Arylcalcium halides

Arylcalcium halides were firstly mentioned by Beckmann in 1905 [53] prepared by the reduction of aryl halides by calcium. However, several years later Gilman and Schulze [54] had difficulties verifying these results and summarized that... 

During the synthesis of arylcalcium halides via direct synthesis the reaction mixture was shaken continuously in order to generate a reactive surface by uninterrupted rubbing of the glass balls against each other. The immediate removal of... 

Recent investigations using this approach finally led to isolation of well-defined arylcalcium halides which could be characterized by X-ray diffi action methods [49, 89-92]. THF is the most widely used solvent in these reactions, but less polar tetrahydropyran (THP) or diethyl ether was found to be suitable, too. The content of formed arylcalcium halides in solution was determined by acidimetric titration of a hydrolyzed aliquot and yields up to 95 % were reported for some substrates in THF (Table 1). In most cases the post-Grignard reagents can be isolated as colorless mononuclear solvent adducts of the type [ArCaX(solv)J from the filtered reaction mixture in THF or THP by cooling. A yield of 70 % of solid [PhCaI(thf)4] was isolated under optimized conditions. The molecular structures of the diethyl ether solvates are not known as of yet. 

Table 1 Synthesis of arylcalcium halides via direct synthesis from haloarene and activated calcium powder...

A Schlenk flask with activated calcium and glass balls is filled with THF (20 mL per 15 mmol calcium) and cooled to 0 °C. Then, 0.8 equivalents of the aryl halide are slowly added. Thereafter, the flask is shaken at 0 °C for 1 h and for additional 6 h at ambient temperamre. The obtained dark-colored suspension is filtered through a Schlenk frit, covered with diatomaceous earth. The conversion is determined by acidic consumption of a hydrolyzed aliquot of the filtrate. The residue on the filter is extracted with additional portions of THF (20 mL) until the extracts are free of further substantial amounts of product as controlled by acidimetric titration of a hydrolyzed aliquot. Cooling of these solutions to —78 °C leads to precipitation of the appropriate arylcalcium halide within one day. The product is collected on a cooled Schlenk frit and dried in vacuo. Depending on the amount of co-crystallized calcium diiodide, recrystallization of the crude product from THF can be necessary in order to isolate pure po%i-Grignard complexes of the type [(thf) Ca(Ar)I]. The amount of bound thf is determined by integration of H NMR signals. 

The highest yields are achieved when iodoarenes are employed for the direct synthesis of arylcalcium halides. Lower yields are observed for the bromoarenes whereas the homologous fluoro- and chloroarenes show no reactivity toward calcium activated with ammonia. 

Mochida K, Ogawa H (1983) Preparation and reactions of solvent-free arylcalcium halides ArCaX (X = F, Cl, Br). J Organomet Chem 243 131-135... 

Fischer R, Gartner M, Gorls H, Westerhausen M (2006) Synthesis and spectroscopic properties of arylcalcium halides. OrganometaEcs 25 3496-3500... 

Arylcalcium complexes are commonly prepared in ethereal solutions via reduction of aryl halides with finely divided and highly reactive calcium powder. Because diethyl ether has a rather low boiling point these exothermic reactions are advantageously carried out in tetrahydrofuran (THF) or tetrahydropyran (TEfP). Due to an operative Schlenk equilibrium usually three species are present in solution heteroleptic [(Ar) CaI(L) ] as well as homoleptic [Cal2(L) ] and [(Ar)2Ca(L)J. Fractionated crystalh-zation and subsequent recrystallization are mandatory in order to remove excess of calcium, imreacted substrates, and ether degradation products. 

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