Mechanistic Studies on the Insertion of Carbonyl Substrates into Cu-H: Different Rate-Limiting Steps as a Function of Electrophilicity

We report mechanistic studies on the insertion reactions of [(NHC)Cu(μ-H)]₂ complexes with carbonyl substrates by UV-vis and ¹H NMR spectroscopic kinetic studies, H/D isotopic labelling, and X-ray crystallography. The results of these comprehensive studies show that the insertion of Cu-H with an aldehyde, ketone, activated ester/amide, and unactivated amide consist of two different rate limiting steps: the formation of Cu-H monomer from Cu-H dimer for more electrophilic substrates, and hydride transfer from a transient Cu-H monomer for less electrophilic substrates. We also report spectroscopic and crystallographic characterization of rare Cu-hemiacetalate and Cu-hemiaminalate moieties from the insertion of an ester or amide into the Cu−H bond.     

Single‐Site Zeolite‐Anchored Organoiridium Carbonyl Complexes: Characterization of Structure and Reactivity by Spectroscopy and Computational Chemistry

A family of HY zeolite‐supported cationic organoiridium carbonyl complexes was formed by reaction of Ir(CO)₂(acac) (acac=acetylacetonate) to form supported Ir(CO)₂ complexes, which were treated at 298 K and 1 atm with flowing gas‐phase reactants, including C₂H₄, H₂, ¹²CO, ¹³CO, and D₂O. Mass spectrometry was used to identify effluent gases, and infrared and X‐ray absorption spectroscopies were used to characterize the supported species, with the results bolstered by DFT calculations. Because the support is crystalline and presents a nearly uniform array of bonding sites for the iridium species, these were characterized by a high degree of uniformity, which allowed a precise determination of the species involved in the replacement, for example, of one CO ligand of each Ir(CO)₂ complex with ethylene. The supported species include the following: Ir(CO)₂, Ir(CO)(C₂H₄)₂, Ir(CO)(C₂H₄), Ir(CO)(C₂H₅), and (tentatively) Ir(CO)(H). The data determine a reaction network involving all of these species.     

On the Role of Pre‐ and Post‐Electron‐Transfer Steps in the SmI2/Amine/H2O‐Mediated Reduction of Esters: New Mechanistic Insights and Kinetic Studies

The mechanism of the SmI₂‐mediated reduction of unactivated esters has been studied using a combination of kinetic, radical clocks and reactivity experiments. The kinetic data indicate that all reaction components (SmI₂, amine, H₂O) are involved in the rate equation and that electron transfer is facilitated by Brønsted base assisted deprotonation of water in the transition state. The use of validated cyclopropyl‐containing radical clocks demonstrates that the reaction occurs via fast, reversible first electron transfer, and that the electron transfer from simple Sm(II) complexes to aliphatic esters is rapid. Notably, the mechanistic details presented herein indicate that complexation between SmI₂, H₂O and amines affords a new class of structurally diverse, thermodynamically powerful reductants for efficient electron transfer to carboxylic acid derivatives as an attractive alternative to the classical hydride‐mediated reductions and as a source of acyl‐radical equivalents for C?C bond forming processes.     

Valence‐to‐Core X‐Ray Emission Spectroscopy of Iron–Carbonyl Complexes: Implications for the Examination of Catalytic Intermediates

Valence‐to‐core X‐ray emission spectroscopy (V2C XES) has been applied to a series of compounds relevant to both homogeneous catalysts and intermediates in heterogeneous reactions, namely [Fe(CO)₅], [Fe₂(CO)₉], [Fe₃(CO)₁₂], [Fe(CO)₃(cod)] (cod=cyclo‐octadienyl), [Fe₂Cp₂(CO)₄] (Cp=cyclo‐pentadienyl), [Fe₂Cp*₂(CO)₄] (Cp*=tetramethylcyclopentadienyl), and [FeCp(CO)₂(thf)][B(ArF)₄] (ArF=pentafluorophenyl). DFT calculations of the V2C XES spectra show very good agreement with experiment, which allows for an in depth analysis of the origins of the observed spectral signatures. It is demonstrated that the observed spectral features can be broken down into specific ligand and metal fragment contributions. The relative intensities of the observed features are further explained through a quantitative investigation of the metal 3p and 4p contributions to the spectra. The ability to use V2C XES to separate carbonyl, hydrocarbon, and solvent contributions is highlighted.     

Comparison of Cu‐ZSM‐5 Zeolites and Cu‐MOF‐505 Metal‐Organic Frameworks as Heterogeneous Catalysts for the Mukaiyama Aldol Reaction: A DFT Mechanistic Study

The density functional theory (DFT) model ONIOM(M06L/6‐311++G(2df,2p):UFF was employed to reveal the catalytic activity of Cu^II in the paddle‐wheel unit of the metal‐organic framework (MOF)‐505 material in the Mukaiyama aldol reaction compared with the activity of Cu‐ZSM‐5 zeolites. The aldol reaction between a silyl enol ether and formaldehyde catalyzed by the Lewis acidic site of both materials takes place through a concerted pathway, in which the formation of the C? C bond and the transfer of the silyl group occurs in a single step. MOF‐505 and Cu‐ZSM‐5 are predicted to be efficient catalysts for this reaction as they strongly activate the formaldehyde carbonyl carbon electrophile, which leads to a considerably lower reaction barrier compared with the gas‐phase system. Both MOF‐505 and Cu‐ZSM‐5 catalysts stabilize the reacting species along the reaction coordinate, thereby lowering the activation energy, compared to the gas‐phase system. The activation barriers for the MOF‐505, Cu‐ZSM‐5, and gas‐phase system are 48, 21, and 61 kJ mol^?1, respectively. Our results show the importance of the enveloping framework by stabilizing the reacting species and promoting the reaction.     

Stepwise Hydride Transfer in a Biological System: Insights into the Reaction Mechanism of the Light‐Dependent Protochlorophyllide Oxidoreductase

Hydride transfer plays a crucial role in a wide range of biological systems. However, its mode of action (concerted or stepwise) is still under debate. Light‐dependent NADPH: protochlorophyllide oxidoreductase (POR) catalyzes the stereospecific trans addition of a hydride anion and a proton across the C₁₇?C₁₈ double bond of protochlorophyllide. Time‐resolved absorption and emission spectroscopy were used to investigate the hydride transfer mechanism in POR. Apart from excited states of protochlorophyllide, three discrete intermediates were resolved, consistent with a stepwise mechanism that involves an initial electron transfer from NADPH. A subsequent proton‐coupled electron transfer followed by a proton transfer yield distinct different intermediates for wild type and the C226S variant, that is, initial hydride attaches to either C₁₇ or C₁₈, but ends in the same chlorophyllide stereoisomer. This work provides the first evidence of a stepwise hydride transfer in a biological system.     

Facile Insertion of Carbon Dioxide into Cu2(μ‐H) Dinuclear Units Supported by Tetraphosphine Ligands

Reactions of meso‐bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm) with Cu^I species in the presence of NaBH₄ afforded di‐ and tetranuclear copper hydride complexes, [Cu₂(μ‐H)(μ‐dpmppm)₂]X ( 1 ) and [Cu₄(μ‐H)₂(μ₄‐H)(μ‐dpmppm)₂]X ( 2 ) (X=BF₄, PF₆). Complex 1 undergoes facile insertion of CO₂ (1 atm) at room temperature, leading to a formate‐bridged dicopper complex [Cu₂(μ‐HCOO)(dpmppm)₂]X ( 3 ). The experimental and DFT theoretical studies clearly demonstrate that CO₂ insertion into the Cu₂(μ‐H) unit occurred with the flexible dicopper platform. Complex 2 also undergoes CO₂ insertion to give a formate‐bridged complex, [Cu₄(μ‐HCOO)₃(dpmppm)₂]X, during which the square Cu₄ framework opened up to a linear tetranuclear chain.     

Mechanistic Studies on the Transformation of Ethanol into Ethene over Fe‐ZSM‐5 Zeolite

Ethanol, through the utilization of bioethanol as a chemical resource, has received considerable industrial attention as it provides an alternative route to produce more valuable hydrocarbons. Using a density functional theory approach incorporating the M06‐L functional, which includes dispersion interactions, a large 34T nanocluster model of Fe‐ZSM‐5 zeolite in which T is a Si or Al atom is employed to examine both the stepwise and concerted mechanisms of the transformation of ethanol into ethene. For the stepwise mechanism, ethanol dehydration commences from the first hydrogen abstraction of the ethanol OH group to form the ethoxide‐hydroxide intermediate with a low activation energy of 17.7 kcal mol^?1. Consequently, the ethoxide‐hydroxide intermediate is decomposed into ethene through hydrogen abstraction from the ethoxide methyl carbon to either the OH group of hydroxide or the oxygen of the ethoxide group with high activation energies of 64.8 and 63.5 kcal mol^?1, respectively. For the concerted mechanism, ethanol transformation into the ethene product occurs in a single step without intermediate formation, with an activation energy of 32.9 kcal mol^?1.     

Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu‐SSZ‐13

Operando X‐ray absorption experiments and density functional theory (DFT) calculations are reported that elucidate the role of copper redox chemistry in the selective catalytic reduction (SCR) of NO over Cu‐exchanged SSZ‐13. Catalysts prepared to contain only isolated, exchanged Cu^II ions evidence both Cu^II and Cu^I ions under standard SCR conditions at 473 K. Reactant cutoff experiments show that NO and NH₃ together are necessary for Cu^II reduction to Cu^I. DFT calculations show that NO‐assisted NH₃ dissociation is both energetically favorable and accounts for the observed Cu^II reduction. The calculations predict in situ generation of Brønsted sites proximal to Cu^I upon reduction, which we quantify in separate titration experiments. Both NO and O₂ are necessary for oxidation of Cu^I to Cu^II, which DFT suggests to occur by a NO₂ intermediate. Reaction of Cu‐bound NO₂ with proximal NH₄⁺ completes the catalytic cycle. N₂ is produced in both reduction and oxidation half‐cycles.     

CHR‐Insertion (RH,CH3) into Cyclohexyl‐Substituted Silsesquioxanes: Reactivity and Decomposition Studies

Amorphous silica plays an important role in heterogeneous catalysis as a support and is frequently presumed to be “inert”. The structure of the supported catalyst is key to understanding the stability and reactivity of catalytic systems. To provide vital insights into the surface reactivity of silica, Polyhedral oligomeric silsesquioxanes (POSSs) can act as realistic homogeneous molecular models for silica surfaces. Here, we report novel reactivities associated with the silica surface, derived from our insights obtained by means of such model systems with potentially significant implications in catalysis when employing silica‐supported catalysts. In this work, the gas‐phase reactivities of two cyclohexyl‐substituted POSSs, namely the completely condensed triganol prism [Si₆cy₆O₉] (a6b0), and the incompletely‐condensed partial cube [Si₇cy₇O₉(OH)₃] (a7b3), with cy=c‐C₆H₁₁, were studied by using atmospheric pressure chemical ionisation (APCI) and collision‐induced decomposition (CID) spectroscopies. Silsesquioxane a6b0, containing three‐membered rings, was found to be much more reactive, undergoing novel CH₂‐insertion on reaction with gas phase molecules—a reaction not observed for a7b3, containing only four‐membered rings. Both silsesquioxanes displayed the ability to trap ammonia formed in situ within the mass spectrometer from N₂ in the instrument. This work also demonstrates the applicability of APCI and the role of CID in elucidating reactive POSS structures, highlighting novel gas‐phase reactivities of POSS.     

Mechanistic Insight into the Intramolecular Benzylic C−H Nitrene Insertion Catalyzed by Bimetallic Paddlewheel Complexes: Influence of the Metal Centers

The intramolecular benzylic C?H amination catalyzed by bimetallic paddlewheel complexes was investigated by using density functional theory calculations. The metal–metal bonding characters were investigated and the structures featuring either a small HOMO–LUMO gap or a compact SOMO energy scope were estimated to facilitate an easier one‐electron oxidation of the bimetallic center. The hydrogen‐abstraction step was found to occur through three manners, that is, hydride transfer, hydrogen migration, and proton transfer. The imido N species are more preferred in the Ru–Ru and Pd–Mn cases whereas coexisting N species, namely, singlet/triplet nitrene and imido, were observed in the Rh–Rh and Pd–Co cases. On the other hand, the triplet nitrene N species were found to be predominant in the Pd–Ni and Pd–Zn systems. A concerted asynchronous mechanism was found to be modestly favorable in the Rh–Rh‐catalyzed reactions whereas the Pd–Co‐catalyzed reactions demonstrated a slight preference for a stepwise pathway. Favored stepwise pathways were seen in each Ru–Ru‐ and Pd–Mn‐catalyzed reactions and in the triplet nitrene involved Pd–Ni and Pd–Zn reactions. The calculations suggest the feasibility of the Pd–Mn, Pd–Co, and Pd–Ni paddlewheel complexes as being economical alternatives for the expensive dirhodium/diruthenium complexes in C?H amination catalysis.     

Enantioselective Palladium‐Catalyzed Carbene Insertion into the N−H Bonds of Aromatic Heterocycles

C3‐substituted indoles and carbazoles react with α‐aryl‐α‐diazoesters under palladium catalysis to form α‐(N‐indolyl)‐α‐arylesters and α‐(N‐carbazolyl)‐α‐arylesters. The products result from insertion of a palladium‐carbene ligand into the N−H bond of the aromatic N‐heterocycles. Enantioselection was achieved using a chiral bis(oxazoline) ligand, in many cases with high enantioselectivity (up to 99 % ee). The method was applied to synthesize the core of a bioactive carbazole derivative in a concise manner.     

Visible‐Light‐Initiated Manganese Catalysis for C−H Alkylation of Heteroarenes: Applications and Mechanistic Studies

A visible‐light‐driven Minisci protocol that employs an inexpensive earth‐abundant metal catalyst, decacarbonyldimanganese Mn₂(CO)₁₀, to generate alkyl radicals from alkyl iodides has been developed. This Minisci protocol is compatible with a wide array of sensitive functional groups, including oxetanes, sugar moieties, azetidines, tert ‐butyl carbamates (Boc‐group), cyclobutanes, and spirocycles. The robustness of this protocol is demonstrated on the late‐stage functionalization of complex nitrogen‐containing drugs. Photophysical and DFT studies indicate a light‐initiated chain reaction mechanism propagated by ^.Mn(CO)₅. The rate‐limiting step is the iodine abstraction from an alkyl iodide by ^.Mn(CO)₅.     

Visible‐Light Photocatalysis of C(sp3)‐H Fluorination by the Uranyl Ion: Mechanistic Insights

The uranyl dication shows photocatalytic activity towards C(sp³)?H bonds of aliphatic compounds, but not towards those of alkylbenzenes or cyclic ketones. Theoretical insights into the corresponding mechanisms are still limited. Multi‐configurational ab initio calculations including relativistic effects reveal the inherent electron‐transfer mechanism for the uranyl catalyzed C?H fluorination under blue light. Along the reaction path of the triplet state it was found that the hydrogen atom abstraction triggered by the electron‐rich oxygen of the uranyl moiety is the rate‐limiting step. The subsequent steps, that is, N?F and O?H bond breakage in a manner of concerted asynchronicity, generation of the targeted fluorinated product, and recovery of the photocatalyst are nearly barrierless. Moreover the single electron transfer between the reactive substrates plays a fundamental role during the whole photocatalytic cycle.     

A DFT study on Cu(I) coordination in Cu‐ZSM‐5: Effects of the functional choice and tuning of the ONIOM approach

The coordination of Cu⁺ at the T1 and T7 positions of the M7 ring of Cu‐ZSM‐5, and the interaction of NO with coordinated Cu⁺ were investigated by means of DFT/ONIOM calculations. The B3LYP, BLYP, PBE1PBE, PBE, M06, and M062X functionals with the def2‐TZVP (def2‐QZVP for Cu) basis set were used in the high‐level part of ONIOM calculations, with the HF/3‐21G, B3LYP/LANL2DZ, M06/LANL2DZ, and M062X/LANL2DZ methods in the low‐level part. The ability of suitable combinations of the above methods to reproduce (i) the crystallographic structure of purely siliceous ZSM‐5, (ii) the tendency of Cu⁺ to be twofold or fourfold coordinated by framework oxygen atoms of Cu‐ZSM‐5, and (iii) the interaction energy and the N? O stretching frequency of adsorbed nitrogen oxide are discussed, showing that different results are obtained depending on the adopted computational approach. With reference to above properties, some considerations about the employment of the ONIOM approximations are also included. © 2015 Wiley Periodicals, Inc.     

Hidden Hydride Transfer as a Decisive Mechanistic Step in the Reactions of the Unligated Gold Carbide [AuC]+ with Methane under Ambient Conditions

The reactivity of the cationic gold carbide [AuC]⁺ (bearing an electrophilic carbon atom) towards methane has been studied using Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS). The product pairs generated, that is, Au⁺/C₂H₄, [Au(C₂H₂)]⁺/H₂, and [C₂H₃]⁺/AuH, point to the breaking and making of C?H, C?C, and H?H bonds under single‐collision conditions. The mechanisms of these rather efficient reactions have been elucidated by high‐level quantum‐chemical calculations. As a major result, based on molecular orbital and NBO‐based charge analysis, an unprecedented hydride transfer from methane to the carbon atom of [AuC]⁺ has been identified as a key step. Also, the origin of this novel mechanistic scenario has been addressed. The mechanistic insights derived from this study may provide guidance for the rational design of carbon‐based catalysts.