Heavy Grignard reagents—Synthesis and reactivity of organocalcium compounds

First investigations regarding the organic chemistry of the heavy alkaline earth metals calcium, strontium, and barium date back more than 100 years. However, challenges in the organocalcium chemistry concern the discrepancy between the unreactive metal itself and the extremely large reactivity of organocalcium compounds. Therefore, the metal has to be activated prior to use via co-condensation of the metal vapour with the substrate or by producing finely divided metal powders. The enormous reactivity of organocalcium compounds initiates side-reactions such as ether cleavage reactions. This fact enforces reaction procedures at very low temperatures. Quite often, the organocalcium compounds are only sparingly soluble due to a salt-like behaviour caused by ionic bonding situations. This problem can be overcome by bulky substituents at the periphery of the molecule or by coordination of Lewis bases such as ethers (e.g. THF, DME, [18]crown-6) to the metal centers. With the understanding of these challenges, many organocalcium compounds were prepared within the last years and characterized structurally. Preparative procedures include the direct synthesis of calcium with alkyl- and arylhalides, the metathesis reaction of calcium diiodides with potassium derivatives, metallation of H-acidic substrates with calcium compounds, transmetallation of mercury compounds with calcium, and reduction of alkenes with calcium. Stabilization of the carbanion and enhancement of the solubility of alkylcalcium compounds often succeeds by phenyl and trimethylsilyl groups in α-position. First applications of organocalcium compounds are also mentioned in this review.