The Stille Reaction of 1,1-Dibromo-1-alkenes: Preparation of Trisubstituted Alkenes and Internal Alkynes.
The Stille reaction of 1,1-dibromo-1-alkenes 1 with aryl- and vinylstannanes produces different products depending on the reaction conditions. When the reaction is run in toluene or 1,4-dioxane with tris(2-furyl)phosphine (TFP) as the ligand, (Z)-bromoalkenes 2 are obtained stereospecifically in good to excellent yields with most substrates. However, 2-aryl-1,1-dibromo-1-alkenes (1e,1g) having an electron-donating methoxy group in the para- or ortho- position give poor yields. This method has been applied to the one-pot syntheses of stereospecifically trisubstituted alkenes 5. When the Stille reaction is conducted in a highly dipolar solvent (DMF), monobromides 2 and/or internal alkynes 4 are the products. The less reactive phenylstannane favors the formation of alkynes 4, regardless of which ligand is used. More reactive organostannanes (vinyl, furyl) require a very electron rich ligand, tris(4-methoxyphenyl)phosphine, for the formation of alkynes 4. This new method for internal alkyne preparation is general, requires very mild reaction conditions, and gives high yields.
Shen
Wang
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Cancer Research, Pharmaceutical Products Division, Abbott Laboratories, Abbott Park, Illinois 60064-6101.
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