Post-Grignard reagents

Keywords Arylcalcium Calcium Diarylcalcium Ether degradation Organylcalcium Post-grignard reagents... 

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. 

Scheme 7 Reaction of iodo-2,4,6-tri(/ert-butyl)benzene with activated calcium in THF. After the insertion of Ca into the C-I bond and formation of post-Grignard reagent a degradation sequence finally yields the C-C coupling product, 2,5-dimethyl-2,5-bis(3,5-di-/ert-butylphenyl)hexane, as a major product...

Temperatures below ambient temperatures have to be maintained in order to avoid side reactions such as ether degradation. For the same reason excess of calcium metal should be removed after the preparation of post-Grignard reagents. 

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