Indenylidene complexes of ruthenium: optimized synthesis, structure elucidation, and performance as catalysts for olefin metathesis--application to the synthesis of the ADE-ring system of nakadomarin A

Repercussion of a 1,3-Hydrogen Shift in a Hydride-Osmium-Allenylidene Complex

An unusual 1,3-hydrogen shift from the metal center to the C_β atom of the C₃-chain of the allenylidene ligand in a hydride-osmium(II)-allenylidene complex is the beginning of several interesting transformations in the cumulene. The hydride-osmium(II)-allenylidene complex was prepared in two steps, starting from the tetrahydride dimer [(Os(H···H){κ³-P,O,P-[xant(PⁱPr₂)₂]})₂(μ-Cl)₂][BF₄]₂ (1). Complex 1 reacts with 1,1-diphenyl-2-propyn-1-ol to give the hydride-osmium(II)-alkenylcarbyne [OsHCl(≡CCH=CPh₂){κ³-P,O,P-[xant(PⁱPr₂)₂]}]BF₄ (2), which yields OsHCl(=C=C=CPh₂){κ³-P,O,P-[xant(PⁱPr₂)₂]} (3) by selective abstraction of the C_β–H hydrogen atom of the alkenylcarbyne ligand with K^tBuO. Complex 3 is metastable. According to results of DFT calculations, the migration of the hydride ligand to the C_β atom of the cumulene has an activation energy too high to occur in a concerted manner. However, the migration can be catalyzed by water, alcohols, and aldehydes. The resulting alkenylcarbyne-osmium(0) intermediate is unstable and evolves into a 7:3 mixture of the hydride-osmium(II)-indenylidene OsHCl(=C_IndPh){κ³-P,O,P-[xant(PⁱPr₂)₂]} (4) and the osmanaphthalene OsCl(C₉H₆Ph){κ³-P,O,P-[xant(PⁱPr₂)₂]} (5). Protonation of 4 with HBF₄ leads to the elongated dihydrogen complex [OsCl(η²-H₂)(=C_IndPh){κ³-P,O,P-[xant(PⁱPr₂)₂]}]BF₄ (6), while the protonation of 5 regenerates 2. In contrast to 4, complex 6 evolves to a half-sandwich indenyl derivative, [Os(η⁵-IndPh)H{κ³-P,O,P-[xant(PⁱPr₂)₂]}][BF₄]Cl (7). Phenylacetylene also provokes the 1,3-hydrogen shift in 3. However, it does not participate in the migration. In contrast to water, alcohols, and aldehydes, it stabilizes the resulting alkenylcarbyne to afford [Os(≡CCH=CPh₂)(η²-HC≡CPh){κ³-P,O,P-[xant(PⁱPr₂)₂]}]Cl (8).

Light-Driven gem Hydrogenation: An Orthogonal Entry into "Second-Generation" Ruthenium Carbene Catalysts for Olefin Metathesis

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.

Grubbs Metathesis Enabled by a Light-Driven gem-Hydrogenation of Internal Alkynes

[(NHC)(cymene)RuCl2 ] (NHC=N-heterocyclic carbene) complexes instigate a light-driven gem-hydrogenation of internal alkynes with concomitant formation of discrete Grubbs-type ruthenium carbene species. This unorthodox reactivity mode is harnessed in the form of a "hydrogenative metathesis" reaction, which converts an enyne substrate into a cyclic alkene. The intervention of ruthenium carbenes formed in the actual gem-hydrogenation step was proven by the isolation and crystallographic characterization of a rather unusual representative of this series carrying an unconfined alkyl group on a disubstituted carbene center.

Hoveyda-Grubbs catalysts with an N→Ru coordinate bond in a six-membered ring. Synthesis of stable, industrially scalable, highly efficient ruthenium metathesis catalysts and 2-vinylbenzylamine ligands as their precursors

A novel and efficient approach to the synthesis of 2-vinylbenzylamines is reported. This involves obtaining 2-vinylbenzylamine ligands from tetrahydroisoquinoline by alkylation and reduction followed by the Hofmann cleavage. The resultant 2-vinylbenzylamines allowed us to obtain new Hoveyda-Grubbs catalysts, which were thoroughly characterised by NMR, ESIMS, and X-ray crystallography. The utility of this chemistry is further demonstrated by the tests of the novel catalysts (up to 10^-2 mol %) in different metathesis reactions such as cross metathesis (CM), ring-closing metathesis (RCM) and ring-opening cross metathesis (ROCM).

Crystal structure of [1,3-bis-(2,4,6-tri-methyl-phen-yl)imidazolidin-2-yl-idene]di-chlorido-{2-[1-(di-methyl-amino)-eth-yl]benzyl-idene}ruthenium including an unknown solvate

The title compound, [RuCl2(C21H26N2)(C11H15N)], is an example of a new generation of N,N-dialkyl metallocomplex ruthenium catalysts with an N→Ru coordination bond as part of a six-membered chelate ring. The Ru atom has an Addison τ parameter of 0.234, which indicates a geometry inter-mediate between square-based pyramidal and trigonal-bipyramidal. The complex shows the usual trans arrangement of the two chloride ligands, with Ru-Cl bond lengths of 2.3397 (8) and 2.3476 (8) Å, and a Cl-Ru- Cl angle of 157.47 (3)°. The crystal structure features C-H⋯Cl, C-H⋯π and π-π stacking inter-actions. The solvent mol-ecules were found to be highly disordered and their contribution to the scattering was removed with the SQUEEZE procedure in PLATON [Spek (2015). Acta Cryst. C71, 9-18], which indicated a solvent cavity of volume 1096 Å3 containing approximately 419 electrons. These solvent mol-ecules are not considered in the given chemical formula and other crystal data.

Synthesis and biological evaluation of fluorinated analogues of ripostatin A

Several monocyclic derivatives of 14,14'-difluororipostatin A were prepared using a catalytic Mukaiyama aldol reaction, a ring-closing metathesis reaction and a late stage click reaction as key steps. The biological activity of the produced compounds was assessed in vivo using a panel of pathogenic microorganisms. Moderate antibiotic activity was observed for 11-OMe-ripostatin A and 11-OMe-14,14'-difluororipostatin A.

Flow Photo-Nazarov Reactions of 2-Furyl Vinyl Ketones: Cyclizing a Class of Traditionally Unreactive Heteroaromatic Enones

Nazarov reactions of 2-furyl vinyl ketones and related heteroaromatic enones, to produce furan-fused cyclopentanones using a flow photochemical approach, are described. Compounds possessing this connectivity between heterocycle and ketone (2-furyl, 2-benzofuryl, 2-thiophene-yl, and 2-benzothiophene-yl) have traditionally proven difficult or impossible to cyclize with typical Brønsted and Lewis acid mediated methods. Using mild flow photochemistry conditions and acetic acid (AcOH) or hexafluoroisopropanol (HFIP) as solvent, these compounds were found to cyclize in 45-97% yields, with typical UV exposure times of 3.4-6.8 min. In all cases, 2-furyl and 2-thiophene-yl enones cyclized, whereas 2-benzofuryl and 2-benzothiophene-yl enones exhibited divergent properties with reactivity patterns tied to the identity of the vinyl group. This report discloses the first photo-Nazarov reactions of tetrahydropyridine-substituted 2-furyl ketones, providing a direct approach to the corresponding fused heterocyclic motifs built around a central cyclopentanone. These motifs constitute the core structures of biologically active natural products, including the marine alkaloid nakadomarin A.

Effect of the bulkiness of indenylidene moieties on the catalytic initiation and efficiency of second-generation ruthenium-based olefin metathesis catalysts

Fast initiation and high efficiency of ruthenium indenylidene catalysts induced by steric repulsion between two ligands (SIMes ligand and bulky indenylidene ligand).

Synthesis and catalytic activity of ruthenium indenylidene complexes bearing unsymmetrical NHC containing a heteroaromatic moiety

New ruthenium(ii) indenylidene catalysts were synthesized and used in olefin metathesis reactions in toluene under air, leading to high conversions and good selectivities.

Olefin metathesis in air

Since the discovery and now widespread use of olefin metathesis, the evolution of metathesis catalysts towards air stability has become an area of significant interest. In this fascinating area of study, beginning with early systems making use of high oxidation state early transition metal centers that required strict exclusion of water and air, advances have been made to render catalysts more stable and yet more functional group tolerant. This review summarizes the major developments concerning catalytic systems directed towards water and air tolerance.

Bis-mixed-carbene ruthenium-thiolate-alkylidene complexes: synthesis and olefin metathesis activity

A series of bis-carbene Ru-hydride species, including (IMes)(Im(OMe)2)(PPh3)RuHCl (1) and (SIMes)(Me2Im(OMe)2)(PPh3)RuHCl (2) were prepared and subsequently shown to react with aryl-vinyl-sulfides to give the bis-carbene-alkylidene complexes: Im(OMe)2(SIMes)RuCl(SR)(=CHCH3) (R = p-FC6H4 (3), p-(NO2)C6H4 (4)), Im(OMe)2(IMes)RuCl(=CHCH3)(SPh) (5), Me2Im(OMe)2(SIMes)RuCl(=CHCH3)(SPh) (6), Im(OMe)2(SIMes)(F5C6S)RuCl(=CHR) (R = C4H9 (9), C5H11 (10)). The activity of these species in the standard Grubbs' tests for ring-opening metathesis polymerization, ring-closing and cross-metathesis are reported. Although these thiolate derivatives are shown to exhibit modest metathesis activities, the reactivity is enhanced in the presence of BCl3.

Synthesis and characterization of non-chelating ruthenium–indenylidene olefin metathesis catalysts derived from substituted 1,1-diphenyl-2-propyn-1-ols

The synthesis and characterization of four first-generation modified non-chelating indenylidene ruthenium catalysts is reported, as well as their catalytic performance in olefin metathesis reactions.

Ruthenium indenylidene complexes bearing N-alkyl/N-mesityl-substituted N-heterocyclic carbene ligands

Three ruthenium indenylidene complexes containing N-alkyl/N-mesityl mixed N-heterocyclic carbene ligands show a significant improvement in their catalytic initiation rate. The smaller sized NHC contributes to a better catalytic performance.

A facile route to Ru-alkylidenes

Reactions of dithioacetal E(CH2CH2S)2CHR (E = O, S, R = Ph, C6F5) and O(C6H4S)2CHPh with either Ru(cod)(cot) or Ru(PPh3)3(H)2 are used to prepare the Ru-alkylidene complexes E(CH2CH2S)2RuCHPh(L), O(C6H4S)2RuCHPh(L) (L = PCy3, SIMes) in high yields.

Cycloalkyl-based unsymmetrical unsaturated (U2)-NHC ligands: flexibility and dissymmetry in ruthenium-catalysed olefin metathesis

Structural features, chemical stabilities and catalytic profiles in olefin metathesis of a new library of low-cost cycloalkyl-based U₂-NHC Ru complexes were disclosed.

Reactions of ruthenium hydrides with ethyl-vinyl sulfide

Ru-hydride precursors containing the OCO-carbene ligand react with ethyl-vinyl-sulfide to give Ru alkyl and vinyl derivatives via an initial insertion of the vinyl-fragment into the Ru–H, subsequent C–H activation and loss of diethyl sulfide.

Catalytic asymmetric synthesis of the pentacyclic core of (−)-nakadomarin A via oxazolidine as an iminium cation equivalent

A catalytic asymmetric synthesis of the pentacyclic core of (−)-nakadomarin A was achieved.

Synthetic Studies Towards the Core Structure of Nakadomarin A by a Thioamide-Based Strategy

The tricyclic BCD substructure of the marine natural product nakadomarin A has been synthesised. The strategy utilised a key carbon-carbon bond-forming reaction between a furan and an N-acyliminium ion derived from a secondary thiolactam. In addition, a novel three-component coupling reaction between a thioamide, an allylic bromide and an isocyanate, leading to the establishment of two new stereogenic centres, is reported. Two key steps in a projected total synthesis of nakadomarin A have been realised by using the unique chemistry of thioamides. Formation of the carbocyclic B ring can be effected by nucleophilic attack of a furan on a thiolactam-derived iminium ion, and the key quaternary centre can be established by a novel three-component coupling reaction.