pubs.acs.org
Lack of Evidence Supporting Widespread Use of 1,2-Dibromoethane as an Activator for Zinc: Alternative Stirring or TMSCl Activation
Decades of protocols use 1,2-dibromoethane as an activating agent for zinc metal powder to promote direct insertion into organohalides, but kinetics experiments examining its effect on reaction rates are absent. Here condition- and concentration-controlled 1H NMR spectroscopy kinetics experiments measure rates, induction periods, and yields─and surprisingly showed that typically reported synthetic conditions and quantities of 1,2-dibromoethane (4 mol %) had no effect on the rate of direct insertion of alkyl iodides. Instead, an acceleration was caused by the physical stirring processes alone that accompanied the published synthetic procedures for the “chemical” activation protocol. This finding was adapted to aryl iodides as an example substrate with longer induction periods, leading to the discovery that relatively short prestir times (1 h) with a magnetic stir bar produced activation of zinc that approached similar degrees (∼85%) in the absence of 1,2-dibromoethane. An origin of the previously observed activation is therefore reassigned, in part, to mechanical activation of the surface during these processes. When 1,2-dibromoethane and trimethylsilyl chloride (TMSCl) were both present, as in many reported synthetic protocols, TMSCl caused full activation, and 1,2-dibromoethane caused no additional effect. The broad replacement of 1,2-dibromoethane activation is thus supported.
