Barbier reactions in the presence of zinc

A few reactions using zinc as the metal in a Barbier process have been published.267 Methallyl bromide and salicylaldehyde sonicated in THF with zinc yield the expected alcohol, which cyclizes by acidic treatment to a chromene.268 

The reaction of 4-bromo-2-sulfolene to aldehydes and ketones in the presence of the zinc-silver couple occurs on carbon-4, but the transposed product is obtained with magnesium, in lower yields (Eq. 59).269 

Starting from a bromodiene (Eq. 60), the reaction takes place in the presence of catalytic amounts of aluminum chloride at the more substituted carbon atom.270 A number of examples involving fluorinated substrates have been published, and a review discusses the essentials of the results with details. 

With zinc as the metal, undesired side reactions that would take place with lithium or magnesium are avoided. In the example of Eq. 61, the sonochemical reaction is easier and cleaner than the silent equivalent,272 but the stereoselectivity remains unaffected.273 

267 Luche, J.L. Sarandeses, L. in Organometallic Reactions Employing Zinc (Knochel, P. Ed.), Oxford University Press, Oxford, UK, in press. 

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