Exploring the Reactivity of Ru-Based Metathesis Catalysts with a π-Acid Ligand Trans to the Ru−Ylidene Bond

Rhenium Alkyne Catalysis: Sterics Control the Reactivity

Metathesis reactions, including alkane, alkene, and alkyne metatheses, have their origins in the fundamental understanding of chemical reactions and the development of specialized catalysts. These reactions stand as transformative pillars in organic chemistry, providing efficient rearrangement of carbon-carbon bonds and enabling synthetic access to diverse and complex compounds. Their impact spans industries such as petrochemicals, pharmaceuticals, and materials science. In this work, we present a detailed mechanistic study of the Re(V) catalyzed alkyne metathesis through density functional theory calculations. Our findings are in agreement with the experimental evidence from Jia and co-workers and unveil critical factors governing catalyst performance. Our work not only enhances our understanding of alkyne metathesis but also contributes to the broader landscape of catalytic processes, facilitating the design of more efficient and selective transformations in organic synthesis.

Decomposition of Ruthenium Olefin Metathesis Catalyst

Ruthenium olefin metathesis catalysts are one of the most commonly used class of catalysts. There are multiple reviews on their uses in various branches of chemistry and other sciences but a detailed review of their decomposition is missing, despite a large number of recent and important advances in this field. In particular, in the last five years several new mechanism of decomposition, both olefin-driven as well as induced by external agents, have been suggested and used to explain differences in the decomposition rates and the metathesis activities of both standard, N-heterocyclic carbene-based systems and the recently developed cyclic alkyl amino carbene-containing complexes. Here we present a review which explores the last 30 years of the decomposition studied on ruthenium olefin metathesis catalyst driven by both intrinsic features of such catalysts as well as external chemicals.

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.

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.

Synthesis and Reactivity of Metallocarbene-Containing Polymers

Metallopolymers are an emerging class of materials with potential utility as semiconductors, catalysts, optical device components, and stimuli responsive networks. While polymer frameworks have been decorated with an array of organometallic moieties, the incorporation of metallocarbenes has been largely overlooked. Here, we report ring-opening metathesis polymerization as a strategy for the synthesis of Fischer carbene-containing polymers. High degrees of polymerization were observed (>800 repeats), and the isolated materials exhibited exceptional solubility and thermal stability. The tungsten carbene subunits were readily incorporated into block copolymers and could be modified through subsequent transformations. Moreover, the metallocarbene polymers were found to release carbon monoxide upon exposure to light or oxygen, which is unusual for tungsten carbene complexes. These metallocarbene-containing polymers could represent new platforms for the development of functional materials.

Electronic effects in mixed N-heterocyclic carbene/phosphite indenylidene ruthenium metathesis catalysts

Five new complexes [RuCl₂(SIMes)(Ind)(O-pXC₅H₄)] bearing different para-substituted triphenylphosphites (X = H, OCH₃, CF₃, Cl, SF₅ and CN) were synthesised and used to study the effect of the electronic properties of the phosphite on olefin metathesis activity. Investigations of the physical properties of the new ligands and complexes were performed using physicochemical and DFT calculations. The catalytic activity of the complexes was benchmarked in challenging ring closing metathesis transformations featuring the formation of tetra-substituted double bonds. Complex [RuCl₂(SIMes)(Ind)P(O-pCF₃C₅H₄)₃] (3c) exhibited a particularly high catalytic activity, superior to state-of-the-art catalysts, and was further tested on a wide range of substrates.

Bimolecular Coupling as a Vector for Decomposition of Fast-Initiating Olefin Metathesis Catalysts

The correlation between rapid initiation and rapid decomposition in olefin metathesis is probed for a series of fast-initiating, phosphine-free Ru catalysts: the Hoveyda catalyst HII, RuCl₂(L)(═CHC₆H₄- o-O ⁱPr); the Grela catalyst nG (a derivative of HII with a nitro group para to O ⁱPr); the Piers catalyst PII, [RuCl₂(L)(═CHPCy₃)]OTf; the third-generation Grubbs catalyst GIII, RuCl₂(L)(py)₂(═CHPh); and dianiline catalyst DA, RuCl₂(L)( o-dianiline)(═CHPh), in all of which L = H₂IMes = N,N'-bis(mesityl)imidazolin-2-ylidene. Prior studies of ethylene metathesis have established that various Ru metathesis catalysts can decompose by β-elimination of propene from the metallacyclobutane intermediate RuCl₂(H₂IMes)(κ²-C₃H₆), Ru-2. The present work demonstrates that in metathesis of terminal olefins, β-elimination yields only ca. 25-40% propenes for HII, nG, PII, or DA, and none for GIII. The discrepancy is attributed to competing decomposition via bimolecular coupling of methylidene intermediate RuCl₂(H₂IMes)(═CH₂), Ru-1. Direct evidence for methylidene coupling is presented, via the controlled decomposition of transiently stabilized adducts of Ru-1, RuCl₂(H₂IMes)L_n(═CH₂) (L_n = py _n'; n' = 1, 2, or o-dianiline). These adducts were synthesized by treating in situ-generated metallacyclobutane Ru-2 with pyridine or o-dianiline, and were isolated by precipitating at low temperature (-116 or -78 °C, respectively). On warming, both undergo methylidene coupling, liberating ethylene and forming RuCl₂(H₂IMes)L_n. A mechanism is proposed based on kinetic studies and molecular-level computational analysis. Bimolecular coupling emerges as an important contributor to the instability of Ru-1, and a potentially major pathway for decomposition of fast-initiating, phosphine-free metathesis catalysts.

Versatile deprotonated NHC: C,N-bridged dinuclear iridium and rhodium complexes

Bearing the versatility of N-heterocyclic carbene (NHC) ligands, here density functional theory (DFT) calculations unravel the capacity of coordination of a deprotonated NHC ligand (pNHC) to generate a doubly C2,N3-bridged dinuclear complex. Here, in particular the discussion is based on the combination of the deprotonated 1-arylimidazol (aryl = mesityl (Mes)) with [M(cod)(μ-Cl)] (M = Ir, Rh) generated two geometrical isomers of complex [M(cod){µ-C₃H₂N₂(Mes)-κC2,κN3}]₂). The latter two isomers display conformations head-to-head (H-H) and head-to-tail (H-T) of C_S and C₂ symmetry, respectively. The isomerization from the H-H to the H-T conformation is feasible, whereas next substitutions of the cod ligand by CO first, and PMe₃ later confirm the H-T coordination as the thermodynamically preferred. It is envisaged the exchange of the metal, from iridium to rhodium, confirming here the innocence of the nature of the metal for such arrangements of the bridging ligands.

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.

A comparison of low and high activity precatalysts: do the calculated energy barriers during the self-metathesis reaction of 1-octene correlate with the precatalyst metathesis activity?

The self-metathesis reaction of 1-octene with several well-known Grubbs-type precatalysts and the new Z-selective Grubbs precatalyst were studied with molecular modeling. The obtained Gibbs-free energy values for all the steps during the productive metathesis of 1-octene were compared to the values obtained for some low catalytic activity precatalysts. Determining how the Gibbs-free energy values of highly active precatalysts compare to that of low catalytic activity precatalysts gave a deeper insight into the mechanism. The questionable correlation of the theoretically observed trends with those obtained experimentally does point to the need to be very cautious when making assumptions from theoretical results without a sufficiently large dataset.

A molecular modeling study of the changes of some steric properties of the precatalysts during the olefin metathesis reaction

The productive self-metathesis of 1-octene with a series of new phosphine ligated Grubbs-type precatalysts was studied. The resulting structures were used to compare some steric properties of the new precatalysts with those of well-known precatalysts. The possibility of α-CC agnostic stabilization as well as the ability of the ligands to shield the metal was studied. A comparison of the obtained data, pointed to the unlikelihood that α-CC agostic stabilization is a major contribution to the stabilization of the various metallacyclobutane rings. The similarity in the ability of the ligands to shield the metal also raised questions about the comparison of experimentally observed trends with those obtained theoretically.

How does the addition of steric hindrance to a typical N-heterocyclic carbene ligand affect catalytic activity in olefin metathesis?

Density functional theory (DFT) calculations were used to predict and rationalize the effect of the modification of the structure of the prototype 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) N-heterocyclic carbene (NHC) ligand. The modification consists in the substitution of the methyl groups of ortho isopropyl substituent with phenyl groups, and here we plan to describe how such significant changes affect the metal environment and therefore the related catalytic behaviour. Bearing in mind that there is a significant structural difference between both ligands in different olefin metathesis reactions, here by means of DFT we characterize where the NHC ligand plays a more active role and where it is a simple spectator, or at least its modification does not significantly change its catalytic role/performance.

Gold(I) carbenes by retro-Buchner reaction: generation and fate

The fate of the aryl gold(I) carbenes generated by retro-Buchner reaction of ortho-substituted 7-aryl-1,3,5-cycloheptatrienes is dependent on the constitution of the ortho substituent. Indenes and fluorenes are obtained by intramolecular reaction of highly electrophilic gold(I) carbenes with alkenes and arenes. According to density functional theory calculations, the gold-catalyzed retro-Buchner process occurs stepwise, although the two carbon-carbon cleavages occur on a rather flat potential energy surface.

Comparing Ru and Fe-catalyzed olefin metathesis

To replace Ru by Fe in olefin metathesis, a dream?

Complete σ* intramolecular aromatic hydroxylation mechanism through O2 activation by a Schiff base macrocyclic dicopper(I) complex

In this work we analyze the whole molecular mechanism for intramolecular aromatic hydroxylation through O2 activation by a Schiff hexaazamacrocyclic dicopper(I) complex, [Cu(I) 2(bsH2m)](2+). Assisted by DFT calculations, we unravel the reaction pathway for the overall intramolecular aromatic hydroxylation, i.e., from the initial O2 reaction with the dicopper(I) species to first form a Cu(I)Cu(II)-superoxo species, the subsequent reaction with the second Cu(I) center to form a μ-η(2):η(2)-peroxo-Cu(II) 2 intermediate, the concerted peroxide O-O bond cleavage and C-O bond formation, followed finally by a proton transfer to an alpha aromatic carbon that immediately yields the product [Cu(II) 2(bsH2m-O)(μ-OH)](2+).

Decomposition pathways of Z-selective ruthenium metathesis catalysts

The decomposition of a Z-selective ruthenium metathesis catalyst and structurally similar analogues has been investigated utilizing X-ray crystallography and density functional theory. Isolated X-ray crystal structures suggest that recently reported C-H activated catalysts undergo decomposition via insertion of the alkylidene moiety into the chelating ruthenium-carbon bond followed by hydride elimination, which is supported by theoretical calculations. The resulting ruthenium hydride intermediates have been implicated in previously observed olefin migration, and thus lead to unwanted byproducts in cross metathesis reactions. Preventing these decomposition modes will be essential in the design of more active and selective Z-selective catalysts.

Studies on CH3CN-assisted decomposition of 1st Grubbs catalyst by electrospray ionization tandem mass spectrometry

The CH(3)CN-assisted decomposition reaction of the 1(st) Grubbs catalyst (1) was studied using electrospray ionization tandem mass spectrometry (ESI-MS/MS). We detected a series of Ru-intermediates and decomposition products by off-line and on-line ESI-MS(/MS) monitoring of the decomposition process. In particular, an on-line microreactor method was applied with ESI-MS/MS to profile the change and relationship of various Ru-intermediates by controlling the reaction within the first 30 s of its time scale. The main fast decomposition mechanism of Ru-catalyst 1 in the presence of CH(3)CN was similar to that proposed by Grubbs, and this was confirmed by detecting the (PhCH(2)PCy(3))(+) ion at m/z 371 as the major decomposition products with ESI-MS. We also studied the time evolution of the transient reactive Ru-intermediate ions step by step with ESI-MS/MS and detected the C-H bond activation products of toluene--dehydrogenated PCy(3), such as P(Cy)(2)(C(6)H(9)), P(Cy)(2)Ph--by analyzing the decomposition reaction solution by gas chromatography (GC)/MS. The mechanism of another minor decomposition pathway involving the phosphine activation of catalyst 1 was proposed on the basis of the ESI-MS(/MS) interception and characterization of the transient reactive Ru-species in the decomposition reaction solution. Finally the coordination effect of the CH(3)CN in assisting the decomposition and stabilizing the transient Ru-complexes is discussed.

Computational methods to predict the reactivity of nanoparticles through structure-property relationships

IMPORTANCE OF THE FIELD

Innovative biomedical techniques operational at the nanoscale level are being developed in therapeutics, including advanced drug delivery systems and targeted nanotherapy. Given the large number of nanoparticles that are being developed for possible biomedical use, the use of computational methods in the assessment of their properties is of key importance.

AREAS COVERED IN THIS REVIEW

Among the in silico methods, quantum mechanics is still used rarely in the study of nanostructured particles. This review provides an overview of some of the main quantum mechanics methods that are already used in the assessment of chemicals. Furthermore, classical tools used in the chemistry field are described, to show their potential also in the pharmacological field.

WHAT THE READER WILL GAIN

The current status of computational methods in terms of availability and applicability to nanoparticles, and recommendations for further research are highlighted.

TAKE HOME MESSAGE

The in silico modelling of nanoparticles can assist in targeting and filling gaps in knowledge on the effects of these particular particles. Computational models of the behaviour of nanoparticles in biological systems, including simulation models for predicting intermolecular interactions and harmful side effects, can be highly valuable in screening candidate particles for potential biomedical use in diagnostics, imaging and drug delivery.