Compounds of the type aryl--M--X, with M=Ca, Sr, Ba and X as any kind of ligand (such as halide, phosphanide, amide, aryl), are presented. The low reactivity of the heavy alkaline earth metals calcium, strontium, and barium enforces an activation prior to use for the direct synthesis. The insertion of these metals into C--I bonds of aryl iodides (direct synthesis) yields aryl metal iodides and has to be performed at low temperatures and in THF. Aryl alkaline-earth-metal compounds show some characteristics: 1) the ease of ether cleavage enforces low reaction temperatures, 2) for Sr and Ba the Schlenk equilibrium is shifted towards homoleptic MI2 and MPh2, 3) high solubility of diaryl alkaline-earth-metal derivatives in THF even at low temperatures initiated quantum chemical investigations on the aggregation behavior, and 4) a strong low field shift of the 13C resonances of the ipso carbon atoms in NMR spectra was observed. First results from quantum chemical calculations on diaryl dicalcium(I) suggest a long Ca--Ca bond with a considerable Ca--Ca bond dissociation energy. Initial results on a selection of applications such as metallation, metathesis, and addition reactions of aryl calcium compounds are presented as well.

Heavy Grignard Reagents: Challenges and Possibilities of Aryl Alkaline Earth Metal Compounds

New developments in the organometallic chemistry of calcium, strontium and barium

The organocalcium chemistry developed vastly during the last decade The preparation of the organocalcium compounds via direct synthesis (insertion of Ca into a C-X bond of phenyl halides, Grignard reaction) affords skilful procedures due to the inertia of the calcium metal and the extreme reactivity of the organocalcium derivatives Further suitable preparative methods include metathesis reactions of CaX2 with KR or LiR, metallation reactions of H-acidic substrates, metal-halogen exchange reactions, and transmetallation of heavy main group atoms in their compounds with calcium metal Possibilities to stabilize organocalcium compounds include steric shielding by bulky ligands at the periphery and electronic reduction of the nucleophilicity of the calcium-bound carbanions Selected applications in catalysis such as hydrophosphination are also mentioned Very recent developments and challenges in the preparation of alkaline earth metal(I) compounds are presented as well Concepts to overcome the rather large atomization energies of the metals are discussed



Die Organocalciumchemie zeigte in den letzten zehn Jahren eine sturmische Entwicklung Der Gegensatz zwischen der Reaktionstragheit des Calciummetalls und der enormen Reaktivitat der Organocalcium-Verbindungen verlangt anspruchsvolle praparative Verfahren bei der Direktsynthese der schweren Grignard-Verbindungen (Einschub von Ca in eine C-X-Bindung bei Phenylhalogeniden) Metathesereaktionen von CaX2 mit KR oder LiR, Metallierung H-acider Substrate, Metall-Halogen-Austauschreaktionen und Transmetallierungen von Verbindungen schwerer Hauptgruppenelemente mit Calciummetall stellen weitere geeignete Syntheseverfahren fur calciumorganische Verbindungen dar Moglichkeiten zur Stabilisierung von Organocalcium-Verbindungen schliesen den sterischen Schutz durch Raum fullende Reste an der Molekulperipherie ebenso ein wie die elektronische Reduzierung der Nucleophilie der Calcium-gebundenen Carbanionen Ausgewahlte katalytische Anwendungen wie beispielsweise Hydrophosphanierung werden vorgestellt Die aktuellen Entwicklungen im Bereich der einwertigen Erdalkalimetall(I)-Verbindungen werden ebenfalls diskutiert Synthesekonzepte konnen nur dann isolierbare Molekule liefern, wenn die grose Verdampfungsenthalpie der Metalle aufgebracht und das Ausfallen von Metall verhindert wird

Recent Developments in the Organic Chemistry of Calcium – An Element with Unlimited Possibilities in Organometallic Chemistry?

Heavy Grignard reagents—Synthesis and reactivity of organocalcium compounds

Organocalcium chemistry is still in its infancy. The direct synthesis of activated calcium and (substituted) iodobenzenes allows for the large-scale and high-yield synthesis of aryl calcium iodides. The influence of the substitution patterns of the phenyl group, halogen atom, and solvent is discussed. Aryl calcium iodides show a Schlenk equilibrium that enables the isolation of diaryl calcium derivatives. Owing to the high reactivity of aryl calcium halides, low temperatures have to be maintained throughout the preparative procedures in order to avoid side reactions. A decrease of reactivity and, hence, an enhanced stability at higher temperatures can be achieved by shielding of the calcium atom by increasing the coordination number of the metal center or by substitution of the iodide anion by bulky groups.

H. Ogawa

Papers

Preparation and reactions of solvent-free arylcalcium halides ArCaX (X = F, Cl, Br)