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The Basics of Zinc Activation

The stubborn carbon-hydrogen bonds of an organic compound can be replaced by more reactive carbon-zinc bonds. The least reactive bonds of organic molecules, single carbon-hydrogen (C–H) bonds, are often the sites where we want to perform a chemical reaction. Metals such as magnesium and lithium can be used to “activate” these bonds by creating organometallic compounds that have more reactive metal-carbon (M–C) bonds (1). If these goals are to be achieved with metalating reagents, which replace a C–H bond with a M–C bond, then these reagents must maximize reactivity, selectivity, and stability—seemingly incompatible goals. Although many useful reactions of this type are well known for basic organometallic species formed from lithium, the activation can sometimes be “too good” and lead to a cascade of unwanted side products. Greater control might result if metals such as Zn, which would form less polar bonds, could be used, but the kinetics for this less reactive metal might be expected to be slow for the task (2–4). However, on page 706 of this issue, Kennedy et al. (5) report the preparation of a well-defined bimetallic zinc compound that, with help from alkali metals, selectively reacts with C–H bonds.