J . Am. Chem. SOC.1993,115, 7426-7433
7426
Mechanisms of the Ullmann Coupling Reaction in Adsorbed Monolayers Ming Xi and Brian E. Bent' Contribution from the Department of Chemistry, Columbia University, New York, New York 10027 Received February 19, 1993 Abstract: Ninety years ago, Ullmann and co-workers discovered that copper powder catalyzes the coupling of aromatic halides to produce biaryls, yet the role of the copper surface in this reaction remains controversial today. Recently, it has been shown that Ullmann coupling of submonolayer coverages of iodobenzene to form biphenyl occurs with 100% selectivity on a Cu( 111) single crystal surface under ultrahigh-vacuum conditions (Xi, M.; Bent, B. E. Surf. Sci. 1992, 278,19). We report here on studies in which surface analysis techniques have been applied to determine the mechanism of this reaction. Two mechanisms are found. At low surface coverages (5 langmuir) and low (
