In Situ Generation of Magnesium‐ and Calcium‐Based Grignard Reagents for Amide Synthesis

The alkaline‐earth metals Mg and Ca are too inert for the direct metalation of primary and secondary amines. Consequently, activation prior to use is required. Alternatively, the Grignard reagents RMgX (R=alkyl, aryl, X=halide) can be applied in metalation of amines. However, such a straightforward procedure for the synthesis of alkylcalcium reagents is disadvantageous due to diverse side reactions, including Wurtz‐type C−C coupling and ether degradation reactions. Therefore, suspensions of magnesium or calcium with amine can be treated in a smooth reaction with ethyl bromide in an ethereal solvent at room temperature. Intermediately formed RAeX (Ae=alkaline‐earth metal, i. e., Mg, Ca) either metalates amines yielding the corresponding amides in an in situ Grignard metalation method (iGMM) or adds across C=N bonds of imines in an in situ Grignard addition method (iGAM). The amides R’2N‐AeX (Ae=Mg: Hauser bases) undergo Schlenk‐type ligand exchange reactions yielding homoleptic Ae(NR’2)2 and potentially sparingly soluble AeX2.