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10.1.1.1.1.1.1.5 2-Halophenols and Alkynes with a Palladium(0)/Copper(I) Catalyst System

DOI: 10.1055/sos-SD-110-00529

Kwiecień, H.Science of Synthesis Knowledge Updates, (2014474.

This method is an efficient synthesis of 2-alkyl- or 2-aryl-substituted benzo[b]furans 63 (R1 = H) and nitrobenzo[b]furans 63 (R1 = NO2) in water by using a palladium-on-carbon based catalyzed reaction of terminal alkynes with 2-iodophenols 62 in the presence of triphenylphosphine, copper(I) iodide, and (S)-prolinol (Scheme 36).[‌115‌] The coupling reactions are carried out using 10% palladium-on-carbon as catalyst and copper(I) iodide as a cocatalyst in water. Apart from the low cost and stability, the use of palladium-on-carbon has the advantage of ease of use and facilitation of reaction mixture workup. It is noteworthy that the use of an organic cosolvent is not required in this case and this eliminates recyclability (waste handling) problems. Moreover, neither a phase-transfer catalyst nor water-soluble phosphine ligands are required for the successful single-step coupling/cyclization sequence.[‌115‌] Prolinol possesses better miscibility with water than commonly used triethylamine and therefore facilitates the coupling reaction, affording better yields of products. Moreover, due to its lower volatility (thereby avoiding internal pressure development as well as ensuring its maximum recovery) in comparison with triethylamine, prolinol has advantages over other amine bases especially for large-scale preparations. This water-based synthesis is safer than those employing organic solvents.[‌115‌] Various substituents, such as alkyl, hydroxy, and phenyl, present in the alkyne are well tolerated and yields are generally high. Mechanistically, the reaction may proceed via in situ generation of a prolinol-stabilized palladium(0) complex, which perhaps facilitates the reaction in aqueous media due to its interaction with the water (via the hydroxy group of prolinol).[‌115‌]

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