Copper Makes the Difference: Visible Light-Mediated Atom Transfer Radical Addition Reactions of Iodoform with Olefins

Herein, we report a visible light-mediated copper-catalyzed protocol enabling the highly economic, vicinal difunctionalization of olefins utilizing the readily available bulk chemical iodoform. This protocol is characterized by high yields under environmentally benign reaction conditions and allows the regioselective and chemoselective functionalization of activated double bonds. Besides the synthetic utility of the shown transformation, this study undergirds the exclusive role of copper in photoredox catalysis as the title transformation is not possible via the most commonly employed ruthenium, iridium, or organic dye-based photocatalysts owing to the ability of copper to stabilize and interact with radical intermediates in its coordination sphere. Furthermore, the protocol can be smoothly scaled to gram quantities of the product, which offers manifold possibilities for further transformations, for example, heterocycle synthesis or intramolecular cyclopropanation

Kinetic and Mechanistic Aspects of Atom Transfer Radical Addition (ATRA) Catalyzed by Copper Complexes with Tris(2-pyridylmethyl)amine

Kinetic and mechanistic studies of atom transfer radical addition (ATRA) catalyzed by copper complexes with tris­(2-pyridylmethyl)­amine (TPMA) ligand were reported. In solution, the halide anions were found to strongly coordinate to [Cu^I(TPMA)]⁺ cations, as confirmed by kinetic, cyclic voltammetry, and conductivity measurements. The equilibrium constant for atom transfer (<i>K</i>_ATRA = <i>k</i>_a/<i>k</i>_d) utilizing benzyl thiocyanate was determined to be approximately 6 times larger for Cu^I(TPMA)­BPh₄ ((1.6 ± 0.2) × 10^–7) than Cu^I(TPMA)Cl ((2.8 ± 0.2) × 10^–8) complex. This difference in reactivity between Cu^I(TPMA)Cl and Cu^I(TPMA)­BPh₄ was reflected in the activation rate constants ((3.4 ± 0.4) × 10^–4 M^–1 s^–1 and (2.2 ± 0.2) × 10^–3 M^–1 s^–1, respectively). The fluxionality of Cu^I(TPMA)­X (X = Cl or Br) in solution was mainly the result of TPMA ligand exchange, which for the bromide complex was found to be very fast at ambient temperature (Δ<i><i>H</i></i>^⧧ = 29.7 kJ mol^–1, Δ<i><i>S</i></i>^<i>‡</i> = −60.0 J K^–1 mol^–1, Δ<i><i>G</i></i>^⧧₂₉₈ = 47.6 kJ mol^–1, and <i>k</i>_obs,298 = 2.9 × 10⁴ s^–1). Relatively strong coordination of halide anions in Cu^I(TPMA)­X prompted the possibility of activation in ATRA through partial TPMA dissociation. Indeed, no visible differences in the ATRA activity of Cu^I(TPMA)­BPh₄ were observed in the presence of as many as 5 equiv of strongly coordinating triphenylphosphine. The possibility for arm dissociation in Cu^I(TPMA)­X was further confirmed by synthesizing tris­(2-(dimethylamino)­phenyl)­amine (TDAPA), a ligand that was structurally similar to currently most active TPMA and Me₆TREN (tris­(2-dimethylaminoethyl)­amine), but had limited arm mobility due to the rigid backbone. Indeed, Cu^I(TDAPA)­Cl complex was found to be inactive in ATRA, and the activity increased only by opening the coordination site around the copper­(I) center by replacing chloride anion with less coordinating counterions such as BF₄^– and BPh₄^–. The results presented in this Article are significant from the mechanistic point of view because they indicate that coordinatively saturated Cu^I(TPMA)­X complexes catalyze the homolytic cleavage of carbon–halogen bond during the activation step in ATRA by prior dissociation of either halide anion or TPMA arm

Kinetic and Mechanistic Aspects of Atom Transfer Radical Addition (ATRA) Catalyzed by Copper Complexes with Tris(2-pyridylmethyl)amine

Kinetic and mechanistic studies of atom transfer radical addition (ATRA) catalyzed by copper complexes with tris­(2-pyridylmethyl)­amine (TPMA) ligand were reported. In solution, the halide anions were found to strongly coordinate to [Cu^I(TPMA)]⁺ cations, as confirmed by kinetic, cyclic voltammetry, and conductivity measurements. The equilibrium constant for atom transfer (<i>K</i>_ATRA = <i>k</i>_a/<i>k</i>_d) utilizing benzyl thiocyanate was determined to be approximately 6 times larger for Cu^I(TPMA)­BPh₄ ((1.6 ± 0.2) × 10^–7) than Cu^I(TPMA)Cl ((2.8 ± 0.2) × 10^–8) complex. This difference in reactivity between Cu^I(TPMA)Cl and Cu^I(TPMA)­BPh₄ was reflected in the activation rate constants ((3.4 ± 0.4) × 10^–4 M^–1 s^–1 and (2.2 ± 0.2) × 10^–3 M^–1 s^–1, respectively). The fluxionality of Cu^I(TPMA)­X (X = Cl or Br) in solution was mainly the result of TPMA ligand exchange, which for the bromide complex was found to be very fast at ambient temperature (Δ<i><i>H</i></i>^⧧ = 29.7 kJ mol^–1, Δ<i><i>S</i></i>^<i>‡</i> = −60.0 J K^–1 mol^–1, Δ<i><i>G</i></i>^⧧₂₉₈ = 47.6 kJ mol^–1, and <i>k</i>_obs,298 = 2.9 × 10⁴ s^–1). Relatively strong coordination of halide anions in Cu^I(TPMA)­X prompted the possibility of activation in ATRA through partial TPMA dissociation. Indeed, no visible differences in the ATRA activity of Cu^I(TPMA)­BPh₄ were observed in the presence of as many as 5 equiv of strongly coordinating triphenylphosphine. The possibility for arm dissociation in Cu^I(TPMA)­X was further confirmed by synthesizing tris­(2-(dimethylamino)­phenyl)­amine (TDAPA), a ligand that was structurally similar to currently most active TPMA and Me₆TREN (tris­(2-dimethylaminoethyl)­amine), but had limited arm mobility due to the rigid backbone. Indeed, Cu^I(TDAPA)­Cl complex was found to be inactive in ATRA, and the activity increased only by opening the coordination site around the copper­(I) center by replacing chloride anion with less coordinating counterions such as BF₄^– and BPh₄^–. The results presented in this Article are significant from the mechanistic point of view because they indicate that coordinatively saturated Cu^I(TPMA)­X complexes catalyze the homolytic cleavage of carbon–halogen bond during the activation step in ATRA by prior dissociation of either halide anion or TPMA arm

Crystal Structures of Au₂ Complex and Au₂₅ Nanocluster and Mechanistic Insight into the Conversion of Polydisperse Nanoparticles into Monodisperse Au₂₅ Nanoclusters

We previously reported a size-focusing conversion of polydisperse gold nanoparticles capped by phosphine into monodisperse [Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺ nanoclusters in the presence of phenylethylthiol. Herein, we have determined the crystal structure of [Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺ nanoclusters and also identified an important side-producta Au(I) complex formed in the size focusing process. The [Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺ cluster features a vertex-sharing bi-icosahedral core, resembling a rod. The formula of the Au(I) complex is determined to be [Au₂(PPh₃)₂(SC₂H₄Ph)]⁺ by electrospray ionization (ESI) mass spectrometry, and its crystal structure (with SbF₆^– counterion) reveals Au–Au bridged by −SC₂H₄Ph and with terminal bonds to two PPh₃ ligands. Unlike previously reported [Au₂(PR₃)₂(SC₂H₄Ph)]⁺ complexes in the solid state, which exist as tetranuclear complexes (i.e., dimers of [Au₂(PR₃)₂(SC₂H₄Ph)]⁺ units) through a Au···Au aurophilic interaction, in our case we found that the [Au₂(PPh₃)₂(SC₂H₄Ph)]⁺ complex exists as a single entity, rather than being dimerized to form a tetranuclear complex. The observation of this Au(I) complex allows us to gain insight into the intriguing conversion process from polydisperse Au nanoparticles to monodisperse Au₂₅ nanoclusters

Crystal Structures of Au₂ Complex and Au₂₅ Nanocluster and Mechanistic Insight into the Conversion of Polydisperse Nanoparticles into Monodisperse Au₂₅ Nanoclusters

We previously reported a size-focusing conversion of polydisperse gold nanoparticles capped by phosphine into monodisperse [Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺ nanoclusters in the presence of phenylethylthiol. Herein, we have determined the crystal structure of [Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺ nanoclusters and also identified an important side-producta Au(I) complex formed in the size focusing process. The [Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺ cluster features a vertex-sharing bi-icosahedral core, resembling a rod. The formula of the Au(I) complex is determined to be [Au₂(PPh₃)₂(SC₂H₄Ph)]⁺ by electrospray ionization (ESI) mass spectrometry, and its crystal structure (with SbF₆^– counterion) reveals Au–Au bridged by −SC₂H₄Ph and with terminal bonds to two PPh₃ ligands. Unlike previously reported [Au₂(PR₃)₂(SC₂H₄Ph)]⁺ complexes in the solid state, which exist as tetranuclear complexes (i.e., dimers of [Au₂(PR₃)₂(SC₂H₄Ph)]⁺ units) through a Au···Au aurophilic interaction, in our case we found that the [Au₂(PPh₃)₂(SC₂H₄Ph)]⁺ complex exists as a single entity, rather than being dimerized to form a tetranuclear complex. The observation of this Au(I) complex allows us to gain insight into the intriguing conversion process from polydisperse Au nanoparticles to monodisperse Au₂₅ nanoclusters