Tandem Intramolecular Silylformylation and Silicon-Assisted Cross-Coupling Reactions. Synthesis of Geometrically Defined α,β-Unsaturated Aldehydes

The palladium- and copper-catalyzed cross-coupling reactions of cyclic silyl ethers with aryl iodides are reported. Silyl ethers 3 were readily prepared by intramolecular silylformylation of homopropargyl silyl ethers 2 under a carbon monoxide atmosphere. The reaction of cyclic silyl ethers 3 with various aryl iodides 7 in the presence of [(allyl)PdCl]₂, CuI, a hydrosilane, and KF·2H₂O in DMF at room temperature provided the α,β-unsaturated aldehyde coupling products 8 in high yields. The need for copper in this process suggested that transmetalation from silicon to copper is an important step in the mechanism. Although siloxane 3 and the product 8 are not stable under basic conditions, KF·2H₂O provided the appropriate balance of reactivity toward silicon and reduced basicity. The addition of a hydrosilane to [(allyl)PdCl]₂ was needed to reduce the palladium(II) to the active palladium(0) form.