Allylmagnesium

Allylmagnesium refers to organomagnesium reagents in which the allyl group, CH2=CH-CH2-, is bonded to magnesium. The most common member is allylmagnesium bromide (CH2=CH-CH2-MgBr); allylmagnesium chloride and allylmagnesium iodide are known but less widely used. These compounds are prepared by reacting allyl halides with magnesium metal in dry ether solvents such as diethyl ether or tetrahydrofuran (THF) under inert atmosphere. Like other Grignard reagents, allylmagnesium species are highly reactive and air- and moisture-sensitive.

Structure and reactivity: Allylmagnesium reagents behave as nucleophiles and carbonyl-adding agents. They are typically used to form new carbon–carbon bonds by adding the allyl group to carbonyl compounds, followed by aqueous workup to furnish alcohols. The allyl moiety tends to survive the addition, allowing downstream transformations that install an extended carbon skeleton with an intact double bond.

Applications: Reaction with formaldehyde yields primary homoallylic alcohols; reaction with other aldehydes yields secondary homoallylic alcohols; reaction with ketones yields tertiary homoallylic alcohols after workup. They also react with carbon dioxide to give allylcarboxylic acids upon acidic hydrolysis, and with epoxides they can open rings to install an allyl-bearing alcohol. In synthesis, allylmagnesium reagents are valued for constructing C–C bonds while preserving the allyl functionality for further transformations.

Safety and handling: These reagents are vigorously reactive toward moisture and air, and are typically prepared and used under strictly dry, inert conditions in appropriate solvents (THF or diethyl ether). They should be handled with appropriate precautions for pyrophoric organomagnesium compounds.