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

Theoretical Studies on O-Insertion Reactions of Nitrous Oxide with Ruthenium Hydride Complexes

DFT calculations have been carried out to study the mechanism of the N2O O-insertion into the Ru−H bonds of ruthenium hydride complexes (dmpe)2Ru(H)(X) (X = OH, H). The reaction pathways for the formation of the monoinsertion product (dmpe)2Ru(H)(OH) and the bis(hydroxo) complex (dmpe)2Ru(OH)(OH), which were obtained directly from the reactions of N2O with the ruthenium hydride complexes, have been investigated in detail. Focus has been made to understand how the kinetically inert N2O is activated by the hydride complexes. It is found that N2O is activated through the hydride ligand nucleophilically attacking the terminal nitrogen of N2O followed by coordination of the activated N2O via the O-end

DFT Studies on the Mechanism of Reactions between N₂O and Cp₂M(η²-alkyne) (M = Ti, Zr)

DFT calculations have been carried out to study the activation of N2O by the transition-metal alkyne complexes Cp2M(η2-alkyne) (M = Ti, Zr). The mechanism for the formation of the five-membered metallacyclic complexes Cp2M(RCCR′NN(O)), which were obtained directly from the reactions of N2O with the metal alkyne complexes, and the conversion of the five-membered metallacyclic complexes Cp2M(RCCR′NN(O)) via N2 loss to the oxametallacyclobutene complexes Cp2M(RCCR′O) have been investigated in detail. An effort has been made to understand how the kinetically inert N2O can be activated. We concluded that N2O is best activated by metal fragments that possess high capability of π-back-bonding interactions with the π* orbitals of N2O

O-Abstraction Reactions of Nitrous Oxide with Cp₂Ti(II) and Other Middle Transition Metal Complexes

DFT calculations have been carried out to study the detailed mechanisms of the O-abstraction reaction of N2O with Cp2Ti(II). The reaction is initiated by coordination of N2O to Cp2Ti via the N-end to form a linear N2O-coordinated species Cp2Ti(N2O), from which the metal center transfers one of its metal d electrons to one π* orbital of the N2O ligand and gives a bent N2O-coordinated intermediate Cp2Ti←NN−O. The intermediate then reacts barrierlessly with another molecule of Cp2Ti to form an N2O-bridged intermediate Cp2Ti←NN−O−TiCp2, from which the singly oxo-bridged product (Cp2Ti)2O is formed with a release of N2. Reactions of N2O with other middle transition metal complexes have also been calculated and discussed. General mechanisms for O-abstraction reactions of N2O with early and middle transition metal complexes have been provided

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)

Theoretical Investigations on Mechanisms of Pd(OAc)₂-Catalyzed Intramolecular Diaminations in the Presence of Bases and Oxidants

DFT calculations have been carried out to study the mechanisms of Pd(OAc)2-catalyzed intramolecular diamination reactions in the presence of bases and oxidants. On the basis of the calculation results, a mechanism involving an anti-aminopalladation/syn-Csp3−N bond formation was proposed

Activity of Different Au<i>_n</i>S_<i>n</i>+1 Staples in the Ligand Exchange of Au₂₃(SR)₁₆^– with a Single Foreign Thiolate Ligand

Ligand exchange has been widely used to synthesize novel thiolated gold nanoclusters and to regulate their specific properties. Herein, density functional theory (DFT) calculations were conducted to investigate the kinetic profiles of the ligand exchange of the [Au23(SCy)16]− nanocluster with an aromatic thiolate (2-napthalenethiol). The three types of staple motifs (i.e., trimetallic Au3S4, monometallic AuS2, and the bridging thiolates) of the Au23 cluster precursor could be categorized into eight groups of S sites with different chemical environments. The ligand exchange of all of them occurs favorably via the SN1-like pathway, with one site starting with the Au–S dissociation and seven other sites starting with the H-transfer steps. By contrast, the SN2-like pathway (i.e., the synergistic SCy-to-SAr exchange prior to the H-transfer step) is unlikely in the target systems. Meanwhile, the Au–S bond on the capping Au atom of the bicapped icosahedral Au15 core is the most active one, while the S sites on Au3S4 (except for the one remote from the metallic core) are all competitive exchanging sites. The ligand exchange activity of the bridging thiolate and the remote S site on Au3S4 is significantly less reactive. The calculation results correlate with the multiple ligand exchange within only a few minutes and the preferential etching of the AuS2 staple with the foreign ligands reported in earlier experiments. The relative activity of different staples might be helpful in elucidating the inherent principles in the ligand exchange-induced size-evolution of metal nanoclusters

DFT Studies on Reactions of Transition Metal Complexes with O₂

DFT calculations have been carried out to study the mechanisms for reactions of O2 with a series of metal complexes, including d6 CpRuL2, d6 ML5, and d8 ML4 complexes. The calculation results indicate that the reaction is initiated by an end-on coordination of O2 to the metal center, which gives an (η1-O2)[M] intermediate. The uncoordinated oxygen atom of the η1-O2 ligand then approaches the metal center to give a new η1-O2 intermediate in which the η1-O2 ligand is oriented approximately the same as the one defined in the product. An intersystem conversion from the triplet to singlet energy surface (MECP) then occurs to enable the metal peroxide product to be formed

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