Cross-Coupling of Aromatic Bromides with Allylic Silanolate Salts

2008-12-03T00:00:00Z (GMT) by Scott E. Denmark Nathan S. Werner
The sodium salts of allyldimethylsilanol and 2-butenyldimethylsilanol undergo palladium-catalyzed cross-coupling with a wide variety of aryl bromides to afford allylated and crotylated arenes. The coupling of both silanolates required extensive optimization to deliver the expected products in high yields. The reaction of the allyldimethylsilanolate takes place at 85 °C in 1,2-dimethoxyethane with allylpalladium chloride dimer (2.5 mol %) to afford 73−95% yields of the allylation products. Both electron-rich and sterically hindered bromides reacted smoothly, whereas electron-poor bromides cross-coupled in poor yield because of a secondary isomerization to the 1-propenyl isomer (and subsequent polymerization). The 2-butenyldimethylsilanolate (<i>E</i>/<i>Z</i>, 80:20) required additional optimization to maximize the formation of the branched (γ-substitution) product. A remarkable influence of added alkenes (dibenzylideneacetone and norbornadiene) led to good selectivities for electron-rich and electron-poor bromides in 40−83% yields. However, bromides containing coordinating groups (particularly in the <i>ortho</i> position) gave lower, and in one case even reversed, selectivity. Configurationally homogeneous (<i>E</i>)-silanolates gave slightly higher γ-selectivity than the pure (<i>Z</i>)-silanolates. A unified mechanistic picture involving initial γ-transmetalation followed by direct reductive elimination or σ−π isomerization can rationalize all of the observed trends.