Palladium-Catalyzed Cyclocarbonylation of Terminal and Internal Alkynols to 2(5H)-Furanones

A series of substituted alkynols, containing alkyl, phenyl, and vinyl groups at the acetylenic terminal, were found to undergo direct carbonylation to the corresponding substituted 2(5H)-furanones in 67-98% yield. This reaction requires catalytic quantities of Pd2(dba)3·CHCl3(4 mol %) and 1,4-bis(diphenylphosphino)butane (dppb) (8 mol %) in dichloromethane under an atmosphere of CO (600 psi) and H2(200 psi) at 95 °C for 36 h. Hydrogen is required for this reaction. Another bidentate ligand such as 1,3-bis(diphenylphosphino)propane, and a monodentate ligand such as PPh3or PCy3, are equally effective for this reaction. Conjugated ene-ynols can also be carbonylated affording 3-alkenyl-2(5H)-furanones in good yield. However, double bond isomerization (cis-trans) occurred if an ene-ynol containing a cis olefinic substituent was used as the substrate. The cyclocarbonylation reaction is believed to proceed via an allenylpalladium intermediate, which is formed by initial insertion of Pd(0) into the C-O bond of the alkynol followed by a rearrangement.