The Activation Mechanism of Ru–Indenylidene Complexes in Olefin Metathesis

Thiel V., Hendann M., Wannowius K., Plenio H.

Grubbs-Hoveyda-type complexes with variable 4-R (complexes 1: 4-R = NEt(2), OiPr, H, F, NO(2)) and 5-R substituents (complexes 2: 5-R = NEt(2), OiPr, Me, F, NO(2)) at the 2-isopropoxy benzylidene ether ligand and with variable 4-R substituents (complexes 3: 4-R = H, NO(2)) at the 2-methoxy benzylidene ether ligand were synthesized and the respective Ru(II/III) redox potentials (ranging from ΔE = +0.46 to +1.04 V), and UV-vis spectra recorded. The initiation kinetics of complexes 1-3 with the olefins diethyl diallyl malonate (DEDAM), butyl vinyl ether (BuVE), 1-hexene, styrene, and 3,3-dimethylbut-1-ene were investigated using UV-vis spectroscopy. Electron-withdrawing groups at the benzylidene ether ligands were found to increase the initiation rates, while electron-donating groups lead to slower precatalyst activation; accordingly with DEDAM, the complex 1(NO(2)) initiates almost 100 times faster than 1(NEt(2)). The 4-R substituents (para to the benzylidene carbon) were found to have a stronger influence on physical and kinetic properties of complexes 1 and 2 than that of 5-R groups para to the ether oxygen. The DEDAM-induced initiation reactions of complexes 1 and 2 are classified as two-step reactions with an element of reversibility. The hyperbolic fit of the k(obs) vs [DEDAM] plots is interpreted according to a dissociative mechanism (D). Kinetic studies employing BuVE showed that the initiation reactions simultaneously follow two different mechanistic pathways, since the k(obs) vs [olefin] plots are best fitted to k(obs) = k(D)·k(4)/k(-D)·[olefin]/(1 + k(4)/k(-D)·[olefin]) + k(I)·[olefin]. The k(I)·[olefin] term dominates the initiation behavior of the sterically less demanding complexes 3 and was shown to correspond to an interchange mechanism with associative mode of activation (I(a)), leading to very fast precatalyst activation at high olefin concentrations. Equilibrium and rate constants for the reactions of complexes 1-3 with the bulky PCy(3) were determined. In general, sterically demanding olefins (DEDAM, styrene) and Grubbs-Hoveyda type complexes 1 and 2 preferentially initiate according to the dissociative pathway; for the less bulky olefins (BuVE, 1-hexene) and complexes 1 and 2 both D and I(a) are important. Activation parameters for BuVE reactions and complexes 1(NEt(2)), 1(H), and 1(NO(2)) were determined, and ΔS(‡) was found to be negative (ΔS(‡) = -113 to -167 J·K(-1)·mol(-1)) providing additional support for the I(a) catalyst activation.

Poater A., Ragone F., Correa A., Cavallo L.

Density functional theory calculations have been used to investigate the activation steps involving three of the most used alkylidene groups in Ru-catalysts for olefins metathesis. Specifically, we compared the benzylidene, the indenylidene and a phosphonium alkylidene groups. Calculations reveal that the benzylidene and the indenylidene groups behave rather similarly, despite their structural differences. The phosphonium alkylidene group seems to have the most favourable activation pathway.

Credendino R., Poater A., Ragone F., Cavallo L.

In recent years olefin metathesis catalyzed by N-heterocyclic carbene ruthenium complexes has attracted remarkable attention as a versatile tool to form new CC bonds. The last developed (pre)catalysts show excellent performances, and this achievement has been possible because of continuous experimental and computational efforts to understand the laws controlling the behavior of these systems. This perspective rapidly traces the ideas and discoveries that computational chemistry contributed to the development of these catalysts, with particular emphasis on catalysts presenting a N-heterocyclic carbene ligand.

Yang H., Huang Y., Lan Y., Luh T., Zhao Y., Truhlar D.G.

As a long-standing puzzle, experimental observations reveal faster organophosphine dissociation in the olefin metathesis by Grubbs’s first-generation precatalyst (Gen I) than by the second-generation precatalyst (Gen II), but Gen I shows less catalytic activity. Here we show by electronic structure calculations with the M06-L density functional that carbene rotamer energetic effects are responsible for the inverse relation between organophosphine dissociation rate and catalytic activity. The carbene rotamer acts as a toggle switch, triggering the dissociative mechanism that produces the active catalyst. The slower catalyst production in Gen II as compared to Gen I is not a pure electronic effect but results from rotameric coupling to the dissociation coordinate speeding up Gen I dissociation more than Gen II dissociation. If organophosphine dissociation were to occur with fixed rotamer orientation, Gen II would be produced faster than Gen I, as originally expected. The rotameric energetics also contribute...

Urbina-Blanco C.A., Leitgeb A., Slugovc C., Bantreil X., Clavier H., Slawin A.M., Nolan S.P.

The synthesis and characterization of two new ruthenium indenylidene complexes [RuCl(2)(SIPr)(Py)(Ind)] 6 and [RuCl(2)(SIPr)(3-BrPy)(Ind)] 7 featuring the sterically demanding N-heterocyclic carbene 1,3-bis(2,6-di isopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr) are reported. Remarkable activity was observed with these complexes in ring closing, enyne, and cross metathesis of olefins at low catalyst loadings. The performance of SIPr-bearing complexes 6 and 7 as well as [RuCl(2)(SIPr)(PCy(3))(Ind)] 5 in ring opening metathesis polymerization is also disclosed. This work highlights the enormous influence of the neutral "spectator" ligands on catalyst activity and stability.

Urbina-Blanco C.A., Manzini S., Gomes J.P., Doppiu A., Nolan S.P.

An efficient synthetic protocol involving reactions between the free carbene and [RuCl(2)(PPh(3))(2)(Ind)] followed by addition of pyridine leads to the isolation of olefin metathesis active [RuCl(2)(L)(Py)(Ind)] (L = SIMes and SIPr) complexes. This novel approach circumvents the use of costly tricyclohexylphosphine.

Urbina-Blanco C.A., Bantreil X., Clavier H., Slawin A.M., Nolan S.P.

The steric and electronic influence of backbone substitution in IMes-based (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) N-heterocyclic carbenes (NHC) was probed by synthesizing the [RhCl(CO)2(NHC)] series of complexes to quantify experimentally the Tolman electronic parameter (electronic) and the percent buried volume (%Vbur, steric) parameters. The corresponding ruthenium–indenylidene complexes were also synthesized and tested in benchmark metathesis transformations to establish possible correlations between reactivity and NHC electronic and steric parameters.

Vorfalt T., Wannowius K.J., Thiel V., Plenio H.

Broggi J., Urbina-Blanco C., Clavier H., Leitgeb A., Slugovc C., Slawin A. ., Nolan S.

van der Eide E.F., Piers W.E.

Ruthenium-catalysed ring-closing metathesis (RCM) is a powerful technique for the preparation of medium-to-large rings in organic synthesis, but the details of the intimate mechanism are obscure. The dynamic behaviour of an RCM-relevant ruthenacyclobutane complex and its reactivity with ethene were studied using low-temperature NMR spectroscopy to illuminate the mechanism of this widely used reaction. These kinetic and thermodynamic experiments allowed for mapping the energy surface of the key steps in the RCM reaction as mediated by Grubbs-type catalysts for alkene metathesis. The highest barrier along the RCM path is only 65 kJ mol−1, which shows that this catalyst has extremely high inherent activity. Furthermore, this transition state corresponds to that connecting the intermediates in this reaction leading to ring opening of the cyclopentene product. This shows that ring closing is kinetically slightly favoured over ring opening, in addition to being driven by the loss of ethene. A kinetic and thermodynamic analysis of the ruthenium-catalysed ring-closing metathesis reaction has been achieved through the study of key intermediates accessible, for the first time, from 14-electron phosphonium alkylidene catalyst precursors. High intrinsic activities and a thermodynamic preference for ring-closing versus ring-opening reactions is observed.

Lozano-Vila A.M., Monsaert S., Bajek A., Verpoort F.

Ragone F., Poater A., Cavallo L.

We present a detailed static and dynamics characterization of 11 N-heterocyclic carbene (NHC) ligands in Ru complexes of the general formula (NHC)Cl(2)Ru horizontal lineCH(2). Analysis of the dynamic trajectories indicates that the nature of the N substituent can result in extremely different flexibilities of the Ru complexes. In almost all the cases the N substituent trans to the Ru-ylidene bond is severely folded so that it protects the vacant coordination position at the Ru center. Limited flexibility is instead associated with the N substituent on the side of the Ru-ylidene bond. NHCs with a single ortho substituent, either a simple Me or a bulkier i-Pr group, have a preferential folding that bends the unsubstituted side of the ring toward the halide-Ru-halide plane. Analysis of the dynamics trajectories in terms of buried volume indicates that the real bulkiness of these systems can be somewhat modulated, and this flexibility is a key feature that allows NHCs to modulate their encumbrance around the metal in order to make room for bulky substrates. Analysis of the buried volume in terms of steric maps showed that NHCs with mesityl or 2,6-diisopropylphenyl N substituents have quite different reactive pockets: rather flat with constant pressure on the halide-Ru-halide plane in the former and vault-shaped with higher pressure on the sides in the latter. Regarding the NHCs with an ortho tolyl or i-Pr group on the N substituent, the steric maps quantify the higher impact of the unsubstituted side of the ligand in the first coordination sphere of the metal and evidence the overall C(s)- and C(2)-symmetric reactive pockets of the corresponding complexes. We believe that a detailed characterization of the differently shaped reactive pockets is a further conceptual tool that can be used to rationalize the experimentally different performances of catalysts bearing these ligands or to devise new applications.

Vougioukalakis G.C., Grubbs R.H.

The fascinating story of olefin (or alkene) metathesis (eq 1) began almost five decades ago, when Anderson and Merckling reported the first carbon-carbon double-bond rearrangement reaction in the titanium-catalyzed polymerization of norbornene. Nine years later, Banks and Bailey reported “a new disproportionation reaction . . . in which olefins are converted to homologues of shorter and longer carbon chains...”. In 1967, Calderon and co-workers named this metal-catalyzed redistribution of carbon-carbon double bonds olefin metathesis, from the Greek word “μeτάθeση”, which means change of position. These contributions have since served as the foundation for an amazing research field, and olefin metathesis currently represents a powerful transformation in chemical synthesis, attracting a vast amount of interest both in industry and academia.

Poli R.

After a reminder of the definition of common terms used in catalysis and reaction mechanisms, different chapters highlight the various approaches used for the elucidation of catalytic mechanisms, each with its own advantages and limitations. Kinetics investigations define the rate law and provide information on certain features of the reaction pathway between the resting state and the transition state of the rate-determining step. The use of isotope labels is useful in three ways: through the measurement of kinetic isotope effects, through the study of the incorporation into the products (regio-, stereochemical features), and through the incorporation into the incompletely converted reactants (reversibility). The investigation of isolated intermediates, if available, gives experimental structural information and thermodynamic/activation data on selected stoichiometric steps of the catalytic cycle. In situ spectroscopic investigations help determine the chemical nature of the resting state and other low-en...

Samojłowicz C., Bieniek M., Grela K.

N-Heterocyclic carbene (NHC) ligands, introduced as analogues to phosphines, are recently getting wide attention in the design of diverse homogeneous catalytic systems.1-6 During recent years, olefin metathesis has gained a position of increasing significance.7-9 The ruthenium complex (PCy3)2(Cl2)RudCHPh 1 (Cy ) cyclohexyl) developed by Grubbs et al.10 constitutes a highly efficient metathesis catalyst11 tolerating most functional groups. In spite of the generally superb application profile of 1, its limited stability and the low activity toward substituted double bonds are major drawbacks. The initial success of olefin metathesis has spurred the intense investigation of new catalysts for this transformation. Inter alia, the recent introduction of NHCs * To whom correspondence should be addressed. E-mail: klgrela@ gmail.com. † Polish Academy of Sciences. ‡ Warsaw University. Cezary Samojłowicz was born in 1983 in Sokołów Podlaski, Poland. He obtained his MSc Eng. degree in chemical technology from the Warsaw University of Technology, studying sigmatropic rearrangements of sulfur ylides under the supervision of Tadeusz Zdrojewski. Before moving to olefin metathesis, he conducted work on supramolecular chemistry with David Reinhoudt at Twente University. Since 2007, he is conducting his PhD study under the supervision of Karol Grela.

Basak T., Trzaskowski B.

Antonova Alexandra S., Zubkov Fedor I.

Catalytic olefin metathesis using Hoveyda-Grubbs type ruthenium complexes is a powerful tool for creating complex molecules possessing a variety of practically useful properties. This method is also applied for obtaining modern polymer materials from low-demand petroleum products. Among all ruthenium complexes containing five- or six-membered chelate rings, the commercially available HG-II catalyst is the most common. In addition, other Hoveyda-Grubbs type complexes, which include a Het→Ru donor–acceptor bond in the chelate ring, often exhibit metathesis activity equal to or superior to that of HG-II. This review considers second-generation N-heterocyclic ruthenium carbene Hoveyda-Grubbs type complexes with donor–acceptor bonds such as O→Ru, S→Ru, Se→Ru, N→Ru, P→Ru and Hal→Ru in the chelate ring. Methods of preparation, analysis of stability and catalytic activity of such complexes are compared, and examples of the application of these organometallic ruthenium derivatives in the synthesis of practically relevant products are provided. The literature from 2010 to 2023 is summarized, making this review useful for a broad audience of chemists working in heterocyclic and organometallic chemistry, as well as practitioners involved in the production of catalysts and polymers.The bibliography includes 174 references.

Freindorf M., Kraka E.

In this study we analyzed the mechanism of [2+2] cycloaddition and cycloreversion reactions between trichlorovinylsilane and one first-generation Grubbs model catalyst M1 with a ligand and two second-generation Grubbs model catalysts M2 with an N-heterocyclic carbene (NHC) ligand and M3 with a 2,5-di-methyl-NHC ligand. As a mechanistic tool, we applied the Unified Reaction Valley Approach (URVA), based on reaction path calculations performed at the B3LYP/6–31G(d,p)/SDD(Ru) level of theory. In addition, for all stationary points of these reactions, we performed a Local Mode Analysis (LMA) and QTAIM analysis of the electron density at the B3LYP/6–31G(d,p)/NESC/Jorge–TZP(Ru) and DLPNO–CCSD(T)/def2–TZVP/ECP(Ru) levels of theory. In all reactions investigated in this work, four target bonds play a key role, either being formed and/or broken or changing from double to single bonds, and vice versa during the catalytic process. As revealed by the URVA analysis in the cycloaddition reactions, the bond forming events leading to the intermediate metallacyclobutane occur in a concerted fashion after the transition state (TS), i.e., this process does not contribute to the energy barrier. The bond breaking events of the following cycloreversion reactions transforming the intermediate into to final product are also concerted, however, they occur before the TS, i.e., this process contributes to the barrier height. In this way URVA rationalizes why all cycloaddition reactions have a lower activation energies than their cycloreversion counterparts. According to our results, M3 is the most effective model catalyst. Its activity is related to a strong stabilization of the metallacyclobutane intermediate and specific interactions between the reacting species and the methyl hydrogen atoms of the 2,5-di-methyl-NHC ligand of the catalyst. Based on LMA, we could also quantify the important role of a 4-center–2-electron α,β-(CCC) agostic interaction in the metallacyclobutane intermediate donating electron density to the Ru coordination center and facilitating the CC bond cleavage of the ring-opening cycloreversion step, lowering in this way the energy barrier. Overall, the new mechanistic details obtained with the URVA and LMA analysis can serve as a roadmap for the optimization of current and the future design of the next generations of Grubbs catalysts and beyond.

Brotons-Rufes A., Bahri-Laleh N., Poater A.

Ruthenium–NHC based catalysts, with a chelated iminium ligand trans to the NHC ligand that polymerize DCPD at different temperatures are studied using DFT calculations to unveil the reaction mechanism.

Martínez J.P., Trzaskowski B.

Stereoselective control of the cross metathesis of olefins is highly desired in synthetic procedures. In this work, guided by thermodynamic and kinetic descriptors calculated using density functional theory methods, we performed an exhaustive exploration of the stereoselectivity of the cross metathesis of allylbenzene and 2-butene-1,4-diyl diacetate, catalyzed by a second-generation Grubbs catalyst. The kinetics of all possible propagation routes resulted in an E/Z 5:1 distribution of the metathesis product, in agreement with the experimental value of 7:1. Structural strain of olefins and activated catalyst was evaluated to reveal insights into the design of stereoselective catalysts within a strain-driven approach. Structural strain of olefins and active catalyst is a key factor that determines the stereoselectivity of the cross-metathesis reaction. • Stereoselectivity of the cross metathesis of olefins is studied using DFT. • The estimated E/Z ratio of 5:1 agrees with the experimental value of 7:1. • Structural strain of olefins and catalyst is crucial to predict stereoselectivity. • A strain-driven approach is a key in the design of stereoselective catalyst.

Rohde L.N., Diver S.T.

Cross metathesis between styrenes and unbranched terminal alkynes is reported. Styrene cross metathesis with terminal alkynes that lack branching at the propargylic position are unknown. In general, unbranched terminal alkynes have been problematic in cross metathesis due to competing alkyne oligomerization. A catalyst screen was performed for a variety of Grubbs catalysts to optimize the yield of the desired 1,3‑diene product. This catalyst screen identified two ruthenium carbene complexes that have been rarely used for cross metathesis applications. Additionally, a side product arising from a competing alkene metathesis was minimized by choice of catalyst and through reaction optimization.

Paredes-Gil K., Sivasamy R., Mendizábal F.

• Reaction mechanism of Z -selective ROMP of norbornadiene were explored using DFT. • Initiation steps revealed that 3a-b+anti was more favored than 3a-b+syn. • The [2+2] cycloaddition control the reaction rate. • The studied MAP-catalyst efficiently yields Z -selective compounds. Herein we present a detailed mechanistic investigation of Z- selective ROMP of norbornadiene (NBD) with MAP-based catalyst 3a-b by DFT (M06) calculations. The studied catalytic reaction mechanism include the initiation, interconversion, and propagation stages with anti and syn NBD olefin attacks. In the initiation, we found the inversion of the metal center from the ( R ) configuration in the initial reactant to the ( S ). Further, we found that the olefin attack anti is kinetically more favorable than syn . Following this, we found that the stabilization order of the ring-opening species (ROS) is vital for the Z -selectivity due to the 3a-b- anti- 1/2 -cis/trans producing cis- polymers, and 3a-b- syn- 1/2 -trans produces trans- polymers. Furthermore, the 3a- anti -2-cis+syn pathways form cis -syndiotactic polymers that must overcome 30.8 kcal mol −1 , respectively, during the propagation stage. Finally, the [2+2] cycloaddition of the propagation stages determines the reaction rate, which is essential for producing stereoregular polymers.

Pump E., Poater A., Bahri-Laleh N., Credendino R., Serra L., Scarano V., Cavallo L.

The examination of cross metathesis reactions leading to the desired product has been conducted to uncover computationally the origin of the chemo-, regio- and stereoselectivity. The comparison between the relative stabilities of all involved intermediates and products, together with the transition states, links to the probability for the respective pathway. Particularly, the respective transition states for each reaction tune the regio- and stereoselectivity because they define the energy barriers needed to be overcome to form the new olefin as final product. The broad range of studied reactions with the 2nd generation Grubbs catalysts allows concluding in detail the points to pay attention and thus helps to understand the chemo-, regio- and stereoselectivity in new olefin metathesis reactions. Here, a web-server joins all these mechanistic insights which is intended to support future predictive olefin metathesis catalysis.

Martínez J.P., Trzaskowski B.

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.

Pablo Martínez J., Solà M., Poater A.

Predictive catalysis must be the tool that does not replace experiments, but acts as a selective agent, so that synthetic strategies of maximum profitability are used in the laboratory in a surgical way. Here, nanotechnology has been used in olefin metathesis from homogeneous Ru-NHC catalysts, specifically annulating a C60 fullerene to the NHC ligand. Based on results with the C60 in the backbone, a sterile change with respect to the catalysis of the metal center, an attempt has been made to bring C60 closer to the metal, by attaching it to one of the two C-N bonds of the imidazole group of the SIMes (1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) ligand (reference NHC ligand of the 2nd generation Grubbs catalysts) to increase the steric pressure of C60 in the first sphere of reactivity of the metal. The DFT calculated thermodynamics and the kinetics of SIMes-derived systems show that they are efficient catalysts for olefin metathesis.

Grela K., Kajetanowicz A., Szadkowska A., Czaban‐Jóźwiak J.

This chapter describes olefin cross‐metathesis (CM) reactions catalyzed by ruthenium, molybdenum, and tungsten complexes and surveys the literature from 1993 through 2020. Cross‐metathesis has been applied to the synthesis of a wide range of functionalized alkenes, including natural and biologically active products. Due to its simplicity and inherit atom economy, CM has found applications in the preparation of fine chemicals and various building blocks, and in transformations of biomass. Tandem reactions involving CM as the key step are also reviewed. Examples discussed in this chapter serve to illustrate how the reactivity of alkene substrates in CM depends on the presence of functional groups and steric congestion in the proximity of the reacting double bonds. A discussion of the mechanism of olefin metathesis is also included.

Patra S.G., Das N.K.

N -heterocyclic carbene (NHC) containing Ru catalysts are highly efficient in olefin metathesis. The computational investigation has been employed to understand the mechanism. NHC increases the catalyst efficiency through easily accessible rotameric conformation. The reaction proceeds through metallacyclobutane intermediate. Using bulky substituents over NHC and other anionic ligands leads to the chemo-, stereo-, and regio-selective product formation. The presence of alcohol, amines leads to catalyst decomposition through β hydride elimination. • Mechanistic study of the olefin metathesis using N -heterocyclic carbene (NHC) containing Ru catalyst. • NHC increases efficiency and stability through trans effect and active conformation. • Associative, dissociative, and interchange pathways of initiation. • A stable metallacyclobutane intermediate is formed, which governs product formation. • NHC types govern the chemo-, stereo, regio-selectivity of the product olefine. The second-generation Grubbs catalyst contains N -heterocyclic carbene (NHC) coordinated to Ru center. Various such complexes have been employed as catalysts for the olefin metathesis reaction. Based on experimental results and theoretical analysis, it has been established that a cyclic four-membered metallacyclobutane is formed as an active intermediate. The use of the right computational recipe is prescribed in the context of the Grubbs catalyst. The carbene electronic structure in the catalyst and the course of the reaction are discussed. A recent computational study examined the possibility of an oxidation state as +2 or +4 in a comparative manner. Further, the role of catalysts in the chemo, regio, and stereoselectivity are also discussed. In particular, in recent years, Z selective catalysts (cyclometalated and dithiolate-containing) were developed through an in-depth understanding of the electronic and steric principles. Besides, the substituents present on the NHC carbene also plays an important role. Furthermore, the presence of a chiral backbone leads to stereoselectivity in the product. Studies also predict a more suitable catalyst for certain types of substrates. In a recent study, the effect of aromaticity while discussing alkene vs. arene substrate has been considered. Catalyst undergoes decomposition in the presence of primary alcohol and Brønsted/Lewis base. Mechanism of such type of deterioration via β hydride elimination of the metallacyclobutane intermediate are discussed. Finally, catalyst decomposition involving chelated complexes are also emphasized. Thus, such theoretical studies may help the experimental chemist reduce their effort in choosing a suitable catalyst. In this review article, recent advancements in the olefin metathesis mechanistic studies by Grubbs catalyst are comprehensively summarized.

He Z., Wang G., Wang C., Guo L., Wei R., Song G., Pan D., Das R., Naik N., Hu Z., Guo Z.

Planer S., Małecki P., Trzaskowski B., Kajetanowicz A., Grela K.

Formation of tetrasubstituted C-C double bonds via olefin metathesis is considered very challenging for classical Ru-based complexes. In the hope to improve this condition, three ruthenium olefin metathesis catalysts bearing sterically reduced N-heterocyclic carbene (NHC) ligands with xylyl "arms" were synthesized, characterized using both computational and experimental techniques, and tested in a number of challenging reactions. The catalysts are predicted to initiate much faster than the analogue with mesityl N-substituents. We also foreboded the rotation of xylyl side groups at ambient temperature and the existence of all four atropoisomers in the solution, which was in agreement with experimental data. These catalysts exhibited high activity at relatively low temperatures (45-60 °C) and at reduced catalyst loadings in various reactions of sterically hindered alkenes, including complex polyfunctional substrates of pharmaceutical interest, such as yangonin precursors, chrysantemic acid derivatives, analogues of cannabinoid agonists, α-terpineol, and finally a thermally unstable peroxide.

Poater A., D'Alterio M.C., Talarico G., Chauvin R.