Catalytic C–S, C–Se, and C–P Cross-Coupling Reactions Mediated by a Cu^I/Cu^III Redox Cycle

A well-defined macrocyclic aryl-Cu^III complex (<b>1</b>) readily reacts with a series of R–SH, Ar–SH, Ar–SeH, and (RO)₂(O)–PH (R = alkyl) nucleophiles to quantitatively afford the corresponding aryl alkyl thioethers, biaryl thioethers, biaryl selenide, and aryl dialkyl phosphonates, respectively. Competition experiments using bifunctional substrates revealed the important impact of lower p<i>K</i>_a values in order to discriminate between functional groups, although other influencing parameters such as steric effects have been identified. The catalytic version of these reactions is achieved using aryl bromide and aryl chloride model substrates, affording C–S, C–Se, and C–P coupling compounds in excellent to moderate yields. Low-temperature UV–vis and NMR monitoring of the reactions of complex <b>1</b> with a variety of nucleophiles support the formation of a ground-state <b>1</b>–nucleophile adduct. A mechanistic proposal for reaction of <b>1</b> with S-nucleophiles involving key nucleophile deprotonation and aryl-nucleophile reductive elimination steps is finally described

Mechanism of the Ullmann Biaryl Ether Synthesis Catalyzed by Complexes of Anionic Ligands: Evidence for the Reaction of Iodoarenes with Ligated Anionic Cu^I Intermediates

A series of experimental studies, along with DFT calculations, are reported that provide a detailed view into the mechanism of Ullmann coupling of phenols with aryl halides in the presence of catalysts generated from Cu­(I) and bidentate, anionic ligands. These studies encompass catalysts containing anionic ligands formed by deprotonation of 8-hydroxyquinoline, 2-pyridylmethyl <i>tert</i>-butyl ketone, and 2,2,6,6-tetramethyl­heptane-3,5-dione. Three-coordinate, heteroleptic species [Cu­(<b>LX</b>)­OAr]⁻ were shown by experiment and DFT calculations to be the most stable complexes in catalytic systems containing 8-hydroxy­quinoline or 2-pyridyl­methyl <i>tert</i>-butyl ketone and to be generated reversibly in the system containing 2,2,6,6-tetramethyl­heptane-3,5-dione. These heteroleptic complexes were characterized by a combination of ¹⁹F NMR, ¹H NMR, and UV–vis spectroscopy, as well as ESI-MS. The heteroleptic complexes generated in situ react with iodoarenes to form biaryl ethers in high yields without evidence for an aryl radical intermediate. Measurements of ¹³C/¹²C isotope effects showed that oxidative addition of the iodoarene occurs irreversibly. This information, in combination with the kinetic data, shows that oxidative addition occurs to the [Cu­(<b>LX</b>)­OAr]⁻ complexes and is turnover-limiting. A Hammett analysis of the effect of phenoxide electronic properties on the rate of the reaction of [Cu­(<b>LX</b>)­OAr]⁻ with iodotoluene also is consistent with oxidative addition of the iodoarene to an anionic phenoxide complex. Calculations by DFT suggest that this oxidative addition is followed by dissociation of I^– and reductive elimination of the biaryl ether from the resulting neutral Cu­(III) complex

Oxidant-Free Au(I)-Catalyzed Halide Exchange and C_sp2–O Bond Forming Reactions

Au has been demonstrated to mediate a number of organic transformations through the utilization of its π Lewis acid character, Au­(I)/Au­(III) redox properties or a combination of both. As a result of the high oxidation potential of the Au­(I)/Au­(III) couple, redox catalysis involving Au typically requires the use of a strong external oxidant. This study demonstrates unusual external oxidant-free Au­(I)-catalyzed halide exchange (including fluorination) and C_sp2–O bond formation reactions utilizing a model aryl halide macrocyclic substrate. Additionally, the halide exchange and C_sp2–O coupling reactivity could also be extrapolated to substrates bearing a single chelating group, providing further insight into the reaction mechanism. This work provides the first examples of external oxidant-free Au­(I)-catalyzed carbon–heteroatom cross-coupling reactions