Synthesis of Sterically Encumbered Alkaline‐Earth Metal Amides Applying the In Situ Grignard Reagent Formation

Magnesium and calcium are too inert to deprotonate amines directly. In addition, N-bound trialkylsilyl groups proved to be beneficial for the synthesis of calcium bis(amides). The addition of hydryl bromides or iodides to suspensions of calcium and imines in THF allows straightforward conversion to silyl-free calcium bis(amides) with bulky substituents.

Abstract

Magnesium and calcium are too inert to deprotonate amines directly. For the synthesis of bulky amides alternative strategies are required and in the past, N-bound trialkylsilyl groups have been used to ease metalation reactions. The in situ Grignard reagent formation in stirred suspensions of magnesium or calcium with hydryl halide and imine in THF allows the synthesis of a plethora of amides with bulky silyl-free substituents. Ball milling protocols partially favor competitive side reactions such as aza-pinacol coupling reactions. Calcium is the advantageous choice for the in situ Grignard reagent formation and subsequent addition onto the imines yielding bulky calcium bis(amides) whereas the stronger reducing heavier alkaline-earth metals strontium and barium are less selective and hence, the aza-pinacol coupling reaction becomes competitive. Exemplary, the solid-state molecular structures of [(Et₂O)Mg(N(Ph)(CHPh₂)₂] and [(Et₂O)₂Ca(N(Ph)(CHPh₂)₂] have been determined.

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