Mechanism of Photocatalytic Cyclization of Bromoalkenes with a Dimeric Gold Complex

In recent years, dimeric gold complexes have been extensively used in photoredox reactions and have successfully mediated a series of traditionally challenging organic reactions. However, little is known about the function of the dimeric gold complexes in these reactions. In this study, we systematically studied the mechanism of the photocatalytic cyclization of bromoalkenes with the dimeric gold complex [Au₂(dppm)₂]²⁺ (dppm denotes bis­(diphenylphosphino)­methane). It is found that the dimeric gold complex acts as the radical initiator and terminator in this radical chain reaction. In the radical initiation step, the gold complex first promotes the electron transfer from amine to the bromoalkene substrate (via a reductive quenching mode) and then accepts the released bromide (from bromoalkene) to stabilize the reaction system. In the radical termination step, the dimeric gold complex mainly works as an unsynchronized bromine and electron donor. In the photocatalytic cyclization of bromoalkenes, the radical propagation step operates between the alkyl radical and the dehydrogenated amine radical. This study sheds light on the function of the dimeric gold complex in photoredox reactions and will hopefully benefit the future understanding of similar synthetic reactions

Mechanism of Photocatalytic Cyclization of Bromoalkenes with a Dimeric Gold Complex

In recent years, dimeric gold complexes have been extensively used in photoredox reactions and have successfully mediated a series of traditionally challenging organic reactions. However, little is known about the function of the dimeric gold complexes in these reactions. In this study, we systematically studied the mechanism of the photocatalytic cyclization of bromoalkenes with the dimeric gold complex [Au₂(dppm)₂]²⁺ (dppm denotes bis­(diphenylphosphino)­methane). It is found that the dimeric gold complex acts as the radical initiator and terminator in this radical chain reaction. In the radical initiation step, the gold complex first promotes the electron transfer from amine to the bromoalkene substrate (via a reductive quenching mode) and then accepts the released bromide (from bromoalkene) to stabilize the reaction system. In the radical termination step, the dimeric gold complex mainly works as an unsynchronized bromine and electron donor. In the photocatalytic cyclization of bromoalkenes, the radical propagation step operates between the alkyl radical and the dehydrogenated amine radical. This study sheds light on the function of the dimeric gold complex in photoredox reactions and will hopefully benefit the future understanding of similar synthetic reactions

Mechanistic Study of Chemoselectivity in Ni-Catalyzed Coupling Reactions between Azoles and Aryl Carboxylates

Itami et al. recently reported the C–O electrophile-controlled chemoselectivity of Ni-catalyzed coupling reactions between azoles and esters: the decarbonylative C–H coupling product was generated with the aryl ester substrates, while C–H/C–O coupling product was generated with the phenol derivative substrates (such as phenyl pivalate). With the aid of DFT calculations (M06L/6-311+G­(2d,p)-SDD//B3LYP/6-31G­(d)-LANL2DZ), the present study systematically investigated the mechanism of the aforementioned chemoselective reactions. The decarbonylative C–H coupling mechanism involves oxidative addition of C­(acyl)–O bond, base-promoted C–H activation of azole, CO migration, and reductive elimination steps (C–H/Decar mechanism). This mechanism is partially different from Itami’s previous proposal (Decar/C–H mechanism) because the C–H activation step is unlikely to occur after the CO migration step. Meanwhile, C–H/C–O coupling reaction proceeds through oxidative addition of C­(phenyl)–O bond, base-promoted C–H activation, and reductive elimination steps. It was found that the C–O electrophile significantly influences the overall energy demand of the decarbonylative C–H coupling mechanism, because the rate-determining step (i.e., CO migration) is sensitive to the steric effect of the acyl substituent. In contrast, in the C–H/C–O coupling mechanism, the release of the carboxylates occurs before the rate-determining step (i.e., base-promoted C–H activation), and thus the overall energy demand is almost independent of the acyl substituent. Accordingly, the decarbonylative C–H coupling product is favored for less-bulky group substituted C–O electrophiles (such as aryl ester), while C–H/C–O coupling product is predominant for bulky group substituted C–O electrophiles (such as phenyl pivalate)

Mechanistic Investigation of Visible-Light-Induced Intermolecular [2 + 2] Photocycloaddition Catalyzed with Chiral Thioxanthone

The recent thioxanthone-sensitizer-catalyzed intermolecular [2 + 2] cycloaddition induced by visible-light irradiation set the stage for the future development of feasible photocycloadditions. Nonetheless, the mechanism of this reaction still remains under debate, especially on the activation mode of the thioxanthone photosensitizer (energy transfer, bielectron exchange, and hydrogen transfer are all possible mechanisms). To settle this issue, systematic density functional theory calculations have been carried out. The results indicate that the energy-transfer pathway is more favorable than the bielectron-exchange and the hydrogen-transfer pathways. Meanwhile, the overall transformations involve the complexation and excitation of photosensitizer, the first C–C bond formation, the dissociation of the sensitizer, the triplet-to-singlet electronic state crossing, and the second C–C bond formation. The first C–C bond formation is the rate- and selectivity-determining step, and synergistic energy and electron transfer from photosensitizer to substrate moieties takes place along this process. On this basis, the effect of olefin substrates (ethyl vinyl ketone vs vinyl acetate) on the stereoselectivity was finally analyzed

Mechanistic Study on Ligand-Controlled Rh(I)-Catalyzed Coupling Reaction of Alkene-Benzocyclobutenone

Recently, Dong’s group [<i>Angew. Chem., Int. Ed.</i> <b>2012</b>, <i>51</i>, 7567–7571; <i>Angew. Chem., Int. Ed.</i> <b>2014</b>, <i>53</i>, 1891–1895] reported the ligand-controlled selectivity of Rh-catalyzed intramolecular coupling reaction of alkene-benzocyclobutenone: the direct coupling product (i.e., fused-rings) was formed in the DPPB-assisted system (DPPB = PPh₂(CH₂)₄PPh₂), while the decarbonylative coupling product (i.e., spirocycles) was generated in the P­(C₆F₅)₃-assited system. To explain this interesting selectivity, density functional theory (DFT) calculations have been carried out in the present study. It was found that the direct and decarbonylative couplings experience the same C­(acyl)–C­(sp²) activation and alkene insertion steps. The following C–C reductive elimination or β-H elimination–decarbonylation–reductive elimination leads to the direct or decarbonylative coupling reaction, respectively. The coordination features of different ligands were found to significantly influence C–C reductive elimination and decarbonylation step. The requisite phosphine dissociation of DPPB ligand from Rh center for the decarbonylation step is disfavored, and thus, the reductive elimination and direct coupling reaction are favored therein. By contrast, a free coordination site is available on the Rh center in the P­(C₆F₅)₃-assisted system, facilitating the decarbonylation process together with the generation of related decarbonylative coupling product

Size Growth of Au₄Cu₄: From Increased Nucleation to Surface Capping

The size conversion of atomically precise metal nanoclusters is fundamental for elucidating structure-property correlations. In this study, copper salt (CuCl)-induced size growth from [Au4Cu4(Dppm)2(SAdm)5]+ (abbreviated as [Au4Cu4S5]+) to [Au4Cu6(Dppm)2(SAdm)4Cl3]+ (abbreviated as [Au4Cu6S4Cl3]+) (SAdmH = 1-adamantane mercaptan, Dppm = bis-(diphenylphosphino)methane) was investigated via experiments and density functional theory calculations. The [Au4Cu4S5]+ adopts a defective pentagonal bipyramid core structure with surface cavities, which could be easily filled with the sterically less hindered CuCl and CuSCy (i.e., core growth) (HSCy = cyclohexanethiol) but not the bulky CuSAdm. As long as the Au4Cu5 framework is formed, ligand exchange or size growth occurs easily. However, owing to the compact pentagonal bipyramid core structure, the latter growth mode occurs only for the surface-capped [Au4Cu6(Dppm)2(SAdm)4Cl3]+ structure (i.e., surface-capped size growth). A preliminary mechanistic study with density functional theory (DFT) calculations indicated that the overall conversion occurred via CuCl addition, core tautomerization, Cl migration, the second [CuCl] addition, and [CuCl]-[CuSR] exchange steps. And the [Au4Cu6(Dppm)2(SAdm)4Cl3]+ alloy nanocluster exhibits aggregation-induced emission (AIE) with an absolute luminescence quantum yield of 18.01% in the solid state. This work sheds light on the structural transformation of Au–Cu alloy nanoclusters induced by Cu(I) and contributes to the knowledge base of metal-ion-induced size conversion of metal nanoclusters

Mechanistic Investigation on Oxygen-Mediated Photoredox Diels–Alder Reactions with Chromium Catalysts

The recent dioxygen-mediated, Cr-complex-catalyzed photoredox Diels–Alder reaction between two electron-rich substrates represents the first example of first-row transition-metal photocatalysis. Motivated by the mechanistic ambiguity (such as the inherent interactions of O₂ with Cr complexes and the origin of the high selectivity), we performed systematic density functional theory (DFT) calculations. The calculation results show that O₂ shuttles an electron via an inner-sphere mechanism (in the form of [CrL₃-O₂] complexes, L = Ph₂phen). Meanwhile, the overall transformations involve the excitation and quenching of the CrL₃³⁺ complex, single-electron transfer from the dienophile to the excited state CrL₃³⁺ catalyst (SET-1), asynchronous cycloaddition (instead of synchronous), and the final single electron transfer from the quintet [CrL₃-O₂]²⁺ complex (instead of superoxide) to the radical cationic cycloadduct. The first C–C bond formation in the asynchronous cycloaddition is the rate-determining and selectivity-determining step. On this basis, the origins of the chemo-, regio-, and stereoselectivity have been identified

Mechanistic Investigation on Oxygen-Mediated Photoredox Diels–Alder Reactions with Chromium Catalysts

The recent dioxygen-mediated, Cr-complex-catalyzed photoredox Diels–Alder reaction between two electron-rich substrates represents the first example of first-row transition-metal photocatalysis. Motivated by the mechanistic ambiguity (such as the inherent interactions of O₂ with Cr complexes and the origin of the high selectivity), we performed systematic density functional theory (DFT) calculations. The calculation results show that O₂ shuttles an electron via an inner-sphere mechanism (in the form of [CrL₃-O₂] complexes, L = Ph₂phen). Meanwhile, the overall transformations involve the excitation and quenching of the CrL₃³⁺ complex, single-electron transfer from the dienophile to the excited state CrL₃³⁺ catalyst (SET-1), asynchronous cycloaddition (instead of synchronous), and the final single electron transfer from the quintet [CrL₃-O₂]²⁺ complex (instead of superoxide) to the radical cationic cycloadduct. The first C–C bond formation in the asynchronous cycloaddition is the rate-determining and selectivity-determining step. On this basis, the origins of the chemo-, regio-, and stereoselectivity have been identified

How a Single Electron Affects the Properties of the “Non-Superatom” Au₂₅ Nanoclusters

In this study, we successfully synthesized the rod-like [Au₂₅(PPh₃)₁₀(SePh)₅Cl₂]^<i>q</i> (<i>q</i> = +1 or +2) nanoclusters through kinetic control. The single crystal X-ray crystallography determined their formulas to be [Au₂₅(PPh₃)₁₀(SePh)₅Cl₂]­(SbF₆) and [Au₂₅(PPh₃)₁₀(SePh)₅Cl₂]­(SbF₆)­(BPh₄), respectively. Compared to the previously reported Au₂₅ coprotected by phosphine and thiolate ligands (i.e., [Au₂₅(PPh₃)₁₀(SR)₅Cl₂]²⁺), the two new rod-like Au₂₅ nanoclusters show some interesting structural differences. Nonetheless, each of these three nanoclusters possesses two icosahedral Au₁₃ units (sharing a vertex gold atom) and the bridging “Au–Se­(S)–Au” motifs. The compositions of the two new nanoclusters were characterized with ESI-MS and TGA. The optical properties, electrochemistry, and magnetism were studied by EPR, NMR, and SQUID. All these results demonstrate that the valence character significantly affects the properties of the “non-superatom” Au₂₅ nanoclusters, and the changes are different from the previously reported “superatom” Au₂₅ nanoclusters. Theoretical calculations indicate that the extra electron results in the half occupation of the highest occupied molecular orbitals in the rod-like Au₂₅⁺ nanoclusters and, thus, significantly affects the electronic structure of the “non-superatom” Au₂₅ nanoclusters. This work offers new insights into the relationship between the properties and the valence of the “non-superatom” gold nanoclusters

Crystal Structures of Two New Gold–Copper Bimetallic Nanoclusters: Cu_<i>x</i>Au_{25–<i>x</i>}(PPh₃)₁₀(PhC₂H₄S)₅Cl₂²⁺ and Cu₃Au₃₄(PPh₃)₁₃(^tBuPhCH₂S)₆S₂³⁺

Herein, we report the synthesis and atomic structures of the cluster-assembled Cu_<i>x</i>­Au_{25–<i>x</i>}­(PPh₃)₁₀­(PhCH₂CH₂S)₅­Cl₂²⁺ and Cu₃­Au₃₄­(PPh₃)₁₃­(^tBuPhCH₂S)₆­S₂³⁺ nanoclusters (NCs). The atomic structures of both NCs were precisely determined by single-crystal X-ray crystallography. The Cu_<i>x</i>­Au_{25–<i>x</i>}­(PPh₃)₁₀­(PhC₂H₄S)₅­Cl₂²⁺ NC was assembled by two icosahedral M₁₃ via a vertex-sharing mode. The Cu atom partially occupies the top and waist sites and is monocoordinated with chlorine or thiol ligands. Meanwhile, the Cu₃­Au₃₄­(PPh₃)₁₃­(^tBuPhCH₂S)₆­S₂³⁺ NC can be described as three 13-atom icosahedra sharing three vertexes in a cyclic fashion. The three Cu atoms all occupy the internal positions of the cluster core. What is more important is that all three Cu atoms in Cu₃Au₃₄ are monocoordinated by the bare S atoms. The absorption spectra of the as-synthesized bimetallic NCs reveal that the additional metal doping and different cluster assemblies affect the electronic structure of the monometallic NCs