Silver I -Mediated Electrocyclic Processes

Department of Chemistry, University of Alberta, Edmonton, Canada 

The halophilic character of the silver cation is traditionally exploited to assist in removal of halides from metal complexes in order to open a coordination site and, in effect, initiate catalyst reactivity. Analogously, silver salts can be used to facilitate the removal of halides from organic molecules to reveal reaction pathways that involve cationic intermediates. In the simplest scenarios, silver has been used to 

Silver in Organic Chemistry Edited by Michael Harmata Copyright 2010 John Wiley Sons, Inc. 

This chapter focuses on the use of silver(I) salts as Lewis acids in the ring-opening reactions of halocyclopropanes. Both the classical and most recent examples of this type of chemistry are examined with emphasis on the use of this simple transformation to build up complex intermediates that are potentially useful in the construction of natural products. 

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Under these conditions, silver-assisted electrocyclic ring opening provided the haloallyl cation, which was subsequently trapped by isocyanate anion. Interception of the cationic species with isocyanate was successful since bromide was removed from the reaction mixture as a precipitate (AgBr). Finally, treatment of intermediate 9 with methanol furnished the desired carbamate in 96% yield. This example demonstrates the usefulness of the silver(I)-mediated process. Removal of free halide from the reaction mixture affords a long-lived cationic species that can be captured by a different nucleophile, such as solvent, the silver(I) counteranion, or an intramolecular nucleophile. This reactivity has been exploited in many different ways throughout the years and is examined in greater detail later in this chapter. 

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