Novel ruthenium-based metathesis catalysts containing electron-withdrawing ligands: synthesis, immobilization, and reactivity

The syntheses and reactivity of seven different ruthenium-based metathesis catalysts are described. Ru(CF3COO)2(PCy3)(=CH-2-(2-PrO)C6H4) (1), Ru(CF3COO)2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) (2), and Ru(CF3COO)2(PCy(3))(1,3-dimesityldihydroimidazolin-2-ylidene)(=CHC6H5) (3) were prepared via chlorine exchange by reacting RuCl2(PCy3)2(=CH-2-(2-PrO)C6H4), RuCl2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4), and RuCl2(PCy3)(1,3-dimesityldihydroimidazolin-2-ylidene)(=CHC6H5), respectively, with silver trifluoroacetate (Cy =cyclohexyl). In analogy, Ru(CF3CF2COO)2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) (4) and Ru(CF3CF2CF2COO)2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) (5) were prepared from RuCl2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) via reaction with CF3CF2COOAg and CF3CF2CF2COOAg, respectively. Ru(C6F5COO)2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) (6) and Ru(C6F5O)2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) (7) were prepared from RuCl2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) via reaction with C6F5COOTl and C6F5OTl, respectively. Supported catalysts Ru(PS-DVB-CH2OOCCF2CF2CF2COO)(CF3COO)(PCy3)(1,3-dimesityldihydroimidazolin-2-ylidene)(=CHC6H5) (8), Ru(PS-DVB-CH2OOCCF2CF2CF2COO)(CF3COO)(PCy3)(=CH-2-(2-PrO)C6H4) (9), and Ru(PS-DVB-CH2OOCCF2CF2CF2COO)(CF3COO)(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4) (10) were synthesized by reaction of RuCl2(PCy3)(1,3-dimesityldihydroimidazolin-2-ylidene)(=CHC6H5), RuCl2(PCy3)(=CH-2-(2-PrO)C6H4), and RuCl2(1,3-dimesityldihydroimidazolin-2-ylidene)(=CH-2-(2-PrO)C6H4), respectively, with a perfluoroglutaric acid-derivatized poly(styrene-co-divinylbenzene) (PS-DVB) support (silver form). Halogen exchange in PCy3-containing systems had to be carried out in dichloromethane in order to suppress precipitation of AgCl.PCy3. The reactivity of all new catalysts in ring-closing metathesis (RCM) of hindered electron-rich and -poor substrates, respectively, at elevated temperature (45 degrees C) was compared with that of existing systems. Diethyl diallylmalonate (DEDAM, 11), diethyl allyl(2-methylallyl)malonate (12), N,N-diallyl-p-toluenesulfonamide (13), N-benzyl-N-but-1-en-4-ylbut-2-enecarboxylic amide (14), and N-allyl-N-(1-carboxymethyl)but-3-en-1-yl-p-toluenesulfonamide (15) were used as educts. Supported catalysts were prepared with high loadings (2.4, 22.1, and 160 mg of catalyst/g PS-DVB for 8, 9, and 10, respectively). Catalyst 8 showed higher and catalysts 9 and 10 sowed significantly reduced activities in RCM compared to their homogeneous analogues. Thus, with 8, turnover numbers (TONs) up to 4200 were realized in stirred-batch (carousel) RCM experiments. To elucidate the nature of the bound species, catalysts 8-10 were subjected to 13C- and 31P-MAS NMR spectroscopy. These investigations provided evidence for the proposed structures. Leaching of ruthenium into the reaction mixture was low, resulting in ruthenium contents <85 ppb (ng/g) in the final RCM-derived products.     

Factors relevant for the ruthenium-benzylidene-catalyzed cyclopolymerization of 1,6-heptadyines

Fourteen metathesis initiators that had been designed for use in the living polymerization of diethyl dipropargylmalonate (DEDPM), including the Hoveyda catalyst [RuCl(2)(IMesH(2))([double bond]CH-2-(2-PrO)[bond]C(6)H(4))] (1 a), as well as [Ru(CF(3)COO)(2)(IMesH(2))([double bond]CH-2-(2-PrO)[bond]C(6)H(4))] (1 b), [Ru(CF(3)CF(2)COO)(2)(IMesH(2))([double bond]CH-2-(2-PrO)[bond]C(6)H(4))] (1 c), [Ru(CF(3)CF(2)CF(2)COO)(2)(IMesH(2))([double bond]CH-2-(2-PrO)[bond]C(6)H(4))] (1 d), [RuCl(2)(IMesH(2))([double bond]CH-2,4,5-(MeO)(3)[bond]C(6)H(2))] (2 a), [Ru(CF(3)COO)(2)(IMesH(2))([double bond]CH-2,4,5-(MeO)(3)[bond]C(6)H(2))] (2 b), [Ru(CF(3)CF(2)COO)(2)(IMesH(2))([double bond]CH-2,4,5-(MeO)(3)[bond]C(6)H(2))] (2 c), [Ru(CF(3)CF(2)CF(2)COO)(2)(IMesH(2))([double bond]CH-2,4,5-(MeO)(3)[bond]C(6)H(2))] (2 d), [RuCl(2)(IMes)([double bond]CH-2-(2-PrO)[bond]C(6)H(4))] (3 a), [Ru(CF(3)COO)(2)(IMes)([double bond]CH-2-(2-PrO)[bond]C(6)H(4))] (3 b), [RuCl(2)(IMesH(2))([double bond]CH-2-(2-PrO)-5-NO(2)[bond]C(6)H(3))] (4 a), [Ru(CF(3)COO)(2)(IMesH(2))([double bond]CH-2-(2-PrO)-5-NO(2)[bond]C(6)H(3))] (4 b), [Ru(CF(3)CF(2)COO)(2)(IMesH(2))([double bond]CH-2-(2-PrO)-5-NO(2)[bond]C(6)H(3))] (4 c), and [Ru(CF(3)CF(2)CF(2)COO)(2)(IMesH(2))([double bond]CH-2-(2-PrO)-5-NO(2)[bond]C(6)H(3))] (4 d) (IMes=1,3-dimesitylimidazol-2-ylidene; IMesH(2)=1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) were prepared. Living polymerization systems could be generated with DEDPM by careful tuning of the electronic nature and steric placement of the ligands. Although 1 a, 2 a, 3 a, 3 b, and 4 a were inactive in the cyclopolymerization of DEDPM, and initiators 1 b-d did not allow any control over molecular weight, initiators 2 b-d and 4 b-d offered access to class VI living polymerization systems. In particular, compounds 2 b and 4 d were superior. The livingness of the systems was demonstrated by linear plots of M(n) versus the number of equivalents of monomer added (N). For initiators 2 b-d and 4 b-d, values for k(p)/k(i) were in the range of 3-7, while 1 b, 1 c, and 1 d showed a k(p)/k(i) ratio of >1000, 80, and 40, respectively. The use of non-degassed solvents did not affect these measurements and underlined the high stability of these initiators. The effective conjugation length (N(eff)) was calculated from the UV/Vis absorption maximum (lambda(max)). The final ruthenium content in the polymers was determined to be 3 ppm.     

1,3-dialkyl- and 1,3-diaryl-3,4,5,6-tetrahydropyrimidin-2-ylidene rhodium(i) and palladium(II) complexes: synthesis, structure, and reactivity

The synthesis of novel 1,3-diaryl- and 1,3-dialkylpyrimidin-2-ylidene-based N-heterocyclic carbenes (NHCs) and their rhodium(i) and palladium(II) complexes is described. The rhodium compounds bromo(cod)[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene]rhodium (7), bromo(cod)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)rhodium (8) (cod=eta(4)-1,5-cyclooctadiene, mesityl=2,4,6-trimethylphenyl), chloro(cod)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)rhodium (9), and chloro(cod)[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene]rhodium (10) were prepared by reaction of [[Rh(cod)Cl](2)] with lithium tert-butoxide followed by addition of 1,3-dimesityl-3,4,5,6-tetrahydropyrimidinium bromide (3), 1,3-dimesityl-3,4,5,6-tetrahydropyrimidinium tetrafluoroborate (4), 1,3-di-2-propyl-3,4,5,6-tetrahydropyrimidinium bromide (6), and 1,3-di-2-propyl-3,4,5,6-tetrahydropyrimidinium tetrafluoroborate, respectively. Complex 7 crystallizes in the monoclinic space group P2(1)/n, and 8 in the monoclinic space group P2(1). Complexes 9 and 10 were used for the synthesis of the corresponding dicarbonyl complexes dicarbonylchloro(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)rhodium (11), and dicarbonylchloro[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene]rhodium (12). The wavenumbers nu(CO I)/nu(CO II) for 11 and 12 were used as a quantitative measure for the basicity of the NHC ligand. The values of 2062/1976 and 2063/1982 cm(-1), respectively, indicate that the new NHCs are among the most basic cyclic ligands reported so far. Compounds 3 and 6 were additionally converted to the corresponding cationic silver(i) bis-NHC complexes [Ag(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(2)]AgBr(2) (13) and [Ag[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene](2)]AgBr(2) (14), which were subsequently used in transmetalation reactions for the synthesis of the corresponding palladium(II) complexes Pd(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(2) (2+)(Ag(2)Br(2)Cl(4) (4-))(1/2) (15) and Pd[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene)(2)]Cl(2) (16). Complex 15 crystallizes in the monoclinic space group P2(1)/c, and 16 in the monoclinic space group C(2)/c. The catalytic activity of 15 and 16 in Heck-type reactions was studied in detail. Both compounds are highly active in the coupling of aliphatic and aromatic vinyl compounds with aryl bromides and chlorides with turnover numbers (TONs) up to 2000000. Stabilities of 15 and 16 under Heck-couplings conditions were correlated with their molecular structure. Finally, selected kinetic data for these couplings are presented.     

Polymerization of phenylacetylene by novel Rh (I)-, Ir (I)- and Ru (IV) 1,3-R2-3,4,5,6-tetrahydropyrimidin-2-ylidenes (R = mesityl, 2-propyl): Influence of structure on activity and polymer structure

The preparation of novel Rh (I) and Ir (I) complexes, i.e. [Rh(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(COD)]⁺[PF₆]⁻ (1), Rh(CF₃SO₃)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(COD) (2) and Ir(CF₃CO₂)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(COD) (3) (COD = 1,5-cyclooctadiene), is described. Compounds 1 and 3 were structurally characterized by X-ray diffraction. In 1, the N-heterocyclic carbene acts as a bidentate ligand with the carbene coordinating to the Rh(I) center and an arene group acting as a homoazallyl ligand. The catalytic activity of complexes 1–3 in the polymerization of phenylacetylene was studied and compared to that of RhCl(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(COD) (4), Rh(CF₃COO)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(COD) (5), [Rh(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(COD)]⁺[BF₄]⁻ (6), IrCl(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(COD) (7), IrCl(1,3-diisopropyl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(COD) (8), IrBr(1,3-di-2-propylimidazolin-2-ylidene)(COD) (9), RuCl₂(PCy₃)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(CH–C₆H₅) (10), RuCl₂(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(CH-2-(2-PrO)-5-NO₂-C₆H₃) (11), Ru(CO₂CF₃)₂(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(CH-2-(2-PrO)-5-NO₂-C₆H₃) (12). Compounds 1–6 were active in the polymerization of phenylacetylene. cis-Poly(phenylacetylene) (PPA) was obtained with the rhodium-based catalysts 1, 2, 4–6, trans-PPA was obtained with the Ir-based catalysts 3 and 8. In addition, compounds 1 and 6 were found to produce highly stereoregular PPA with a cis-content of 100% in the presence of water. Finally, the Ru-based metathesis initiator 12 allowed for the synthesis of trans-PPA, representing the first example of a ruthenium complex being active in the polymerization of a terminal alkyne.     

Synthesis and reactivity of homogeneous and heterogeneous ruthenium-based metathesis catalysts containing electron-withdrawing ligands

The synthesis and heterogenization of new Grubbs-Hoveyda type metathesis catalysts by chlorine exchange is described. Substitution of one or two chlorine ligands with trifluoroacetate and trifluoromethanesulfonate was accomplished by reaction of [RuCl(2)([double bond]CH-o-iPr-O-C(6)H(4))(IMesH(2))] (IMesH(2) = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) with the silver salts CF(3)COOAg and CF(3)SO(3)Ag, respectively. The resulting compounds, [Ru(CF(3)SO(3))(2)([double bond]CH-o-iPr-O-C(6)H(4))(IMesH(2))] (1), [RuCl(CF(3)SO(3))([double bond]CH-o-iPr-O-C(6)H(4))(IMesH(2))] (2), and [Ru(CF(3)CO(2))(2)([double bond]CH-o-iPr-O-C(6)H(4))(IMesH(2))] (3) were found to be highly active catalysts for ring-closing metathesis (RCM) at elevated temperature (45 degrees C), exceeding known ruthenium-based catalysts in catalytic activity. Turn-over numbers (TONs) up to 1800 were achieved in RCM. Excellent yields were also achieved in enyne metathesis and ring-opening cross metathesis using norborn-5-ene and 7-oxanorborn-5-ene-derivatives. Even more important, 3 was found to be highly active in RCM at room temperature (20 degrees C), allowing TONs up to 1400. Heterogeneous catalysts were synthesized by immobilizing [RuCl(2)([double bond]CH-o-iPr-O-C(6)H(4))(IMesH(2))] on a perfluoroglutaric acid derivatized polystyrene-divinylbenzene (PS-DVB) support (silver form). The resulting supported catalyst [RuCl(polymer-CH(2)-O- CO-CF(2)-CF(2)-CF(2)-COO)([double bond]CH-o-iPr-O-C(6)H(4))(IMesH(2))] (5) showed significantly reduced activities in RCM (TONs = 380) compared with the heterogeneous analogue of 3. The immobilized catalyst, [Ru(polymer-CH(2)-O-CO-CF(2)-CF(2)-CF(2)-COO)(CF(3)CO(2))([double bond]CH-o-iPr-O-C(6)H(4))(IMesH(2))] (4) was obtained by substitution of both Cl ligands of the parent Grubbs-Hoveyda catalyst by addition of CF(3)COOAg to 5. Compound 4 can be prepared in high loadings (160 mg catalyst g(-1) PS-DVB) and possesses excellent activity in RCM with TONs up to 1100 in stirred-batch RCM experiments. Leaching of ruthenium into the reaction mixture was unprecedentedly low, resulting in a ruthenium content <70 ppb (ng g(-1)) in the final RCM-derived products.     

Simply assembled and recyclable polymer-supported olefin metathesis catalysts

Polymer-supported ruthenium catalysts (PCy(3))(2)Ru(=C(H)Ph)Cl(2), (PCy(3))Ru(IMes)(=C(H)Ph)Cl(2), and (PCy(3))Ru(SIMes)(=C(H)Ph)Cl(2), where IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene and SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene, have been prepared and found to be effective "boomerang" catalysts for ring-closing metathesis. They are recyclable, show comparable or better reactivity than their homogeneous counterparts, tolerate functional groups, and perform very well with dienes and moderately well with highly hindered substrates.     

Molybdenum Alkylidyne Silyloxy N-Heterocyclic Carbene Complexes – Highly Active Alkyne Metathesis Catalysts that can be Handled in Air

A series of molybdenum alkylidyne silyloxy N-heterocyclic carbene (NHC) complexes of the general formula [Mo(≡C(R))(OSiPh₃)₃(NHC)] (R=tBu, 4-methoxyphenyl, 2,4,6-trimethylphenyl; NHC = 1,3-diisopropylimidazol-2-ylidene, 1,3-dicyclohexylimidazol-2-ylidene, 1,3-dicyclohexyl-4,5-dihydroimidazol-2-ylidene, 1,3-dimethylimidazol-2-ylidene, 1,3-dimethyl-4,5-dichloroimidazol-2-ylidene) was synthesized. Single crystal X-ray analyses revealed that with increasing steric demand of the alkylidyne group, enhanced air-stability of the complexes in the solid-state is achieved with the most stable complex (R=2,4,6-trimethylphenyl, NHC = 1,3-diisopropylimidazol-2-ylidene) being stable in air for 24 h without showing signs of decomposition in ¹H NMR. In contrast to previously reported air-stable molybdenum-based complexes, the novel catalysts proved to be highly active in alkyne metathesis, allowing for turnover numbers (TONs) of up to 6000 without further activation, and tolerant towards several functional groups such as tosyl, ether, ester, thioether and nitro moieties. Their air stability allows for facile handling of the catalysts in air and even after exposure to ambient atmosphere for one week, the most stable representative still displayed high productivity in alkyne metathesis.     

N-heterocyclic carbene and phosphine ruthenium indenylidene precatalysts: a comparative study in olefin metathesis

Kinetic studies on ring-closing metathesis of unhindered and hindered substrates using phosphine and N-heterocyclic carbene (NHC)-containing ruthenium-indenylidene complexes (first and second generation precatalysts, respectively) have been carried out. These studies reveal an appealing difference, between the phosphine and NHC-containing catalysts, associated with a distinctive rate-determining step in the reaction mechanism. These catalysts have been compared with the benzylidene generation catalysts and their respective representative substrates. Finally, the reaction scope of the two most interesting precatalysts, complexes that contain tricyclohexylphosphine and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (SIMes), has been investigated for the ring-closing and enyne metathesis for a large range of olefins. Owing to their high thermal stability, the SIMes-based indenylidene complexes were more efficient than their benzylidene analogues in the ring-closing metathesis of tetrasubstituted dienes. Importantly, none of the indenylidene precatalysts were found to be the most efficient for all of the substrates, indeed, a complementary complex-to-substrate activity relationship was observed.     

Copper (I) 1,3-R2-3,4,5,6-tetrahydropyrimidin-2-ylidenes (R=mesityl, 2-propyl): synthesis, X-ray structures, immobilization and catalytic activity

The synthesis of novel copper (I) N-heterocyclic carbene complexes is described. Thus, reaction of CuX with 1,3-di(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene yields CuX(1,3-di(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene) (X=Cl, (1a), Br (1b)); however, reaction of CuCl with 1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene yields the bis-N-heterocylcic carbene complex Cu(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)₂⁺CuBr₂⁻ (2). A supported version of 1, i.e. PS-DVB-CH₂-OCO-CF₂-CF₂-CF₂-COOCu(1,3-di(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene) (3) was prepared from 1 and PS-DVB-CH₂-OCO-CF₂-CF₂-CF₂-COOAg. A copper loading of 4.15 μmol/g was realized. The new compounds were used as catalysts in carbonyl hydrosilylation and cyanosilylation reactions. Excellent reactivity was observed, giving raise to turn-over numbers (TONs) of up to 100,000. Compounds 1a, 1b, and 2 have also been used as catalysts for the atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA). A linear conversion of monomer with time was observed, however, no control over molecular weight of PMMA was observed.     

Chelating Ruthenium Phenolate Complexes: Synthesis,General Catalytic Activity,and Applications in Olefin Metathesis Polymerization

Cyclic Ru‐phenolates were synthesized, and these compounds were used as olefin metathesis catalysts. Investigation of their catalytic activity pointed out that, after activation with chemical agents, these catalysts promote ring‐closing metathesis (RCM), enyne and cross‐metathesis (CM) reactions, including butenolysis, with good results. Importantly, these latent catalysts are soluble in neat dicyclopentadiene (DCPD) and show good applicability in ring‐opening metathesis polymeriyation (ROMP) of this monomer.     

Synthesis and activity of ruthenium olefin metathesis catalysts coordinated with thiazol-2-ylidene ligands

A new family of ruthenium-based olefin metathesis catalysts bearing a series of thiazole-2-ylidene ligands has been prepared. These complexes are readily accessible in one step from commercially available (PCy3)2Cl2Ru=CHPh or (PCy3)Cl2Ru=CH(o-iPrO-Ph) and have been fully characterized. The X-ray crystal structures of four of these complexes are disclosed. In the solid state, the aryl substituents of the thiazole-2-ylidene ligands are located above the empty coordination site of the ruthenium center. Despite the decreased steric bulk of their ligands, all of the complexes reported herein efficiently promote benchmark olefin metathesis reactions such as the ring-closing of diethyldiallyl and diethylallylmethallyl malonate and the ring-opening metathesis polymerization of 1,5-cyclooctadiene and norbornene, as well as the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene and the macrocyclic ring-closing of a 14-membered lactone. The phosphine-free catalysts of this family are more stable than their phosphine-containing counterparts, exhibiting pseudo-first-order kinetics in the ring-closing of diethyldiallyl malonate. Upon removing the steric bulk from the ortho positions of the N-aryl group of the thiazole-2-ylidene ligands, the phosphine-free catalysts lose stability, but when the substituents become too bulky the resulting catalysts show prolonged induction periods. Among five thiazole-2-ylidene ligands examined, 3-(2,4,6-trimethylphenyl)- and 3-(2,6-diethylphenyl)-4,5-dimethylthiazol-2-ylidene afforded the most efficient and stable catalysts. In the cross metathesis reaction of allyl benzene with cis-1,4-diacetoxy-2-butene increasing the steric bulk at the ortho positions of the N-aryl substituents results in catalysts that are more Z-selective.     

Olefin metathesis catalysts bearing a pH-responsive NHC ligand: a feasible approach to catalyst separation from RCM products

Two novel ruthenium-based olefin metathesis catalysts, H(2)ITap(PCy(3))Cl(2)Ru[double bond, length as m-dash]CH-Ph and H(2)ITapCl(2)Ru[double bond, length as m-dash]CH-(C(6)H(4)-O-iPr) (H(2)ITap = 1,3-bis(2',6'-dimethyl-4'-dimethylaminophenyl)-4,5-dihydroimidazol-2-ylidene), were synthesized bearing a pH-responsive NHC ligand with two aromatic NMe(2) groups. The crystal structures of complexes and were determined via X-ray crystallography. Both catalysts perform ring opening metathesis polymerization (ROMP) of cyclooctene (COE) at faster rates than their commercially available counterparts H(2)IMes(PCy(3))Cl(2)Ru[double bond, length as m-dash]CH-Ph and H(2)IMesCl(2)Ru[double bond, length as m-dash]CH-(C(6)H(4)-O-iPr) (H(2)IMes = 1,3-bis(2',4',6'-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) and perform at similar rates during ring closing metathesis (RCM) of diethyldiallylmalonate (DEDAM). Upon addition of 2 equiv. of HCl, catalyst is converted into a mixture of several mono and diprotonated Ru-carbene species 12' which are soluble in methanol but degrade within a few hours at room temperature. Catalyst can be protonated with 2 equiv. of HCl and the resulting complex is moderately water-soluble. The complex is stable in aqueous solution in air for >4 h, but over prolonged periods of time shows degradation in acidic media due to hydrolysis of the NHC-Ru bond. Catalysts and perform RCM of diallylmalonic acid in acidic protic media with only moderate activity at 50 degrees C and do not produce polymer in the ROMP of cationic 7-oxanorbornene derivative under the same conditions. Catalyst was used for Ru-seperation studies when RCM of DEDAM or 3,3-diallypentadione (DAP) was conducted in low-polar organic solution and the Ru-species was subsequently precipitated by addition of strong acid. The Ru-species were removed by (1) filtration and (2) filtration and subsequent extraction with water. The residual Ru-levels could be reduced to as far as 11 ppm (method 2) and 24 ppm (method 1) without the use of chromatography or other scavenging methods.     

Rh(1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene)(COD) tetrafluoroborate, an unsymmetrical Rh-homoazallylcarbene: synthesis, X-ray structure and reactivity in carbonyl arylation and hydrosilylation reactions

The synthesis of novel Rh(1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene)(COD) tetrafluoroborate (1, COD = η⁴-1,5-cyclooctadiene) is described. The N-heterocyclic carbene acts as a bidentate ligand with the carbene coordinating to the Rh(I) center and an arene group acting as a homoazallyl ligand. 1 was used in various carbonyl arylation and hydrosilylation reactions allowing the formation of the desired products with unprecedented selectivity and efficiency. Thus, turn-over numbers (TONs) up to 2000 were achieved.     

Stoichiometric and catalytic reactivity of the N-heterocyclic carbene ruthenium hydride complexes [Ru(NHC)(L)(CO)HCl] and [Ru(NHC)(L)(CO)H(eta2-BH4)] (L=NHC, PPh3)

Thermolysis of [Ru(AsPh3)3(CO)H2] with the N-aryl heterocyclic carbenes (NHCs) IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) or the adduct SIPr.(C6F5)H (SIPr=1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene), followed by addition of CH2Cl2, affords the coordinatively unsaturated ruthenium hydride chloride complexes [Ru(NHC)2(CO)HCl] (NHC=IMes , IPr , SIPr ). These react with CO at room temperature to yield the corresponding 18-electron dicarbonyl complexes . Reduction of and [Ru(IMes)(PPh3)(CO)HCl] () with NaBH4 yields the isolable borohydride complexes [Ru(NHC)(L)(CO)H(eta2-BH4)] (, L=NHC, PPh3). Both the bis-IMes complex and the IMes-PPh3 species react with CO at low temperature to give the eta1-borohydride species [Ru(IMes)(L)(CO)2H(eta1-BH4)] (L=IMes , PPh3), which can be spectroscopically characterised. Upon warming to room temperature, further reaction with CO takes place to afford initially [Ru(IMes)(L)(CO)2H2] (L=IMes, L=PPh3) and, ultimately, [Ru(IMes)(L)(CO)3] (L=IMes , L=PPh3). Both and lose BH3 on addition of PMe2Ph to give [Ru(IMes)(L)(L')(CO)H2](L=L'=PMe2Ph; L=PPh3, L'=PMe2Ph). Compounds and have been tested as catalysts for the hydrogenation of aromatic ketones in the presence of (i)PrOH and H2. For the reduction of acetophenone, catalytic activity varies with the NHC present, decreasing in the order IPr>IMes>SIMes.     

Predictive Catalysis in Olefin Metathesis with Ru-based Catalysts with Annulated C60 Fullerenes in the N-heterocyclic Carbenes

Predictive catalysis must be the tool that does not replace experiments, but acts as a selective agent, so that synthetic strategies of maximum profitability are used in the laboratory in a surgical way. Here, nanotechnology has been used in olefin metathesis from homogeneous Ru-NHC catalysts, specifically annulating a C₆₀ fullerene to the NHC ligand. Based on results with the C₆₀ in the backbone, a sterile change with respect to the catalysis of the metal center, an attempt has been made to bring C₆₀ closer to the metal, by attaching it to one of the two C−N bonds of the imidazole group of the SIMes (1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) ligand (reference NHC ligand of the 2^nd generation Grubbs catalysts) to increase the steric pressure of C₆₀ in the first sphere of reactivity of the metal. The DFT calculated thermodynamics and the kinetics of SIMes-derived systems show that they are efficient catalysts for olefin metathesis.     

Isocyanate‐ and Isothiocyanate‐Derived RuIV‐Based Alkylidenes: Synthesis,Structure, and Activity

The synthesis and characterization of a series of isocyanate‐ and isothiocyanate‐derived second generation Grubbs–Hoveyda‐type ruthenium–alkylidene complexes, that is, [Ru(N?C?O)₂(IMesH₂)(?CH‐2‐(2‐PrO)‐C₆H₄)] ( 1 ), [Ru(N?C?O)₂(1,3‐dimesityl‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene)(=CH‐2‐(2‐PrO)‐C₆H₄)] ( 2 ), [Ru(N?C?S)₂(IMesH₂)(?CH‐2‐(2‐PrO)‐C₆H₄)] ( 3 ), and [Ru(N?C?S)₂(1,3‐dimesityl‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene)(?CH‐2‐(2‐PrO)‐C₆H₄)] ( 4 ), and their activity in various metathesis reactions are described. Compounds 1 – 4 were prepared by reaction of the parent complexes [RuCl₂(IMesH₂)(?CH‐2‐(2‐PrO)C₆H₄)] ( 5 ) (IMesH₂=1,3‐bis‐(2,4,6‐trimethylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene) and [RuCl₂(1,3‐dimesityl‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene)(?CH‐2‐(2‐PrO)‐C₆H₄)] ( 6 ) with silver cyanate and thiocyanate, respectively. The X‐ray structure of 1 was determined, confirming the isocyanate‐type bonding of the ligand. The isothiocyanate‐type bonding in 3 and 4 was unambiguously confirmed by IR and ¹³C NMR spectroscopy. The isocyanate‐derived complexes 1 and 2 were found to be excellent catalysts for the ring‐opening metathesis polymerization (ROMP) of cis‐cycloocta‐1,5‐diene (COD). Both 1 and 2 yielded poly(COD) with a trans‐content of about 80 %. First‐order kinetics with unprecedentedly high rate constants of polymerization (k_p=0.068 and 0.26 s^?1, respectively) were observed. Compounds 3 and 4 were also active initiators for the ROMP of COD, however, they generated poly(COD) with a cis‐content of 80 and 67 %, respectively. Complexes 1 and 2 also showed good catalytic activity in cross‐metathesis (CM) reactions. Finally, 1 – 4 were also found to be excellent catalysts for the regioselective cyclopolymerization of diethyl 2,2‐dipropargylmalonate (DEDPM), resulting in poly(DEDPM) almost entirely based on five‐membered repeat units, that is, cyclopent‐1‐ene‐1,2‐vinylenes.     

Unsaturated dinickel-molybdenum clusters with N-heterocyclic carbene ligands

The first examples of mixed metal trinuclear clusters carrying N-heterocyclic carbene (NHC) ligands were isolated from reactions of the complexes [Ni(NHC)ClCp] [NHC = bis-(2,6-diisopropylphenyl)- or bis-(2,4,6-trimethylphenyl)-imidazol-2-ylidene] with [Mo(CO)(3)Cp](-); the unsaturated 46-electron clusters have triangular MoNi(2) cores and the reaction pathway activates usually inert Ni-Cp and Ni-NHC bonds.     

Ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene ligands: synthesis, structure, and catalytic activity

A series of ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene (NHC) ligands has been prepared and fully characterized. These complexes are readily accessible in one or two steps from commercially available [(PCy(3))(2)Cl(2)Ru==CHPh]. All of the complexes reported herein promote the ring-closing of diethyldiallyl and diethylallylmethallyl malonate, the ring-opening metathesis polymerization of 1,5-cyclooctadiene, and the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene, in some cases surpassing in efficiency the existing second-generation catalysts. Especially in the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene, all new catalysts demonstrate similar or higher activity than the second-generation ruthenium catalysts and, most importantly, afford improved E/Z ratios of the desired cross-product at conversion above 60 %. The influence of the unsymmetrical NHC ligands on the initiation rate and the activation parameters for the irreversible reaction of these ruthenium complexes with butyl vinyl ether were also studied. Finally, the synthesis of the related chlorodicarbonyl(carbene) rhodium(I) complexes allowed for the study of the electronic properties of the new unsymmetrical NHC ligands that are discussed in detail.     

The pivotal role of symmetry in the ruthenium-catalyzed ring-closing metathesis of olefins

The synthesis of Ru-based precatalysts with N-heterocyclic carbene (NHC) ligands bearing syn- and anti-methyl groups on the NHC backbone and aryl N-substituents with differing steric bulk was carried out. The catalytic behavior of the monophospine Ru precatalysts (7a, 7b, 8a, and 8b) was compared to the corresponding family of phosphine-free catalysts (9a, 9b, 10a and 10b) in the ring-closing metathesis (RCM) of olefins. These catalysts showed high efficiency in RCM reactions and the syn-isomers 7a and 9a, in particular, proved to be among the most active catalysts in the formation of tetrasubstituted olefins through RCM. DFT studies on the entire RCM catalytic cycle of hindered olefins were performed to rationalize the different behaviors of catalysts with syn- and anti-methyl groups on the NHC backbone. Theoretical results not only disclosed how NHC symmetry influences the overall activity of the catalyst, but also gave relevant and more general indications on the crucial steps of the RCM of olefins.     

Synthesis and activity of ruthenium alkylidene complexes coordinated with phosphine and N-heterocyclic carbene ligands

This paper reports the synthesis and characterization of a variety of ruthenium complexes coordinated with phosphine and N-heterocyclic carbene (NHC) ligands. These complexes include several alkylidene derivatives of the general formula (NHC)(PR(3))(Cl)(2)Ru=CHR', which are highly active olefin metathesis catalysts. Although these catalysts can be prepared adequately by the reaction of bis(phosphine) ruthenium alkylidene precursors with free NHCs, we have developed an alternative route that employs NHC-alcohol or -chloroform adducts as "protected" forms of the NHC ligands. This route is advantageous because NHC adducts are easier to handle than their free carbene counterparts. We also demonstrate that sterically bulky bis(NHC) complexes can be made by reaction of the pyridine-coordinated precursor (NHC)(py)(2)(Cl)(2)Ru=CHPh with free NHCs or NHC adducts. Two crystal structures are presented, one of the mixed bis(NHC) derivative (H(2)IMes)(IMes)(Cl)(2)Ru=CHPh, and the other of (PCy(3))(Cl)(CO)Ru[eta(2)-(CH(2)-C(6)H(2)Me(2))(N(2)C(3)H(4))(C(6)H(2)Me(3))], the product of ortho methyl C-H bond activation. Other side reactions encountered during the synthesis of new ruthenium alkylidene complexes include the formation of hydrido-carbonyl-chloride derivatives in the presence of primary alcohols and the deprotonation of ruthenium vinylcarbene ligands by KOBu(t). We also evaluate the olefin metathesis activity of NHC-coordinated complexes in representative RCM and ROMP reactions.