Effect of Fe and Cu Salts in the Heteroannulative Synthesis of Benzo[b]furans

Significance

Reported is the synthesis of benzo-[b]furans from the corresponding aryl 2-bromo-benzyl ketones catalyzed either by iron or copper (or both) salts. This report was prompted by previous studies in the iron-catalyzed synthesis of various heterocycles. Since the FeCl₃ used was shown by atom absorption spectroscopy to contain 0.0088 mol% of CuCl₂, a detailed study concerning catalysis either by extremely low amounts of a high purity copper salt (0.0088 mol% of CuCl₂, 99.995% purity) or with higher catalyst loadings by FeCl₃ (10 mol%, 99.995% purity) was undertaken. The reaction functioned in both cases but, interestingly, it was shown that in many instances the yield was higher using 10 mol% of FeCl₃ than an ultralow percentage of CuCl₂. Control reactions showed that in the absence of catalyst no product was formed at the reaction temperature; however, at 130 ˚C the product was formed in 51% yield (R^¹ = H, R^² = 4-MeOC₆H₄).