Alkaline Earth Catalyzed CO₂ Hydroboration into Acetal Derivatives Leading to C–S Bond Formation

The use of Mg and Ca hydride catalysts for the 4e– reduction of CO2 is reported. Borabicyclo[3.3.1]nonane (9-BBN) and dicyclohexylborane (HBCy2), as reductants, led to the generation of the corresponding bis(boryl)acetal (BBA) which in situ reacted with thiol to afford new borylated hemithioacetal compounds (R2BOCH2SR) under mild neutral conditions. These hemithioacetals were then converted into dithioacetal (RSCH2SR) with the addition of a second equivalent of thiol under acidic activation, or amino-methyl sulfide (RSCH2NR2) upon reaction with secondary amine

Reactivity Studies of [(thf)₂Mg{μ-C(CH₃)₂C₂H₄C(CH₃)₂}]₂: Scrambling Reactions and Diverse Reactions with Dichlorophenylphosphane

In THF solution [{(thf)₂Mg­{μ-C­(CH₃)₂C₂H₄(CH₃)₂C}}₂] (<b>1</b>) exchanges the alkanediide ligand with [{(thf)₂Mg­{μ-CH₂)₅}}₂] in an equilibrium leading to the formation of [{(thf)₂Mg}₂{μ-C­(CH₃)₂C₂H₄C­(CH₃)₂}­{μ-(CH₂)₅}] (<b>2</b>). Depending on the crystallization temperature, homoleptic <b>1</b> or heteroleptic <b>2</b> crystallizes from THF solutions, verifying a temperature-dependent Schlenk equilibrium. Irradiation of a solution of <b>1</b> in [D₈]­THF with UV light yields magnesium hydride and alkene via a β-hydride elimination reaction. In a metathetical approach dichlorophenylphosphane reacts with <b>1</b> in THF to give the intermediate “PhP­(Cl)­{C­(CH₃)₂C₂H₄(CH₃)₂CMgCl” (<b>3-MgCl</b>), which forms three subsequent products. In order to ease handling and characterization of these compounds, hydrolysis and oxidation with sulfur has been performed. This product mixture was separated by column chromatography, yielding the chlorophosphane sulfide [Ph­(S)­P­(Cl)­{C­(CH₃)₂C₂H₄(CH₃)₂CH)}] (<b>3-S</b>), the cyclic phosphane sulfide [Ph­(S)­P­{C­(CH₃)₂C₂H₄(CH₃)₂C)}] (<b>4-S</b>), and the cyclic 1,1-diphosphane disulfide [{(Ph­(S)­P}₂{μ-C­(CH₃)₂C₂H₄(CH₃)₂C}] (<b>6-S₂</b>). Furthermore, traces of the acyclic 1,1-diphosphane disulfides [{PhP­(S)­(C­(CH₃)₂C₂H₄CH­(CH₃)₂)}­{PhP­(S)­(C­(CH₃)₂C₂H₄C­(CH₃)­(CH₂)}] (<b>8-S₂</b>) and <i>meso</i>-[{Ph­(S)­P­(C­(CH₃)₂C₂H₄(CH₃)₂CH)}₂] (<b>7-S₂</b>) have also been isolated. Compounds <b>6</b>–<b>8</b> represent the phosphorus-containing products of indirect Grignard reductions

Reactivity Studies of [(thf)₂Mg{μ-C(CH₃)₂C₂H₄C(CH₃)₂}]₂: Scrambling Reactions and Diverse Reactions with Dichlorophenylphosphane

In THF solution [{(thf)₂Mg­{μ-C­(CH₃)₂C₂H₄(CH₃)₂C}}₂] (<b>1</b>) exchanges the alkanediide ligand with [{(thf)₂Mg­{μ-CH₂)₅}}₂] in an equilibrium leading to the formation of [{(thf)₂Mg}₂{μ-C­(CH₃)₂C₂H₄C­(CH₃)₂}­{μ-(CH₂)₅}] (<b>2</b>). Depending on the crystallization temperature, homoleptic <b>1</b> or heteroleptic <b>2</b> crystallizes from THF solutions, verifying a temperature-dependent Schlenk equilibrium. Irradiation of a solution of <b>1</b> in [D₈]­THF with UV light yields magnesium hydride and alkene via a β-hydride elimination reaction. In a metathetical approach dichlorophenylphosphane reacts with <b>1</b> in THF to give the intermediate “PhP­(Cl)­{C­(CH₃)₂C₂H₄(CH₃)₂CMgCl” (<b>3-MgCl</b>), which forms three subsequent products. In order to ease handling and characterization of these compounds, hydrolysis and oxidation with sulfur has been performed. This product mixture was separated by column chromatography, yielding the chlorophosphane sulfide [Ph­(S)­P­(Cl)­{C­(CH₃)₂C₂H₄(CH₃)₂CH)}] (<b>3-S</b>), the cyclic phosphane sulfide [Ph­(S)­P­{C­(CH₃)₂C₂H₄(CH₃)₂C)}] (<b>4-S</b>), and the cyclic 1,1-diphosphane disulfide [{(Ph­(S)­P}₂{μ-C­(CH₃)₂C₂H₄(CH₃)₂C}] (<b>6-S₂</b>). Furthermore, traces of the acyclic 1,1-diphosphane disulfides [{PhP­(S)­(C­(CH₃)₂C₂H₄CH­(CH₃)₂)}­{PhP­(S)­(C­(CH₃)₂C₂H₄C­(CH₃)­(CH₂)}] (<b>8-S₂</b>) and <i>meso</i>-[{Ph­(S)­P­(C­(CH₃)₂C₂H₄(CH₃)₂CH)}₂] (<b>7-S₂</b>) have also been isolated. Compounds <b>6</b>–<b>8</b> represent the phosphorus-containing products of indirect Grignard reductions

Concept for Enhancement of the Stability of Calcium-Bound Pyrazolyl-Substituted Methanides

Metalation of bis­(3-thiophen-2-ylpyrazol-1-yl)­phenylmethane [<b>2</b>, which is accessible from the reaction of bis­(3-thien-2-ylpyrazol-1-yl)­methanone (<b>1</b>) with triphosgene] with [(thf)₂Ca­{N­(SiMe₃)₂}₂] in tetrahydrofuran and subsequent crystallization from a mixture of toluene and 1,2-dimethoxyethane yield [(dme)­Ca­{C­(Pz^th)₂Ph}­{N­(SiMe₃)₂}] (<b>3</b>). The α,α-bis­(3-thiophen-2-ylpyrazol-1-yl)­benzyl ligand exhibits a κ²<i>N</i>,κ<i>C</i>-coordination mode with a Ca–C σ-bond length of 262.8(2) pm. The crystalline compound is stable if air and moisture is strictly excluded; however, in solution; this calcium complex slowly degrades

Directed Ortho Calciation of 1,3-Bis(3-isopropylimidazol-2-ylidene)benzene

The deprotonation of 1,3-bis­(3-isopropylimidazol-2-ylidene)­benzene with Me₃SiCH₂CaX (X = Br, I) in tetrahydrofuran (THF) yields the ether adducts of the corresponding 2,6-bis­(3-isopropylimidazol-2-ylidene)­phenylcalcium halides (X = Br (<b>1</b>·2thf), I (<b>2</b>·2thf)). The crystallization behavior of <b>2</b> can be improved via substitution of ligated thf molecules by tetrahydropyran (thp) ligands, leading to <b>2</b>·2thp. These heteroleptic complexes <b>1</b>·2thf and <b>2</b>·2thp show very small Ca–C_ipso bond lengths to the ipso-carbon atoms of the aryl groups. Calciation of 1,3-bis­(3-isopropylimidazol-2-ylidene)­benzene with Ca­(CH₂SiMe₃)₂ in THP leads to the formation of ether-free homoleptic bis­[2,6-bis­(3-isopropylimidazol-2-ylidene)­phenyl]calcium (<b>3</b>). Intramolecular steric strain causes an elongation of the Ca–C_ipso bonds to the aryl groups. In all of these complexes, the Ca–C_carbene distances are significantly larger than those to the ipso-carbon atoms of the aryl groups

1,2-Bis(anilido)ethane Complexes of Calcium and Potassium: Synthesis, Structures, and Catalytic Activity

The metalation of 1,2-bis­(anilino)­ethane with excess KH leads to the formation of the potassium complex [(thf)₃K₂{1,2-(PhN)₂C₂H₄}] (<b>1</b>). Complex <b>1</b> quantitatively reacts with anhydrous CaI₂ in THF yielding insoluble KI and dinuclear [(thf)₅Ca₂{1,2-(PhN)₂C₂H₄}₂] (<b>2</b>) after crystallization from a mixture of THF and hexane. Addition of <i>N</i>,<i>N</i>,<i>N′</i>,<i>­N′′</i>,<i>N′′′</i>,<i>N′′′</i>-hexa­methyl­triethylene­tetraamine (hmteta) yields [(hmteta)­Ca­{1,2-(PhN)₂C₂H₄}] (<b>3</b>). The complexes <b>1</b> and <b>2</b> proved to be inactive as catalysts in hydroamination reactions of diphenylbutadiyne with secondary amines. However, a mixture of <b>1</b> and <b>2</b> (K:Ca ratio of 2:1) mediated the addition of <i>N</i>-methyl-aniline and 1,2-bis­(anilino)­ethane to one of the CC triple bonds of diphenylbutadiyne. Addition of 18-crown-6 ether (18C6) leads to the formation of the sparingly soluble potassium complex [{(18C6)­K}₂{1,2-(PhN)₂C₂H₄}] (<b>5</b>) and the insoluble calcium complex [(18C6)­Ca­{1,2-(PhN)₂C₂H₄}] (<b>6</b>). The calciate-mediated hydroamination reaction is regiocontrolled, but <i>E</i>- and <i>Z</i>-isomeric addition products are observed, regardless of whether the reaction is performed at daylight or in the dark. If toluene is used as solvent for this s-block metal-mediated hydroamination catalysis, (<i>Z,Z</i>)-1,4,5,8,9,12-hexaphenyl-5,8-diazadodeca-3,9-diene-1,11-diyne (<i>Z,Z</i>-<b>8</b>) precipitates, allowing isolation and characterization of this isomer. In solution, this compound isomerizes upon irradiation yielding an equilibrium between (<i>Z,Z</i>)-, (<i>E,Z</i>)- and (<i>E,E</i>)-isomers. The determination of the crystal structures of (<i>Z,Z</i>)- and (<i>E,E</i>)-1,4,5,8,9,12-hexaphenyl-5,8-diazadodeca-3,9-diene-1,11-diyne unequivocally allows the assignment of the NMR parameters to specific isomers

Trimethylsilylmethylcalcium Iodide, an Easily Accessible Grignard-Type Reagent of a Heavy Alkaline Earth Metal

The direct synthesis of iodomethyl­trimethylsilane with calcium and magnesium in ether yields the corresponding ether adducts [(thf)₄Ca­(I)­(CH₂SiMe₃)] (<b>1a</b>) and [(Et₂O)₂Mg­(I)­(CH₂SiMe₃)] (<b>2a</b>). The 1,4-dioxane method allows shifting the Schlenk equilibrium toward bis­(trimethylsilylmethyl)­magnesium <b>2b</b>. After substitution of ligated thf ligands by tetrahydropyran (thp) crystalline [(thp)₄Ca­(I)­(CH₂SiMe₃)] (<b>1b</b>) can be isolated. The Ca–C and Ca–I bond lengths of 252.7(3) and 319.11(3) pm represent characteristic values. Steric repulsion between ligated thp ligands and the rather bulky trimethylsilylmethyl group widens the Ca–C–Si angle to 131.19(14)°. NMR data and quantum chemical studies support that hyperconjugative effects might be operative, leading to a shortened Si–C bond of this Ca–C–Si fragment

Synthesis and Molecular Structures of Meta-Substituted Arylcalcium Iodides

The reduction of <i>meta</i>-methyl-substituted iodobenzene with activated calcium in tetrahydrofuran (THF) yields [(3-MeC₆H₄)­CaI­(thf)₄] (<b>1</b>) and [(3,5-Me₂C₆H₃)­CaI­(thf)₄] (<b>2</b>). The reaction of 3-halo-1-iodobenzene with calcium powder leads to the formation of the corresponding post-Grignard reagents [(3-XC₆H₄)­CaI­(thf)₄] [X = F (<b>3</b>), Cl (<b>4</b>), Br (<b>5</b>), and I (<b>6</b>)]. The synthesis of the thf adducts of 3-methoxyphenylcalcium iodide (<b>7</b>) and β-naphthylcalcium iodide (<b>8</b>) follows the same strategy. All post-Grignard reagents show a characteristic low-field shift for the calcium-bound carbon atoms in ¹³C NMR spectra. The molecular structures of <b>1</b>, <b>2</b>, <b>4</b>, and <b>8</b> show distorted octahedral environments for the calcium centers with the aryl and halide anions in a trans arrangement

Solution Stability of Organocalcium Compounds in Ethereal Media

Organocalcium compounds (post-Grignard reagents) of the type [Ca­(R)­(X)­(L)_<i>n</i>] are very reactive and able to degrade ethers with α- and β-deprotonation as possible first reaction step. In this systematic study, the durability of phenyl-, α-naphthyl-, and 1,2-dihydronaphth-4-ylcalcium derivatives in cyclic ethers such as tetrahydrofuran (THF), tetrahydropyran (THP), and α-methyltetrahydrofuran (Me-THF) is investigated. The temperature, solvent, and the nature of the second anionic ligand X [iodide, bis­(trimethylsilyl)­amide, α-naphthyl] significantly influence the durability of their ethereal solutions

Alkaline Earth Metal–Carbene Complexes with the Versatile Tridentate 2,6-Bis(3-mesitylimidazol-2-ylidene)pyridine Ligand

Diffusion of 2,6-bis­(3-mesitylimidazol-2-ylidene)­pyridine (Car^MesPyCar^Mes, <b>2</b>) into a solution of CaI₂ in THF leads to microcrystalline [(Car^MesPyCar^Mes)­(thf)­CaI₂] (<b>3</b>), in one case containing a few single crystals of the unique separated ion pair [(Car^MesPyCar^Mes)₂(thf)­Ca]­I₂ (<b>4</b>) with four Ca–C bonds. However, isolation of single-crystalline [(Car^MesPyCar^Mes)­(thf)­CaI₂] (<b>3</b>) succeeds via the addition of <b>2</b> to a solution of [(thf)₅CaI]⁺[BPh₄]⁻ due to a subsequent dismutation. Two modifications with the shapes of needles and cubes crystallize simultaneously. In contrast to this finding, the reaction of [(thf)₅CaI]⁺[AlPh₄]⁻ with <b>2</b> yields solvent separated [(Car^MesPyCar^Mes)­(thf)₂CaI]⁺[AlPh₄]⁻ (<b>5</b>). The Ca–C_NHC bond lengths lie in a typical range of Ca–C_Aryl σ bonds and represent the shortest Ca–C_NHC bonds known to date. Soluble [(Car^MesPyCar^Mes)­(thf)­Ca­(NPh₂)₂] (<b>6</b>) can be prepared via a metathetical approach from <b>4</b> and KNPh₂ as well as via the addition of <b>2</b> to Ca­(NPh₂)₂ in tetrahydrofuran. The bulkier amido ligands lead to elongated bonds between the calcium center and the ligand <b>2</b>. Furthermore, the reactions of MI₂ (M = Sr, Ba) with <b>2</b> yield [(Car^MesPyCar^Mes)­(thf)₂MI₂] (M = Sr (<b>7</b>), Ba (<b>8</b>))