Absolute Asymmetric Synthesis of “Chiral‐at‐Metal” Grignard Reagents and Transfer of the Chirality to Carbon

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Abstract
Two new six-coordinate Grignard reagents, cis-[(p-CH3C6H4)MgBr(dme)2] (1) and cis-[MgCH3(thf)(dme)2]I (2), have been synthesized and their crystal structures have been determined. Both reagents are cis-octahedral and therefore chiral. They crystallize as conglomerates and racemize rapidly in solution. By utilizing these properties, the absolute asymmetric synthesis of specifically the Δ or the Λ enantiomer was achieved for both Grignard reagents. Enantiopure 1 and 2 were then reacted with butyraldehyde or benzaldehyde to give the corresponding alcohol in up to 22 % enantiomeric excess. At −60 °C, the Grignard reagents crystallize as racemic phases instead of conglomerates. Consequently, the crystal structures of rac-cis-[(p-CH3C6H4)MgBr(dme)2]⋅DME (3) and rac-cis-[MgCH3(thf)(dme)2]I (4) could be determined.