REGIOSELECTIVE BROMINATION OF BENZOFURAN IN AN NH_4Br/AcOH/H_2O ELECTROLYSIS SYSTEM

摘要

Regioselective electro-bromination of benzofuran 2 was achieved successfully by proper choice of bromide salts and solvents to afford 5-bromobenzofuran 1, 5,7-dibromobenzofuran 3, and 2,3-dibromo-2,3-dihydrobenzofuran 4, respectively. Electrolysis of benzofuran 2 in CH_2Cl_2/H_2O (1/1) and/or AcOH/H_2O (10/1) in the presence of NaBr and/or NH_4Br afforded 2,3-dibromo-2,3-dihydrobenzofuran 4 while in AcOH/H_2O (100/1) containing NH_4Br, substitution at the C(5)-position of benzofuran proceeded smoothly to afford 5-bromobenzofuran 1. Continuing electrolysis in the latter system afforded 5,7-dibromobenzofuran 3, exclusively.