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A series of a novel CAAC ligands featuring a spiro‐fluorene group have been synthesized and complexed with ruthenium alkylidenes, yielding the corresponding Hoveyda‐type derivatives as a new family of olefin metathesis catalysts. The novel complexes have been characterized by XRD, HRMS and NMR measurements. The synthetised complexes were tested in catalysis and showed good activity in olefin metathesis, as demonstrated on diethyl diallylmalonate and allyl acetate substrates. The unique backbone in the ligand with the large, yet inflexible condensed system renders interesting properties to the catalyst, exemplified by the good catalytic performance and improved Z‐selectivity. In addition, the complex can also serve as a hydrogenation catalyst in a consecutive (one‐pot) reaction. The latter reaction can convert allyl acetate to butane‐1,4‐diol, a valuable chemical intermediate for biodegradable polybutylene succinate (PBS). [ABSTRACT FROM AUTHOR]

The syntheses of the neutral molybdenum imido alkylidene N‐heterocyclic carbene (NHC) complexes of the general formula [Mo(NAr)(CHCMe2Ph)(NHC)XY] (Ar=2‐tBu‐C6H4, 2‐CF3‐C6H4, 2,6‐Me2‐C6H3, 2,6‐Cl2‐C6H3, adamantyl; X, Y=OTf, OC(CF3)3, OCH(CF3)2, OC6F5, SC6F5, 2,5‐bis(pentafluorophenyl)phen‐1‐yl) bearing electron‐withdrawing NHCs (1,3‐dimethyl‐4,5‐dichloroimidazol‐2‐ylidene (IMeCl2), 1,3,4‐triphenyl‐1,2,4‐triazol‐5‐ylidene (TPT)) are reported. Complementary, the corresponding cationic molybdenum imido alkylidene NHC complexes of the general formula [Mo(NAr)(CHCMe2R)(NHC)X+][B(ArF)4−/Al(OC(CF3)3)4−] (R=Me, Ph; B(ArF)4−=tetrakis (3,5‐bis(trifluoromethyl)phenyl)borate) have been prepared. Aiming at tungsten oxo complexes, reaction of [W(O)Cl2(CHCMe2Ph)(PMe2Ph)2] with [1,3‐dimethyl‐4,5‐dichloroimidazol‐2‐ylidene⋅AgI] (IMeCl2⋅AgI) followed by the addition of lithium terphenoxide yields [W(O)(CHCMe2Ph)(IMeCl2)(DPPO)2]. For comparison, [W(O)Cl(CHCMe2Ph)(IMes)(OSi(OtBu)3)] was prepared via reaction of [W(O)Cl2(CHCMe2Ph)(PMe2Ph)(IMes)] with KOSi(OtBu)3. [W(O)(CHCMe2Ph)(IMeCl2)(DPPO)(Et2O)+][B(ArF)4−] (DPPO=2,6‐diphenylphenoxide) became accessible via reaction of [W(O)(DPPO)2(CHCMe2Ph)(IMeCl2)] with anilinium B(ArF)4−. The structural peculiarities of selected complexes are reported. Benchmark ring‐closing metathesis and homometathesis reactions revealed that the neutral complexes bearing weakly coordinating NHCs such as IMeCl2 and TPT possessed only moderate activity, which could, however, be improved by preparing the corresponding cationic metal alkylidene complexes. [ABSTRACT FROM AUTHOR]

Neutral three‐coordinate iron alkylidenes of the form PN−Fe=CHR have been proposed as viable candidates for alkene metathesis. Indeed, during the final stages of preparing this current study, a separate report disclosed that dearomatized PN−Fe‐alkyl complexes are active precatalysts for ring‐opening metathesis polymerization (ROMP) of norbornene implicating PN−Fe=CHR species as possible intermediates. In yet another separate report, we prepared Zn analogues of PN−Fe‐alkyl complexes and herein provide an account for the synthesis, characterization, and reactivity of some new iron complexes with the same tBu substituted PN platform. [ABSTRACT FROM AUTHOR]

In the present study, we investigated the catalytic power of boric acid used for the synthesis of 2-alkylidene/arylidene derivatives resulting from active methylene compounds and 4-chlorobenzaldehyde in the presence of 10 mol% of boric acid in ethanol under conventional conditions. We achieved good-to-excellent yields of synthesized products and then characterized them using conventional spectroscopic techniques. [ABSTRACT FROM AUTHOR]

In recent years, the concept of transition metal-catalyzed functionalization of unsaturated hydrocarbons has emerged as a promising strategy for developing new and valuable reactions, and has attracted considerable attention because it is efficient and atomic economical. In this critical review, we describe recent advances that have been made in the development of transition metal-catalyzed functionalization of 1,3-enynes via alkylene-π-allylpalladium intermediates. [ABSTRACT FROM AUTHOR]

3,3′,5,5′-Tetra-tert-butyl-2′-sulfanyl[1,1′-biphenyl]-2-ol (H2[tBu4OS]) was prepared in 24 % yield overall from the analogous biphenol using standard techniques. Addition of H2[tBu4OS] to Mo(NAr)(CHCMe2Ph)(2,5-dimethylpyrrolide)2 led to formation of Mo(NAr)(CHCMe2Ph)[tBu4OS], which was trapped with PMe3 to give Mo(NAr)(CHCMe2Ph)[tBu4OS](PMe3) (1(PMe3)). An X-ray crystallographic study of 1(PMe3) revealed that two structurally distinct square pyramidal molecules are present in which the alkylidene ligand occupies the apical position in each. Both 1(PMe3)A and 1(PMe3)B are disordered. Mo(NAd)(CHCMe2Ph)(tBu4OS)(PMe3) (2(PMe3); Ad=1-adamantyl) and W(NAr)(CHCMe2Ph)(tBu4OS)(PMe3) (3(PMe3)) were prepared using analogous approaches. 1(PMe3) reacts with ethylene (1 atm) in benzene within 45 minutes to give an ethylene complex Mo(NAr)(tBu4OS)(C2H4) (4) that is isolable and relatively stable toward loss of ethylene below 60 °C. An X-ray study shows that the bond distances and angles for the ethylene ligand in 4 are like those found for bisalkoxide ethylene complexes of the same general type. Complex 1(PMe3) in the presence of one equivalent of B(C6F5)3 catalyzes the homocoupling of 1-decene, allyltrimethylsilane, and allylboronic acid pinacol ester at ambient temperature. 1(PMe3), 2(PMe3), and 3(PMe3) all catalyze the ROMP of rac-endo,exo-5,6-dicarbomethoxynorbornene (rac-DCMNBE) in the presence of B(C6F5)3, but the polyDCMNBE that is formed has a random structure.

We describe the development of an efficient method for the olefination of hydrazones and oximes. The key design approach that enables this transformation is tuning of the energy/polarity of C=N π‐bonds by employing heteroatom functionalities (NR2, OR). The resulting hydrazones or oximes facilitate olefination with ruthenium alkylidenes. Through this approach, we show that air‐stable, commercially available ruthenium alkylidenes provide access to functionalized alkenes (20 examples) in ring‐closing reactions with yields up to 88 %. [ABSTRACT FROM AUTHOR]

In this work, we study the ligand exchange process between an alkane and a series of silica supported metal alkylidenes, which may occur by different pathways: C–H addition, σ-bond metathesis, and α-H abstraction. The results indicate that the α-H abstraction pathway is the preferred one, regardless of the catalyst and ligands. This is in contrast to the expected preference for the C–H addition route. When looking for the origin of this preference, our calculations revealed that the α-H abstraction pathway is driven by entropy, which favors the initial dissociation of the alkyl ligand from the catalyst. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Tungsten oxo adamantylidenes (=Ad) are a new class of readily accessible alkylidenes, that opens the possibility to evaluate a broader class of molecular and supported olefin metathesis catalysts. In this context, W(O)(=Ad)X2, with X=pyrrolide and perfluoro‐tert‐butoxide, were grafted on partially dehydroxylated silica, to generate the corresponding silica‐supported tungsten oxo adamantylidenes. The high thermal stability of the molecular perfluoro‐tert‐butoxide complex, that exists as a dimer in the solid state, even enables its grafting through sublimation. Evaluation of their catalytic activities toward olefin metathesis using standardized protocols show that these supported species display high activity toward internal and terminal olefins. In particular, avoiding the use of solvent during grafting of the highly electrophilic perfluoro‐tert‐butoxide compounds, that can bind to the W center, enable to generate significantly more active catalysts. [ABSTRACT FROM AUTHOR]

A series of cationic tungsten oxo alkylidene N‐heterocyclic carbene (NHC) complexes was synthesized and structurally characterized by single crystal X‐ray diffraction. The 13C NMR chemical shifts of the alkylidene C atoms of these complexes were correlated with the diamagnetic, paramagnetic and spin‐orbit chemical shifts calculated by DFT. A good correlation (R2=0.90) between the DFT isotropic chemical shifts and the experimental chemical shift as well as a strong correlation between the DFT isotropic chemical shifts and the TOF1min for the RCM of 1,7‐octadiene was found. Further, a comparison of the catalyst geometries allowed for assigning tetracoordinate pseudotetrahedral catalysts to the most deshielded alkylidenes and to the highest TOF1min, pentacoordinate square‐planar catalysts to the intermediate deshielded alkylidenes and intermediate TOF1min, and hexacoordinate and octahedral catalyst to the most shielded alkylidene and lowest TOF1min. Analysis of the magnetic shielding tensors allowed for ascribing variations in the chemical shifts to electronic transitions between occupied molecular orbitals corresponding to the alkylidene‐C and alkylidene‐H σ‐bonds and the empty molecular orbital corresponding to the W‐alkylidene σ*‐bond. [ABSTRACT FROM AUTHOR]

Lewis acids increase the catalytic activity of classical heterogeneous catalysts and molecular d0 tungsten oxo alkylidenes in a variety of olefin metathesis processes. The formation of labile adducts between the metal complex and the Lewis acid has been observed experimentally and suggested to be involved in the catalyst activity increase. In this contribution, DFT (M06) calculations have been performed to determine the role of Lewis acids on catalyst activity, Z-/E- selectivity and stability by comparing three W(E)(CHR)(2,5-dimethylpyrrolide)(O-2,6-dimesithylphenoxide) (E = oxo, imido or oxo-Lewis acid adduct) alkylidenes. Results show that the formation of the alkylidene—Lewis acid adducts influences the reactivity of tungsten oxo alkylidenes due to both steric and electronic effects. The addition of the Lewis acid on the E group increases its bulkiness and this decreases catalyst Z-selectivity. Moreover, the interaction between the oxo ligand and the Lewis acid decreases the donating ability of the former toward the metal. This is important when the oxo group has either a ligand in trans or in the same plane that is competing for the same metal d orbitals. Therefore, the weakening of oxo donating ability facilitates the cycloaddition and cycloreversion steps and it stabilizes the productive trigonal bipyramid metallacyclobutane isomer. The two factors increase the catalytic activity of the complex. The electron donating tuneability by the coordination of the Lewis acid also applies to catalyst deactivation and particularly the key β-hydride elimination step. In this process, the transition states show a ligand in pseudo trans to the oxo. Therefore, the presence of the Lewis acid decreases the Gibbs energy barrier significantly. Overall, the optimization of the E group donating ability in each step of the reaction makes tungsten oxo alkylidenes more reactive and this applies both for the catalytic activity and catalyst deactivation. [ABSTRACT FROM AUTHOR]

Catalytic olefin metathesis based on the second- and third-row transition metals has become one of the most powerful transformations in modern organic chemistry. The shift to first-row metals to produce fine and commodity chemicals would be an important achievement to complement existing methods with inexpensive and greener alternatives. In addition, those systems can offer unusual reactivity based on the unique electronic structure of the base metals. In this Minireview, we summarize the progress of the development of alkylidenes and metallacycles of first-row transition metals from scandium to nickel capable of performing cycloaddition and cycloreversion steps, crucial reactions in olefin metathesis. In addition, we will discuss systems capable of performing olefin metathesis; however, the nature of active species is not yet known. [ABSTRACT FROM AUTHOR]

Schrock alkylidenes are highly versatile, very active olefin metathesis catalysts, but their pronounced sensitivity to air still hinders their applications. Converting them into more robust but inactive 18-electron adducts was suggested previously to facilitate their handling. Generating the active species from the inactive adducts, however, required a high-temperature Lewis acid treatment and resulted in an insoluble by-product, limiting the practicality of the methodology. Herein, we introduce an approach to circumvent the inconvenient, costly, and environmentally taxing activation process. We show that 18-electron adducts of W- and Mo-based Schrock catalysts with finite stability constants (typically K = 200–15,000 M−1) can readily be prepared and isolated in excellent yields. The adducts display enhanced air-stability in the solid state, and in solution they dissociate spontaneously, hence liberating the active alkylidenes without chemical assistance. Air-sensitive molybdenum olefin metathesis catalysts are known to form stable but inactive adducts with appropriate chelating ligands, but regenerating the active catalyst requires the use of Lewis acids and vigorous conditions. Here, tuning the electron donor capacity of the ligands leads to air-stable solid adducts of tungsten and molybdenum alkylidenes that spontaneously release the active catalysts in solution. [ABSTRACT FROM AUTHOR]

Two novel 18‐electron titanium germylene complexes, Cp2Ti(L)=Ge[Si3(SiMetBu2)4] 3 b (L=Me3P) and 3 c (L=XylNC), were synthesized, isolated, and structurally characterized. The length of the titanium‐germanium bonds of 2.5387(3) Å and 2.5276(3) Å (in 3 b and 3 c, respectively) well match those expected for the double bond, which was further supported by the DFT study. Based on their structural characteristics, as well as their atomic charges calculations which revealed Ti(δ+)=Ge(δ−) bond polarization, both 3 b and 3 c are classified as the Schrock‐type titanium germylidenes, as the germanium analogues of the widely known titanium alkylidenes. By contrast, germylene complexes of the group 6 metals 8 a (M=Mo) and 8 b (M=W) are better described as Fischer‐type germylene complexes. [ABSTRACT FROM AUTHOR]

Despite their excellent selectivities and activities, Mo‐and W‐based catalysts for olefin metathesis have not gained the same widespread use as Ru‐based systems, mainly due to their inherent air sensitivity. Herein, we describe the synthesis of air‐stable cationic‐at‐metal molybdenum and tungsten imido alkylidene NHC nitrile complexes. They catalyze olefin metathesis reactions of substrates containing functional groups such as (thio‐) esters, (thio‐) ethers and alcohols without the need for prior activation, for example, by a Lewis acid. The presence of a nitrile ligand was found to be essential for their stability towards air, while no decrease in activity and productivity could be observed upon coordination of a nitrile. Variations of the imido and anionic ligand revealed that alkoxide complexes with electron‐withdrawing imido ligands offer the highest reactivities and excellent stability compared to analogous triflate and halide complexes. [ABSTRACT FROM AUTHOR]

• Reported experimental RAIRS measurements suggesting alkylidene formation in Mo 2 C are confirmed by DFT simulations. • Surface C adatoms play an active role in the deoxygenation process, hosting alkylidene adsorption. • Alkylidene formation via deoxygenation can provide initiator sites for heterogeneously catalyzed metathesis reactions. There is evidence that there is a potential overlap between olefin metathesis and deoxygenation chemistries on molybdenum carbide catalysts. Ketones and aldehydes are known to undergo selective deoxygenation on Mo 2 C to form surface alkylidenes, and metal alkylidenes are initiator and propagator sites in olefin metathesis reactions. Moreover, a recent report correlated increased olefin metathesis activity with molybdenum carbide or oxycarbide formation on pre-treatment of a supported molybdenum oxide catalyst in methane. Taken together, these three strands prompt a DFT study of the elementary carbonyl bond scission step of selected aldehydes and ketones to form surface alkylidenes on Mo 2 C. This theoretical study is extended to an analysis of previously reported experimental reflectance infrared data for the interaction of cyclopentanone and acetaldehyde with a polycrystalline bulk hexagonal Mo 2 C sample. [ABSTRACT FROM AUTHOR]

Transition‐metal alkylidenes have exhibited wide applications in organometallic chemistry and synthetic organic chemistry, however, cyclic Schrock‐carbene‐like bis‐alkylidenes of group 4 metals with a four‐electron donor from an alkylidene have not been reported. Herein, the synthesis and characterization of five‐membered cyclic bis‐alkylidenes of titanium (4 a,b) and zirconium (5 a,b) are reported, as the first well‐defined group 4 metallacyclopentatrienes, by two‐electron reduction of their corresponding titana‐ and zirconacyclopentadienes. DFT analyses of 4 a show a four‐electron donor (σ‐donation and π‐donation) from an alkylidene carbon to the metal center. The reaction of 4 a with N,N′‐diisopropylcarbodiimide (DIC) leads to the [2+2]‐cycloaddition product 6. Compound 4 a reacted with CO, affording the oxycyclopentadienyl titanium complex 7. These reactivities demonstrate the multiple metal–carbon bond character. The reactions of 4 a or 5 a with cyclooctatetraene (COT) or azobenzene afforded sandwich titanium complex 8 or diphenylhydrazine‐coordinated zirconacyclopentadiene 9, respectively, which exhibit two‐electron reductive ability. [ABSTRACT FROM AUTHOR]

Reaction of CrCl2(N‐tBu)2 with 1,3‐dimethylimidazol‐2‐ylidene (IMe), 1,3‐dimethyl‐4,5‐dichloroimidazol‐2‐ylidene (IMeCl2), 1,3‐di(2‐propyl)imidazol‐2‐ylidene (IPr), 1,3‐dimesitylimidazol‐2‐ylidene (IMes) and 1,3‐bis(2,6‐(2‐Pr)2C6H3)imidazol‐2‐ylidene (IDipp) yields the corresponding N‐heterocyclic carbene (NHC) adducts CrCl2(IMe)(N‐tBu)2 (1), CrCl2(IMeCl2)(N‐tBu)2 (2), CrCl2(IPr)(N‐tBu)2 (3), CrCl2(IMes)(N‐tBu)2 (4) and CrCl2(IDipp)(N‐tBu)2 (5). Likewise, reaction of CrCl2(N‐2,6‐(2‐Pr)2C6H3)2 and CrCl2(N‐adamantyl)2 with IMes yields CrCl2(N‐2,6‐(2‐Pr)2C6H3)2(IMes) (6) and CrCl2(N‐adamantyl)2(IMes) (7), respectively. Reaction of CrCl2(N‐tBu)2 with the bidentate NHCs 1‐R‐3‐(1‐(2‐LiO‐C6H4))imidazol‐2‐ylidene yields the corresponding pentacoordinated Cr(VI) complexes CrCl2(1‐R‐3‐(1‐(2‐O‐C6H4))imidazol‐2‐ylidene)2C6H3)2(IMes) (R = 2,4,6‐(CH3)3C6H2, 8), (R = tBu, 9), (R = 2‐phenyl‐C6H4, 10). Reaction of the chromium(VI) complex Cr(N‐2,6‐(2‐Pr)2‐C6H3)2(CH2C(CH3)3)2 with 1,3‐dimethylimidazol‐2‐ylidene·AgI yields the bimetallic silver adduct of the chromium alkylidene complex (11) along with the tetrahedral chromium(V) complex CrI(N‐2,6‐(2‐Pr)2‐C6H3)2(1,3‐dimethylimidazol‐2‐ylidene) (12). Compounds 1–4, 7, 9–12 were characterized by single‐crystal X‐ray analysis. Finally, the chromium(VI) bisimido‐amido complexes 13–14 bearing the N‐6‐(2‐(diethylboryl)phenyl)pyridyl‐2‐yl‐motif are reported. [ABSTRACT FROM AUTHOR]

Despite their excellent selectivities and activities, Mo‐and W‐based catalysts for olefin metathesis have not gained the same widespread use as Ru‐based systems, mainly due to their inherent air sensitivity. Herein, we describe the synthesis of air‐stable cationic‐at‐metal molybdenum and tungsten imido alkylidene NHC nitrile complexes. They catalyze olefin metathesis reactions of substrates containing functional groups such as (thio‐) esters, (thio‐) ethers and alcohols without the need for prior activation, for example, by a Lewis acid. The presence of a nitrile ligand was found to be essential for their stability towards air, while no decrease in activity and productivity could be observed upon coordination of a nitrile. Variations of the imido and anionic ligand revealed that alkoxide complexes with electron‐withdrawing imido ligands offer the highest reactivities and excellent stability compared to analogous triflate and halide complexes. [ABSTRACT FROM AUTHOR]

Here we present our observations on a peculiar kinetic behavior observed at low catalyst loadings for the metathesis of internal olefins on a highly active silica‐supported tungsten oxo alkylidene prepared through surface organometallic chemistry (SOMC). The observed kinetic profile is comprised of two consecutive regimes, which support the presence of two different active sites on the surface of such catalysts. We propose that the more active site is operational in the beginning of the reaction but deactivates very fast, and the second one initiates over time due to the presence of ethylene formed subsequently under the reaction conditions. A combination of spectroscopic techniques and computations was used to identify the possible nature of these sites. Solid‐state NMR, EXAFS and DFT calculations are consistent with the esence of two distinct surface species that likely originate from the occurrence of slightly different local environments on the surface, that is, metal sites with a different number of nearby siloxane bridges. This data highlights the effects that may arise from the amorphous nature of the supports like silica and the difficulties to obtain truly single‐site catalysts – one of the ultimate goals of SOMC. [ABSTRACT FROM AUTHOR]

Grafting a molybdenum oxo alkylidene on silica (partially dehydroxylated at 700 °C) affords the first example of a well‐defined silica‐supported Mo oxo alkylidene, which is an analogue of the putative active sites in heterogeneous Mo‐based metathesis catalysts. In contrast to its tungsten analogue, which shows poor activity towards terminal olefins because of the formation of a stable off‐cycle metallacyclobutane intermediate, the Mo catalyst shows high metathesis activity for both terminal and internal olefins that is consistent with the lower stability of Mo metallacyclobutane intermediates. This Mo oxo metathesis catalyst also outperforms its corresponding neutral silica‐supported Mo and W imido analogues. [ABSTRACT FROM AUTHOR]

Formally dianionic ligands such as alkylidenes or organoimidos play a major role in the organometallic chemistry of transition metals and are an emerging topical area of f‐element chemistry. The pursuit and development of main‐group‐metal congeners has been tackled sporadically but is clearly lacking behind. The pronounced ionic bonding in particular, prevailing in alkali and alkaline‐earth (Ae) metal derivatives, proved cumbersome. Recent substantial progress in the respective field of divalent Ae chemistry has been triggered by the implementation of new synthesis strategies involving new AeII precursors and tailor‐made ligands. The main emphasis of this Minireview will be on the synthesis and reactivity of well‐defined Group 2 alkylidenes, organoimides, silylenes, and phosphandiides. [ABSTRACT FROM AUTHOR]

Five 21‐alkylidenes (2‐6) and a 21‐alkylol (7) derivatives of uscharin (1) were synthesized by aldol condensation reaction and aldol reaction of uscharin with aromatic aldehydes, respectively. We investigated the ability of uscharin and its derivatives to interfere with the intracellular calcium level by flow cytometry and contractile characteristics at the single cell level by time‐lapse microscopy in H9c2 cells. Changes of spontaneous calcium oscillations in beating HL‐1 cells were also studied by a high‐throughput Fluorometric Imaging Plate Reader (FLIPR) assay. We reported for the first time that uscharin could produce increased contraction strength in cardiac muscle cells while it showed a significant cytotoxicity to atrial cells. Unlike uscharin, 6 induced a marked increase of myocardial contractility, whereas 6 neither caused irregular frequency of oscillations nor cardiac cytotoxic effects under tested conditions. Structural modification on the butenolide moiety of uscharin showed a reduction of cardiac cytotoxicity and exhibited potential for use as an inotropic agent for the treatment of heart failure. [ABSTRACT FROM AUTHOR]

[Display omitted] • Phenylacetylene reactions mediated by the 3rd generation Grubbs catalyst are studied. • Pathways for phenylacetylene dimerization and cyclotrimerization are calculated. • Ruthenium vinylidene species can be the key intermediate in dimerization. • [RuCl 2 (H 2 IMes)] can be the active species in cyclotrimerization. • Overall mechanism for terminal arylalkyne transformations is proposed. Transformations of alkynes are of great importance in organic synthesis, including formation of natural compounds. Products of C C coupling of terminal alkynes, such as conjugated 1,3- or 1,4-enynes and tri-substituted benzene derivatives are key units of variety natural and biologically active products. Ruthenium catalysts can be effective for such synthesis, however, the mechanisms of these reactions are still not well understood. In this work, dimerization and cyclotrimerization of terminal arylalkyne (phenylacetylene) initiated by the third-generation Grubbs catalyst [RuCl 2 (CHPh)(3-Br-py) 2 (H 2 IMes)] have been theoretically and experimentally studied. It is shown that ruthenium vinylidene species might be the key intermediate in the catalytic cycle of phenylacetylene dimerization, whereas [RuCl 2 (H 2 IMes)] can be the active species in phenylacetylene cycylotrimerization. Ruthenium alkyne species [RuCl 2 (η2 PhC CH)(H 2 IMes)] is a common intermediate for both competitive reactions. An overall mechanism for terminal arylalkyne transformations is proposed. It includes alkyne-induced decomposition of the phosphine-free ruthenium alkylidene catalyst via ruthenium vinylcarbene to ruthenium vinylidene and [RuCl 2 (H 2 IMes)], followed by the dimerization and cyclotrimerization reactions. [ABSTRACT FROM AUTHOR]

Isolable cationic PtII and PtIV alkylidenes, proposed intermediates in catalytic organic transformations, are reported. The bonding in these species was probed by experimental, structural, spectroscopic, electrochemical and computational methods, providing direct evidence for π‐bonding, the often‐theorized relativistic stabilization of these species, and the influence of oxidation state. It's all relative: A homologous series of Group 10 PCcarbeneP complexes were prepared and the bonding in the M=C moiety probed by experimental and computational techniques to show that relativistic stabilization is significant in the platinum carbene. [ABSTRACT FROM AUTHOR]

Thiolate-coordinated ruthenium alkylidene complexes can give high Z-selectivity and stereoretentivity in olefin metathesis. To investigate their applicability as heterogeneous catalysts, we have successfully developed a methodology to easily immobilize prototype ruthenium alkylidenes onto hybrid mesostructured silica via a thiolate tether. In contrast, the preparation of the corresponding molecular complexes appeared very challenging in solution. These prototype supported complexes contain small thiolates but still, they are slightly more Z-selective than their molecular analogues. These results open the door to more active and selective heterogeneous catalysts by supporting more advanced thiolate Ru-complexes. [ABSTRACT FROM AUTHOR]

The metal-free carboxylative cyclization of propargylic amides with CO2 to oxazolidine-2,4-diones was achieved for the first time employing 1,5,7-triaza-bicyclo-[4.4.0]dec-5-ene (TBD) as an organocatalyst. This method allows for the efficient and selective synthesis of a variety of (Z) 5-alkylidene 1,3-oxazolidine-2,4-diones, and a variety of functional groups are well-tolerated under mild reaction conditions. Theoretical studies reveal that the bifunctional activity (base/H-bond donor) of TBD plays a key role in accelerating this reaction. [ABSTRACT FROM AUTHOR]

The synthesis of 3‐alkyliden‐2,3‐dihydro‐4‐quinolones has been accomplished in a domino fashion through a three‐step sequence that comprised an initial aza‐Baylis‐Hillman reaction, followed by a 1,3‐rearrangement and an intramolecular amination. Starting from readily available aryl vinyl ketones and N‐tosyl imines, the reaction with PPh3, CsOAc and CuI in CH3CN gave rise, in good overall yields, to final 3‐alkyliden‐4‐quinolone derivatives, valuable scaffolds in medicinal chemistry. The simultaneous addition of two bases, PPh3 and CsOAc, was found to be crucial for the success of the process. While PPh3 promoted the reversible aza‐Baylis‐Hillman reaction, CsOAc triggered the subsequent 1,3‐rearrangement, which shifted the initial equilibrium and allowed to complete the synthetic sequence upon the addition of CuI. [ABSTRACT FROM AUTHOR]

Facile α‐H elimination from tetrakis(trimethylsilylmethyl)titanium precursors to give adducts of (alkylidene)bis(alkyl)titanium complexes is induced by light alkali metal amides of the NNNN‐type macrocyclic anionic ligand Me3TACD [(Me3TACD)H=1,4,7‐trimethyl‐1,4,7,10‐tetraazacyclododecane]. In the crystal, the alkali metal interacts with the carbene carbon atom or with the CH2 group of the trimethylsilymethyl ligand. The nucleophilic character of the carbene carbon atom was shown by the reaction with benzophenone and terminal acetylenes. Facile α‐H elimination from tetrakis(trimethylsilylmethyl)titanium is induced by alkali metal amides to give adducts of (alkylidene)bis(alkyl)titanium complexes. In the crystal, the alkali metal interacts with the carbene carbon atom or with the CH2 group of the trimethylsilymethyl ligand. The nucleophilic character of the carbene carbon atom was shown by reactions with benzophenone and terminal acetylenes. [ABSTRACT FROM AUTHOR]

In this personal account, our recent studies of novel synthetic methods of 3,4‐fused tricyclic indole derivatives using 3‐alkylidene indoline derivatives as versatile precursors are discussed. Two types of cascade reactions producing 3,4‐fused tricyclic 3‐alkylidene indolines were developed based on a palladium‐catalyzed intramolecular Heck insertion to an allene‐allylic amination cascade and a platinum‐catalyzed intramolecular Friedel‐Crafts type C−H coupling‐allylic amination cascade. Furthermore, three types of 3,4‐fused tricyclic indoles were accessible from a single 3‐alkylidene indoline precursor via acid‐promoted olefin isomerization or oxidative treatments. The application of the developed methods to the synthesis of natural products bearing a 3,4‐fused tricyclic indole skeleton, (−)‐aurantioclavine, fargesine, and synthetic studies of dragmacidin E are also highlighted. Our recent studies of novel synthetic methods of 3,4‐fused tricyclic indoles using 3‐alkylidene indoline derivatives as versatile precursors are discussed. Two types of cascade cyclizations producing 3,4‐fused tricyclic 3‐alkylidene indolines were developed using palladium catalysis and platinum catalysis. These compounds could be efficiently converted into the tricyclic indole derivatives. The developed reactions were successfully applied to the synthesis of related indole alkaloids. [ABSTRACT FROM AUTHOR]

Numerous organic transformations are based on the use of stoichiometric amounts of phosphorus reagents. The formation of phosphane oxides from phosphanes is usually the thermodynamic driving force for these reactions. The stoichiometric amounts of phosphane oxide which are formed as by-products often significantly hamper the product purification. Organophosphorus catalysis based on P(III)/P(V) redox cycling aims to address these problems. Herein we present our recent advances in developing catalytic Wittig-type reactions. More specifically, we reported our results on catalytic Wittig reactions based on readily available Bu3P=O as pre-catalyst as well as the first microwave-assisted version of this reaction and the first enantioselective catalytic Wittig reaction utilizing chiral phosphane catalysts. Further developments led to the implementation of catalytic base-free Wittig reactions yielding highly functionalized alkylidene and arylidene succinates. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A-Ring fragment's synthesis started from cyclohexanone derived from quinic acid. • 19-Norcalcitriol analogs with pegylated 2-alkylidene groups were synthesized. • 2-Alkylidene chain is terminated by hydroxyl or 2-(pyridin-2′-yl)ethylamino group. • The affinities of the synthesized 19-norcalcitriol analogs to VDR were assessed. Abstract The results presented in this paper constitute an extension of our synthetic efforts focused on 19-norvitamin D compounds possessing elongated 2-alkylidene substituents. Based on our previous results, molecular modeling studies, and docking experiments, we selected a novel 19-norcalcitriol analog with long chain at C-2 containing several ether moieties and terminated by 2-(pyridin-2′-yl)ethylamino fragment. It was expected that such structural modification might allow binding of transition metal by the ligand, increase solubility of the formed complexes as well as improve their affinity to the VDR. For comparison, a 19-norcalcitriol analog was also obtained with the terminal hydroxyl group at its pegylated 2-alkylidene substituent. The synthesis of the target vitamin D compounds described in this work was performed using the Wittig-Horner approach. The respective A-ring phosphine oxide was obtained starting from the D -(-)-quinic acid and then coupled with the known Grundmann ketone. [ABSTRACT FROM AUTHOR]

Highly active catalysts for the ring-opening metathesis polymerization (ROMP) of cyclic olefins (norbornene, cyclopentene, cycloheptene) can be generated in situ by premixing V(CHSiMe3)(NC6F5)(O-2,6-iPr2C6H3)(PMe3)2 (1) with 1.0 equiv. of C6F5OH or C6Cl5OH via the immediate phenoxy exchange; the activity was affected by the kind of phenol added [activity (TOF) in the ROMPs of norbornene: 46 200 min−1 (upon addition of C6F5OH) vs. 37.3 min−1 (by 1)]. [ABSTRACT FROM AUTHOR]

In this study, stereoselective conjugate addition of ketones to alkylidene malonates using organocatalyst has been developed. The reaction in the presence of 20 mol% of a novel thiourea‐sulfonamide organocatalyst afforded conjugate adducts in moderate to high yields (up to 81%) under mild reaction conditions. Excellent diastereoselectivity (up to 98:2 dr) and enantioselectivity (up to 88% ee) were achieved. [ABSTRACT FROM AUTHOR]

• Kinetics of ring-opening metathesis polymerization with vanadium initiators varies greatly as a function of ligand environment. • Non-covalent interactions play a previously unknown role in stabilizing or destabilizing transition states. • Anti -alkylidenes react faster than syn -alkylidenes. • Functional groups are poorly tolerated in ring-opening metathesis polymerization with vanadium initiators. Ring-opening metathesis polymerization (ROMP) with vanadium(V) alkylidene initiators bearing diverse ligand sets is presented, along with analyses of factors governing the activity and kinetics for each. The first DFT calculations on ROMP with vanadium are executed and show that non-covalent interactions between supporting ligands and the alkylidene have drastic effects on rates of initiation and propagation, specifically with both stabilizing and destabilizing interactions occurring in transition states. This allows for rationalization of the kinetics inferred from the molar mass distributions. Furthermore, we demonstrate that the metallocyclobutane forms in a manner quite distinct from group 6 analogues. NMR experiments empirically support the computational findings and allow for important insights for future initiator design. Termination of growing polymer chains is shown to not play any role in the observed broad molar mass distributions, which has been assumed but not proven experimentally until now, and for the first time with vanadium the syn- alkylidenes are shown experimentally to be less reactive than their anti- counterparts, analogous to group 6 alkylidenes. Functional group tolerance, which has yet to be investigated in ROMP with vanadium-based initiators, is also explored for each class of initiators. In general, the more electron poor chlorido ligated complexes were less tolerant to functional groups. [Display omitted] [ABSTRACT FROM AUTHOR]

Silica‐supported cationic Mo‐imido alkylidene N‐heterocyclic carbene catalysts, prepared by surface organometallic chemistry, display contrasting olefin metathesis activity for terminal and internal olefins. The high metathesis activity towards terminal alkenes is attributed to the strong σ‐donating property of the NHC ancillary ligand, which disfavors the formation of the parent square‐planar metallacyclobutane, an off‐cycle reaction intermediate resulting from the reaction with ethylene, one of the metathesis products. This tailored ligand environment also allowed the first trigonal bipyramidal (TBP) metallacyclobutane reaction intermediate for supported Mo metathesis catalysts to be identified. [ABSTRACT FROM AUTHOR]

Cyclopropenylcarbinyl cyanates, generated in situ by dehydration of the corresponding carbamates, underwent an efficient and stereoselective [3,3]‐sigmatropic rearrangement leading to the corresponding alkylidene(isocyanatocyclopropanes), which could be converted into various alkylidene(aminocyclopropane) derivatives in a one‐pot manner. This transformation complements the repertoire of sigmatropic rearrangements involving cyclopropenylcarbinol derivatives and in particular, the previously reported Overman rearrangement of cyclopropenylcarbinyl trichloroacetimidates. Cyclopropenylcarbinyl cyanates, generated in situ by dehydration of the corresponding carbamates, undergo an efficient and stereoselective [3,3]‐sigmatropic rearrangement leading to the corresponding alkylidene(isocyanatocyclopropanes) that can be readily converted into various alkylidene(aminocyclopropane) derivatives in a one‐pot manner. [ABSTRACT FROM AUTHOR]

The α-methylene-γ-butyrolactone skeleton is part of many natural products with remarkable biological properties. C75, a fatty acid synthase (FAS) inhibitor, is a synthetic α-methylene-γ-butyrolactone which is cytotoxic for different cell lines in vitro and in vivo against human cancer xenografts. We synthesized a panel of C75 derivatives through modifications of the a-methylene moiety at the C-ß/C-γ carbons of the γ-butyrolactone skeleton and tested their effects on different cancer models. The in vitro effects of the compounds on pancreatic, colon and breast cancer cell viability (PANC-1, HTC-116, Lovo, MCF-7, MDA-231-MB) were evaluated. Two compounds showed significant cytotoxicity. The structures of all synthesized compounds obtained were identified on the basis of their spectral data (IR, MS, NMR). [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Novel synthesis of functionalized cyclopenta[ c ]chromene derivatives. • One-pot and multi-component diastereoselective synthesis of cyclopenta[ c ]chromenes. • Synthesis under mild conditions, excellent atom economy, ease of work-up and high yields of the products. Abstract A novel one-pot, multi-component diastereoselective synthesis of functionalized cyclopenta[ c ]chromenes is described. This transformation proceeds via a Michael addition-cyclization reaction sequence between trialkyl phosphites, dialkyl acetylenedicarboxylates and in situ generated 2-(1-(2-oxo-2 H -chromen-3-yl)alkylidene)malononitriles from the condensation of 3-acyl-2 H -chromen-2-ones and malononitrile. [ABSTRACT FROM AUTHOR]

Abstract: This article outlines the fundamental concept to molybdenum imido, tungsten imido and tungsten oxo alkylidene N‐heterocyclic carbene (NHC) catalysts for olefin metathesis. Starting from Mo(NR)(CHCMe2R′)(OTf)2⋅DME (R=alkyl, aryl; R′=CH3, Ph; OTf=CF3SO3; DME=1,2‐dimethoxyethane), W(NR)(CHCMe2Ph)X2 (X=pyrrolide, OR′′, SiPh3) and W(O)Cl2(CHCMe2Ph)(PMe2Ph)2, respectively, the neutral, pentacoordinated 16‐valence electron (VE) complexes became accessible. These serve as precursors for the corresponding cationic, tetracoordinated 14‐VE catalysts. This class of olefin metathesis catalysts based on group 6 metal alkylidene NHCs is characterized by a broad structural variability, high activity, high productivity and, in tailored systems, high functional group tolerance even vs. protic groups including alcohols and carboxylic acids. Also, high regio‐ and stereoselectivity is achieved, both in ring‐opening metathesis polymerization (ROMP) and in the cyclopolymerization of α,ω‐diynes. Fundamentals concerning the influence of the imido/oxo ligand, the different NHCs and the anionic ligands on catalyst performance and stability as well as reaction pathways are summarized. Also, the concepts for catalyst immobilization, whether via the NHC or by substitution of an alkoxide, are presented. Finally, special features such as the use of betaine‐type anionic ligands (alkoxides and sulfonates), which offer access to ionic catalysts for use biphasic reaction setups, are briefly outlined. [ABSTRACT FROM AUTHOR]

Abstract: A series of monomeric rare‐earth metal silyl‐thiophosphinoyl‐alkylidene complexes [LLn{C(SiR3)PPh2S}] (5: Ln=Lu, R=Me; 6: Ln=Lu, R=Ph; 7: Ln=Y, R=Me; 8: Ln=Y, R=Ph; 9: Ln=Sm, R=Ph; 10: Ln=Sm, R=Me; 11: Ln=La, R=Ph; L=[MeC(NDIPP)CHC(Me)(NCH2CH2N(Me)2)]−, DIPP=2,6‐(iPr)2C6H3) have been synthesized and structurally characterized. The influences of rare‐earth metal ions, ancillary ligands, and alkylidene groups on the reactivity of complexes 5–11 and the related scandium complexes [LSc{C(SiR3)PPh2S}] (1: R=Me; 2: R=Ph) and [L′Sc{C(SiR3)PPh2S}] (3: R=Me; 4: R=Ph; L′=[MeC(NDIPP)CHC(Me)(NCH2CH2N(iPr)2)]−) have been studied. Reactions of these rare‐earth metal alkylidene complexes with PhCN give four kinds of products, the formation of which is dependent on the rare‐earth metal ions, ancillary ligands, and alkylidene groups of the complexes. In the reactions with tBuNC, unusual C−P bond cleavage of the alkylidene group and C≡C triple bond formation occur. Complexes 10 and 11 also react with PhSiH3 to form hydrides, which subsequently undergo Ln−H addition to the C=N bond of the ancillary ligand L. DFT calculations have been used to analyze the bonding in complex 10, which exhibits a polarized three centers Sm−C−P π interaction, and to rationalize the reactivity by computing reaction mechanisms. The difference in reactivity of PhCN and tBuNC is due to the electron density delocalization that is enabled by the phenyl group rather than the tBu group. [ABSTRACT FROM AUTHOR]

Abstract: The pentacoordinated, 16‐valence electron (VE) Mo imido alkylidene N‐heterocyclic carbene (NHC) complexes I1–I5 and the hexacoordinated 18‐VE Mo imido alkylidene NHC complexes 1–4, 8, 10 and 12 containing a chelating ligand have been prepared and used as thermally latent catalysts in the ring‐opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD). Both 10 and 12 are the first Mo imido alkylidene complexes with a chelating alkylidene featuring a carboxylate group. Complexes I1–I3 and I5 as well as 1–4 proved to be fully thermally latent in the presence of DCPD. With the changes in both the electronic and steric situation at the imido ligand provided by these pre‐catalysts, different temperatures of the onset of polymerization (Tonset=65–140 °C) and for the exothermic maximum of the curing curve (Texo,max=98–183 °C) of DCPD were achieved. Also, the degree of crosslinking was successfully varied as indicated by swelling experiments in toluene, which revealed degrees of swelling between 0 and 50 %. While the introduction of a chelating alkylidene increases Tonset, the introduction of more electron‐donating anionic ligands (tert‐butoxide, phenoxide) resulted in a drastic reduction in Tonset, underlining the high flexibility of these systems. The hexacoordinated high‐oxidation state molybdenum imido alkylidene NHC complexes 2, 3 and 4 were stable under air for at least twelve hours in the solid state. [ABSTRACT FROM AUTHOR]

FeCl 3 ·6H 2 O/triethylsilane composite catalyst system is successfully developed for the selective conjugate reduction of carbon-carbon double bond of Michael acceptor-alkylidene- β -keto esters and alkylidene-1,3-diketones under mild reaction conditions to afford the corresponding saturated β -keto esters and 1,3-diketones. The process involves the iron-catalyzed hydrosilylation, followed by in situ hydrolysis of silyl enol ether. The optimal reaction conditions include 20 mol% of FeCl 3 ·6H 2 O and triethylsilane in dichloromethane at room temperature. A broad range of substrates undergoes the reduction in 1, 4-selective manner to afford the corresponding saturated compounds in excellent yields. [ABSTRACT FROM AUTHOR]

Although generally the ruthenium based homogeneous catalysts are used to initiate metathesis reactions, immobilization of homogeneous ruthenium metathesis catalysts on various solid supports has attracted a lot of interest from the scientific community in recent years. In this article, the synthesis and applications of highly active polyHIPE-supported N-heterocyclic carbene ligand (NHC)-bearing ruthenium alkylidene metathesis catalysts were described. Open-celled porous polyHIPE foams having surface area of 575 m2/g were synthesized by polymerization of the continuous phase of w/o high internal phase emulsions. Then RuCl2(PCy3)(H2IMes)(=CHPh) complex was attached to porous polyHIPE solid supports via alkylidene exchange reactions to obtain new olefin metathesis catalysts. The amount of initiator loadings was determined by ruthenium analysis via ICP-OES and found to be 0.0336-0.0514 mmol/g. Subsequently, the activity of new catalysts was tested in ring opening metathesis polymerization of norbornene and a series of its derivatives.Graphical Abstract: [ABSTRACT FROM AUTHOR]

Abstract: Helical chirality is a novel enantioselectivity‐inducing property in transition‐metal‐catalyzed transformations. The principle is illustrated herein for the example of asymmetric olefin metathesis. This work reports the synthesis of the first helically chiral Ru‐NHC alkylidene complex from an aminohelicene‐derived imidazolium salt, which was ligated to the first generation Hoveyda–Grubbs catalyst. Kinetic data were acquired for benchmark test reactions and compared to an achiral catalyst. The helically chiral Ru‐catalyst was evaluated in asymmetric ring‐closing metathesis (RCM) and ring‐opening metathesis–cross‐metathesis (ROM/CM) reactions, which proceeded with promising levels of enantioselectivity. Extensive NMR‐spectroscopic investigations and a DFT geometry optimization were performed. These results led to a topographic steric map and calculation of percent‐buried‐volume values for each quadrant around the metal center. [ABSTRACT FROM AUTHOR]

Abstract: A 1,4‐addition reaction of 5H‐oxazol‐4‐ones to alkylidene β‐ketoesters, which was catalyzed by using chiral guanidines via a dynamic kinetic resolution process that involved geometric isomerization of the alkylidene β‐ketoesters, was developed. This method allowed using E,Z isomeric mixture of various acyclic alkylidene β‐ketoesters to obtain the products stereoselectively. The relation between the geometry and the stereoselectivity of products was investigated, and the derivatization of the products to the corresponding α‐acyl‐γ‐lactone was performed. [ABSTRACT FROM AUTHOR]

Chiral β‐lactams and cyclobutanones are present in numerous natural and pharmaceutical products. The stereoselective construction of chiral four‐membered cyclic compounds is an ongoing challenge for the chemical community. Herein, we report a highly stereocontrolled construction of four‐membered ring (mini‐sized) β‐lactams and cyclobutanones via an Ir/In‐BiphPHOX‐catalyzed asymmetric hydrogenation, providing the corresponding optically active four‐membered ring carbonyl products bearing an α‐chiral carbon center with excellent yields (up to 99%) and enantioselectivities (up to 98%) under mild reaction conditions (1.0—2.5 bar H2 for 1.0—10 h). The reaction presents wide substrate scope. Diverse transformations of the catalyzed products were also conducted to show the potential utility of this protocol. [ABSTRACT FROM AUTHOR]