Aryl calcium halide

Scheme 1 A generalized decomposition route for aryl calcium halide complexes. The acidic proton from the THE co-ligand is abstracted, yielding benzene, followed by liberation of ethene and formation of an aryl calcium ethenolate. The above mechanism is proposed based on observations from NMR experiments [71] and is in agreement with other estabhshed ether cleavage decomposition routes [99, 100]...

Alkyl halides Aryl halides Anhydrous calcium chloride anhydrous sodium, magnesium or calcium sulphate phosphorus pentoxide. 

Reactions of aryl halides with the reactive calcium required slightly higher temperatures, up to -30°C for aryl bromides and up to -20°C for aryl chlorides.1 2 3 4 5 Surprisingly, the active calcium reacted readily with fluorobenzene at room temperature to form the corresponding organocalcium reagent in near quantitative yield. 

Table VIII shows the conversions and selectivities obtained in this reaction with the various bases. Both calcium and barium hydroxide, as well as potassium carbonate, gave good yields and selectivities, particularly barium hydroxide, which gave conversions near 100% after 14h in a batch reactor. Ba(OH)2 was tested as a basic catalyst for the reaction with other aryl halides such as iodobenzene and chlorobenzene, and in both cases the yield and selectivity were also excellent.

Aryl halides Calcium chloride, calcium sulphate, magnesium sulphate. 

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... 

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. 

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. 

A study of this carbonylation reaction catalysed by tetracarbonylnickel in polar aprotic solvents, e.g. dimethyl sulphoxide or iVA-dimethylformamide, in the presence of calcium hydroxide reveals that the reaction can proceed at one atmosphere pressure of carbon monoxide and at 100 °C. The reaction probably involves oxidation-addition of aryl halide to the nickel followed by carbon monoxide insertion. Rates decrease in the order I > Br > Cl > F, Chloride ions and bromide ions enhance the reaction and this explains the autocatalytic behaviour observed as these ions are released from the aryl halide during reaction. ... 

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