Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

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.

Al-Enezi M.Y., John E., Ibrahim Y.A., Al-Awadi N.A.

Three novel phosphine-free Ru-alkylidenes (7a-7c) have been synthesized and utilized as efficient catalysts for ring closing metathesis (RCM) reaction. Spectroscopic data, i.e. NMR and HRMS, along with single crystal X-ray diffraction analysis, were used to confirm their chemical structures. The tosylated carbenoid 7b showed the highest efficiency in cyclizing different acyclic diene substrates. RCM of various (un)substituted N,N-diallylaniline derivatives and stereoselective RCM of different macromolecular dienes were well tolerated using only a catalytic amount (0.5-2.0 mol%) of the additive catalyst (7b) as compared to the well-known Grubbs (II) and Hoveyda-Grubbs (II) catalysts.

Sherstobitov I.A., Kiselev S.A., Lyapkov A.A., Yusubov M.S., Zhdankin V.V., Yu B., Verpoort F.

The synthesis and characterization of a novel latent ruthenium catalyst bearing a chelate ligand containing an oxadiazole fragment for the olefin metathesis are described. The main advantages of the utilization of this ligand are the straightforwardness of preparation, mild reaction conditions, and availability of the feedstock. Preliminary study of the catalytic activity of the Ru complex for metathesis reactions demonstrated its inactive nature for the ring-closing metathesis (RCM), but an excellent thermo-switchability for the ring-opening metathesis polymerization of norbornene.

Park S., Byun S., Ryu H., Hahm H., Lee J., Hong S.

Azobenzene-bearing photoswitchable ruthenium catalysts were developed for olefin metathesis reactions. These catalysts exhibited an on–off switching ability in ring-closing metathesis depending on ...

Monsigny L., Cejas Sánchez J., Piątkowski J., Kajetanowicz A., Grela K.

Herein, we describe a study of the synthesis, characterization, and catalytic properties of a cis-dichlorido seleno-chelated Hoveyda-Grubbs type complex (Ru8). Such a complex has been obtained through a straightforward and high-yielding synthetic protocol in three steps from the commercially available 2-bromobenzaldehyde in good overall yield (54%). The catalytic profile, especially the latency of this complex, has been probed through selected olefin metathesis reactions such as ring-closing metathesis (RCM), self-cross-metathesis (self-CM) and ring-opening metathesis polymerization (ROMP). In addition to its high latency, the selenium Hoveyda-type complex Ru8 exhibits a switchable behavior upon thermal activation. Of interest, while the corresponding sulfur-chelated Hoveyda type catalyst is reported to be only activated by heat, the selenium analogue was found to be active upon both heat and light irradiation.

Kravchenko A., Timmer B.J., Inge A.K., Biedermann M., Ramström O.

Balance for dynamics: The efficiency of catalysts based on cyclic (alkyl)(amino) carbenes (CAACs) has been evaluated in benchmark olefin metathesis reactions under mild conditions. The catalysts displayed a delicate balance between activity and stability, where good performance and functional group tolerance could be identified. Well-balanced catalysts allowed for the generation of dynamic systems of olefins.

Monsigny L., Piątkowski J., Trzybiński D., Woźniak K., Nienałtowski T., Kajetanowicz A., Grela K.

Two EWG-activated Hoveyda-Grubbs-type ruthenium complexes (Sil-II and Sil-II’) were obtained, characterized, and screened in a set of olefin metathesis reactions. These catalysts were conveniently synthesized from a commercially available pharmaceutical building block – Sildenafil aldehyde – in two steps only. Stability and catalytic activity tests disclosed that the bulkier NHC-ligand bearing catalyst Sil-II’ is visibly more stable and productive than its smaller NHC-analogue Sil-II. Good application profile of catalyst Sil-II’ was confirmed in a set of diverse metathesis reactions including ring-closing metathesis (RCM) and cross-metathesis (CM) of complex polyfunctional substrates of medicinal chemistry interest, including a challenging macrocyclization of the Pacritinib precursor. Compatibility of the new catalyst with various green solvents was checked and metathesis of Sildenafil and Tadalafil-based substrates was successfully conducted in acetone. The mechanism of Sil-II’ initiation has been investigated through kinetic experiments unveiling that the decrease of the steric hindrance of the chelating alkoxy moiety (from iPrO to EtO) favors the interchange initiation pathway over the typical dissociation pathway for other popular 2nd generation Hoveyda-Grubbs catalysts.

Monsigny L., Kajetanowicz A., Grela K.

This review describes a distinct class of ruthenium olefin metathesis catalysts featuring unsymmetrical N-heterocyclic carbene (uNHC) ligands, from its historical beginning to the present state of the art. Thanks to advantageous traits, such as pronounced thermodynamic stability, chemical latency, outstanding selectivity, and compatibility with green solvents, these catalysts led to good results in a number of specialized metathesis transformations. Therefore, while being a niche, the uNHC complexes can potentially be implemented in a number of industrial processes, such as valorization of Fischer-Tropsch olefin fractions, ethenolysis of renewable products, and modern pharmaceutical production.

Blanco C.O., Nascimento D.L., Fogg D.E.

Clean, high-yielding routes are described to ruthenium–diiodide catalysts that were recently shown to enable high productivity in olefin metathesis. For the second-generation Grubbs and Hoveyda catalysts (GII: RuCl2(H2IMes)(PCy3)(═CHPh); HII: RuCl2(H2IMes)(═CHAr), Ar = C6H4-2-OiPr), slow salt metathesis is shown to arise from the low lability of the ancillary PCy3 or ether ligands, which retards access to the four-coordinate intermediate required for efficient halide exchange. To exploit the lability of the first-generation catalysts, the diiodide complex RuI2(PCy3)(═CHAr) HI-I2 was prepared by treating “Grubbs I” (RuCl2(PCy3)2(═CHPh), GI) with NaI, H2C═CHAr (1a), and a phosphine-scavenging Merrifield iodide (MF-I) resin. Subsequent installation of H2IMes or cyclic (alkyl)(amino)carbene (CAAC) ligands afforded the second-generation iodide catalysts in good to excellent yields. Given the incompatibility of the nitro group with a free carbene, the iodo-Grela catalyst RuI2(H2IMes)(═CHAr′) (nG-I2: Ar′ = C6H3-2-OiPr-4-NO2) was instead accessed by sequential salt metathesis of GI with NaI, installation of H2IMes, and finally cross-metathesis with the nitrostyrenyl ether H2C═CHAr′ (1b), with MF-I as the phosphine scavenger. The bulky iodide ligands improve the selectivity for macrocyclization in ring-closing metathesis.

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.

Morvan J., Mauduit M., Bertrand G., Jazzar R.

Discovered in 2005, cyclic (alkyl)(amino)carbenes (CAACs) have led to numerous discoveries in the field of ruthenium olefin metathesis, until then largely dominated by the well-known N-heterocyclic...

Blanco C.O., Sims J., Nascimento D.L., Goudreault A.Y., Steinmann S.N., Michel C., Fogg D.E.

Ruthenium catalysts for olefin metathesis are widely viewed as water-tolerant. Evidence is presented, however, that even low concentrations of water cause catalyst decomposition, severely degrading yields. Of 11 catalysts studied, fast-initiating examples (e.g., the Grela catalyst RuCl2(H2IMes)(═CHC6H4-2-OiPr-5-NO2) were most affected. Maximum water tolerance was exhibited by slowly initiating iodide and cyclic (alkyl)(amino)carbene (CAAC) derivatives. Computational investigations indicated that hydrogen bonding of water to substrate can also play a role, by retarding cyclization relative to decomposition. These results have important implications for olefin metathesis in organic media, where water is a ubiquitous contaminant, and for aqueous metathesis, which currently requires superstoichiometric “catalyst” for demanding reactions.

Kumandin P.A., Antonova A.S., Alekseeva K.A., Nikitina E.V., Novikov R.A., Vasilyev K.A., Sinelshchikova A.A., Grigoriev M.S., Polyanskii K.B., Zubkov F.I.

An efficient approach to the synthesis of new types of Hoveyda–Grubbs catalysts containing an N→Ru bond in a six-membered chelate ring is proposed. The synthesis of the organometallic compounds is based on the interaction of ready accessible 2-vinylbenzylamines and 1,3-bis(2,4,6-trimethylphenyl)-2-trichloromethylimidazolidine ligands with dichloro(3-phenyl-1H-inden-1-ylidene)bis(tricyclohexylphosphane)ruthenate, and it afforded the target ruthenium complexes in 70–80% yields. Areas of practical utility and potential applications of the obtained chelates were highlighted by tests of the catalysts in different olefin cross-metathesis (CM) and ring-closing-metathesis (RCM) reactions. These experiments revealed a high catalytic performance (up to 10–2 mol %) of all the synthesized structures in a broad temperature range. The structural peculiarities of the resultant ruthenium catalysts were thoroughly investigated by X-ray crystallography, which allowed making a reliable correlation between the structure of the metallo-complexes and their catalytic properties. It was proved that the bond length between ruthenium and nitrogen in the six-membered chelate ring has the greatest effect on the stability and efficiency of the catalyst. As a rule, the shorter and stronger the N→Ru bond, the higher the stability of the complex and the worse its catalytic characteristics. In turn, the coordination N→Ru bond length can be finely tuned and varied over a wide range of values by changing the steric volume of the cyclic substituents at the nitrogen atom, which will make it possible, as appropriate, to obtain in the future metal complexes with predictable stability and the required catalytic activity. Also, it was found that complexes in which the nitrogen atom is included in the morpholine or isoquinoline rings are the most efficient catalysts in this series. An attempt to establish a correlation between the N→Ru bond length and the 1H and 13C chemical shifts in the Ru═CH fragment has been made.

Xu Y., Wong J.J., Samkian A.E., Ko J.H., Chen S., Houk K.N., Grubbs R.H.

The efficient Z-selective cross-metathesis between acrylamides and common terminal olefins has been developed by the use of novel cyclometalated ruthenium catalysts with bulky N-heterocyclic carbene (NHC) ligands. Superior reactivity and stereoselectivity are realized for the first time in this challenging transformation, allowing streamlined access to an important class of cis-Michael acceptors from readily available feedstocks. The kinetic preference for cross-metathesis is enabled by a pivalate anionic ligand, and the origin of this effect is elucidated by density functional theory calculations.

Morvan J., Vermersch F., Zhang Z., Falivene L., Vives T., Dorcet V., Roisnel T., Crévisy C., Cavallo L., Vanthuyne N., Bertrand G., Jazzar R., Mauduit M.

An expedient access to the first optically pure ruthenium complexes containing C1-symmetric cyclic (alkyl)(amino)carbene ligands is reported. They demonstrate excellent catalytic performances in asymmetric olefin metathesis with high enantioselectivities (up to 92% ee). Preliminary mechanistic insights provided by density functional theory models highlight the origin of the enantioselectivity.

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.