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45.4.2.1.1.3 Variation 3: Photochemical Friedel–Crafts Alkylation of Arenes with Alkyl Halides

DOI: 10.1055/sos-SD-045-00097

Zysman-Colman, E.Science of Synthesis, (201045161.

Primary alkyl iodides can be photolyzed (irradiation at 254nm) and the alkyl radicals and/or carbenium ions generated can be trapped by arene solvents such as benzene or anisole to afford alkylarenes in poor yields (<7%);[‌27‌] the major products in these reactions are elimination products. The reaction is thought to proceed via radical generation followed by electron transfer to generate the carbenium ion, followed by trapping of the electrophile by the arene. These poorly solvated alkyl carbenium ions will also isomerize, thus generating complicated mixtures of products, including other alkylarenes formed by arene capture of these carbenium ions. This isomerization can be arrested with the addition of triethylamine, which acts as a hydrogen iodide sponge. Secondary alkyl halides are even less effective substrates, producing scant quantities of alkylarenes.

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