Palladium-Catalyzed Reaction of o-Ethynylphenols, o-((Trimethylsilyl)ethynyl)phenyl Acetates, and o-Alkynylphenols with Unsaturated Triflates or Halides: A Route to 2-Substituted-, 2,3-Disubstituted-, and 2-Substituted-3-acylbenzo[b]furans

The reaction of o -ethynylphenols 3 with a wide variety of unsaturated halides or triflates 6 in the presence of Pd(OAc) 2 (PPh 3 ) 2, CuI, and Et 3 N (procedure A) gives 2-vinyl- and 2-arylbenzo[ b ]furans 7, in good to high yield, through a palladium-catalyzed coupling followed by an in situ cyclization step. Small amounts of 2,3-disubstituted-benzo[ b ]furans 8 are usually isolated as side products. In some cases, however, compounds 8 are generated in significant yield or even as the main products. The formation of 8 can be prevented by employing alternative procedures (B and C) that use o -((trimethylsilyl)ethynyl)phenyl acetates 5 as starting building blocks. Procedure B is based on the palladium-catalyzed reaction of 5 with 6 in the presence of Pd(PPh 3 ) 4, Et 3 N, and n -Bu 4 NF, followed by the hydrolysis of the resultant coupling derivative 12 under basic conditions. Procedure C affords 7 through an in situ coupling/cyclization of 5 with 6 in the presence of Pd(PPh 3 ) 4 and KOBu t . The utilization of o -alkynylphenols 9 as the starting alkynes in the palladium-catalyzed reaction with 6 leads to the formation of 2,3-disubstituted-benzo[ b ]furans 13 through an annulation process promoted by σ-vinyl- and σ-arylpalladium complexes generated in situ . The best results in this case are obtained by using KOAc and Pd(PPh 3 ) 4 . In the presence of KOAc and Pd(PPh 3 ) 4, and under an atmosphere of carbon monoxide, the reaction of o -alkynylphenols with 6 provides 2-vinyl- and 2-aryl-3-acylbenzo[ b ]furans 14 .