A further method to synthesize Grignard reagents involves reaction of Mg with an organozinc compound.  This method has been used to make adamantane-based Grignard reagents, which are, due to C-C coupling side reactions, difficult to make by the conventional method from the alkyl halide and Mg. The reductive transmetalation achieves: 
:AdZnBr + Mg  →  AdMgBr  +  Zn