Fluorinated biphenyls from aromatic arylations with pentafluorobenzenediazonium and related cations. Competition between arylation and azo coupling
High yields of the mixed perfluorinated biaryls (C6F5-Ar) are obtained by the catalytic dediazonlatlon of the pentafluorobenzenediazonium salt (C6F5N2+BF4-) in acetonitrile solutions containing various aromatic substrates (ArH) together with small amounts of iodide salts. Activated (electron-rich) as well as deactivated (electron-poor) arenes are successfully pentafluorophenylated by this method. The arylation is distinct from the azo coupling of the same substrates, which takes place in the absence of the iodide catalyst and yields the corresponding diazene (C6F5N=N-Ar) as product. The catalytic role of iodide, and the isomeric product distributions obtained with this procedure indicate that the arylation proceeds via the pentafluorophenyl radical in a efficient homolytic chain process. Since azo coupling involves electrophilic aromatic substitution of electron-rich ArH by C6F5N2+, the two competing pathways are distinct and do not have reactive intermediates in common.
Kosynkin, Dmitry,Bockman, T. Michael,Kochi, Jay K.
p. 2003 - 2012
(2007/10/03)
Isolation and Structure Elucidation of Transient (Colored) Complexes of Arenediazonium with Aromatic Hydrocarbons as Intermediates in Arylations and Azo Couplings
Brightly colored (1:1) complexes are observed immediately upon the mixing of electron-poor arenediazonium (ArN2+) salts and aromatic hydrocarbons (ArH). Binding energies of these intermolecular electron donor-acceptor or EDA complexes are typically ΔGEDA -1 in acetonitrile. As such, the weak donor-acceptor interaction in [ArN2+ ArH] is reminiscent of that in other transient complexes which exhibit charge-transfer (CT) absorption bands of the type originally described by Mulliken. Successful isolation of crystalline (colored) complexes between various ArH/ArN2+ pairs allows X-ray crystallography to identify the pertinent donor-acceptor interaction responsible for the distinctive (CT) colors. Indeed, the cofacial stacking of the aromatic donor on top of the aromatic moiety of ArN2+ is uniquely different from the usual bonding modes of electron-rich anions directly to the diazono (N2+) center. The mechanistic implication of such π-π interactions of ArH/ArN2+ pairs to the transition state for electron-transfer arylation is discussed in its relationship to electrophilic azo coupling.
Bockman,Kosynkin,Kochi
p. 5811 - 5820
(2007/10/03)
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