A Highly Efficient Synthesis of Z-Macrocycles Using Stereoretentive, Ruthenium-Based Metathesis Catalysts

Preserving precise choreography of bonds in Z-stereoretentive olefin metathesis by using quinoxaline-2,3-dithiolate ligand

The Z-alkene geometry is prevalent in various chemical compounds, including numerous building blocks, fine chemicals, and natural products. Unfortunately, established Mo, W, and Ru Z-selective catalysts lose their selectivity at high temperatures required for industrial processes like reactive distillation, which limits their synthetic applications. To address this issue, we develop a catalyst capable of providing Z-alkenes with high selectivity under harsh conditions. Our research reveals a dithiolate ligand that, stabilised by resonance, delivers high selectivity at temperatures up to 150 °C in concentrated mixtures. This distinguishes the dithioquinoxaline complex from existing Z-selective catalysts. Notably, this trait does not compromise the new catalyst's usability under classical conditions, matching the activity of known stereoretentive catalysts. Density Functional Theory calculations were employed to understand the reaction mechanism and selectivity, and to investigate the poisoning that the catalyst may undergo and how it competes with catalytic activity. Furthermore, the quinoxaline-based catalyst enables the valorisation of bio-sourced alkene feedstocks and the production of agricultural sex pheromones for pest control.

Unexpected Latency of Z-Stereoretentive Ruthenium Olefin Metathesis Catalysts Bearing Unsymmetrical N-heterocyclic Carbene or Cyclic(alkyl)(amino)carbene Ligands

A set of ruthenium complexes bearing a CAAC or uNHC ligand and a dithiocatechol fragment have been obtained and characterized spectroscopically. The activity and Z-selectivity of the newly obtained catalysts were studied in selected model CM, self-CM, and RCM olefin metathesis reactions. Intriguingly, and in contrast to structurally related NHC-bearing catalysts Ru4a and Ru4b, the CAAC and uNHC analogues showed no or only very little activity in olefin metathesis. Interestingly, despite being not productive in metathesis reactions conducted in solution, Ru8 enabled the synthesis of a model 16-membered macrocyclic lactone of valuable musk smell with excellent chemoselectivity (no C-C double-bond migration was observed) at a concentration 40 times higher than that typically used by organic chemists in similar macrocyclizations (200 mM instead of 5 mM) with excellent Z-selectivity. Unfortunately, also here the conversion was low.

Taking Olefin Metathesis to the Limit: Stereocontrolled Synthesis of Trisubstituted Alkenes

ConspectusIn this Account, we share the story of the development of catalytic olefin metathesis processes that efficiently deliver a wide range of acyclic and macrocyclic E- or Z-trisubstituted alkenes. The tale starts with us unveiling, in collaboration with Richard Schrock and his team, the blueprint in 2009 for the design of kinetically controlled Z-selective olefin metathesis reactions. This paved the way for the development of Mo-, W-, and Ru-based catalysts and strategies for synthesizing countless linear and macrocyclic Z-olefins. Six years later, in 2015, we found that abundant Z-alkene feedstocks, such as oleic acid, can be directly transformed to high-value and more difficult-to-access alkenes through a cross-metathesis reaction promoted by a Ru-catechothiolate complex that we had developed; the approach, later coined stereoretentive olefin metathesis, was extended to the synthesis of E-alkenes.It was all about disubstituted alkenes until when in 2017 we addressed the challenge of accessing stereodefined Z- and E-trisubstituted alkenes, key to medicine and materials research. These transformations can be most effectively catalyzed by Mo monoaryloxides pyrrolide (MAP) and chloride (MAC) complexes. A central aspect of the advance is the merging of olefin metathesis, which delivered trisubstituted alkenyl fluorides, chlorides, and bromides with cross-coupling. These catalytic and stereoretentive transformations can be used in various combinations, thereby enabling access to assorted Z- or E-trisubstituted alkene. Ensuing work led to the emergence of other transformations involving substrates that can be purchased with high stereoisomeric purity, notably E- and Z-trihalo alkenes. Trisubstituted olefins, Z or E, bearing a chemoselectively and stereoretentively alterable F,Cl-terminus or B(pin),Cl-terminus may, thus, be easily and reliably synthesized. Methods for stereoretentive preparation of other alkenyl bromide regioisomers and α,β-unsaturated carboxylic and thiol esters, nitriles, and acid fluorides followed, along with stereoretentive ring-closing metathesis reactions that afford macrocyclic trisubstituted olefins. Z- and E-Macrocyclic trisubstituted olefins, including those that contain little or no entropic support for cyclization (minimally functionalized) and/or are disfavored under substrate-controlled conditions, can now be synthesized. The utility of this latest chapter in the history of olefin metathesis has been highlighted by applications to the synthesis of several biologically active compounds, as well as their analogues, such as those marked by one or more site-specifically incorporated fluorine atoms or more active but higher energy and otherwise unobtainable conformers.The investigations discussed here, which represent every stereoretentive method that has been reported thus far for preparing a trisubstituted olefin, underscore the inimitable power of Mo-based catalysts. This Account also showcases a variety of mechanistic attributes─some for the first time, and each instrumental in solving a problem. Extensive knowledge of mechanistic nuances will be needed if we are to address successfully the next challenging problem, namely, the development of catalysts and strategies that may be used to synthesize a wide range of tetrasubstituted alkenes, especially those that are readily modifiable, with high stereoisomeric purity.

E- and Z-trisubstituted macrocyclic alkenes for natural product synthesis and skeletal editing

Many therapeutic agents are macrocyclic trisubstituted alkenes but preparation of these structures is typically inefficient and non-selective. A possible solution would entail catalytic macrocyclic ring-closing metathesis, but these transformations require high catalyst loading, conformationally rigid precursors and are often low yielding and/or non-stereoselective. Here we introduce a ring-closing metathesis strategy for synthesis of trisubstituted macrocyclic olefins in either stereoisomeric form, regardless of the level of entropic assistance. The goal was achieved by addressing several unexpected difficulties, including complications arising from pre-ring-closing metathesis alkene isomerization. The power of the method is highlighted by two examples. The first is the near-complete reversal of substrate-controlled selectivity in the formation of a macrolactam related to an antifungal natural product. The other is a late-stage stereoselective generation of an E-trisubstituted alkene in a 24-membered ring, en route to the cytotoxic natural product dolabelide C.

Transformation of the sp2 Carbanion to Carbene with Subsequent 1,1-Migratory Insertion and Nucleophilic Substitution in Rare-Earth Metal Chemistry

The development of Fischer-type electrophilic carbene chemistry with early transition metals has been a great challenge due to the fact that such metals in their high oxidation states lack the d electrons to stabilize the electrophilic carbene. Herein, we disclose the first experimental and theoretical findings of in situ transformation of an sp² carbanion to a Fischer-type electrophilic carbene with rare-earth metals in their high oxidation state with a d⁰ electron via electron transfer. The carbene may undergo 1,1-migratory insertion into an adjacent RE-C(sp³) bond, and an unprecedented ring opening of the indole ring of the ligand occurs when the carbenes undergo nucleophilic substitution with a special organolithium reagent o-Me₂NC₆H₄CH₂Li. The key to success is the uniquely tailored novel ligand systems featuring a suitable conjugate building block (^-C═C-C═N) bearing an sp² carbanion connected to the rare-earth metal center.

Total Synthesis of (+)-Jatrophalactam

The first asymmetric total synthesis of (+)-jatrophalactam was reported, which unambiguously determined the absolute configuration of the titled natural product. The key features entail a conformationally controlled cyclopropanation, a Meldrum's acid adduct-engaged macrolactam formation, and a Pd(II)-mediated oxidative cyclization.

Living β-selective cyclopolymerization using Ru dithiolate catalysts

Cyclopolymerization (CP) of 1,6-heptadiyne derivatives is a powerful method for synthesizing conjugated polyenes containing five- or six-membered rings via α- or β-addition, respectively. Fifteen years of studies on CP have revealed that user-friendly Ru-based catalysts promoted only α-addition; however, we recently achieved β-selective regiocontrol to produce polyenes containing six-membered-rings, using a dithiolate-chelated Ru-based catalyst. Unfortunately, slow initiation and relatively low catalyst stability inevitably led to uncontrolled polymerization. Nevertheless, this investigation gave us some clues to how successful living polymerization could be achieved. Herein, we report living β-selective CP by rational engineering of the steric factor on monomer or catalyst structures. As a result, the molecular weight of the conjugated polymers from various monomers could be controlled with narrow dispersities, according to the catalyst loading. A mechanistic investigation by in situ kinetic studies using 1H NMR spectroscopy revealed that with appropriate pyridine additives, imposing a steric demand on either the monomer or the catalyst significantly improved the stability of the propagating carbene as well as the relative rates of initiation over propagation, thereby achieving living polymerization. Furthermore, we successfully prepared diblock and even triblock copolymers with a broad monomer scope.

4-Coordinated, 14-electron ruthenium(ii) chalcogenolate complexes: synthesis, electronic structure and reactions with PhICl2 and organic azides

The 4-coordinated Ru^II chalcogenolate complexes [Ru(STipp)₂(PPh₃)₂] (Tipp = 2,4,6-triisopropylphenyl, 1) and [Ru(SeMes)₂(PPh₃)₂] (Mes = 2,4,6-trimethylphenyl, 2) have been synthesized, and their reactions with PhICl₂ and organic azides have been studied. Complex 2 synthesized from [Ru^II(PPh₃)₃Cl₂] and NaSeMes displays a seesaw structure with P-Ru-P and Se-Ru-Se bond angles of 103.43(13) and 145.26(6)°, respectively. Natural bond order analyses revealed that in each of 1 and 2, there are two n →σ* (donor-acceptor) π interactions between the chalcogen lone pairs and the Ru-P antibonding molecular orbitals. The calculated second-order perturbation interaction energies of the two interactions for 1 (20.5 and 18.3 kcal mol^-1) are stronger than those of 2 (13.6 and 11.0 kcal mol^-1), suggesting the thiolate ligand (TippS^-) is a stronger π-donor than the selenolate ligand (MesSe^-) with respect to Ru^II. Chlorination of 1 with PhICl₂ afforded the dichloride complex [Ru(STipp)₂Cl₂(PPh₃)] (3), which was hydrolyzed to the hydroxo complex [Ru(STipp)₂(OH)Cl(PPh₃)] (4) after column chromatography on silica in air. Treatment of 4 with HCl and methyl triflate gave 3 and [Ru(STipp)₂(OH)(OTf)(PPh₃)] (OTf = triflate, 5), respectively. Reactions of 1 and 2 with p-tolyl azide (p-tolN₃) afforded the tetrazene complexes [Ru{N₄(p-tol)₂}(ER)₂(PPh₃)] (ER = STipp (6), SeMes (7)), whereas that with tosyl azide (TsN₃) gave the imido complexes [Ru(κ²-NTs)(STipp)₂(PPh₃)] (ER = STipp (8), SeMes (10)). The short Ru-N_imido distances in 8 [1.883(3) Å] and 10 [1.892(2) Å] are indicative of multiple bond character. Treatment of 8 with TsN₃ afforded the tetrazene complex [Ru(N₄Ts₂)(STipp)₂(PPh₃)] (9), but no cycloaddition was found between 10 and TsN₃. Nucleophilic attack of the imido ligand in 10 with methyl triflate yielded the amido complex [Ru(κ²-NMeTs)(SeMes)₂(PPh₃)](OTf) (11). The crystal structures of 2, 4, 6, and 8-11 have been determined.

A concise route to virginiamycin M2

Modular, fully synthetic routes to structurally complex natural products provide useful avenues to access chemical diversity. Herein we report a concise route to virginiamycin M2, a member of the group A streptogramin class of natural products that inhibits bacterial protein synthesis. Our approach features a longest linear sequence of six steps from 7 simple building blocks, and is the shortest and highest yielding synthesis of any member of the streptogramin class reported to date. We believe this route will enable access to unexplored structural diversity and may serve as a useful tool to improve the therapeutic potential of the streptogramin class of antibiotics.

A 3,4-dimercapto-3-cyclobutene-1,2-dione-chelated ruthenium carbene catalyst for Z-stereoretentive/stereoselective olefin metathesis

A ruthenium carbene catalyst chelated with a 3,4-dioxocyclobut-1-ene-1,2-dithiolate ligand was synthesized and its molecular structure was determined by single-crystal X-ray diffraction. The Ru catalyst had excellent catalytic activity with high yields and good Z/E ratios for the ring opening metathesis polymerization (ROMP) of norbornene (yield: 96%/Z/E: 86 : 14) and 1,5-cyclooctadiene (yield: 86%/Z/E: 91 : 9) and for ring opening cross metathesis (ROCM) reactions of norbornene/5-norbornene-2-exo, 3-exo-dimethanol with styrene (yields: 64%-92%/Z/E: 97 : 3-98 : 2) or 4-fluorostyrene (yield: 46%-94%/Z/E: 98 : 2). The catalyst also had high Z-stereoretentivity (91 : 9-98 : 2) for cross-metathesis (CM) reactions of terminal olefins with (Z)-2-butene-1,4-diol. More importantly, the catalyst had moderate Z-stereoselectivity for homometathesis reactions of terminal olefins giving cis-olefins as the major products (Z/E ratios of 70 : 30-77 : 23). Like other Ru carbene complexes, the catalyst tolerates many different functional groups. The presented data, supported by DFT calculations, show that our catalyst, bearing a chelating 3,4-dioxocyclobut-1-ene-1,2-dithiolate ligand, exhibits higher stability towards air than Hoveyda's stereoretentive complex systems.

Concise Syntheses of Δ12-Prostaglandin J Natural Products via Stereoretentive Metathesis

Δ¹²-Prostaglandin J family is recently discovered and has potent anticancer activity. Concise syntheses of four Δ¹²-prostaglandin J natural products (7-8 steps in the longest linear sequences) are reported, enabled by convergent stereoretentive cross-metathesis. Exceptional control of alkene geometry was achieved through stereoretention.

A tutorial review of stereoretentive olefin metathesis based on ruthenium dithiolate catalysts

Stereoretentive olefin metathesis based on ruthenium dithiolate complexes has become a very active field of research within the past years. This unique catalyst class is able to kinetically produce both Z- and E-alkenes in high stereochemical purity (typically >95:5) starting from stereochemically pure Z- or E-alkenes. The aim of this tutorial review is to organize the reported information concerning ruthenium dithiolate catalysts in a logic manner, thus providing an "operators handbook" for chemists who wish to apply this methodology in synthesis.

Preparation of Musk-Smelling Macrocyclic Lactones from Biomass: Looking for the Optimal Substrate Combination

Macrocyclic musk belongs to a well-known and valued class of the fragrance family. Originally, natural musks were obtained from rectal musk glands which often led to the death of the animals. Recently, a lot of effort was invested to obtain such macrocycles in a synthetic way. This research presents a study on the preparation of macrocyclic lactones with the musk scent by ring-closing metathesis (RCM) using biomass-derived starting materials: oleic and 9-decenoic acid. An experimental rule correlating the C-C double bond substitution pattern in the starting diene and the yield for the RCM macrocyclization was proposed.

Using stereoretention for the synthesis of E-macrocycles with ruthenium-based olefin metathesis catalysts

The synthesis of E-macrocycles is achieved using stereoretentive, Ru-based olefin metathesis catalysts supported by dithiolate ligands.

The synthesis of E-macrocycles is achieved using stereoretentive, Ru-based olefin metathesis catalysts supported by dithiolate ligands. Kinetic studies elucidate marked differences in activity among the catalysts tested, with catalyst 4 providing meaningful yields of products in much shorter reaction times than stereoretentive catalysts 2 and 3. Macrocycles were generated with excellent selectivity (>99% E) and in moderate to high yields (47–80% yield) from diene starting materials bearing two E-configured olefins. A variety of rings were constructed, ranging from 12- to 18-membered macrocycles, including the antibiotic recifeiolide.