Haloallyl cations

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. 

Figure 4.7. Termination of the reaction via desilylation of the haloallyl cation.

The construction of complex intermediates from simple and readily available starting materials has been accomplished using the electrocyclic ring-opening reaction of halocyclopropanes. This is typically achieved through interception of the cationic haloallyl intermediate by solvent, the silver(I) counteranion, or some alternate tethered heteroatom or carbon-based nucleophile. Examples of these processes are described below. 

The gem-dihalocyclopropane type of structure, considered highly stable, is attacked by certain electrophilic reagents or simply by heating, and rearranges by ring opening. Thus, with Lewis-acid catalysts, the dihalocyclopropane derivative opens with formation of a haloallylic car-bo cation (32) which, in presence of a nucleophilic ion, gives either a haloallylic derivative or a 1,3-diene. 

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