Mesomeric Acceleration Counters Slow Initiation of Ruthenium–CAAC Catalysts for Olefin Metathesis (CAAC = Cyclic (Alkyl)(Amino) Carbene)

Ruthenium catalysts bearing cyclic (alkyl)(amino)carbene (CAAC) ligands can attain very high productivities in olefin metathesis, owing to their resistance to unimolecular decomposition. Because the propagating methylidene species RuCl₂(CAAC)(=CH₂) is extremely susceptible to bimolecular decomposition, however, turnover numbers in the metathesis of terminal olefins are highly sensitive to catalyst concentration, and hence loadings. Understanding how, why, and how rapidly the CAAC complexes partition between the precatalyst and the active species is thus critical. Examined in a dual experimental-computational study are the rates and basis of initiation for phosphine-free catalysts containing the leading CAAC ligand C1 ^Ph , in which a CMePh group α to the carbene carbon helps retard degradation. The Hoveyda-class complex HC1 ^Ph (RuCl₂(L)(=CHAr), where L = C1 ^Ph , Ar = C₆H₃-2-O ⁱ Pr-5-R; R = H) is compared with its nitro-Grela analogue (nG-C1 ^Ph ; R = NO₂) and the classic Hoveyda catalyst HII (L = H₂IMes; R = H). t-Butyl vinyl ether (tBuVE) was employed as substrate, to probe the reactivity of these catalysts toward olefins of realistic bulk. Initiation is ca. 100× slower for HC1 ^Ph than HII in C₆D₆, or 44× slower in CDCl₃. The rate-limiting step for the CAAC catalyst is cycloaddition; for HII, it is tBuVE binding. Initiation is 10-13× faster for nG-C1 ^Ph than HC1 ^Ph in either solvent. DFT analysis reveals that this rate acceleration originates in an overlooked role of the nitro group. Rather than weakening the Ru-ether bond, as widely presumed, the NO₂ group accelerates the ensuing, rate-limiting cycloaddition step. Faster reaction is caused by long-range mesomeric effects that modulate key bond orders and Ru-ligand distances, and thereby reduce the trans effect between the carbene and the trans-bound alkene in the transition state for cycloaddition. Mesomeric acceleration may plausibly be introduced via any of the ligands present, and hence offers a powerful, tunable control element for catalyst design.

Electrophilicity of Hoveyda-Grubbs Olefin Metathesis Catalysts as the Driving Force that Controls Initiation Rates

The dissociative mechanism of initiation for a series of Hoveyda-Grubbs type metathesis catalysts modified at the para and meta positions in the isopropoxybenzylidene ligand is investigated by means of DFT calculations. The electron donating/withdrawing capacity of the ligand was screened through the incorporation of various substituents such as halogens, nitro, alkoxides, ketones, esters, amines, and amides. Variations in structural parameters, energy barriers for the Ru-O bond dissociation, and Ru-O bond strength were examined as a function of the Hammett constant. It was found that electronic properties of the catalysts such as chemical potential, hardness, and electrophilicity correlate linearly with the dissociative energy barriers. These findings enable a systematic rationalization and prediction of rate of precatalyst initiation through the calculation of only the HOMO-LUMO gap of catalysts, as the faster the initiation, the more electrophilic the catalyst.

Self-Supported Polymeric Ruthenium Complexes as Olefin Metathesis Catalysts in Synthesis of Heterocyclic Compounds

New ruthenium olefin metathesis catalysts containing N-heterocyclic carbene (NHC) connected by a linker tether to a benzylidene ligand were studied. Such obtained self-chelated Hoveyda–Grubbs type complexes existed in the form of an organometallic polymer but could still catalyze olefin metathesis after being dissolved in an organic solvent. Although these polymeric catalysts exhibited a slightly lower activity compared to structurally related nonpolymeric catalysts, they were successfully used in a number of ring-closing metathesis reactions leading to a variety of heterocyclic compounds, including biologically and pharmacologically related analogues of cathepsin K inhibitor and sildenafil (Viagra™). In the last case, a good solubility of a polymeric catalyst in toluene allowed the separation of the product from the catalyst via simple filtration.

Olefin Metathesis Catalyzed by a Hoveyda-Grubbs-like Complex Chelated to Bis(2-mercaptoimidazolyl) Methane: A Predictive DFT Study

Although highly selective complexes for the cross-metathesis of olefins, particularly oriented toward the productive metathesis of Z-olefins, have been reported in recent years, there is a constant need to design and prepare new and improved catalysts for this challenging reaction. In this work, guided by density functional theory (DFT) calculations, the performance of a Ru-based catalyst chelated to a sulfurated pincer in the olefin metathesis was computationally assessed. The catalyst was designed based on the Hoveyda–Grubbs catalyst (SIMes)Cl₂Ru(=CH–o–OiPrC₆H₄) through the substitution of chlorides with the chelator bis(2-mercaptoimidazolyl)methane. The obtained thermodynamic and kinetic data of the initiation phase through side- and bottom-bound mechanisms suggest that this system is a versatile catalyst for olefin metathesis, as DFT predicts the highest energy barrier of the catalytic cycle of ca. 20 kcal/mol, which is comparable to those corresponding to the Hoveyda–Grubbs-type catalysts. Moreover, in terms of the stereoselectivity evaluated through the propagation phase in the metathesis of propene–propene to 2-butene, our study reveals that the Z isomer can be formed under a kinetic control. We believe that this is an interesting outcome in the context of future exploration of Ru-based catalysts with sulfurated chelates in the search for high stereoselectivity in selected reactions.

Impact of the Carbene Derivative Charge on the Decomposition Rates of Hoveyda-Grubbs-like Metathesis Catalysts

Hoveyda–Grubbs metathesis catalysts undergo a relatively fast decomposition in the presence of olefins. Using a computational density functional theory approach, we show that positively charged derivatives of N-heterocyclic carbenes have little impact on the degradation/deactivation rates of such catalysts with respect to neutral carbenes. On the other hand, the hypothetical anionic Hoveyda–Grubbs-like catalysts are predicted to less likely undergo degradation in the presence of the olefin, while being as active as standard, neutral Hoveyda–Grubbs catalysts.

Mechanistic Study on Palladium-Catalyzed Regioselective Oxidative Amination: Roles of Ammonium Salts

Anti-Markovnikov selective oxidative amination reaction with simple alkenes is particularly promising but challenging because of the inherent electronic effect of the alkene substrate which is in favor of the Markovnikov product. In a recently reported Pd-catalyzed anti-Markovnikov oxidative amination reaction, the addition of quaternary ammonium salts is shown to be critical. We performed a comprehensive DFT study to elucidate the reaction mechanism and the origin of the regioselectivity, as well as the roles of the ammonium salts. Our results show that without and with the ammonium salts the reaction mechanisms are different. Detailed analyses indicate that the steric effects account for the switch of regioselectivity. The roles of the quaternary ammonium salts have been elucidated: (1) Me₄NOAc plays the role of base in deprotonating the phthalimide and allows the reaction to proceed through a trans-aminopalladation mechanism; (2) Me₄NCl facilitates the thermodynamically favorable transformation of Pd(OAc)₂ to the palladate ([Pd(AcO)₂Cl₂]^2-), which lessens the polarity of the carbon-carbon double bond, minimizes the inherent electronic effects, and leads to a steric-effect-controlled reaction; (3) Me₄NCl is essential in decreasing the activation barrier in the rate-determining ligand exchange step by Cl^- acting as a better leaving group (compared to AcO^-).

Impact of the olefin structure on the catalytic cycle and decomposition rates of Hoveyda-Grubbs metathesis catalysts

A relatively fast degradation of ruthenium catalysts in the presence of selected olefins, and ethylene in particular, is one of the bottlenecks in their use in metathesis reactions. Here we explore the structure-activity relationships between the rate of degradation of Hoveyda-Grubbs catalysts and the structure of olefins by means of DFT calculations. We show that (Z)-1,2-dichloroethene can't form stable complexes with a 14-electron active complex due to a strong inductive electron withdrawal effect. Hoveyda-Grubbs catalysts can be, however, used to convert (Z)-1,2-dichloroethene to (E)-1,2-dichloroethene due to differences in crucial barriers in the catalytic cycle for E/Z isomers. Hoveyda-Grubbs catalysts in the presence of both isomers of 1,2-dimethoxyethene and 1,2-dichloroethene are predicted to be very stable in the unproductive metathesis, while for monosubstituted olefins the methoxyethene presence gives relatively low barriers for crucial degradation transition states and can readily undergo decomposition.

A Macrocyclic Ruthenium Carbene for Size-Selective Alkene Metathesis

The synthesis of a macrocyclic Ru carbene catalyst for selective cross alkene metathesis is reported. The new catalyst showed different reactivity for various type 1 alkenes in homodimerization which correlated with the aggregrate size of the allylic substituent. The altered reactivity profile allowed for selective product formation in competition cross alkene metathesis between two different type 1 alkenes and tert-butyl acrylate. Selectivity in these reactions is attributed to the ability of the macrocyclic catalyst to differentiate alkenes based on their size. Two preparative examples of cross metathesis with the macrocyclic catalyst are also provided.

Second-coordination sphere effects on the reactivities of Hoveyda–Grubbs-type catalysts: a ligand exchange study using phenolic moiety-functionalized ligands

The reactivities of Hoveyda–Grubbs-type complexes are tunable through second-coordination sphere effects caused by a functional group in the ligand.

Kinetics of initiation of the third generation Grubbs metathesis catalyst: convergent associative and dissociative pathways

The kinetics of the nominally irreversible reaction of the third generation Grubbs catalyst G-III-Br (4.6 μM) with ethyl vinyl ether (EVE) in toluene at 5 °C have been re-visited. There is a rapid equilibrium between the bispyridyl form of G-III-Br, 1, and its monopyridyl form, 2 (K ≈ 0.001 M). The empirical rate constants (kobs.) for the reaction with EVE, determined UV-vis spectrophotometrically under optimised anaerobic stopped-flow conditions, are found by testing the quality of fit of a series of steady-state approximations. The kinetics do not correlate with solely dissociative or associative pathways, but do correlate with a mechanism where these pathways converge at an alkene complex primed to undergo metathesis. In the presence of traces of air there is a marked increased in the rate of decay of G-III-Br due to competing oxidation to yield benzaldehyde; a process that appears to be very efficiently catalysed by trace metal contaminants. The apparent acceleration of the initiation process may account for the rates determined herein being over an order of magnitude lower than previously estimated.

Formation of active species from ruthenium alkylidene catalysts-an insight from computational perspective

Ruthenium alkylidene complexes are commonly used as olefin metathesis catalysts. Initiation of the catalytic process requires formation of a 14-electron active ruthenium species via dissociation of a respective ligand. In the present work, this initiation step has been computationally studied for the Grubbs-type catalysts (H₂IMes)(PCy₃)(Cl)₂Ru=CHPh, (H₂IMes)(PCy₃)(Cl)₂Ru=CH-CH=CMe₂ and (H₂IMes)(3-Br-py)₂(Cl)₂Ru=CHPh, and the Hoveyda-Grubbs-type catalysts (H₂IMes)(Cl)₂Ru=CH(o-OiPrC₆H₄), (H₂IMes)(Cl)₂Ru=CH(5-NO₂-2-OiPrC₆H₃), and (H₂IMes)(Cl)₂Ru=CH(2-OiPr-3-PhC₆H₃), using density functional theory (DFT). Additionally, the extended-transition-state combined with the natural orbitals for the chemical valence (ETS-NOCV) and the interacting quantum atoms (IQA) energy decomposition methods were applied. The computationally determined activity order within both families of the catalysts and the activation parameters are in agreement with reported experimental data. The significance of solvent simulation and the basis set superposition error (BSSE) correction is discussed. ETS-NOCV demonstrates that the bond between the dissociating ligand and the Ru-based fragment is largely ionic followed by the charge delocalizations: σ(Ru-P) and π(Ru-P) and the secondary CH^…Cl, CH^…π, and CH^…HC interactions. In the case of transition state structures, the majority of stabilization stems from London dispersion forces exerted by the efficient CH^…Cl, CH^…π, and CH^…HC interactions. Interestingly, the height of the electronic dissociation barriers is, however, directly connected with the prevalent (unfavourable) changes in the electrostatic and orbital interaction contributions despite the favourable relief in Pauli repulsion and geometry reorganization terms during the activation process. According to the IQA results, the isopropoxy group in the Hoveyda-Grubbs-type catalysts is an efficient donor of intra-molecular interactions which are important for the activity of these catalysts.

Disentangling Ligand Effects on Metathesis Catalyst Activity: Experimental and Computational Studies of Ruthenium-Aminophosphine Complexes

Second-generation ruthenium olefin metathesis catalysts bearing aminophosphine ligands were investigated with systematic variation of the ligand structure. The rates of phosphine dissociation ( k₁; initiation rate) and relative phosphine reassociation ( k_-1) were determined for two series of catalysts bearing cyclohexyl(morpholino)phosphine and cyclohexyl(piperidino)phosphine ligands. In both cases, incorporating P-N bonds into the architecture of the dissociating phosphine accelerates catalyst initiation relative to the parent [Ru]-PCy₃ complex; however, this effect is muted for the tris(amino)phosphine-ligated complexes, which exhibit higher ligand binding constants in comparison to those with phosphines containing one or two cyclohexyl substituents. These results, along with X-ray crystallographic data and DFT calculations, were used to understand the influence of ligand structure on catalyst activity. Especially noteworthy is the application of phosphines containing incongruent substituents (PR₁R'₂); detailed analyses of factors affecting ligand dissociation, including steric effects, inductive effects, and ligand conformation, are presented. Computational studies of the reaction coordinate for ligand dissociation reveal that ligand conformational changes contribute to the rapid dissociation for the fastest-initiating catalyst of these series, which bears a cyclohexyl-bis(morpholino)phosphine ligand. Furthermore, the effect of amine incorporation was examined in the context of ring-opening metathesis polymerization, and reaction rates were found to correlate well with catalyst initiation rates. The combined experimental and computational studies presented in this report reveal important considerations for designing efficient ruthenium olefin metathesis catalysts.

Theoretical evidence for bond stretch isomerism in Grubbs olefin metathesis

A comprehensive density functional theory study on the dissociative and associative mechanisms of Grubbs first and second generation olefin metathesis catalysis reveals that ruthenacyclobutane intermediate (RuCB) observed in the Chauvin mechanism is not unique as it can change to a non-metathetic ruthenacyclobutane (RuCB') via the phenomenon of bond stretch isomerism (BSI). RuCB and RuCB' differ mainly in RuC_α , RuC_β , and C_α C_β bond lengths of the metallacycle. RuCB is metathesis active due to the agostic type bonding-assisted simultaneous activation of both C_α C_β bonds, giving hypercoordinate character to C_β whereas an absence of such bonding interactions in RuCB' leads to typical CC single bond distances and metathesis inactivity. RuCB and RuCB' are connected by a transition state showing moderate activation barrier. The new mechanistic insights invoking BSI explains the non-preference of associative mechanism and the requirement of bulky ligands in the Grubbs catalyst design. The present study lifts the status of BSI from a concept of largely theoretical interest to a phenomenon of intense importance to describe an eminent catalytic reaction. © 2017 Wiley Periodicals, Inc.

Hoveyda–Grubbs catalyst analogues bearing the derivatives of N-phenylpyrrol in the carbene ligand – structure, stability, activity and unique ruthenium–phenyl interactions

N-Phenylpyrrole-2,6-diisopropylphenyl ruthenium complex and its perbrominated derivative are active in ring-closing metathesis at 80 °C, but inactive at room temperature.

From Resting State to the Steady State: Mechanistic Studies of Ene-Yne Metathesis Promoted by the Hoveyda Complex

The kinetics of intermolecular ene-yne metathesis (EYM) with the Hoveyda precatalyst (Ru1) has been studied. For 1-hexene metathesis with 2-benzoyloxy-3-butyne, the experimental rate law was determined to be first-order in 1-hexene (0.3-4 M), first-order in initial catalyst concentration, and zero-order for the terminal alkyne. At low catalyst concentrations (0.1 mM), the rate of precatalyst initiation was observed by UV-vis and the alkyne disappearance was observed by in situ FT-IR. Comparison of the rate of precatalyst initiation and the rate of EYM shows that a low, steady-state concentration of active catalyst is rapidly produced. Application of steady-state conditions to the carbene intermediates provided a rate treatment that fit the experimental rate law. Starting from a ruthenium alkylidene complex, competition between 2-isopropoxystyrene and 1-hexene gave a mixture of 2-isopropoxyarylidene and pentylidene species, which were trappable by the Buchner reaction. By varying the relative concentration of these alkenes, 2-isopropoxystyrene was found to be 80 times more effective than 1-hexene in production of their respective Ru complexes. Buchner-trapping of the initiation of Ru1 with excess 1-hexene after 50% loss of Ru1 gave 99% of the Buchner-trapping product derived from precatalyst Ru1. For the initiation process, this shows that there is an alkene-dependent loss of precatalyst Ru1, but this does not directly produce the active catalyst. A faster initiating precatalyst for alkene metathesis gave similar rates of EYM. Buchner-trapping of ene-yne metathesis failed to deliver any products derived from Buchner insertion, consistent with rapid decomposition of carbene intermediates under ene-yne conditions. An internal alkyne, 1,4-diacetoxy-2-butyne, was found to obey a different rate law. Finally, the second-order rate constant for ene-yne metathesis was compared to that previously determined by the Grubbs second-generation carbene complex: Ru1 was found to promote ene-yne metathesis 62 times faster at the same initial precatalyst concentration.

Structural analogues of Hoveyda–Grubbs catalysts bearing the 1-benzofuran moiety or isopropoxy-1-benzofuran derivatives as olefin metathesis catalysts

We have used the DFT/M06-D3 computational method to study structures and activation free energies for a series of Hoveyda–Grubbs-like catalysts with the isopropoxybenzene part replaced by 1-benzofuran and ten derivatives of isopropoxy-1-benzofuran.

Nitrenium ions and trivalent boron ligands as analogues of N-heterocyclic carbenes in olefin metathesis: a computational study

We used the density functional theory to evaluate the suitability of nitrenium ions and trivalent boron ligands as analogues of N-heterocyclic carbenes in ruthenium-based metathesis catalysts. We demonstrate that these analogues induce only minor structural changes in Hoveyda-Grubbs-like precatalysts, but have major impact on precatalyst initiation. Nitrenium ion-modified precatalysts are characterized by a weak Ru-N bond resulting in a relatively strong Ru-O bond and large free energy barriers for initiation, making them good candidates for efficient latent Ru-based catalysts. On the other hand the trivalent boron ligand, bearing a formal -1 charge, binds strongly to the ruthenium ion, weakening the Ru-O bond and facilitating its dissociation, to promote fast reaction initiation. We show that the calculated bond dissociation energy of the Ru-C/N/B bond may serve as an accurate indicator of the Ru-O bond strength and the rate of metathesis initiation.

Key processes in ruthenium-catalysed olefin metathesis

While the fundamental series of [2+2]cycloadditions and retro[2+2]cycloadditions that make up the pathways of ruthenium-catalysed metathesis reactions is well-established, the exploration of mechanistic aspects of alkene metathesis continues. In this Feature Article, modern mechanistic studies of the alkene metathesis reaction, catalysed by well-defined ruthenium complexes, are discussed. Broadly, these concern the processes of pre-catalyst initiation, propagation and decomposition, which all have a considerable impact on the overall efficiency of metathesis reactions.

DFT study on the reaction mechanism of the ring closing enyne metathesis (RCEYM) catalyzed by molybdenum alkylidene complexes

DFT calculations show that the RCEYM reaction catalyzed by Mo-based catalysts proceeds preferentially through an yne-then-ene mechanism and that the endo-/exo- selectivity mainly depends on sterics.

Does the rate of competing isomerisation during alkene metathesis depend on pre-catalyst initiation rate?

Detailed kinetic experiments reveal that the rates of competing isomerisation processes in alkene metathesis reactions are independent of pre-catalyst initiation rate, but are reduced by the presence of phosphine.

Computation and experiment reveal that the ring-rearrangement metathesis of Himbert cycloadducts can be subject to kinetic or thermodynamic control

Unusual observations in the ring-rearrangement metathesis of Himbert arene/allene cycloadducts to form fused polycylic lactams led to a more in-depth experimental study that yielded conflicting results. Differences in reactivity within related systems and unexpected changes in diastereoselectivity among other similar substrates were not readily explained on the basis of the experimental results. Computational investigations demonstrated substrate-dependent changes in reaction pathways (ring-opening metathesis/ring-closing metathesis [ROM/RCM] cascade vs ring-closing metathesis/ring-opening metathesis [RCM/ROM] cascade). Furthermore, some reactions were judged to be under thermodynamic control and others under kinetic control. The greater understanding of the most likely reaction pathways and their energetics provides a reasonable explanation for the previously irreconcilable results.