Copper(II) Bromide/BoronTrifluoride Cocatalyzed Synthesis of Benzofurans

Significance

A formal [3+2] cycloaddition ­between CuBr₂-activated α-EWG ketene dithio­acetals 1 and BF₃-activated p-quinones 2 provides benzofurans. Optimized reaction conditions were established using CuBr₂ (20 mol%)/BF₃ (10 mol%) in MeCN at room temperature. p-Quinones containing both electron-withdrawing and -donating substituents proved to be effective and gave ­benzofurans in good yields. On the other hand, o-quinone proved not to be a suitable substrate. Ketene dithioacetals 1 with a range of EWG were shown to be effective reaction partners. A possible mechanism for the catalytic activation of CuBr₂ on α-EWG ketene dithioacetals 1 was ­proposed.