Cyclic (Alkyl)(amino)carbenes (CAACs) in Ruthenium Olefin Metathesis

Ring Contraction by Rearrangement of Sterically Congested Cyclic (Amino)(aryl)carbenes

The rearrangement of sterically congested cyclic (amino)(aryl)carbenes (CAArCs) by the reaction of related iminium salts with potassium bis(trimethylsilyl)amide is reported, allowing for forming benzocyclobutanimines via a ring contraction process. Mechanistic studies by theoretical calculations indicate that the formation of conjugated ketenimines as intermediates could be considered, in which steric hindrance caused by N-alkyl motifs of CAArCs plays an important role in promoting the ring-opening by the cleavage of C-N bond.

Structural and thermodynamic insights into the coordination preference of norbornadiene with the initiator complex [RuCl2(PPh3)2(piperidine)] in polymerization via olefin metathesis

The metathesis reaction has been an important tool in both organic and inorganic synthetic chemistry. More specifically in polymer chemistry, ring opening metathesis polymerization (ROMP), via the formation of an active metal-carbene species (MCHR), has been widely used. The elucidation of the mechanism for ROMP opened the way for the development of well-defined catalysts, suited to local conditions. In the present study, we employed density functional theory (DFT) to investigate three reaction pathways for the formation of a species capable of activating ROMP. The active species is formed from the [RuCl2(PPh3)2(pip)] complex in the presence of norbornadiene (NBD) and the carbene source ethyl diazoacetate (EDA). Formation of a hexacoordinated intermediate [RuCl2(PPh3)2(pip)(NBD)] is favored in the first step, with NBD doubly coordinated to the [RuCl2(PPh3)2(pip)] moiety. Analysis of donation (X → Ru) and back-donation (Ru → X) processes in the [RuCl2(PPh3)2(pip)(NBD)] complex shows that piperidine behaves as a σ donor, while NBD behaves as a π donor and the PPh3 groups act as π acceptors. The intensity of the orbital component is predominant in relation to the steric component in the complex. Thus, we propose that the reaction occurs through the formation of a hexacoordinated complex, followed by the dissociation of a PPh3 group, thus forming a complex where NBD is doubly coordinated to the metal center. Coordination of EDA leads finally to the catalyst capable of forming the metallocyclobutane intermediate required for the ROMP reaction.

"Inverted" Cyclic(Alkyl)(Amino)Carbene (CAAC) Ruthenium Complex Catalyzed Isomerization Metathesis (ISOMET) of Long Chain Olefins to Propylene at Low Ethylene Pressure

Isomerization Metathesis (ISOMET) reaction is an emerging tool for "open loop" chemical recycling of polyethylene to propylene. Novel, latent N-Alkyl substituted Cyclic(Alkyl)(Amino)Carbene (CAAC)-ruthenium catalysts (5a-Ru, 3b-Ru - 6c-Ru) are developed rendering "inverted" chemical structure while showing enhanced ISOMET activity in combination with (RuHCl)(CO)(PPh3)3 (RuH) double bond isomerization co-catalyst. Systematic investigations reveal that the steric hindrance of the substituents on nitrogen and carbon atom adjacent to carbene moiety in the CAAC ligand have significantly improved the catalytic activity and robustness. In contrast to the NHC-Ru and CAAC-Ru catalyst systems known so far, these systems show higher isomerization metathesis (ISOMET) activity (TON: 7400) on the model compound 1-octadecene at as low as 3.0 bar optimized pressure, using technical grade (3.0) ethylene. The propylene content formed in the gas phase can reach up to 20% by volume.

An isolable, chelating bis[cyclic (alkyl)(amino)carbene] stabilizes a strongly bent, dicoordinate Ni(0) complex

Chelating ligands have had a tremendous impact in coordination chemistry and catalysis. Notwithstanding their success as strongly σ-donating and π-accepting ligands, to date no chelating bis[cyclic (alkyl)(amino)carbenes] have been reported. Herein, we describe a chelating, C2-symmetric bis[cyclic (alkyl)(amino)carbene] ligand, which was isolated as a racemic mixture. The isolation and structural characterization of its isostructural, pseudotetrahedral complexes with iron, cobalt, nickel, and zinc dihalides featuring eight-membered metallacycles demonstrates the binding ability of the bis(carbene). Reduction of the nickel(II) dibromide with potassium graphite produces a dicoordinate nickel(0) complex that features one of the narrowest angles measured in any unsupported dicoordinate transition metal complexes.

Two-Coordinate Thermally Activated Delayed Fluorescence Coinage Metal Complexes: Molecular Design, Photophysical Characters, and Device Application

Since the emergence of the first green light emission from a fluorescent thin-film organic light emitting diode (OLED) in the mid-1980s, a global consumer market for OLED displays has flourished over the past few decades. This growth can primarily be attributed to the development of noble metal phosphorescent emitters that facilitated remarkable gains in electrical conversion efficiency, a broadened color gamut, and vibrant image quality for OLED displays. Despite these achievements, the limited abundance of noble metals in the Earth's crust has spurred ongoing efforts to discover cost-effective electroluminescent materials. One particularly promising avenue is the exploration of thermally activated delayed fluorescence (TADF), a mechanism with the potential to fully harness excitons in OLEDs. Recently, investigations have unveiled TADF in a series of two-coordinate coinage metal (Cu, Ag, and Au) complexes. These organometallic TADF materials exhibit distinctive behavior in comparison to their organic counterparts. They offer benefits such as tunable emissive colors, short TADF emission lifetimes, high luminescent quantum yields, and reasonable stability. Impressively, both vacuum-deposited and solution-processed OLEDs incorporating these materials have achieved outstanding performance. This review encompasses various facets on two-coordinate TADF coinage metal complexes, including molecular design, photophysical characterizations, elucidation of structure-property relationships, and OLED applications.

Cyclic (amino)(barrelene)carbene Ru-complexes: synthesis and reactivity in olefin metathesis

The synthesis of ruthenium-complexes with cyclic (amino)(barrelene)carbenes (namely CABCs) as ligands is reported. Isolated in moderate to good yields, these new complexes showed impressive thermal stability at 110 °C over several days. Good catalytic performances were demonstrated in various ring-closing metathesis (RCM), macrocyclic-RCM, ring-closing enyne metathesis (RCEYM), cross-metathesis (CM), and ring-opening cross metathesis (ROCM) reactions.

Biodiesel as a Sustainable Platform Chemical Enabled by Selective Partial Hydrogenation: Compounds Outplace Combustion?!

The hydrogenation of polyunsaturated fatty acids (PUFAs) in vegetable oils and their derivatives is essential for their use in many areas, such as biofuels and food chemistry. However, no attempts have been made to adapt this technology to the requirements of further chemical utilization of fatty acid methyl esters as molecular building blocks, especially for particularly promising double-bond reactions. In this work, we, therefore, use three homogeneous catalytic model reactions (hydroformylation, isomerizing methoxycarbonylation, and ethenolysis) to show, firstly, that it is already known from the literature that high PUFA contents have a negative impact on activity and selectivity. Subsequently, using the example of soybean and canola biodiesel, we demonstrate that these key figures can be drastically improved by a preceding selective partial hydrogenation. This makes it possible to first reduce the share of PUFAs to <1 w % without causing significant overhydrogenation and then to carry out hydroformylation, methoxycarbonylation, and ethenolysis with significantly increased activity (up to twentyfold) and selectivity (up to 80 % increase). With these findings, we hope to convince the scientific and industrial world of the potential of selective partial hydrogenation as a key technology for utilizing renewable raw materials and to encourage its effective use in future work.

Bis-[cyclic(alkyl)(amino)carbene]-derived diradicals

Crystalline polymeric structures of trans-1,4-cyclohexylene bridged N-tethered bis-CAACs in the form of their LiOTf adducts were synthesized and isolated. These were further used as building blocks for the synthesis of crystalline (amino)(carboxy)-based diradicals. The triplet diradical character of these compounds was unambiguously confirmed by the presence of a half-field signal in their EPR spectra. Theoretical calculations show that the singlet state is marginally more stable than the triplet state.

Aldiminium and 1,2,3-triazolium dithiocarboxylate zwitterions derived from cyclic (alkyl)(amino) and mesoionic carbenes

The synthesis of zwitterionic dithiocarboxylate adducts was achieved by deprotonating various aldiminium or 1,2,3-triazolium salts with a strong base, followed by the nucleophilic addition of the in situ-generated cyclic (alkyl)(amino) or mesoionic carbenes (CAACs or MICs) onto carbon disulfide. Nine novel compounds were isolated and fully characterized by 1H and 13C NMR, FTIR, and HRMS techniques. Moreover, the molecular structures of two CAAC·CS2 and two MIC·CS2 betaines were determined by X-ray diffraction analysis. The analytical data recorded for all these adducts were compared with those reported previously for related NHC·CS2 betaines derived from imidazolinium or (benz)imidazolium salts. Due to the absence of electronic communication between the CS2 unit and the orthogonal heterocycle, all the CAAC·CS2, MIC·CS2, and NHC·CS2 zwitterions displayed similar electronic properties and featured the same bite angle. Yet, their steric properties are liable to ample modifications by varying the exact nature of their cationic heterocycle and its substituents.

Inhibition of the Decomposition Pathways of Ruthenium Olefin Metathesis Catalysts: Development of Highly Efficient Catalysts for Ethenolysis

Ruthenium-based Hoveyda-type olefin metathesis catalysts bearing novel rigid spirocyclic alkyl amino carbenes (CAACs) have been developed. They are characterized by exceptional stability toward decomposition through β-elimination and bimolecular pathways, thus enabling unprecedented efficiency in the cross-metathesis of seed oil-derived fatty acid esters with ethylene (ethenolysis). Catalyst loading as low as 100 ppb was applied to the ethenolysis of the model substrate methyl oleate, leading to a remarkable turnover number (TON) of 2.6 million, significantly higher than previously reported (TON 340 000 at 1 ppm and 744 000 at 0.5 ppm catalyst loading). Ethenolysis of methyl esters derived from high oleic sunflower oil and rapeseed oil, readily available on an industrial scale, inexpensive, and renewable feedstocks, was for the first time effectively carried out with 0.5 ppm catalyst loading with TON as high as 964 000.

Uncovering Nitrosyl Reactivity at N-Heterocyclic Carbene Center

N-heterocyclic carbenes (NHCs) have garnered much attention due to their unique properties, such as strong σ-donating and π-accepting abilities, as well as their transition-metal-like reactivity toward small molecules. In 2015, we discovered that NHCs can react with nitric oxide (NO) gas to form radical adducts that resemble transition metal nitrosyl complexes. To elucidate the analogy between NHC and transition metal NO adducts, here we have undertaken a systematic investigation of the electron- and proton-transfer chemistry of [NHC-NO]⋅ (N-heterocyclic carbene nitric oxide radical) compounds. We have accessed a suite of compounds, comprised of [NHC-NO]+ , [NHC-NO]- , [NHC-NOH]0 , and [NHC-NHOH]+ species. In particular, [NHC-NO]- was isolated as potassium and lithium ion adducts. Most interestingly, a monomeric potassium [NHC-NO]- compound was isolated with the assistance of 18-crown-6, which is the first instance of a monomeric alkali N-oxyl compound to the best of our knowledge. Our results demonstrate that [NHC-NO]⋅ exhibits redox behavior broadly similar to metal nitrosyl complexes, which opens up more possibilities for utilizing NHCs to build on the known reactivity of metal complexes.

Development of Highly Enantio- and Z-Selective Grubbs Catalysts via Controllable C-H Bond Activation

Asymmetric olefin metathesis is a powerful strategy for stereocontrolled synthesis that allows the formation of chiral elements in conjunction with carbon-carbon double bonds. Here, we report a new series of cyclometalated stereogenic-at-Ru catalysts that enable highly efficient asymmetric ring opening/cross-metathesis (AROCM) and asymmetric ring-closing metathesis (ARCM) reactions. Single enantiomers of these catalysts with either right-handed or left-handed configurations at the Ru center can be easily accessed via highly stereoselective C-H bond activation-based cyclometalation. Right-handed chiral Ru catalysts enabled the Z- and enantioselective AROCM of a wide range of norbornenes and terminal alkenes, generating densely functionalized cyclopentanes with excellent stereo- and enantioselectivities (99:1 Z/E, up to 99% ee). Left-handed chiral Ru catalysts enabled the facile ARCM of sterically unhindered, all-terminal prochiral trienes, which had not been achieved by previous Ru catalysts, providing simple cyclic ethers and amides with tertiary or quaternary carbon stereocenters with excellent enantioselectivities (up to 99% ee).

A tunable family of CAAC-ruthenium olefin metathesis catalysts modularly derived from a large-scale produced ibuprofen intermediate

A series of tunable CAAC-based ruthenium benzylidene complexes with increased lipophilicity derived from a ketone being a large-scale produced key substrate for a popular nonsteroidal anti-inflammatory drug-ibuprofen was obtained and tested in various olefin metathesis transformations. As a group, these catalysts exhibited higher activity than their known analogues containing a smaller and less lipophilic phenyl substituent on the α-carbon atom, but in individual reactions, the size of the N-aryl moiety was revealed as a decisive factor. For example, in the cross-metathesis of methyl oleate with ethylene (ethenolysis)-a reaction with growing industrial potential-the best results were obtained when the N-aryl contained an isopropyl or tert-butyl substituent in the ortho position. At the same time, in the RCM, CM, and self-CM transformations involving larger olefinic substrates, the catalysts with smaller aryl-bearing CAAC ligands, where methyl and ethyl groups occupy ortho, ortho' positions performed better. This offers a great deal of tunability and allows for selection of the best catalyst for a given reaction while keeping the general structure (and manufacturing method) of the ibuprofen-intermediate derived CAAC ligand the same.

IPr*Oxa - a new class of sterically-hindered, wingtip-flexible N,C-chelating oxazole-donor N-heterocyclic carbene ligands

N-heterocyclic carbenes (NHCs) have emerged as a major direction in ancillary ligand development for stabilization of reactive metal centers in inorganic and organometallic chemistry. In particular, wingtip-flexible NHCs have attracted significant attention due to their unique ability to provide a sterically-demanding environment for transition metals in various oxidation states. Herein, we report a new class of sterically-hindered, wingtip-flexible NHC ligands that feature N,C-chelating oxazole donors. These ligands are readily accessible through a modular arylation of oxazole derivatives. We report their synthesis and complete structural and electronic characterization. The evaluation of steric, electron-donating and π-accepting properties and coordination chemistry to Ag(I), Pd(II) and Rh(I) is described. Preliminary studies of catalytic activity in Ag, Pd and Rh-catalyzed coupling and hydrosilylation reactions are presented. This study establishes the fluxional behavior of a freely-rotatable oxazole unit, wherein the oxazolyl ring adjusts to the steric and electronic environment of the metal center. Considering the tremendous impact of sterically-hindered NHCs and their potential to stabilize reactive metals by N-chelation, we expect that this class of NHC ligands will be of broad interest in inorganic and organometallic chemistry.

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.

Synthesis Strategies towards Tagged Homogeneous Catalysts To Improve Their Separation

The recycling of homogeneous catalysts while keeping them in the homogeneous matrix is an ongoing challenge many reactions face if they are to find industrial applications. While a plethora of different synthetic approaches towards better, recyclable homogeneous catalysts exist, the literature shows a gap when one searches for a concise overview of the different catalyst modifications. This Review is designed to close that gap by summarising the existing synthesis pathways towards polar, non-polar, fluorous, and molecular-weight-enlarged catalysts and by examining their respective synthesis routes with a focus on modular and late-stage approaches. Furthermore, we map out the potential for a generally applicable tag library that allows straightforward catalyst modifications to tune them for each desired recycling strategy.

Steric Modulation of CAACs Controls Orientation and Ethenolysis Performance

In this issue of Chem Catalysis, Sytniczuk, Kajetanowicz, and Grela report sterically tuned Cyclic(Alkyl)(Amino)Carbene (CAAC) ligands to protect the requisite Ru-methylidene ([Ru]=CH2) intermediate present during ethenolysis of renewable fatty acid methyl esters (FAME). Surprising structural characteristics of the Ru-CAAC complexes resulted in TON up to 740,000 and sub-ppm catalyst loadings.

CuH-Catalyzed Selective N-Methylation of Amines Using Paraformaldehyde as a C1 Source

Copper hydride (CuH) complexes have been proposed as key intermediates in synthesis and catalysis. Herein, we developed a highly efficient strategy for CuH-catalyzed N-methylation of aromatic and aliphatic amines using paraformaldehyde and polymethylhydrosiloxane (PMHS) under mild reaction conditions. The reaction proceeded smoothly without additives to furnish the corresponding N-methylated products using cyclic(alkyl)(amino)carbene (CAAC)CuH as a reaction intermediate, which results from a reaction between PMHS and (CAAC)CuCl.

Mesomeric Acceleration Counters Slow Initiation of Ruthenium–CAAC Catalysts for Olefin Metathesis (CAAC = Cyclic (Alkyl)(Amino) Carbene)

Ruthenium catalysts bearing cyclic (alkyl)(amino)carbene (CAAC) ligands can attain very high productivities in olefin metathesis, owing to their resistance to unimolecular decomposition. Because the propagating methylidene species RuCl₂(CAAC)(=CH₂) is extremely susceptible to bimolecular decomposition, however, turnover numbers in the metathesis of terminal olefins are highly sensitive to catalyst concentration, and hence loadings. Understanding how, why, and how rapidly the CAAC complexes partition between the precatalyst and the active species is thus critical. Examined in a dual experimental-computational study are the rates and basis of initiation for phosphine-free catalysts containing the leading CAAC ligand C1 ^Ph , in which a CMePh group α to the carbene carbon helps retard degradation. The Hoveyda-class complex HC1 ^Ph (RuCl₂(L)(=CHAr), where L = C1 ^Ph , Ar = C₆H₃-2-O ⁱ Pr-5-R; R = H) is compared with its nitro-Grela analogue (nG-C1 ^Ph ; R = NO₂) and the classic Hoveyda catalyst HII (L = H₂IMes; R = H). t-Butyl vinyl ether (tBuVE) was employed as substrate, to probe the reactivity of these catalysts toward olefins of realistic bulk. Initiation is ca. 100× slower for HC1 ^Ph than HII in C₆D₆, or 44× slower in CDCl₃. The rate-limiting step for the CAAC catalyst is cycloaddition; for HII, it is tBuVE binding. Initiation is 10-13× faster for nG-C1 ^Ph than HC1 ^Ph in either solvent. DFT analysis reveals that this rate acceleration originates in an overlooked role of the nitro group. Rather than weakening the Ru-ether bond, as widely presumed, the NO₂ group accelerates the ensuing, rate-limiting cycloaddition step. Faster reaction is caused by long-range mesomeric effects that modulate key bond orders and Ru-ligand distances, and thereby reduce the trans effect between the carbene and the trans-bound alkene in the transition state for cycloaddition. Mesomeric acceleration may plausibly be introduced via any of the ligands present, and hence offers a powerful, tunable control element for catalyst design.

How To Enhance the Efficiency of Breslow Intermediates for SET Catalysis

Oxidative carbene organocatalysis, which proceeds via single electron transfer (SET) pathways, has been limited by the moderately reducing properties of deprotonated Breslow intermediates BI^-s derived from thiazol-2-ylidene 1 and 1,2,4-triazolylidene 2. Using computational methods, we assess the redox potentials of BI^-s based on ten different types of known stable carbenes and report our findings concerning the key parameters influencing the steps of the catalytic cycle. From the calculated values of the first oxidation potential of BI^-s derived from carbenes 1 to 10, it appears that, apart from the diamidocarbene 7, all the others are more reducing than thiazol-2-ylidene 1 and the 1,2,4-triazolylidene 2. We observed that while the reducing power of BI^-s significantly decreases with increasing solvent polarity, the redox potential of the oxidant can increase at a greater rate, thus facilitating the reaction. The cation, associated with the base, also plays an important role when a nonpolar solvent is used; large and weakly coordinating cations such as Cs⁺ are beneficial. The radical-radical coupling step is probably the most challenging step due to both electronic and steric constraints. Based on our results, we predict that mesoionic carbene 3 and abnormal NHC 4 are the most promising candidates for oxidative carbene organocatalysis.

Fast Initiating Furan-Containing Hoveyda-Type Complexes: Synthesis and Applications in Metathesis Reactions

Two new ruthenium complexes with chelating-ether benzylidene ligands bearing a furan moiety were synthesized and characterized, including X-ray crystallography. They initiated fast, also at 0 °C, and were found to be highly active in a variety of ring-closing, ene-yne, and cross-metathesis reactions, including an active pharmaceutical ingredient (API) model, which makes them good candidates for the transformation of complex polyfunctional compounds that require mild reaction conditions.

N-Heterocyclic carbene and cyclic (alkyl)(amino)carbene complexes of vanadium(III) and vanadium(V)

[VCl₃(THF)₃] offers a convenient entrance point into the chemistry of carbene stabilized V(III) complexes. Herein we report the paramagnetic mono- and biscarbene complexes [VCl₃(cAAC^Me)] 1, [VCl₃(cAAC^Me)(THF)] 1(thf), [VCl₃(IMes)] 2, [{VCl₂(IiPr^Me)(μ-Cl)}₂] 3, [VCl₃(IDipp)] 4, [VCl₃(SIDipp)] 5, [VCl₃(SIDipp)(THF)] 5(thf), [VCl₃(ItBu)] 6, [VCl₃(cAAC^Me)₂] 7 and [VCl₃(IiPr^Me)₂] 8. Reaction of 1 with MesMgCl, MesLi and LiNPh₂ afforded the complexes [VCl₂(Mes)(cAAC^Me)] 9, [cAAC^MeH]⁺[VCl₂Mes₂]^-10 and [VCl₂(NPh₂)(cAAC^Me)] 11. The V(V) complexes [V(O)Cl₃(IDipp)] 12 and [V(O)Cl₃(SIDipp)] 13 were selectively prepared from oxygen oxidation of 4 and 5. [V(O)Cl₃(IDipp)] 12 and [V(O)Cl₃(IMes)] react with isocyanates to yield the NHC-ligated imido complexes [V(N-p-CH₃C₆H₄)Cl₃(IDipp)] 14, [V(N-p-FC₆H₄)Cl₃(IDipp)] 15, [V(N-p-CH₃C₆H₄)Cl₃(SIDipp)] 16, [V(N-p-FC₆H₄)Cl₃(SIDipp)] 17, [V(N-p-CH₃C₆H₄)Cl₃(IMes)] 18 and [V(N-p-FC₆H₄)Cl₃(IMes)] 19.

Catalytic Decomposition of Long-Chain Olefins to Propylene via Isomerization-Metathesis Using Latent Bicyclic (Alkyl)(Amino)Carbene-Ruthenium Olefin Metathesis Catalysts

One of the most exciting scientific challenges today is the catalytic degradation of non-biodegradable polymers into value-added chemical feedstocks. The mild pyrolysis of polyolefins, including high-density polyethylene (HDPE), results in pyrolysis oils containing long-chain olefins as major products. In this paper, novel bicyclic (alkyl)(amino)carbene ruthenium (BICAAC-Ru) temperature-activated latent olefin metathesis catalysts, which can be used for catalytic decomposition of long-chain olefins to propylene are reported. These thermally stable catalysts show significantly higher selectivity to propylene at a reaction temperature of 75 °C compared to second generation Hoveyda-Grubbs or CAAC-Ru catalysts under ethenolysis conditions. The conversion of long-chain olefins (e.g., 1-octadecene or methyl oleate) to propylene via isomerization-metathesis is performed by using a (RuHCl)(CO)(PPh₃ )₃ isomerization co-catalyst. The reactions can be carried out at a BICAAC-Ru catalyst loading as low as 1 ppm at elevated reaction temperature (75 °C). The observed turnover number and turnover frequency are as high as 55 000 and 10 000 mol_propylene  mol_catalyst ^-1  h^-1 , respectively.

(CAAC)Copper Catalysis Enables Regioselective Three-Component Carboboration of Terminal Alkynes

Cyclic(alkyl)(amino)carbene (CAAC) ligands are found to perturb regioselectivity of the copper-catalyzed carboboration of terminal alkynes, favoring the less commonly observed internal alkenylboron regiosomer through an α-selective borylcupration step. A variety of carbon electrophiles participate in the reaction, including allyl alcohols derivatives and alkyl halides. The method provides a straightforward and selective route to versatile tri-substituted alkenylboron compounds that are otherwise challenging to access.

Bis[cyclic (alkyl)(amino)carbene] isomers: Stable trans-bis(CAAC) versus facile olefin formation for cis-bis(CAAC)

Isomeric bis(aldiminium) salts with a 1,4-cyclohexylene framework were synthesized. The first isolable bis(CAAC) was prepared from the trans-stereoisomer and its ditopic ligand competency was proven by conversion to iridium(I) and rhodium(I) complexes. Upon deprotonation, the cis-isomer yielded an electron rich olefin via a classic, proton-catalyzed pathway. The CC bond formation from the desired cis-bis(CAAC) was shown to be thermodynamically very favorable and to involve a small activation barrier. Compounds that can be described as insertion products of the cis-bis(CAAC) into the E-H bonds of NH₃, CH₃CN and H₂O were also identified.

Thiazol-2-ylidenes as N-Heterocyclic carbene ligands with enhanced electrophilicity for transition metal catalysis

Over the last 20 years, N-heterocyclic carbenes (NHCs) have emerged as a dominant direction in ligand development in transition metal catalysis. In particular, strong σ-donation in combination with tunable steric environment make NHCs to be among the most common ligands used for C-C and C-heteroatom bond formation. Herein, we report the study on steric and electronic properties of thiazol-2-ylidenes. We demonstrate that the thiazole heterocycle and enhanced π-electrophilicity result in a class of highly active carbene ligands for electrophilic cyclization reactions to form valuable oxazoline heterocycles. The evaluation of steric, electron-donating and π-accepting properties as well as structural characterization and coordination chemistry is presented. This mode of catalysis can be applied to late-stage drug functionalization to furnish attractive building blocks for medicinal chemistry. Considering the key role of N-heterocyclic ligands, we anticipate that N-aryl thiazol-2-ylidenes will be of broad interest as ligands in modern chemical synthesis.

Testing enabling techniques for olefin metathesis reactions of lipophilic substrates in water as a diluent

Olefin metathesis reactions of diverse polyfunctional substrates were conducted in water emulsions using two hydrophobic ruthenium catalysts in the presence of air. Instead of using surfactants to increase the efficiency of the metathesis reaction in water, ultrasound and microwave techniques were tested on a small-scale reaction, whereas conventional heating and mechanical stirring were effective enough to provide high conversion and selectivity on a larger scale. The developed conditions extend known protocols for the aqueous metathesis methodology, utilizing relatively low catalyst loadings and allowing for simple product isolation and purification. The established synthetic protocol was successfully adopted in the large-scale synthesis of a pharmaceutically related product – sildenafil (Viagra) derivative.

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Sustainable approach for metathesis reaction in water emulsion system on air.

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Utilization of enabling techniques for boosting metathesis under aqueous conditions.

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RCM of medically important sildenafil derivative.

Bonding and stability of dinitrogen-bonded donor base-stabilized Si(0)/Ge(0) species [(cAACMe-Si/Ge)2(N2)]: EDA-NOCV analysis

Recently, dinitrogen (N₂) binding and its activation have been achieved by non-metal compounds like intermediate cAAC-borylene as (cAAC)₂(B-Dur)₂(N₂) [cAAC = cyclic alkyl(amino) carbene; Dur = aryl group, 2,3,5,6-tetramethylphenyl; B-Dur = borylene]. It has attracted a lot of scientific attention from different research areas because of its future prospects as a potent species towards the metal free reduction of N₂ into ammonia (NH₃) under mild conditions. Two (cAAC)(B-Dur) units, each of which possesses six valence electrons around the B-centre, are shown to accept σ-donations from the N₂ ligand (B ← N₂). Two B-Dur further provide π-backdonations (B → N₂) to a central N₂ ligand to strengthen the B–N₂–B bond, providing maximum stability to the compound (cAAC)₂(B-Dur)₂(N₂) since the summation of each pair wise interaction accounted for the total stabilization energy of the molecule. (cAAC)(B-Dur) unit is isolobal to cAAC–E (E = Si, Ge) fragment. Herein, we report on the stability and bonding of cAAC–E bonded N₂-complex (cAAC–E)₂(N₂) (1–2; Si, Ge) by NBO, QTAIM and EDA-NOCV analyses (EDA-NOCV = energy decomposition analysis coupled with natural orbital for chemical valence; QTAIM = quantum theory of atoms in molecule). Our calculation suggested that syntheses of elusive (cAAC–E)₂(N₂) (1–2; Si, Ge) species may be possible with cAAC ligands having bulky substitutions adjacent to the C_cAAC atom by preventing the homo-dimerization of two (cAAC)(E) units which can lead to the formation of (cAAC–E)₂. The formation of E Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> E bond is thermodynamically more favourable (E = Si, Ge) over binding energy of N₂ inbetween two cAAC–E units.

Dinitrogen (N₂) binding and its activation have been achieved by non-metal compounds like intermediate cAACborylene with the general formula of (cAAC)₂(B-Dur)₂(N₂) [cAAC = cyclic alkyl(amino)carbene; Dur = aryl group, 2,3,5,6-tetramethylphenyl; B-Dur = aryl-borylene].

Bi- and trimetallic complexes with macrocyclic xanthene-4,5-diNHC ligands

Three different types of bimetallic NHC-metal complexes were synthesized, whose NHC units are attached at the 4,5-positions of xanthene. The NHC units are in close proximity and are designed such that each carbene coordinates one ML unit, while the chelation of one metal by two NHC is not possible. Several xanthene-((NHC)ML)₂ complexes with ML = RhCl(cod), IrCl(cod), RhCl(CO)₂, IrCl(CO)₂, AuCl, AgCl, CuCl and Pd(allyl)Cl were synthesized and investigated.

Recent advances in the domain of Cyclic (alkyl)(amino) carbenes

Isolation of cyclic (alkyl) amino carbenes (cAACs) in 2005 has been a major achievement in the field of stable carbenes due to their better electronic properties. cAACs and bicyclic(alkyl)(amino)carbene (BicAAC) in essence are the most electrophilic as well as nucleophilic carbenes are known till date. Due to their excellent electronic properties in terms of nucleophilic and electrophilic character, cAACs have been utilized in different areas of chemistry, including stabilization of low valent main group and transition metal species, activation of small molecules, and catalysis. The applications of cAACs in catalysis have opened up new avenues of research in the field of cAAC chemistry. This review summarizes the major results of cAAC chemistry published until August 2021.

Cationic molybdenum oxo alkylidenes stabilized by N-heterocyclic carbenes: from molecular systems to efficient supported metathesis catalysts

The first cationic molybdenum oxo complexes were synthesized and immobilized on partially dehydroxylated silica. Vastly enhanced catalytic activity for terminal olefins was found compared to their neutral congeners.

Olefin Metathesis in Confined Spaces: The Encapsulation of Hoveyda-Grubbs Catalyst in Peanut, Square, and Capsule Shaped Hollow Silica Gels

In this study, Hoveyda-Grubbs 2nd generation (HG2) catalyst was encapsulated in hollow mesoporous silica gels with various morphologies (peanut, square, and capsule) by reducing the pore size of the mesoporous...

Highly selective ethenolysis with acyclic-aminooxycarbene ruthenium catalysts

Acyclic carbene–ruthenium catalysts were developed for the ethenolysis. Remarkable catalytic efficiency (turnover numbers of 100 000) and excellent α-olefin selectivity (up to 98%) were exhibited.

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.

Cross-metathesis of Allenes. Mechanistic Analysis and Identification of a Ru-CAAC as the Most Effective Catalyst

The first examples of cross-metathesis between two different allenes is disclosed. First- and second-generation Ru complexes were found to be ineffective, at most affording only oligomeric products. The exception was a first-generation complex bearing a bidentate phenyl isopropoxy ligand (i.e., PCy₃ is not released upon initiation), reactions with which afforded a 1,3-disubstituted allenyl boronate in 22% yield. On the basis of mechanistic studies designed to gain deeper understanding of the reasons for the ineffectiveness of different Ru catalysts, it was discovered that phosphine-free Ru-CAAC complexes have the steric and electronic attributes to be highly effective. The results of these investigations pave the way for development of additional olefin metathesis reactions that generate allenes.