Activated Zinc Dust

Introduction

Traditionally, zinc dust [¹] has been employed as a reducing agent in organic synthesis. Notably, zinc dust or an amalgam thereof has been used to reduce carbonyls [²] (Clemmensen reduction), carbon-oxygen bonds, [³] carbon-halide bonds, [4] alkynes, [5] heteroatom bonds (N-N and N-O bonds), [6] and to prepare numerous organozinc reagents. In addition to various reduction reactions, zinc has been employed in several named reactions including the Reformatsky reaction, [7] Serini reaction, [8] the Knorr pyrrole synthesis, [9] and the Simmons-Smith cyclopropanation. [¹0] Other notable reactions that are dependent upon zinc for the preparation of the active species are the Oshima-­Lombardo olefination, [¹¹] the Corey-Fuchs alkynylation, [¹²] and the Negishi cross-coupling. [¹³]

Organozinc reagents have also proven highly amenable to asymmetric variants of a number of reactions, as zinc species are mild in comparison to either organolithium or ­organomagnesium reagents. These reactions include the copper-mediated conjugate addition of dialkylzinc ­reagents to enones, [¹4] asymmetric Simmons-Smith cyclopropanations, [¹5] and enantioselective Reformatsky reactions, [¹6] amongst others.

Activated zinc dust is commercially available from a number of sources. Alternatively it can be activated by stirring with dilute HCl, then washing with distilled water, ethanol, and absolute diethyl ether before rigorous drying. This procedure removes oxides from the surface of zinc, which form slowly upon standing in air. There are no known toxic properties associated with zinc or organo­metallics thereof.

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