highlights that, in a related manner to the ortho-tolyl to benzyl isomerization, the requisite C-C coupling may be facilitated in an "across dimer" fashion by the experimentally-observed polyhapto engagement of the aryl substituents with each calcium.

Organocalcium-Complex-Catalyzed Dehydrogenative Silylation and Mono/Dihydrosilylation Tandem Reactions of Terminal Alkynes

In principle, catalytic dehydrogenative silylation and mono/dihydrosilylation tandem reactions of terminal alkynes with hydrosilanes provide gem-disilylated alkenes or gem-trisilylated alkanes, but very little progress has been made. Herein, we report organocalcium-complex-catalyzed dehydrogenative silylation and mono/dihydrosilylation tandem reactions of terminal alkynes with hydrosilanes in one pot, which produce gem-disilylated alkenes in moderate yields and gem-trisilylated alkanes in high yields. We also briefly demonstrate that the synthesized gem-disilylated alkenes can be easily transformed into other organosilanes.

Organocalcium Hydride-Catalyzed Intramolecular C(sp3)-H Annulation of Functionalized 2,6-Lutidines

This work reports an intramolecular C(sp³)-H annulation of functionalized 2,6-lutidines catalyzed by an organocalcium hydride [{(^DIPPnacnac)CaH(thf)}₂] (^DIPPnacnac = CH{(CMe)(2,6-ⁱPr₂-C₆H₃N)}₂). This reaction constitutes a streamlined approach for producing a new family of tetrahydro-1,5-naphthyridines and hexahydropyrido[3,2-b]azocines derivatives in good to excellent yields with high atom efficiency and broad substrates scope. A calcium alkyl complex was isolated from the stoichiometric reaction between calcium hydride and the substrate through deprotonation, which was structurally characterized and confirmed as the catalytic intermediate.

Calcium-catalysed synthesis of amines through imine hydrosilylation: an experimental and theoretical study

A method to reduce aldimines through hydrosilylation is reported. The catalytic system involves calcium triflimide (Ca(NTf₂)₂) and potassium hexafluorophosphate (KPF₆) which have been shown to act in a synergistic manner. The expected amines are obtained in fair to very high yields (40-99%) under mild conditions (room temperature in most cases). To illustrate the potential of this method, a bioactive molecule with antifungal properties was prepared on the gram scale and in high yield in environmentally friendly 2-methyltetrahydrofuran. Moreover, it is shown in this example that the imine can be prepared in situ from the aldehyde and the amine without isolating the imine. The mechanism involved has been explored experimentally and through DFT calculations, and the results are in accordance with an electrophilic activation of the silane by the calcium catalyst.

Calcium Hydride Cation Dimer Catalyzed Hydrogenation of Unactivated 1-Alkenes and H2 Isotope Exchange: Competitive Ca-H-Ca Bridges and Terminal Ca-H Bonds

Recently, it was shown that the double Ca-H-Ca bridged calcium hydride cation dimer complex [LCaH₂ CaL]²⁺ (macrocyclic ligand L=NNNN-tetradentate Me₄ TACD) exhibited remarkable activity in catalyzing the hydrogenation of unactivated 1-alkenes as well as the H₂ isotope exchange under mild conditions, tentatively via the terminal Ca-H bond of cation monomer LCaH⁺ . In this DFT mechanistic work, a novel substrate-dependent catalytic mechanism is disclosed involving cooperative Ca-H-Ca bridges for H₂ isotope exchange, competitive Ca-H-Ca bridges and terminal Ca-H bonds for anti-Markovnikov addition of unactivated 1-alkenes, and terminal Ca-H bonds for Markovnikov addition of conjugation-activated styrene. THF-coordination plays a key role in favoring the anti-Markovnikov addition while strong cation-π interactions direct the Markovnikov addition to terminal Ca-H bonds.

Origin of the Ligand Ring-Size Effect on the Catalytic Activity of Cationic Calcium Hydride Dimers in the Hydrogenation of Unactivated 1-Alkenes

Recently, it was shown that the double Ca-H-Ca-bridged calcium hydride cation dimer [LCaH2 CaL]2+ when stabilized by a larger macrocyclic N,N',N'',N''',N''''-pentadentate ligand showed evidently higher activity than when stabilized by a smaller N,N',N'',N'''-tetradentate ligand in the catalytic hydrogenation of unactivated 1-alkenes. In this DFT-mechanistic work, the origin of the observed ring-size effect is examined in detail using 1-hexene, CH2 =CH2 and H2 as substrates. It is shown that, at room temperature, both the N,N',N'',N''',N''''-stabilized dimer and the monomer are not coordinated by THF in solution, while the corresponding N,N',N'',N'''-stabilized structures are coordinated by one THF molecule mimicking the fifth N-coordination. Catalytic 1-alkene hydrogenation may occur via anti-Markovnikov addition over the terminal Ca-H bonds of transient monomers, followed by faster Ca-C bond hydrogenolysis. The higher catalytic activity of the larger N,N',N'',N''',N''''-stabilized dimer is due to not only easier formation of but also due to the higher reactivity of the catalytic monomeric species. In contrast, despite unfavorable THF-coordination in solution, the smaller N,N',N'',N'''-stabilized dimer shows a 3.2 kcal mol-1 lower barrier via a dinuclear cooperative Ca-H-Ca bridge for H2 isotope exchange than the large N,N',N'',N''',N''''-stabilized dimer, mainly due to less steric hindrance. The observed ring-size effect can be understood mainly by a subtle interplay of solvent, steric and cooperative effects that can be resolved in detail by state-of-the-art quantum chemistry calculations.

Chemo- and Stereoselective Synthesis of Substituted Thiazoles from tert-Alcohols Bearing Alkene and Alkyne Groups with Alkaline Earth Catalysts

We report here that thioamides can distinguish C-C double bonds and C-C triple bonds chemoselectively when subjected to a reaction with pent-1-en-4-yn-3-ol derivatives in the presence of Ca(OTf)₂. This protocol offers a fast, efficient, and high-yielding synthesis of functionalized thiazoles. Interestingly, this reaction offers a time-dependent formation of kinetic and thermodynamic products. The products showed stereoselectivity concerning the alkene geometry. Further, we extended this protocol to synthesize oxazoles from propargyl alcohols and ibuprofen (NSAID) was converted into amide and then subjected to oxazole formation with tert-propargyl alcohols.

Unified Approach to Diverse Fused Fragments via Catalytic Dehydrative Cyclization

A range of highly functionalized polycyclic fragments have been synthesized, employing a catalytic dehydrative cyclization. A range of nucleophiles are shown to be successful, with the reaction producing numerous high value motifs.

Synthesis, characterisation and reactivity of group 2 complexes with a thiopyridyl scorpionate ligand

Herein we report the reactivity of the proligand tris(2-pyridylthio)methane (HTptm) with various Alkaline Earth (AE) reagents: (1) dialkylmagnesium reagents and (2) AE bis-amides (AE = Mg-Ba). Heteroleptic complexes of general formulae [Mg(Tptm)(R)] (R = Me, ⁿBu; Tptm = {C(S-C₅H₄N)₃}^-) and [AE(Tptm)(N'')] (AE = Mg-Ba; N'' = {N(SiMe₃)₂}^-) were targeted from the reaction of HTptm with R₂Mg or [AE(N'')₂]₂. Reaction of the proligand with dialkylmagnesium reagents led to formation of [{Mg(κ³C,N,N-C{Bu}{S-C₅H₄N}₂)(μ-S-C₅H₄N)}₂] (1) and [{Mg(κ³C,N,N-C{Me}{S-C₅H₄N}₂)(μ-OSiMe₃)}₂] (2) respectively, as a result of a novel transfer of an alkyl group onto the methanide carbon with concomitant C-S bond cleavage. However, reactivity of bis-amide precursors for Mg and Ca did afford the target species [AE(Tptm)(N'')] (3-AE; AE = Mg-Ca), although these proved susceptible to ligand degradation processes. DFT calculations show that alkyl transfer in the putative [Mg(Tptm)(ⁿBu)] (1m') system and amide transfer in 3-Ca is a facile process that induces C-S bond cleavage in the Tptm ligand. 3-Mg and 3-Ca were also tested as catalysts for the hydrophosphination of selected alkenes and alkynes, including the first example of mono-hydrophosphination of 4-ethynylpyridine which was achieved with high conversions and excellent regio- and stereochemical control.

Calcium Bistriflimide-Mediated Sulfur(VI)-Fluoride Exchange (SuFEx): Mechanistic Insights toward Instigating Catalysis

We report a mechanistic investigation of calcium bistriflimide-mediated sulfur(VI)–fluoride exchange (SuFEx) between sulfonyl fluorides and amines. We determine the likely pre-activation resting state—a calcium bistriflimide complex with ligated amines—thus allowing for corroborated calculation of the SuFEx activation barrier at ∼21 kcal/mol, compared to 21.5 ± 0.14 kcal/mol derived via kinetics experiments. Transition state analysis revealed: (1) a two-point calcium-substrate contact that activates the sulfur(VI) center and stabilizes the leaving fluoride and (2) a 1,4-diazabicyclo[2.2.2]octane additive that provides Brønsted-base activation of the nucleophilic amine. Stable Ca–F complexes upon sulfonamide formation are likely contributors to inhibited catalytic turnover, and a proof-of-principle redesign provided evidence that sulfonamide formation is feasible with 10 mol % calcium bistriflimide.

We report a computational and experimental mechanistic study of sulfur(VI)-fluoride exchange mediated by a calcium salt. Ca²⁺ activates the substrate via two critical Lewis acid−base interactions. Stable fluoride-ligated Ca²⁺ complexes are formed upon sulfonamide formation, and computations indicate that heat and/or fluoride scavengers will facilitate regeneration of the active Ca²⁺ species. These calculations guided an experimental redesign, thus demonstrating improved catalytic efficiency using 10 mol % Ca²⁺.

Teaming up main group metals with metallic iron to boost hydrogenation catalysis

Hydrogenation of unsaturated bonds is a key step in both the fine and petrochemical industries. Homogeneous and heterogeneous catalysts are historically based on noble group 9 and 10 metals. Increasing awareness of sustainability drives the replacement of costly, and often harmful, precious metals by abundant 3d-metals or even main group metals. Although not as efficient as noble transition metals, metallic barium was recently found to be a versatile hydrogenation catalyst. Here we show that addition of finely divided Fe⁰, which itself is a poor hydrogenation catalyst, boosts activities of Ba⁰ by several orders of magnitude, enabling rapid hydrogenation of alkynes, imines, challenging multi-substituted alkenes and non-activated arenes. Metallic Fe⁰ also boosts the activity of soluble early main group metal hydride catalysts, or precursors thereto. This synergy originates from cooperativity between a homogeneous, highly reactive, polar main group metal hydride complex and a heterogeneous Fe⁰ surface that is responsible for substrate activation.

Elemental iron turns alkaline-earth metal complexes into highly active catalysts for the hydrogenation of alkenes, alkynes, imines and arenes. The proposed mechanism combines homogeneous catalysis by a soluble main group metal hydride complex with heterogeneous catalysis at the iron surface.

Synthesis of Molecular Phenylcalcium Derivatives: Application to the Formation of Biaryls

Hydrocarbon-soluble β-diketiminato phenylcalcium derivatives, which display various modes of Ca-μ2 -Ph-Ca bridging, are accessible from reactions of Ph2 Hg and [(BDI)CaH]2 . Although the resultant compounds are inert toward the C-H bonds of benzene, they yield selective and uncatalyzed biaryl formation when reacted with readily available aryl bromides.

Rare Earth Starting Materials and Methodologies for Synthetic Chemistry

The number of rare earth (RE) starting materials used in synthesis is staggering, ranging from simple binary metal-halide salts to borohydrides and "designer reagents" such as alkyl and organoaluminate complexes. This review collates the most important starting materials used in RE synthetic chemistry, including essential information on their preparations and uses in modern synthetic methodologies. The review is divided by starting material category and supporting ligands (i.e., metals as synthetic precursors, halides, borohydrides, nitrogen donors, oxygen donors, triflates, and organometallic reagents), and in each section relevant synthetic methodologies and applications are discussed.

s-Block Metal Catalysts for the Hydroboration of Unsaturated Bonds

The addition of a B-H bond to an unsaturated bond (polarized or unpolarized) is a powerful and atom-economic tool for the synthesis of organoboranes. In recent years, s-block organometallics have appeared as alternative catalysts to transition-metal complexes, which traditionally catalyze the hydroboration of unsaturated bonds. Because of the recent and rapid development in the field of hydroboration of unsaturated bonds catalyzed by alkali (Li, Na, K) and alkaline earth (Mg, Ca, Sr, Ba) metals, we provide a detailed and updated comprehensive review that covers the synthesis, reactivity, and application of s-block metal catalysts in the hydroboration of polarized as well as unsaturated carbon-carbon bonds. Moreover, we describe the main reaction mechanisms, providing valuable insight into the reactivity of the s-block metal catalysts. Finally, we compare these s-block metal complexes with other redox-neutral catalytic systems based on p-block metals including aluminum complexes and f-block metal complexes of lanthanides and early actinides. In this review, we aim to provide a comprehensive, authoritative, and critical assessment of the state of the art within this highly interesting research area.