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The germylone dimNHCGe (5, dimNHC=diimino N‐heterocyclic carbene) undergoes a [2+2] cycloaddition with isocyanates RNCO (R=4‐tolyl or 3,5‐xylyl) to furnish novel alkyl carboxamido germylenes 7 (R=4‐tolyl) and 8 (R=3,5‐xylyl), featuring a C−C bond between the former carbene carbon and the isocyanate moiety. Heating a mixture of 8 with 4‐tolyl isocyanate to 100 °C results in isocyanate metathesis, demonstrating reversible C−C bond formation on the reduced germanium compound. DFT calculations suggest that this process occurs via the reductive dissociation of isocyanate from 8 that regenerates the parent Ge(0) compound 5. [ABSTRACT FROM AUTHOR]

We describe the facile synthesis of [(Me3Si)2CH]2E=PMes* (E=Ge, Sn) from the reaction of the tetrylenes with the phospha‐Wittig reagent, Me3P−PMes*. Their reactivity towards a range of substrates with protic and hydridic E−H bonds (E=N, O, Si) is described. In addition to hydroelementation reactions of the E=P bonds, we show that these compounds, particularly [(Me3Si)2CH]2Sn=PMes*, also act as base‐stabilized phosphinidenes, allowing phosphinidene transfer to other nucleophiles. [ABSTRACT FROM AUTHOR]

Covalent organic frameworks (COFs) offer vast structural and chemical diversity enabling a wide and growing range of applications. While COFs are well-established as heterogeneous catalysts, so far, their high and ordered porosity has scarcely been utilized to its full potential when it comes to spatially confined reactions in COF pores to alter the outcome of reactions. Here, we present a highly porous and crystalline, large-pore COF as catalytic support in α,ω-diene ring-closing metathesis reactions, leading to increased macrocyclization selectivity. COF pore-wall modification by immobilization of a Grubbs-Hoveyda-type catalyst via a mild silylation reaction provides a molecularly precise heterogeneous olefin metathesis catalyst. An increased macro(mono)cyclization (MMC) selectivity over oligomerization (O) for the heterogeneous COF-catalyst (MMC:O=1.35) of up to 51 % compared to the homogeneous catalyst (MMC:O=0.90) was observed along with a substrate-size dependency in selectivity, pointing to diffusion limitations induced by the pore confinement.

Terminal aluminium and gallium imides of the type K[(NON)M(NR)], bearing heteroatom substituents at R, have been synthesised via reactions of anionic aluminium(I) and gallium(I) reagents with silyl and boryl azides (NON=4,5‐bis(2,6‐diisopropyl‐anilido)‐2,7‐di‐tert‐butyl‐9,9‐dimethyl‐xanthene). These systems vary significantly in their lability in solution: the N(SiiPr3) and N(Boryl) complexes are very labile, on account of the high basicity at nitrogen. Phenylsilylimido derivatives provide greater stabilization through the π‐acceptor capabilities of the SiR3 group. K[(NON)AlN(SitBuPh2)] offers a workable compromise between stability and solubility, and has been completely characterized by spectroscopic, analytical and crystallographic methods. The silylimide species examined feature minimal π‐bonding between the imide ligand and aluminium/gallium, with the HOMO and HOMO‐1 orbitals effectively comprising orthogonal lone pairs centred at N. Reactivity‐wise, both aluminium and gallium silylimides can act as viable sources of nitride, [N]3−, with systems derived from either metal reacting with CO to afford cyanide complexes. By contrast, only the gallium system K[(NON)Ga{N(SiPh3)}] is capable of effecting a similar transformation with N2O to yield azide, N3−, via formal oxide/nitride metathesis. The aluminium systems instead generate RN3 via transfer of the imide fragment [RN]2−. [ABSTRACT FROM AUTHOR]

As the first known example of ring‐opening cross metathesis (ROCM) of polyfluorinated strained cyclobutenes, ROCM of 3,3,4,4‐tetrafluorocyclobutene with electronically rich alkenes, catalyzed by Grubbs or Hoveyda‐Grubbs 2nd generation precatalysts, gave a small library of non‐symmetrical isolated dienes bearing a tetrafluoroethylene spacer between the double bonds. 1‐Butoxy‐3,3,4,4‐tetrafluorohexa‐1,5‐diene thus formed underwent subsequent regioselective cross metathesis (CM) with a series of styrenes, catalyzed by Hoveyda‐Grubbs 2nd generation precatalyst, leading to non‐symmetrically substituted dienes. 6,6‐Dibutoxy‐3,3,4,4‐tetrafluorohex‐1‐ene, formed by regioselective butoxylation of 1‐butoxy‐3,3,4,4‐tetrafluorohexa‐1,5‐diene, was dihydroxylated and cyclized to the corresponding 3,3,4,4‐tetrafluorohexopyranose. [ABSTRACT FROM AUTHOR]

The reactions of a terminal aluminium imide with a range of oxygen‐containing substrates have been probed with a view to developing its use as a novel main group transfer agent for the [NR]2− fragment. We demonstrate transfer of the imide moiety to [N2], [CO] and [Ph(H)C] units driven thermodynamically by Al−O bond formation. N2O reacts rapidly to generate the organoazide DippN3 (Dipp=2,6‐iPr2C6H3), while CO2 (under dilute reaction conditions) yields the corresponding isocyanate, DippNCO. Mechanistic studies, using both experimental and quantum chemical techniques, identify a carbamate complex K2[(NON)Al‐{κ2−(N,O)−N(Dipp)CO2}]2 (formed via [2+2] cycloaddition) as an intermediate in the formation of DippNCO, and also in an alternative reaction leading to the generation of the amino−dicarboxylate complex K2[(NON)Al{κ2−(O,O′)−(O2C)2N‐(Dipp)}] (via the take‐up of a second equivalent of CO2). In the case of benzaldehyde, a similar [2+2] cycloaddition process generates the metallacyclic hemi‐aminal complex, Kn[(NON)Al{κ2−(N,O)−(N(Dipp)C(Ph)(H)O}]n. Extrusion of the imine, PhC(H)NDipp, via cyclo‐reversion is disfavoured thermally, due to the high energy of the putative aluminium oxide co‐product, K2[(NON)Al(O)]2. However, addition of CO2 allows the imine to be released, driven by the formation of the thermodynamically more stable aluminium carbonate co‐product, K2[(NON)Al(κ2−(O,O′)−CO3)]2. [ABSTRACT FROM AUTHOR]

In this work an approach for the synthesis of furanocembranoid natural products containing the C‐7,8‐diol moiety is disclosed. This culminated in the first total synthesis of the natural product molestin E, together with ent‐sinulacembranolide A and ent‐sinumaximol A as well as a thorough exploration of their chemistry. Late‐stage ring‐closure of the C‐7,8‐diols to the corresponding epoxides was also demonstrated. Key features of this synthetic strategy include a stereoselective Baylis‐Hillman reaction, ring‐closing metathesis and Shiina macrolactonisation. Chiral‐pool materials were deployed to ensure the desired absolute stereochemistry which was confirmed by late‐stage single crystal X‐ray diffraction. [ABSTRACT FROM AUTHOR]

Boron Lewis acid‐catalyzed and catalyst‐free hydroboration reactions of imines are attractive due to the mild reaction conditions. In this work, the mechanistic details of the hydroboration reactions of two different kinds of imines with pinacolborane (HBpin) are investigated by combining density functional theory calculations and some experimental studies. For the hydroboration reaction of N‐(α‐methylbenzylidene)aniline catalyzed by tris[3,5‐bis(trifluoromethyl)phenyl]borane (BArF3), our calculations show that the reaction proceeds through a boron Lewis acid‐promoted hydride transfer mechanism rather than the classical Lewis acid activation mechanism. For the catalyst‐ and solvent‐free hydroboration reaction of imine, N‐benzylideneaniline, our calculations and experimental studies indicate that this reaction is difficult to occur under the reaction conditions reported previously. With a combination of computational and experimental studies, we have established that the commercially available BH3 ⋅ SMe2 can serve as an efficient catalyst for the hydroboration reactions of N‐benzylideneaniline and similar imines. The hydroboration reactions catalyzed by BH3 ⋅ SMe2 are most likely to proceed through a hydroboration/B−H/B−N σ‐bond metathesis pathway, which is very different from that of the reaction catalyzed by BArF3. [ABSTRACT FROM AUTHOR]

The catalytic metathesis of C=C bonds is a textbook reaction that has no parallel in the widely studied area of multiple bonds involving heavier p‐block elements. A high‐yielding P=C bond metathesis of phosphaalkenes (ArP=CPh2, Ar=Mes, o‐Tol, Ph) has been discovered that is catalyzed by N‐heterocyclic carbenes (NHC=Me2IMe, Me2IiPr). The products are cyclic oligomers formally derived from ArP=PAr [i. e. cyclo‐(ArP)n; n=3, 4, 5, 6] and Ph2C=CPh2. Preliminary mechanistic studies of this remarkable transformation have established NHC=PAr (Ar=Mes, o‐Tol, Ph) as key phosphinidene transfer agents. In addition, novel cyclic intermediates, such as, cyclo‐(ArP)2CPh2 and cyclo‐(ArP)4CPh2 have also been observed. This work represents a rare application of non‐metal‐based catalysts for transformations involving main‐group elements. [ABSTRACT FROM AUTHOR]

The newly discovered light‐driven gem hydrogenation of alkynes opens an unconventional yet efficient entry into five‐coordinate Grubbs‐type ruthenium carbene complexes with cis‐disposed chloride ligands. Representatives of this class featuring a chelate substructure formed by an iodo‐substituted benzylidene unit react with (substituted) 2‐isopropoxystyrene to give prototypical "second‐generation" Grubbs‐Hoveyda complexes for olefin metathesis. The new approach to this venerable catalyst family is safe and versatile as it uses a triple bond rather than phenyldiazomethane as the ultimate carbene source and does not require any sacrificial phosphines. [ABSTRACT FROM AUTHOR]

Vanadium‐based catalysts have shown activity and selectivity in ring‐opening metathesis polymerization of strained cyclic olefins comparable to those of Ru, Mo, and W catalysts. However, the application of V alkylidenes in routine organic synthesis is limited. Here, we present the first example of ring‐closing olefin metathesis catalyzed by well‐defined V chloride alkylidene phosphine complexes. The developed catalysts exhibit tolerance to various functional groups, such as an ether, an ester, a tertiary amide, a tertiary amine, and a sulfonamide. The size and electron‐donating properties of the imido group and the phosphine play a crucial role in the stability of active intermediates. Reactions with ethylene and olefins suggest that both β‐hydride elimination of the metallacyclobutene and bimolecular decomposition are responsible for catalyst degradation. [ABSTRACT FROM AUTHOR]

2,3‐Diarylbenzo[b]arsoles were synthesized from zirconacycles and diiodophenylarsine. The structural modification to the luminophore was attained through diarylacetylene precursors, Suzuki–Miyaura coupling, and oxidation of the arsenic atom. The emission properties were controlled according to these modifications. The 2,3‐diarylbenzo[b]arsoles showed aggregation‐induced emission enhancement; the stronger emission was observed in the solid states than in solutions. In addition, Suzuki–Miyaura polycondensation and olefin metathesis polymerization produced main‐ and side‐chain polymers, respectively. The resultant polymers showed different emission behaviors such as aggregation caused quenching and aggregation induced emission enhancement. [ABSTRACT FROM AUTHOR]

The transition metal catalysed formation and cleavage of C−C bonds is of utmost importance in synthetic chemistry. While most of the existing homogeneous catalysts are mononuclear, knowledge of the behaviour of polynuclear species is much more limited. By using computational methods, here we shed light into the mechanistic details of the thermally‐induced isomerization of Cp*3Ru3(μ‐H)2(μ3‐η2‐pentyne)(μ3‐pentylidyne) (2) into Cp*3Ru3(μ‐H)2(μ3‐η2‐octyne)(μ3‐ethylidyne) (3), a process that involves the migration of a C3 fragment between the hydrocarbyl ligands and across the plane formed by the three Ru centres. Our results show this to be a complex transformation that comprises of five individual rearrangements in an A→B→A→B→A order. Each so‐called rearrangement A consists of the CH migration from the μ3‐η2‐alkyne into the μ3‐alkylidine ligand in the other side of the Ru3 plane. This process is facilitated by the cluster's ability to adopt open‐core structures in which one Ru−Ru bond is broken and a new C−C bond is formed. In contrast, rearrangements B do not involve the formation or cleavage of C−C bonds, nor do they require the opening of the cluster core. Instead, they consist of the isomerization of the μ3‐η2‐alkyne and μ3‐alkylidyne ligands on each side of the triruthenium plane into μ3‐alkylidyne and μ3‐η2‐alkyne, respectively. Such transformation implies the migration of three H atoms within the hydrocarbyl ligands, and in this case, it is aided by the cluster's ability to behave as a H reservoir. All in all, this study highlights the plasticity of these Ru3 clusters, whereby Ru−Ru, Ru−C, Ru−H, C−C, and C−H bonds are formed and broken with surprising ease. [ABSTRACT FROM AUTHOR]

Ruthenium vinyl carbenes derived from Cp/Cp*RuCl‐based complexes (Cp=cyclopentadiene, Cp*=1,2,3,4,5‐pentamethylcyclopentadiene) have been routinely invoked as key intermediates in tandem reactions involving a carbene/alkyne metathesis (CAM). A priori, these intermediates resemble the Grubbs‐type family of catalysts, but they exhibit a completely different reactivity pattern that few, if any, other catalytic system can reproduce so far. The reactivity of these species with α‐unsubstituted and α‐substituted alkynals showcases the peculiarities of these intermediates. Although Z‐vinyl dihydrooxazines are preferentially obtained with the former, Z‐vinyl epoxypyrrolidines are obtained with the latter. A combination of spectroscopic and computational data now prove that a η3‐coordination mode of the ruthenium vinyl carbene and the presence of a Lewis basic chloride ligand give rise to two markedly different stereoelectronic faces, which are responsible for the unconventional reactivity of these species. [ABSTRACT FROM AUTHOR]

A novel approach of electrolysis using alternating current was applied in the sulfur–sulfur bond metathesis of symmetrical disulfides towards unsymmetrical disulfides. As initially expected, a statistical distribution in disulfides was obtained. Furthermore, the influence of electrode polarisation by alternating current was investigated on a two‐disulfide matrix. The highly dynamic nature of this chemistry resulted in the creation of dynamic disulfide libraries by expansion of the matrices, consisting of up to six symmetrical disulfides. In addition, mixing of matrices and stepwise expanding of a matrix by using alternating current electrolysis were realised. [ABSTRACT FROM AUTHOR]

Compared with the ripeness of olefin metathesis, exploration of the construction of carbon–carbon double bonds through the catalytic carbonyl–olefin metathesis reaction remains stagnant and has received scant attention. Herein, a highly efficient AuCl3‐catalyzed intramolecular ring‐closing carbonyl–olefin metathesis reaction is described. This method features easily accessible starting materials, simple operation, good functional‐group tolerance and short reaction times, and provides the target cyclopentenes, polycycles, benzocarbocycles, and N‐heterocycle derivatives in good to excellent yields. [ABSTRACT FROM AUTHOR]

Drugs in the chemical space beyond the rule of 5 (bRo5) can modulate targets with difficult binding sites while retaining cell permeability and oral absorption. Reviewing the syntheses of bRo5 drugs approved since 1990 highlights synthetic chemistry's contribution to drug discovery in this space. Initially, bRo5 drugs were mainly natural products and semi‐synthetic derivatives. Later, peptidomimetics and de novo designed compounds, that include up to seven chiral centres and macrocyclic rings became dominant. These drugs are prepared by total synthesis, sometimes by routes of more than 25 steps with stereocentres originating from the chiral pool, or being installed by chiral induction or enzymatic resolution. Interestingly, ring‐closing metathesis proved to be the method of choice for macrocyclisation in hepatitis C virus protease inhibitors. We conclude that structural simplification, planning of synthetic routes regarding incorporation of stereocentres and macrocyclisation, as well as incorporation of structural knowledge and consideration of chameleonic properties in design, should facilitate drug discovery in bRo5 space. [ABSTRACT FROM AUTHOR]

The lanthanum oxonitridophosphate La21P40O46N57 was synthesized by high‐pressure metathesis from partially hydrolysed LiPN2 and LaCl3 at 750–950 °C and 7–9 GPa. The combination of transmission electron microscopy (TEM) and diffraction using microfocused synchrotron radiation revealed a monoclinic crystal structure (space group P21/n, a=14.042(4), b=7.084(3), c=41.404(10) Å, β=97.73(3)° and Z=2), which is characterized by loop‐branched 21 member single chains of P(O,N)4 tetrahedra that extend along [2 0 1]. These chains are related to the loop‐branched dreier single chains with dreier‐ring loops in stillwellite (CeBSiO5). In La21P40O46N57, these chains are characterized by a complex long‐periodic conformation and exhibit disorder that involves La/N and P split positions. This is an extraordinarily long periodicity with respect to branched single chains of tetrahedra. La21P40O46N57 constitutes the first rare‐earth oxonitridophosphate exhibiting a chain structure. Single‐crystal data are consistent with electron and powder X‐ray diffraction. [ABSTRACT FROM AUTHOR]

In the search of smarter routes to control the conditions of N‐heterocyclic carbene (NHCs) formation, a two‐component air‐stable NHC photogenerating system is reported. It relies on the irradiation at 365 nm of a mixture of 2‐isopropylthioxanthone (ITX) with 1,3‐bis(mesityl)imidazoli(ni)um tetraphenylborate. The photoinduced liberation of NHC is evidenced by reaction with a mesitoyl radical to form an NHC‐radical adduct detectable by electron spin resonance spectroscopy. The NHC yield can be determined by 1H NMR spectroscopy through the formation of a soluble and stable NHC–carbodiimide adduct. To deprotonate the azolium salt and liberate the NHC, a mechanism is proposed in which the role of base is played by ITX radical anion formed in situ by a primary photoinduced electron‐transfer reaction between electronically excited ITX (oxidant) and BPh4− (reductant). An NHC yield as high as 70 % is achieved upon starting with a stoichiometric ratio of ITX and azolium salt. Three different photoNHC‐mediated polymerizations are described: synthesis of polyurethane and polyester by organocatalyzed step‐growth polymerization and ring‐opening copolymerization, respectively, and generation of polynorbornene by ring‐opening metathesis polymerization using an NHC‐coordinated Ru catalyst formed in situ. [ABSTRACT FROM AUTHOR]

Isomerizing olefin metathesis is currently undergoing a transformation from laboratory curiosity to powerful synthetic concept at the heart of orthogonal tandem catalysis. In this process, an isomerization catalyst continuously moves double bonds along carbon chains, while a metathesis catalyst scrambles the residues at the C−C double bonds. This cooperative action of two catalysts can be used to access single, defined products from a complex mixture of compounds. Alternatively, it enables the transformation of uniform starting materials into complex product blends with defined, tunable properties. This concept article highlights recent developments and potential applications of this fascinating reaction concept. [ABSTRACT FROM AUTHOR]

The reaction system GeCl2⋅dioxane/LiSTsi (Tsi=C(SiMe3)3) opens a fruitful area in germanium chemistry, depending on the stoichiometry and solvent used during the reaction. For example, the reaction of GeCl2⋅dioxane in toluene with two equivalents of the thiolate gives the expected germylene Ge(STsi)2 in excellent yield. This germylene readily reacts with hydrogen and acetylene, however, in a non‐selective way. By using an excess amount of the thiolate and toluene as the solvent, the germanide [Ge(STsi)3][Li(thf)] is obtained. Performing the same reaction in thf leads to a C−H activation of thf to give (H7C4O)Ge[STsi](μ2‐S)2Ge[STsi]2, in which the thf molecule is still intact. Using a sub‐stoichiometric amount of the thiolate leads to the heteroleptic compound [ClGe(STsi)]2 and to the insertion product (thf)Ge[S‐GeCl2‐Tsi]2, in which additional GeCl2 molecules insert into the C−S bonds of Ge(STsi)2. The synthesis and the experimentally determined structures of all compounds are presented together with first reactivity studies of Ge(STsi)2. [ABSTRACT FROM AUTHOR]

A highly efficient and flexible protocol for intramolecular annulation of exocyclic β‐enaminones has been disclosed for the synthesis of carbazolones and imidazo[1,2‐a]pyridines through a counter‐anion‐controlled free‐radical mechanism promoted by hypervalent iodine(III). The cooperative behavior of HTIB and AgSbF6 plays a crucial role in the intramolecular annulation process through C−C and C−N bond formation to give the desired products. The mechanistic insights suggest that the two competitive reactions involved in the system are guided by the nature of the counteranion, which determines the formation of the final products. A wide variety of carbazolones and imidazo[1,2‐a]pyridine molecules have been prepared and isolated in good to excellent yields. [ABSTRACT FROM AUTHOR]

A short and efficient synthesis of the C2‐symmetric antibiotic (−)‐vermiculine by utilizing an enantioselective catalytic one‐step dimerization methodology as key‐step to construct the core structure is reported. The late‐stage modifications feature a double metathesis homologation followed by a double Wacker‐type oxidation. These key‐steps allowed the synthesis of vermiculine in only seven steps, starting from commercially available building blocks. A short and efficient synthesis of the C2‐symmetric antibiotic (−)‐vermiculine utilizing an enantioselective catalytic one‐step dimerization methodology as key‐step to construct the core structure is reported. This key step allowed the synthesis of vermiculine in only seven steps starting from commercially available building blocks (see scheme). [ABSTRACT FROM AUTHOR]

Although alkyne metathesis has been known for 50 years, rapid progress in this field has mostly occurred during the last two decades. In this article, the development of several highly efficient and thoroughly studied alkyne metathesis catalysts is reviewed, which includes novel well‐defined, in situ formed and heterogeneous systems. Various alkyne metathesis methodologies, including alkyne cross‐metathesis (ACM), ring‐closing alkyne metathesis (RCAM), cyclooligomerization, acyclic diyne metathesis polymerization (ADIMET), and ring‐opening alkyne metathesis polymerization (ROAMP), are presented, and their application in natural product synthesis, materials science as well as supramolecular and polymer chemistry is discussed. Recent progress in the metathesis of diynes is also summarized, which gave rise to new methods such as ring‐closing diyne metathesis (RCDM) and diyne cross‐metathesis (DYCM). Making and breaking triple bonds: The development of highly active alkyne metathesis catalysts and their recent applications in organic and natural product synthesis as well as supramolecular and polymer chemistry are highlighted in this Minireview. [ABSTRACT FROM AUTHOR]

A short and efficient total synthesis of the C2‐symmetric (−)‐cylindrocyclophane F is presented, using a cross olefin metathesis dimerization strategy for construction of the [7,7]‐paracyclophane macrocycle. The synthesis of the dimerization building block includes a Pd‐catalyzed sp3‐sp2 Negishi cross coupling of a sterically hindered Zn‐reagent with an aromatic triflate, an enantiospecific Zn‐catalyzed sp3‐sp3 cross coupling of an α‐hydroxy ester triflate with a Grignard reagent and the application of an enantioselective Rh‐catalyzed C‐allylation of an electron rich arene. Asymmetric C−C bond formation using novel catalytic methods enabled a short and concise synthesis of the cylindrocyclophane F, a potent cytotoxin, in high overall yield. [ABSTRACT FROM AUTHOR]

Three mono‐N‐heterocyclic carbene (NHC) ruthenium 2‐isopropoxybenzylidene (10 a–c) and one bis(NHC) indenylidene complex (8) bearing an unsymmetrical N‐heterocyclic carbene ligand were synthesized and structurally characterized by single‐crystal X‐ray diffraction. The catalytic activity of the newly obtained complexes were evaluated in ring‐closing metathesis (RCM) and ene–yne (RCEYM) reactions in toluene and environmentally friendly 2‐MeTHF under air. The results confirmed that although all tested reactions can be successfully mediated by catalysts 10 a–c, their general reactivity is lower than the benchmark all‐purpose Ru catalysts with symmetrical NHC ligands. However, the latter cannot compete with specialized ruthenium complex 10 a in industrially relevant self‐CM of terminal olefins in neat conditions. Joining the elite: New "special tasks" catalysts have been developed. They exhibited much better reactivity than the commercially available complexes in self‐metathesis reactions of α‐olefins, enabling high yields and selectivities to be reached. [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: 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]

Abstract: Metal‐catalyzed borylative cyclization reactions of polyunsaturated compounds allow concomitant formation of C−C and C−B bonds to provide cyclic boronates, which constitute synthetically useful intermediates. Recent developments using first‐row transition metals as catalysts provide fully atom‐economical reactions that follow mechanisms involving σ‐bond metathesis. [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: We report successful utilization of sequential alkene isomerization and ring‐closing metathesis of dec‐9‐enoic acid based dienes in synthesis of macrocyclic lactones that possess a strong scent of musk. This catalytic sequence was essential to trim the chain length of starting dienes to yield macrocycles of the right size. Dec‐9‐enoic acid is conveniently obtainable from oleic esters by Ru‐catalysed ethenolysis. [ABSTRACT FROM AUTHOR]

Abstract: The site‐selective and regioselective allylation of 2‐substituted indoles was performed by using a ruthenium(IV) precatalyst containing a phosphine–sulfonate chelate. Mono‐, di‐, and triallylated indoles were selectively obtained depending on the reaction conditions with the formation of water as the only byproduct. The preparation of 3‐oxindole derivatives was then successfully performed owing to air oxidation of the corresponding allylated indoles. Diallylated pseudoindoxyls were proven to be good synthons to perform cyclization through a ring‐closing metathesis reaction to afford the corresponding tricyclic adducts. The photophysical properties of the 3‐oxindoles were measured, and some of the compounds showed strong fluorescence in water. [ABSTRACT FROM AUTHOR]

Abstract: The dimerization of a saturated N‐heterocyclic carbene (NHC) to tricyclic piperazine in preference to the commonly observed Wanzlick dimerization is presented. Mechanistic investigations revealed that the N‐fluorene substituent of the heterocycle is implicated in both ring opening of corresponding carbene dimer and tautomerization of NHC to an azomethine ylide. This has consequences for the fate of the NHC when generated from either an azolinium salt or a pentafluorophenyl adduct. The insights gained permitted the synthesis of a new indenylidene metathesis precatalyst, which exhibits exceptional selectivity and high TONS in self‐metathesis of 1‐octene. [ABSTRACT FROM AUTHOR]

Highly stereoselective conjugate b-borylation, using a new chiral NHC-based copper catalyst, has been achieved. The chiral NHC copper complex was prepared in gram scale and showed high enantioselectivity and activity (up to 10000 turnovers at 100 ppm of catalyst loading). This method was employed in the synthesis of a chiral b-borylated ester from simple unconjugated alkenes though an unprecedented one-pot cross metathesis-asymmetric borylation sequence. [ABSTRACT FROM AUTHOR]

Abstract: 1,3‐Bis(mesityl)imidazolium tetraphenylborate (IMesH+ BPh4−) can be synthesized in one step by anion metathesis between the corresponding imidazolium chloride and sodium tetraphenylborate. In the presence of 2‐isopropylthioxanthone (sensitizer), an IMes N‐heterocyclic carbene (NHC) ligand can be photogenerated under irradiation at 365 nm through coupled electron/proton transfer reactions. By combining this tandem NHC photogenerator system with metathesis inactive [RuCl2(p‐cymene)]2 precatalyst, the highly active RuCl2(p‐cymene)(IMes) complex can be formed in situ, enabling a complete ring‐opening metathesis polymerization (ROMP) of norbornene in the matter of minutes at room temperature. To the best of our knowledge, this is the first example of a photogenerated NHC. Its exploitation in photoROMP has resulted in a simplified process compared to current photocatalysts, because only stable commercial or easily synthesized reagents are required. [ABSTRACT FROM AUTHOR]

The first synthetic protocol to high oxidation state molybdenum(VI) N-heterocyclic carbene (NHC) alkylidyne complexes (NHC=1,3-diisopropylimidazol-2-ylidene, 1,3-dimethyl-4,5-R2-imidazol-2-ylidene, R2=H, Cl, CN) is reported. Steric limitations of the NHCs and the benzylidyne are described. All novel complexes were characterized by single crystal X-ray diffraction and solution NMR techniques. It was shown that all complexes presented here show activity in the self-metathesis of 1-phenyl-1-propyne at room temperature. To identify mechanistic differences, an experimental sequence to detect dissociation of ligands was developed. Results reveal dissociation of less electron-donating NHCs in course of the reaction. Mechanistic and reactivity differences were attributed to electronic and steric effects through Tolman's electronic parameter and the percentage of buried volume. Furthermore, Mo-1 containing the 1,3-dimethylimidazol-2-ylidene ligand showed good activity in self-metathesis reactions of p-substituted 1-phenyl-1-propynes with electron-donating moieties at room temperature. [ABSTRACT FROM AUTHOR]

Cyclophane-containing bis(2-amino-1,8-naphthyridine) moieties attached to variable linkers at the C2-position (linker B) were synthesized as cyclic mismatch-binding ligands (CMBLs). Ring-closing metathesis (RCM) is used as a key step for the introduction of double bonds at the linker B. Decreasing the size of the linker of the substrate, formation of the RCM products with an increasing trans/ cis ( E/ Z) ratio was observed with moderate to high overall yields. Concentration-dependent fluorescence spectra were observed for CMBLs with longer linkers ( n=3), whereas concentration-independent spectra were observed for CMBLs with shorter linkers ( n=2 and/or 1) with a marked exception of the E-alkene 6 a. Concomitant changes in the absorption as well as in the fluorescence spectra were also observed for the CMBLs with an increasing hydrophobicity of the solvent. Absorption and fluorescence spectra of the CMBLs in solutions containing 99-100 % methanol resembled to that of the monomer. The binding behavior of these CMBLs with repeat DNA structures was investigated by using a surface plasmon resonance (SPR) assay and circular dichroism (CD) spectra. The cyclic E-alkenes 1 a ( n=3) and 3 a ( n=2) show an orthogonal binding relationship with d(CCTG)9 and d(CAG)9. However, the selectivity for the cyclic Z-alkenes increased with decreasing the length of the linker from compound 2 b ( n=3) to compound 7 b ( n=1). These compounds display a large molecular diversity, which allowed the tuning of the binding affinity and selectivity of the CMBLs by varying the linkers towards various biologically significant repeat DNA structures. [ABSTRACT FROM AUTHOR]

Inspired by recent reports of low-valent iron-complex-catalyzed formal [2+2] cycloaddition of olefins, we demonstrate computationally that with such low-valent iron complexes and with 'strong' ligands, the olefin metathesis is also preferred over the undesired cyclopropanation side-reaction, competition already studied by Hoffmann and co-workers almost 40 years ago ( J. Am. Chem. Soc. 1981, 103, 5582). The [2+2] cycloaddition step in metathesis propagation, which gives a Chauvin-type metallacyclobutane intermediate, is proposed to proceed either via a planar four-electron Craig-Möbius aromatic [π2s+π2s] transition-state structure with a low barrier of 4.7 kcal mol−1 or, alternatively, via a twisted Zimmerman-Möbius aromatic [π2s+π2a] transition state with a 5.5 kcal mol−1 activation-energy barrier, with respect to an 'encounter' π-complex minimum obtained from an FeII alkylidene and the entering olefin, while the corresponding triplet pathways are all disfavored. [ABSTRACT FROM AUTHOR]

Ionic Mo- and W-imido alkylidene N-heterocyclic carbene (NHC) olefin metathesis catalysts, [Mo{ N-2,6-(Me2)C6H3}(CHCMe2Ph)(IMesH2)(OTf)(PPS)]OTf ( 3), [Mo( N-2,6-(Me2)C6H3)(CHCMe2Ph)(IMesH2)(OC6F5)(PPS)][B(ArF)4] ( 5), [Mo(N tBu)(CHCMe2Ph)(4,5-dichloro-1,3-dimethylimidazol-2-ylidene)(OTf)(2,6-Ph-4-{2,4,6-Ph-pyridinium}phenolate)][OTf] ( 9), [Mo(N tBu)(CHCMe2Ph)(4,5-dichloro-1,3-dimethylimidazol-2-ylidene)(2,6-Ph-4-{2,4,6-Ph-pyridinium}phenolate)][B(ArF)4]2 ( 10, PPS=pyridiniumpropanesulfonate, IMesH2=1,3-dimesitylimidazolin-2-ylidene, OTf=CF3SO3, B(ArF)4=tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) were prepared from betaine-type ligands. Also, the first bis-NHC and a nitron-based bis(amido) bistriflate imido alkylidene complex, [Mo(N tBu)(CHCMe2Ph)(4,5-dichloro-1,3-dimethylimidazol-2-ylidene)2(THF)(2,6-Ph-4-{2,4,6-Ph-pyridinium}phenolate)][OTf]2 ( 11) and [Mo( N-2,6-Me2-C6H3)(CHCMe2Ph)( N-{1,4-diphenyl-1,3,4-triazol-2-ylium}- N-phenyl-amido)2][OTf]2 ( 14) along with ionic [W( N-2,6- iPr2-C6H3)(CHCMe2Ph)( N-2,5-Me2C4H2)(I iPr)(2,6- tBu-4-PPh3-phenolate)] (17, I iPr=1,3-diisopropylimidazol-2-ylidene) are reported. With these new catalysts, the first biphasic reaction setup with Group 6 metal alkylidene NHC complexes was successfully established using a pyrrole/heptane mixture as a liquid phase. Productivities under biphasic conditions were comparable to those of reactions in 1,2-dichloroethane or toluene. Metal concentrations of <2 ppm migrated into the nonpolar heptane phase as measured by inductively coupled plasma-optical emission spectroscopy. [ABSTRACT FROM AUTHOR]

The silicon analogue of the potent spirocyclic δ-damascone odorant 6 was synthesized from allyltrichlorosilane ( 15) and but-2-en-1-ol ( 16). The latter was transformed to 3-methylpen-4-enenitrile ( 11) by Saucy-Marbet reaction with ethoxyethane and subsequent treatment with HONH2 ⋅HCl. The resulting γ,δ-unsaturated nitrile 11 was silylated with 1-allyl-1-chlorosilolane ( 14), which was prepared from allyltrichlorosilane ( 15) and the bis-Grignard reagent of 1,4-dichlorobutane. Metathetic ring closure employing the Grubbs I catalyst, followed by DIBAL reduction with non-aqueous work up, Grignard reaction with prop-1-en-1-ylmagnesium bromide, and Attenburrow MnO2 oxidation concluded the synthesis. The target compound was found to be olfactorily related to the spiro[4.5]-δ-damascone lead, but approximately 900 times weaker. In a type of enol Brook rearrangement, it thermally decomposes however to 3,6-dihydro-2 H-1,2-oxasilocine ( 20), which surprisingly is a damascone odorant as well; although, 12 000 times weaker. [ABSTRACT FROM AUTHOR]

A structural revision of clavilactone D, a potent inhibitor of protein tyrosine kinases, was achieved by total syntheses of two newly proposed structures. The syntheses relied on ring-opening/ring-closing metathesis, which transformed a cyclobutenecarboxylate into a γ-butenolide. The syntheses confirmed that the correct structure of clavilactone D has an amino group at C-3 instead of a hydroxy group at C-2 in the originally proposed structure. [ABSTRACT FROM AUTHOR]

N-Heterocyclic carbene (NHC) ligands IMes [ABSTRACT FROM AUTHOR]