Ligand Isomerization in Sulfur-Chelated Ruthenium Benzylidenes

Zirngast M., Pump E., Leitgeb A., Albering J.H., Slugovc C.

The cationic pyridine adduct of a ruthenium complex bearing a chelating benzylidene and an N-heterocyclic carbene was identified as an intermediate during the activation of cis dichloro species and a novel triggering concept for olefin metathesis catalysts based on cationic species was disclosed.

Poater A., Ragone F., Correa A., Szadkowska A., Barbasiewicz M., Grela K., Cavallo L.

Kanai Y., Srinivasan V., Meier S.K., Vollhardt K.P., Grossman J.C.

Bantreil X., Schmid T.E., Randall R.A., Slawin A.M., Cazin C.S.

The synthesis, characterisation and catalytic behaviour of ruthenium indenylidene complexes bearing an N-heterocyclic carbene and triisopropylphosphite are described.

Peeck L.H., Leuthäusser S., Plenio H.

Grubbs−Hoveyda and Grubbs III type complexes with ferrocenyl- or −NEt2-substituted NHC ligands were synthesized according to standard procedures. The electron donation of the NHC ligands in the respective ruthenium complexes can be modulated by oxidation of the ferrocenyl moiety or by protonation of the amino group. The neutral and the respective cationic (oxidized or protonated) ruthenium complexes were tested in the ROMP of norbornene. The change in the electron donation of the NHC ligands upon protonation leads to a significant change in the double-bond geometry (from E/Z ratio = 0.78 to E/Z = 1.04) and in the microstructure of the resulting polynorbornene. Consequently, addition of acid and protonation of the living catalyst attached to the polymer chain during the polymerization reaction allows fine-tuning the E/Z ratio of the resulting polynorbornene.

Song A., Parker K.A., Sampson N.S.

Catalysis of alternating ROMP with (H(2)IMes)Cl(2)Ru=CHPh(OiPr), the second generation Hoveyda-Grubbs catalyst, provided an entirely cyclic alternating polymer. Conditions for the cyclic AROMP were used to prepare a polymer in which one of the repeat units bore a primary alkyl chloride that was used for further elaboration.

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

Vorfalt T., Wannowius K., Plenio H.

Tzur E., Szadkowska A., Ben-Asuly A., Makal A., Goldberg I., Woźniak K., Grela K., Lemcoff N. .

A short overview on the structural design of the Hoveyda-Grubbs-type ruthenium initiators chelated through oxygen, nitrogen or sulfur atoms is presented. Our aim was to compare and contrast O-, N- and S-chelated ruthenium complexes to better understand the impact of electron-withdrawing and -donating substituents on the geometry and activity of the ruthenium complexes and to gain further insight into the trans-cis isomerisation process of the S-chelated complexes. To evaluate the different effects of chelating heteroatoms and to probe electronic effects on sulfur- and nitrogen-chelated latent catalysts, we synthesised a series of novel complexes. These catalysts were compared against two well-known oxygen-chelated initiators and a sulfoxide-chelated complex. The structures of the new complexes have been determined by single-crystal X-ray diffraction and analysed to search for correlations between the structural features and activity. The replacement of the oxygen-chelating atom by a sulfur or nitrogen atom resulted in catalysts that were inert at room temperature for typical ring-closing metathesis (RCM) and cross-metathesis reactions and showed catalytic activity only at higher temperatures. Furthermore, one nitrogen-chelated initiator demonstrated thermo-switchable behaviour in RCM reactions, similar to its sulfur-chelated counterparts.

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

Leitao E.M., van der Eide E.F., Romero P.E., Piers W.E., McDonald R.

Initiation processes in a family of ruthenium phosphonium alkylidene catalysts, some of which are commercially available, are presented. Seven 16-electron zwitterionic catalyst precursors of general formula (H(2)IMes)(Cl)(3)Ru=C(H)P(R(1))(2)R(2) (R(1) = R(2) = C(6)H(11), C(5)H(9), i-C(3)H(7), 1-Cy(3)-Cl, 1-Cyp(3)-Cl, 1-(i)Pr(3)-Cl; R(1) = C(6)H(11), R(2) = CH(2)CH(3), 1-EtCy(2)-Cl; R(1) = C(6)H(11), R(2) = CH(3), 1-MeCy(2)-Cl; R(1) = i-C(3)H(7), R(2) = CH(2)CH(3), 1-Et(i)Pr(2)-Cl; R(1) = i-C(3)H(7), R(2) = CH(3), 1-Me(i)Pr(2)-Cl) were prepared. These compounds can be converted to the metathesis active 14-electron phosphonium alkylidenes by chloride abstraction with B(C(6)F(5))(3). The examples with symmetrically substituted phosphonium groups exist as monomers in solution and are rapid initiators of olefin metathesis reactions. The unsymmetrically substituted phosphonium alkylidenes are observed to undergo reversible dimerization, the extent of which is dependent on the steric bulk of the phosphonium group. Kinetic and thermodynamic parameters of these equilibria are presented, as well as experiments that show that metathesis is only initiated through the monomers; thus dedimerization is required for initiation. In another detailed study, the series of catalysts 1-R(3) were reacted with o-isopropoxystyrene under pseudo-first-order conditions to quantify second-order olefin binding rates. A more complex initiation process was observed in that the rates were accelerated by catalytic amounts of ethylene produced in the reaction with o-isopropoxystyrene. The ability of the catalyst to generate ethylene is related to the nature of the phosphonium group, and initiation rates can be dramatically increased by the intentional addition of a catalytic amount of ethylene.

Albright T., Dosa P., Grossmann T., Khrustalev V., Oloba O., Padilla R., Paubelle R., Stanger A., Timofeeva T., Vollhardt K.â.

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.

Dunbar M., Balof S., LaBeaud L., Yu B., Lowe A., Valente E., Schanz H.

The effect of the addition of H(3)PO(4) on the ROMP activity of cyclooctene (COE) with first- [Cl(2)(PCy(3))(2)Ru=CHPh] and second-generation [(H(2)IMes)Cl(2)(PCy(3))Ru=CHPh] Grubbs' catalysts 1 and 4 (Cy=cyclohexyl, Ph=phenyl, Mes=2,4,6-trimethylphenyl (mesityl)), their inhibited mixtures with 1-methylimidazole (MIM), as well as their isolated bis-N,N'-dimethylaminopyridine (DMAP) derivatives [Cl(2)(PCy(3))(DMAP)(2)Ru=CHPh)] (5 b) and [Cl(2)(H(2)IMes)(DMAP)(2)Ru=CHPh] (7 b) (DMAP=dimethylaminopyridine), a novel catalyst, has been investigated. The studies include the determination of their initiation rates, as well as a determination of the molecular weights and molecular weight distributions of the polymers obtained with these catalysts and catalyst mixtures from the exo-7-oxanorbornene derivative 11. The structure of catalyst 7 b was confirmed by means of X-ray diffraction. All N-donor-bearing catalysts or N-donor-containing catalyst mixtures not only exhibited elevated activity in the presence of acid, but also increased initiation rates. Using the reversible inhibition/activation protocol with MIM and H(3)PO(4) enabled us to conduct controlled ROMP with catalyst 4 producing the isolated exo-7-oxanorbornene-based polymer 12 with predetermined molecular weights and narrow molecular weight distributions. This effect was based on fast and efficient catalyst initiation in contrast to the parent catalyst 4. Hexacoordinate complex 5 b also experienced a dramatic increase in initiation rates upon acid-addition and the ROMP reactions became well-controlled in contrast to the acid-free reaction. In contrast, complex 7 b performs well-controlled ROMP in the absence of acid, whereas the polymerization of the same monomer becomes less controlled in the presence of H(3)PO(4). The closer evaluation of catalysts 5 b and 7 b demonstrated that their initiation rates exhibit a linear dependency on the substrate concentration in contrast to catalysts 1 and 4. As a consequence, their initiation rates are determined by an associative step, not a dissociative step as seen for catalysts 1 and 4. A feasible associative metathesis initiation mechanism is proposed.

Diesendruck C.E., Tzur E., Ben-Asuly A., Goldberg I., Straub B.F., Lemcoff N.G.

Gradient-corrected (BP86) and hybrid (M06-L) density functional calculations were used to study the relative stability of cis and trans-dichloro X-chelated benzylidene ruthenium complexes (X = O, S, Se, N, P). Calculations in the gas phase differed from experimental results, predicting the trans-dichloro configuration as being more stable in every case. The addition of Poisson-Boltzmann (PBF) continuum approximation (dichloromethane) corrected the disagreement and afforded energies consistent with experimental results. Novel N, Se, and P chelated ruthenium olefin metathesis complexes were synthesized to evaluate calculation predictions. These findings reinforce the importance of including solvent corrections in DFT calculations of ruthenium metathesis catalysts and predict that stronger sigma donors as chelating atoms tend to electronically promote the unusual and less active cis-dichloro configuration.

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.

Kinugawa T., Matsuo T.

For regulating olefin metathesis (OM) activities of Hoveyda–Grubbs second-generation complex (HG-II), the structural modification of the benzylidene ligand is a useful strategy. This paper reports the effect of a chalcogen...

Nechmad N.B., Iudanov K., Tarannam N., Kobernik V., Kozuch S., Lemcoff N.G.

Alassad N., Nechmad N.B., Phatake R.S., Reany O., Lemcoff N.G.

In this work, the structure, latency, and activity of twelve sulfur-chelated ruthenium precatalysts were studied by systematically varying their ligand shell.

Phatake R.S., Nechmad N.B., Reany O., Lemcoff N.G.

A selective ring-closing metathesis (RCM) reaction for the formation of large macrocycles by using latent sulfur chelated ruthenium iodide benzylidenes, readily activated by thermal and photochemical (UV-A and visible light) stimuli, is reported. For dienes having one terminal alkene and one internal double bond, the specific affinity of diiodo ruthenium alkylidenes for the unhindered terminus, combined with their reluctance to react with internal olefins, favors RCM over oligomerization, providing high macrocyclic yields even at relatively high concentrations. Alternatively, for substrates containing two internal double bonds, a sacrificial methylene donor can be used to obtain the desired products. With this methodology, lactones, lactams, and macrocyclic ketones ranging from 13- to 22-membered rings could be synthesized in moderate to high yields. In addition, synthetic applications for a one-pot cyclization/reduction sequence to produce Exaltolide, a natural macrolide (commercial musk), Dihydrocivetone, and other saturated macrocycles have been explored. Thus, we disclose herein an important advantage for diiodo ruthenium benzylidene catalysts over their less selective dichloro counterparts and provide a more profound understanding of the mechanisms that provide the enhanced cyclization outcome.

Singh R.K., Khan T.K., Misra S., Singh A.K.

• CAACs form strong bonds with transition metals as well as main group elements. • CAAC ligand families (CAAC-5, CAAC-6, BiCAACs, FunCAACs, CAArCs, CAAmCs, ChiCAACs) are described. • CAAC-based transition metal complexes are shown to be robust catalysts. • Complexes with CAAC ligands find applications in coordination chemistry, catalysis, bio-inorganic chemistry, medicinal, and material science. CAACs have emerged as an attractive class of ancillary ligands, forming strong bonds with transition metals as well as main-group elements and stabilise both low and high oxidation states. These strongly nucleophilic and electrophilic carbenes have a small HOMO−LUMO energy gap compared to N -heterocyclic carbenes (NHCs). The robust metal complexes formed with CAAC ligands have been utilised in different areas such as catalysis, small-molecule activation, medicinal chemistry, and bioinorganic chemistry. This review describes recent developments in the synthesis of various types of CAAC ligands, their binding ability to transition metals and applications of CAAC−based metal complexes in homogeneous catalysis via hydrogenation, hydroboration, coupling reactions, olefin metathesis and asymmetric transformations.

Wei W., Jia G.

The coordination chemistry of ruthenium, osmium, technetium and rhenium with carbon-based ligands covering the literature from 2003 to 2018 is discussed. These metals form a vast number of coordination compounds with carbon-based ligands ranging from a single carbon atom (a carbido ligand) to polydentate hydrocarbon chains, and from monodentate organometallic ligands such as carbenes to multidentate ligands such as pincers and macrocycles. Particular focus of this review is given to syntheses, properties and applications of complexes with popular carbon-based ligands such as monodentate N-heterocyclic carbenes (NHCs), cyclometallated bidentate ligands, tridentate pincers and porphyrin-like macrocycles.

Nechmad N.B., Kobernik V., Tarannam N., Phatake R., Eivgi O., Kozuch S., Lemcoff N.G.

A diiodo trifluoromethyl sulfur-chelated ruthenium benzylidene complex was synthesized as precatalyst for olefin metathesis, allowing ring-opening metathesis of dicyclopentadiene in the presence of a methylene donor to produce 1,3-divinyl-hexahydropentalenes and depolymerization of polybutadiene upon green light irradiation. The selectivity displayed by the catalyst is attained through a fine balance between its rate of pre-activation and deactivation.

Nechmad N.B., Kobernik V., Tarannam N., Phatake R., Eivgi O., Kozuch S., Lemcoff N.G.

Stawiasz K.J., Paul J.E., Schwarz K.J., Sottos N.R., Moore J.S.

In this work, a simple method is reported for control over initiation in frontal ring-opening metathesis polymerization (FROMP). This noncontact approach uses 375 nm light to excite Grubbs' second-generation catalyst in the presence of a phosphite inhibitor. Photoinitiated FROMP of dicylcopentadiene (DCPD) displays a similar cure profile to that of its thermally initiated counterpart, yielding a robust polymer with high glass transition temperature. Furthermore, this system is applied to enhance reaction rates in conventional ring-closing metathesis reactions.

Lemcoff N.G., Nechmad N.B.

This Account summarizes the historical development of latent sulfur-chelated ruthenium precatalysts from the Lemcoff group’s perspective. The most unique feature of this family of complexes is that they appear in the more stable cis-dichloro configuration, which is latent towards olefin metathesis reactions. Activation of the precatalyst, brought about by isomerization from the cis-dihalo to the trans-dihalo forms, can be achieved either by thermal or light stimuli. Modifications of the ligand sphere bestows unique properties upon the catalysts, which have been used in diverse applications, from 3D printing of metathesis polymers to orthogonally divergent synthetic pathways.1 Introduction2 Effect of Sulfur Substituents3 Effect of Benzylidene Ligands4 Effect of the NHC Ligands5 Effect of the Anionic Ligands6 Conclusions

Segalovich-Gerendash G., Rozenberg I., Alassad N., Nechmad N.B., Goldberg I., Kozuch S., Lemcoff N.G.

Herein we show the design and synthesis of an electron rich, sulfoxide-chelated, ruthenium benzylidene. In contrast to previously reported sulfoxide-chelated ruthenium benzylidenes, this complex is...

Phillips J.H.

Over the past few decades, alkene metathesis has shown great potential in the chemical industry. It has seen an increased adoption across various sectors from pharmaceutical to materials, primarily driven by advances in the development of well-defined metathesis catalysts. This chapter will highlight a few of the commercial uses of well-defined metathesis catalysts, with an extra emphasis on the challenges encountered at the industrial scale. Furthermore, recommendations for reaction design and strategies, for troubleshooting problems that are encountered in metathesis transformations, will also be discussed.

Nechmad N.B., Phatake R., Ivry E., Poater A., Lemcoff N.G.

The development of selective olefin metathesis catalysts is crucial to achieving new synthetic pathways. Herein, we show that cis-diiodo/sulfur-chelated ruthenium benzylidenes do not react with strained cycloalkenes and internal olefins, but can effectively catalyze metathesis reactions of terminal dienes. Surprisingly, internal olefins may partake in olefin metathesis reactions once the ruthenium methylidene intermediate has been generated. This unexpected behavior allows the facile formation of strained cis-cyclooctene by the RCM reaction of 1,9-undecadiene. Moreover, cis-1,4-polybutadiene may be transformed into small cyclic molecules, including its smallest precursor, 1,5-cyclooctadiene, by the use of this novel sequence. Norbornenes, including the reactive dicyclopentadiene (DCPD), remain unscathed even in the presence of terminal olefin substrates as they are too bulky to approach the diiodo ruthenium methylidene. The experimental results are accompanied by thorough DFT calculations.

Nechmad N.B., Phatake R., Ivry E., Poater A., Lemcoff N.G.