Hydrogenation and Transfer Hydrogenation Promoted by Tethered Ru−S Complexes: From Cooperative Dihydrogen Activation to Hydride Abstraction/Proton Release from Dihydrogen Surrogates

Hydrogenation and transfer hydrogenation of imines with cyclohexa‐1,4‐dienes, as well as with a representative Hantzsch ester dihydrogen surrogate, are reported. Both processes are catalyzed by tethered Ru?S complexes but differ in the activation mode of the dihydrogen source: cooperative activation of the H?H bond at the Ru?S bond leads to the corresponding Ru?H complex and protonation of the sulfur atom, whereas the same cationic Ru?S catalyst abstracts a hydride from a donor‐substituted cyclohexa‐1,4‐diene to form the neutral Ru?H complex and a low‐energy Wheland intermediate. A sequence of proton and hydride transfers on the imine substrate then yields an amine. The reaction pathways are analyzed computationally, and the established mechanistic pictures are in agreement with the experimental observations.     

Iron Borohydride Pincer Complexes for the Efficient Hydrogenation of Ketones under Mild,Base‐Free Conditions: Synthesis and Mechanistic Insight

The new, structurally characterized hydrido carbonyl tetrahydridoborate iron pincer complex [(iPr‐PNP)Fe(H)(CO)(η¹‐BH₄)] ( 1 ) catalyzes the base‐free hydrogenation of ketones to their corresponding alcohols employing only 4.1 atm hydrogen pressure. Turnover numbers up to 1980 at complete conversion of ketone were reached with this system. Treatment of 1 with aniline (as a BH₃ scavenger) resulted in a mixture of trans‐[(iPr‐PNP)Fe(H)₂(CO)] ( 4 a ) and cis‐[(iPr‐PNP)Fe(H)₂(CO)] ( 4 b ). The dihydrido complexes 4 a and 4 b do not react with acetophenone or benzaldehyde, indicating that these complexes are not intermediates in the catalytic reduction of ketones. NMR studies indicate that the tetrahydridoborate ligand in 1 dissociates prior to ketone reduction. DFT calculations show that the mechanism of the iron‐catalyzed hydrogenation of ketones involves alcohol‐assisted aromatization of the dearomatized complex [(iPr‐PNP*)Fe(H)(CO)] ( 7 ) to initially give the Fe⁰ complex [(iPr‐PNP)Fe(CO)] ( 21 ) and subsequently [(iPr‐PNP)Fe(CO)(EtOH)] ( 38 ). Concerted coordination of acetophenone and dual hydrogen‐atom transfer from the PNP arm and the coordinated ethanol to, respectively, the carbonyl carbon and oxygen atoms, leads to the dearomatized complex [(iPr‐PNP*)Fe(CO)(EtO)(MeCH(OH)Ph)] ( 32 ). The catalyst is regenerated by release of 1‐phenylethanol, followed by dihydrogen coordination and proton transfer to the coordinated ethoxide ligand.     

Iridicycle‐Catalysed Imine Reduction: An Experimental and Computational Study of the Mechanism

The mechanism of imine reduction by formic acid with a single‐site iridicycle catalyst has been investigated by density functional theory (DFT), NMR spectroscopy, and kinetic measurements. The NMR and kinetic studies suggest that the transfer hydrogenation is turnover‐limited by the hydride formation step. The calculations reveal that, amongst a number of possibilities, hydride formation from the iridicycle and formate probably proceeds by an ion‐pair mechanism, whereas the hydride transfer to the imino bond occurs in an outer‐sphere manner. In the gas phase, in the most favourable pathway, the activation energies in the hydride formation and transfer steps are 26–28 and 7–8 kcal mol^?1, respectively. Introducing one explicit methanol molecule into the modelling alters the energy barrier significantly, reducing the energies to around 18 and 2 kcal mol^?1 for the two steps, respectively. The DFT investigation further shows that methanol participates in the transition state of the turnover‐limiting hydride formation step by hydrogen‐bonding to the formate anion and thereby stabilising the ion pair.     

Reaction Mechanism for the Dual Gold‐Catalyzed Synthesis of Dibenzopentalene: A DFT Study

A wide range of gold‐catalyzed reactions based on a dual activation mechanism has recently been reported in the literature. Herein, we present a computational investigation of the mechanism for the formation of dibenzopentalenes from 1‐ethynyl‐2‐(phenylethynyl)benzene. Transition states have been found, which substantiate the dual activation mechanism previously published and furthermore point towards a continuous presence of two gold moieties throughout the mechanistic cycle, an observation of high importance for all reactions in the field of dual activation. The initial activation of the diyne has been shown to proceed via an intermolecular transfer of a cationic gold catalyst from the thermodynamically preferred geminal‐σ,π‐acetylide complex to the active non‐geminal analogue. Furthermore, the regioselectivity of a 5‐endo versus a 6‐endo cyclization has been addressed, and the 5‐endo cyclization was found to be most favorable both thermodynamically and with regard to the activation barrier.     

Mechanism of the Asymmetric Hydrogenation of Exocyclic α,β‐Unsaturated Carbonyl Compounds with an Iridium/BiphPhox Catalyst: NMR and DFT Studies

The mechanism of the asymmetric hydrogenation of exocyclic α,β‐unsaturated carbonyl compounds with the (aS)‐Ir/iPr‐BiphPhox catalyst was studied by NMR experiments and DFT computational analyses. Computed optical yields of the asymmetric hydrogenation proceeding by an iridium(I)/iridium(III) mechanism involving a transition state stabilized through two intramolecular hydrogen bonds are in good accordance with the experimental ee values.     

Behaviour of [PdH(dppe)2]X (X=CF3SO3-, SbF6-, BF4-) as proton or hydride donor: relevance to catalysis

The synthesis, characterization and properties of [PdH(dppe)(2)](+)CF(3)SO(3) (-).0.125 THF (1; dppe=1,2-bis(diphenylphosphanyl)ethane) and its SbF(6) (-) (1') and BF(4) (-) (1") analogues, the missing members of the [MH(dppe)(2)](+)X(-) (M=Ni, Pd, Pt) family, are described. The Pd hydrides are not stable in solution and can react as proton or hydride donors with formation of dihydrogen, [Pd(dppe)(2)](2+) and [Pd(dppe)(2)]. Complexes 1-1" react with carbocations and carbanions by transferring a hydride and a proton, respectively. Such H(-) or H(+) transfer occurs also towards unsaturated compounds, for example, hydrogenation of a C=C double bond. Accordingly, 1 can hydrogenate methyl acrylate to methyl propionate. Complex 1" is an effective (hourly turnover frequency=16) and very selective (100 %) catalyst for the hydrogenation of cyclohexen-2-one to cyclohexanone with dihydrogen under mild conditions. Density functional calculations coupled with a dielectric continuum model were carried out to compute the energetics of the hydride/proton transfer reactions, which were used to rationalize some of the experimental findings. Theory provides strong support for the thermodynamic and kinetic viability of a tetracoordinate Pd complex as an intermediate in the reactions.     

Ligand‐Controlled Regiodivergent Pathways of Rhodium(III)‐Catalyzed Dihydroisoquinolone Synthesis: Experimental and Computational Studies of Different Cyclopentadienyl Ligands

Rh^III‐catalyzed directed C?H functionalizations of arylhydroxamates have become a valuable synthetic tool. To date, the regioselectivity of the insertion of the unsaturated acceptor into the common cyclometalated intermediate was dependent solely on intrinsic substrate control. Herein, we report two different catalytic systems that allow the selective formation of regioisomeric 3‐aryl dihydroisoquinolones and previously inaccessible 4‐aryl dihydroisoquinolones under full catalyst control. The differences in the catalysts are computationally examined using density functional theory and transition state theory of different possible pathways to elucidate key contributing factors leading to the regioisomeric products. The stabilities of the initially formed rhodium complex styrene adducts, as well as activation barrier differences for the migratory insertion, were identified as key contributing factors for the regiodivergent pathways.     

The Role of Free N‐Heterocyclic Carbene (NHC) in the Catalytic Dehydrogenation of Ammonia–Borane in the Nickel NHC System

Enders' N‐heterocyclic carbene (NHC) dehydrogenates ammonia–borane with a relatively low barrier, producing NH₂BH₂ and NHC–(H)₂. The nickel NHC catalyst present in the reaction media can activate the NHC–(H)₂ produced to regenerate the free NHC and release H₂. The release of free NHC enables further dehydrogenation of ammonia–borane.

     

Transition States of Binap–Rhodium(I)‐Catalyzed Asymmetric Hydrogenation: Theoretical Studies on the Origin of the Enantioselectivity

By using the hybrid IMOMM(B3LYP:MM3) method, we examined the binap–Rh^I‐catalyzed oxidative‐addition and insertion steps of the asymmetric hydrogenation of the enamide 2‐acetylamino‐3‐phenylacrylic acid. We report a path that is energetically more favorable for the major enantiomer than for the minor enantiomer. This path follows the “lock‐and‐key” motif and leads to the major enantiomeric product via an energetically favorable binap–dihydride–Rh^III–enamide complex. Our theoretical results are consistent with the mechanism that takes place via Rh^III dihydride formation, that is, oxidative addition of H₂ followed by enamide insertion.     

Modeling Active Centers in Ammonia Synthesis. DFT Study of Dissociative Adsorption of N2 on Ru Clusters

Models of possible active centers (AC) of nitrogen adsorption on ruthenium clusters are suggested. An AC is represented by a labile Ru_n (n = 6, 7) cluster stabilized in carbon nanotubes. Nanotubes are modeled by the C₆₀ cluster. Density functional theory (DFT) is used to calculate the stationary points of the reaction path of dissociative adsorption of N₂ on the suggested AC. Optimal structures and transition state (TS) energies are determined. The effect of alkali metal (Cs) additions on the activation energy is investigated.     

Bifunctional (Cyclopentadienone)Iron–Tricarbonyl Complexes: Synthesis,Computational Studies and Application in Reductive Amination

Reductive amination under hydrogen pressure is a valuable process in organic chemistry to access amine derivatives from aldehydes or ketones. Knölker’s complex has been shown to be an efficient iron catalyst in this reaction. To determine the influence of the substituents on the cyclopentadienone ancillary ligand, a series of modified Knölker’s complexes was synthesised and fully characterised. These complexes were also transformed into their analogous acetonitrile iron–dicarbonyl complexes. Catalytic activities of these complexes were evaluated and compared in a model reaction. The scope of this reaction is also reported. For mechanistic insights, deuterium‐labelling experiments and DFT calculations were undertaken and are also presented.     

Enantioselective Nitroso‐Diels–Alder Reaction and Its Application for the Synthesis of (−)‐Peracetylated Conduramine A‐1

Cu^I‐catalyzed enantioselective nitroso‐Diels–Alder reactions (NDA reactions) of 2‐nitrosopyridine with various dienes are presented. The [CuPF₆(MeCN)₄]/Walphos‐CF₃ catalyst system is best suited to catalyze the NDA reaction of various dienes by using 2‐nitrosopyridine as a dienophile. In most of the cases studied, cycloadducts are obtained in quantitative yield with very good to excellent enantioselectivities. Based on DFT calculations, a model to explain the stereochemical outcome of the NDA reaction is presented. Finally, an efficient short synthesis of (?)‐peracetylated conduramine A‐1 by applying the enantioselective NDA reaction as a key step is described.