Palladium-Catalyzed Oxidative Coupling of 2-Alkylfurans with Olefins through C−H Activation: Synthesis of Difurylalkanes

The course of the palladium-catalyzed coupling of 2-alkylfurans with olefins through C−H activation is influenced by the nature of the solvent. At room temperature, with acetic acid as solvent and benzoquinone as oxidant, the usual Heck product, 2-cinnamyl-5-methylfuran, is obtained in low yield. The use of a AcOH/CH₃CN mixture as solvent induced the formation of difurylalkanes in fair to high yields. Allylarenes and styrenes led to the formation of β,β-difurylalkanes as major compounds, while acrylates afforded selectively β,β-difuryl esters. Mechanism studies have shown that these transformations do not occur through the processes usually involved in the formation of diarylalkanes. According to ESI-MS studies and labeling experiments, two consecutive C−H activations of 2-alkylfurans by Pd(II) are followed by an insertion of the alkene, a migration of a furan ring involving β-elimination/insertion steps, and finally a reductive elimination reaction.