Imido Alkylidene Complexes Research Articles

Cationic Tungsten Imido Alkylidene N-Heterocyclic Carbene Complexes That Contain Bulky Ligands

Neutral and cationic tungsten imido alkylidene complexes of the general formulas W(NtBu)(CHR1)(OR2)Cl(NHC), W(N-2,6-bis(2,4,6-tri-iPr-C6H4)phenyl)(CHR1)Cl2(NHC), [W(NtBu)(CHR1)(OR2)(NHC)][B(ArF)4] and [W(N-2,6-bis(2,4,6-tri-iPr-C6H4)phenyl)(CHR1)Cl(NHC)][B(ArF)4] (R1 = CMe3, CMe2Ph; R2 = sterically demanding alkoxide; B(ArF)4 = tetrakis(3,5-(CF3)2-C6H3)borate; NHC = N-heterocyclic carbene) were prepared. Two electronically different NHCs, namely 1,3-dimethylimidazol-2-ylidene (IMe) and 1,3-dimethyl-4,5-dichloroimidazol-2-ylidene (IMeCl), as well as a variety of terphenolates and a chiral biphenolate were employed. Z-selective homometathesis (HM) of unfunctionalized olefins was achieved with a selectivity of up to 90% and decent turnover numbers (TON) of up to 480 in the HM of 1-dodecene. Additionally, the reactivity of the cationic tungsten tert-butylimido complexes in the reaction with vinyltrimethylsilane and ethylene was investigated, which yielded the corresponding silyl-alkylidene complex and, for the first time, a fully characterized cationic tungsten(IV) NHC ethylene complex.

Understanding Synthetic Peculiarities of Cationic Molybdenum(VI) Imido Alkylidene N‐Heterocyclic Carbene Complexes

We report on novel synthetic approaches to molybdenum imido alkylidene N‐heterocyclic carbene (NHC) complexes including the first NHC alkylidene complexes featuring pentafluorophenyl‐substituted imido ligands, highly basic 6‐Mes (6‐Mes = 1,3‐dimesityl‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene) NHC ligands and the preparation of the first representatives of cationic 14‐electron Mo imido alkylidene NHC catalysts bearing sterically demanding anionic terphenoxide ligands. Also, the Mo imido alkylidene and alkylidyne NHC bisalkoxide complexes [Mo(NR)(CHCMe2Ph)(NHC)(OR′)2], [Mo(NH‐C6F5)(CCMe2Ph)(aIMes){OCMe(CF3)2}2] (aIMes = 1,3‐dimesitylimidazol‐5‐ylidene, an abnormally C5‐bound imidazolylidene) and [Mo(NH‐2‐CF3‐C6H4)(CCMe2Ph)(6‐Mes){OCMe(CF3)2}2] are presented. Anionic Mo amido alkylidyne complexes have been found to form preferentially in the presence of sterically demanding NHC and alkoxide ligands. The first Mo imido alkylidene NHC complexes containing the strongly basic 6‐Mes ligand, namely [Mo(N‐C6F5)(CHCMe2Ph)(6‐Mes)(Br)2], [Mo(N‐2‐CF3‐C6H4)(CHCMe2Ph)(6‐Mes)(Br)2] and [Mo(N‐2,6‐Me2‐C6H3)(CHCMe2Ph)(6‐Mes)(Br)2], were synthesized from the corresponding Mo imido dibromo alkylidene‐DME complexes (DME = 1,2‐dimethoxyethane) and transformed into their cationic monobromo or monoalkoxide derivatives. Finally, we developed the high yield synthesis of Mo imido alkylidene NHC dichloro complexes via direct protonation of [Mo(N‐Adamantyl)2(CH2CMe2Ph)2] with HCl followed by stabilization with NHCs.

Latent and Air-Stable Pre-Catalysts for the Polymerization of Dicyclopentadiene: From Penta- to Hexacoordination in Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes.

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.

Molybdenum and Tungsten Imido Alkylidene N-Heterocyclic Carbene Catalysts Bearing Cationic Ligands for Use in Biphasic Olefin Metathesis.

Ionic Mo- and W-imido alkylidene N-heterocyclic carbene (NHC) olefin metathesis catalysts, [Mo{N-2,6-(Me2 )C6 H3 }(CHCMe2 Ph)(IMesH2 )(OTf)(PPS)]OTf (3), [Mo(N-2,6-(Me2 )C6 H3 )(CHCMe2 Ph)(IMesH2 )(OC6 F5 )(PPS)][B(ArF )4 ] (5), [Mo(NtBu)(CHCMe2 Ph)(4,5-dichloro-1,3-dimethylimidazol-2-ylidene)(OTf)(2,6-Ph-4-{2,4,6-Ph-pyridinium}phenolate)][OTf] (9), [Mo(NtBu)(CHCMe2 Ph)(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=CF3 SO3 , 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(NtBu)(CHCMe2 Ph)(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 -C6 H3 )(CHCMe2 Ph)(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 -C6 H3 )(CHCMe2 Ph)(N-2,5-Me2 C4 H2 )(IiPr)(2,6-tBu-4-PPh3 -phenolate)] (17, IiPr=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.

Molybdenum and Tungsten Alkylidene and Metallacyclobutane Complexes That Contain a Dianionic Biphenolate Pincer Ligand

Molybdenum imido alkylidene and tungsten oxo alkylidene complexes that contain a tridentate “pincer” [ONO]2– ligand have been prepared and treated with ethylene to give unsubstituted metallacyclobutane complexes that have a 16e count. Both Mo and W metallacyclobutane complexes exchange C2D4 into the metallacyclobutane ring at 22 °C at a rate that is first order in metal and zero order in C2D4. These metallacycles lose ethylene at least 104–105 times slower than reported 14e unsubstituted Mo and W metallacyclobutane complexes that have been explored in the literature that have a TBP geometry with the metallacyclobutane ring bound in the equatorial positions. Our studies suggest that breaking up the metallacyclobutane ring in these 16e d0 Mo or W complexes is slow because a 14e TBP metallacyclobutane complex cannot be accessed readily.

Application of imidazolinium salts and N-heterocyclic olefins for the synthesis of anionic and neutral tungsten imido alkylidene complexes.

The synthesis, single crystal X-ray structure and activity in olefin metathesis of novel anionic tungsten imido alkylidene complexes [1,3-bis-(2,4,6-trimethylphenyl)imidazolinium](+) [W(N-2,6-iPr2C6H3)(CHCMe2Ph)(2,5-Me2Pyr)2Cl](-), [1,3-bis-(2,4,6-trimethylphenyl)imidazolinium](+) [W(N-2,6-iPr2C6H3)(CHCMe2Ph)(2,5Me2Pyr)2(OC6F5)](-), and [1,3-bis-(2,6-diisopropylphenyl)imidazolinium](+) [W(N-2,6-iPr2C6H3)(CHCMe2Ph)(2,5-Me2Pyr)Cl2](-) are reported. Additionally, the first example of a bis(N-heterocyclic olefinium) alkylidene tungstate, W(N-2,6-iPr2C6H3)(CHCMe2Ph)(2-methylene-1,3,4,5-tetramethyl-imidazoline)2(OTf)2, is described, including preparation, crystal structure and catalytic activity.

Formation of Alternating trans-A-alt-B Copolymers through Ring-Opening Metathesis Polymerization Initiated by Molybdenum Imido Alkylidene Complexes

Ring-opening metathesis polymerization (ROMP) is used to prepare trans-poly(A-alt-B) polymers from a 1:1 mixture of A and B where A is a cyclic olefin such as cyclooctene (A1) or cycloheptene (A2) and B is a large norbornadiene or norbornene derivative such as 2,3-dicarbomethoxy-7-isopropylidenenorbornadiene (B1) or dimethylspirobicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate-7,1′-cyclopropane (B2). The most successful initiators that were examined are of the type Mo(NR)(CHCMe2Ph)[OCMe(CF3)2]2 (R = 2,6-Me2C6H3 (1) or 2,6-i-Pr2C6H3 (2)). The trans configuration of the AB linkages is proposed to result from the steric demand of B. Both anti-MB and syn-MB alkylidenes are observed during the copolymerization, where B was last inserted into a Mo═C bond, although anti-MB dominates as the reaction proceeds. anti-MB is lower in energy than syn-MB, does not react readily with either A or B, and interconverts slowly with syn-MB through rotation about the Mo═C bond. Syn-MB does not readily react with B, but it does ...

Quantitatively analyzing metathesis catalyst activity and structural features in silica-supported tungsten imido-alkylidene complexes.

A broad series of fully characterized, well-defined silica-supported W metathesis catalysts with the general formula [(≡SiO)W(═NAr)(═CHCMe2R)(X)] (Ar = 2,6-iPr2C6H3 (AriPr), 2,6-Cl2C6H3 (ArCl), 2-CF3C6H4 (ArCF3), and C6F5 (ArF5); X = OC(CF3)3 (OtBuF9), OCMe(CF3)2 (OtBuF6), OtBu, OSi(OtBu)3, 2,5-dimethylpyrrolyl (Me2Pyr) and R = Me or Ph) was prepared by grafting bis-X substituted complexes [W(NAr)(═CHCMe2R)(X)2] on silica partially dehydroxylated at 700 °C (SiO2-(700)), and their activity was evaluated with the goal to obtain detailed structure-activity relationships. Quantitative influence of the ligand set on the activity (turnover frequency, TOF) in self-metathesis of cis-4-nonene was investigated using multivariate linear regression analysis tools. The TOF of these catalysts (activity) can be well predicted from simple steric and electronic parameters of the parent protonated ligands; it is described by the mutual contribution of the NBO charge of the nitrogen or the IR intensity of the symmetric N-H stretch of the ArNH2, corresponding to the imido ligand, together with the Sterimol B5 and pKa of HX, representing the X ligand. This quantitative and predictive structure-activity relationship analysis of well-defined heterogeneous catalysts shows that high activity is associated with the combination of X and NAr ligands of opposite electronic character and paves the way toward rational development of metathesis catalysts.

Synthesis of Molybdenum and Tungsten Alkylidene Complexes That Contain Sterically Demanding Arenethiolate Ligands.

Imido alkylidene complexes of Mo and W and oxo alkylidene complexes of W that contain thiophenoxide ligands of the type S-2,3,5,6-Ph4C6H (STPP) and S-2,6-(mesityl)2C6H3 (SHMT = S-hexamethylterphenyl) have been prepared in order to compare their metathesis activity with that of the analogous phenoxide complexes. All thiolate complexes were significantly slower (up to ∼10× slower) for the metathesis homocoupling of 1-octene or polymerization of 2,3-dicarbomethoxynorbornene, and none of them was Z-selective. The slower rates could be attributed to the greater σ-donating ability of a thiophenoxide versus the analogous phenoxide and consequently a higher electron density at the metal in the thiophenoxide complexes.

Synthesis of stereoregular polymers through ring-opening metathesis polymerization.

Some of the most readily available and inexpensive monomers for ring-opening metathesis polymerization (ROMP) are norbornenes or substituted norbornadienes. Polymers made from them have tacticities (the stereochemical relationship between monomer units in the polymer chain) that remain after the C═C bonds in the polymer backbone are hydrogenated. Formation of polymers with exclusively a single structure (one tacticity) was rare until approximately 20 years ago, when well-defined ROMP catalysts based on molybdenum imido alkylidene complexes that contain a chiral biphenolate or binaphtholate ligand were shown to yield cis,isotactic-poly(2,3-dicarbomethoxynorbornadiene) and related polymers through addition of the monomer to the same side of the M═C bond in each step. Over the past few years, molybdenum and tungsten monoaryloxide pyrrolide (MAP) imido alkylidene initiators have been found to produce cis,syndiotactic polynorbornenes and substituted norbornadienes through addition of the monomer to one side of the M═C bond in one step followed by addition to the other side of the M═C bond in the next step. This "stereogenic metal control" is possible as a consequence of the fact that the configuration of the stereogenic metal center switches with each step in the polymerization. Stereogenic metal control also allows syndiotactic polymers to be prepared from racemic monomers in which enantiomers of the monomer are incorporated alternately into the main chain. Because pure trans polymers have not yet been prepared through some predictable mechanism of stereochemical control, it seems unlikely that all four basic polymer structures from a single given monomer can be prepared simply by choosing the right initiator. However, because tactic, and relatively oxygen-stable, hydrogenated polymers are often a desirable goal, the ability to form pure cis,isotactic polymers (through enantiomorphic site control) and cis,syndiotactic polymers (through stereogenic metal control) is sufficient for preparing hydrogenated polymers with a single structure. It is hoped that the principles of forming polymers that have a single structure through ring-opening metathesis polymerization will be general for a relatively large number of monomers and that some important problems in ROMP polymer chemistry can benefit from knowledge of polymer structure at a molecular level. With an increase in knowledge concerning the mechanistic details of polymerization by well-defined initiators, more elaborate ROMP polymers and copolymers with stereoregular structures may be possible.

New Boron-Containing Molybdenum Imido Alkylidene Complexes for Linear Olefin Homometathesis

The new molybdenum imido alkylidene complex Mo(N(2,6-iPr2C6H3))(CHCMe2Ph)(NC4H2Me2)(OB(Mes)2) (1; Mes = 2,4,6-MePh) containing both boroxide and pyrrolide ligands is reported. Its formation results from the reaction between bis(mesityl)borinic acid ((Mes)2BOH) and the bis-pyrrolide Schrock-type precursor Mo(N-2,6-iPr2C6H3)(CHCMe2Ph)(NC4H2Me2)2. The complex was fully characterized by 1H, 13C, 11B, and 95Mo NMR spectroscopy, X-ray diffraction, and elemental analysis. Complex 1 proved to be active for homometathesis reactions of 1- and 2-octene at 0.1 mol % loading. The synthesis of mixed pyrrolide boroxide imido molybdenum alkylidene complexes was extended to other borinic acids. The catalytic activity of these new complexes was evaluated in the homometathesis of linear olefins.

Molybdenum and Tungsten Monoalkoxide Pyrrolide (MAP) Alkylidene Complexes That Contain a 2,6-Dimesitylphenylimido Ligand

Molybdenum and tungsten bispyrrolide alkylidene complexes that contain a 2,6-dimesitylphenylimido (NAr*) ligand have been prepared, in which the pyrrolide is the parent pyrrolide or 2,5-dimethylpyrrolide. Monoalkoxide pyrrolide (MAP) complexes were prepared through addition of 1 equiv of an alcohol to the bispyrrolide complexes. MAP compounds that contain the parent pyrrolide (NC4H4–) are pyridine adducts, while those that contain 2,5-dimethylpyrrolide are pyridine free. Molybdenum and tungsten MAP 2,5-dimethylpyrrolide complexes that contain O-t-Bu, OCMe(CF3)2, or O-2,6-Me2C6H3 ligands were found to have approximately equal amounts of syn and anti alkylidene isomers, which allowed a study of the interconversion of the two employing 1H–1H EXSY methods. The Keq values ([syn]/[anti]) are all 2–3 orders of magnitude smaller than those observed for a large number of Mo bisalkoxide imido alkylidene complexes, as a consequence of the destabilization of the syn isomer by the sterically demanding NAr* ligand. The rates of interconversion of syn and anti isomers were found to be 1–2 orders of magnitude faster for W MAP complexes than for Mo MAP complexes.

Syntheses of Variations of Stereogenic-at-Metal Imido Alkylidene Complexes of Molybdenum

In this paper we describe the syntheses of several new stereogenic-at-metal imido alkylidene complexes of molybdenum, Mo(NR)(CHR′)(X)(Y), many of which had to be prepared through selective nucleophilic displacement reactions in imido alkylidene complexes. The reported compounds include Mo(NAd)(CHCMe2Ph)(MesPyr)2 (1a; MesPyr = 2-mesitylpyrrolide, Ad = 1-adamantyl), Mo(NAd)(CHCMe2Ph)(2-CNPyr)2 (1b; 2-CNPyr = 2-cyanopyrrolide), Mo(NAd)(CHCMe2Ph)(MesPyr)(OTPP) (2a; OTPP = 2,3,5,6-tetraphenylphenoxide), Mo(NAd)(CHCMe2Ph)(MesPyr)(OBr2Bitet) (2b; OBr2Bitet = (R)-3,3′-dibromo-2′-(tert-butyldimethylsilyloxy)-5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-binaphthyl-2-olate), Mo(NAd)(CHCMe2Ph)(OHIPT)(2-Mespyr) (2c; HIPT = 2,6-(2,4,6-iPr3C6H2)2C6H3), Mo(NAd)(CHCMe2Ph)(OTf)(OHIPT) (3), Mo(NAd)(CHCMe2Ph)(OTf)(OHIPT)(PMe3) (3(PMe3)), Mo(NAd)(CHCMe2Ph)(2-CNPyr)(OHIPT) (4), Mo(NAd)(CHCMe2Ph)(OHIPT)(OCMe3) (5), Mo(NR)(CHCMe2Ph)(ORF6)(OHMT) (ORF6 = OCMe(CF3)2; HMT = 2,6-Mes2C6H3; R = 2,6-iPr2C6H3 (Ar, 6a), 2,6-Me2C6H3 (Ar′, 6b), 2-iPrC6H4 (AriPr, 6c), Ad (6d)), Mo(NR)(CHCMe2Ph)(ORF6)[N(H)HMT] (7a (R = Ar′) and 7b (R = AriPr)), and Mo(NAd)(CHCMe2Ph)(ORF6)(HMT) (8). X-ray structural studies were carried out on 1b, 2a–c, 3(PMe3), 4, 5, 6d, 7b, and 8. Compound 1b is an octamer in which two η1-pyrrolides are trans to one another at each metal center and cyano groups bind from neighboring Mo centers trans to the alkylidene and imido ligands.

Bipyridine Adducts of Molybdenum Imido Alkylidene and Imido Alkylidyne Complexes.

Seven bipyridine adducts of molybdenum imido alkylidene bispyrrolide complexes of the type Mo(NR)(CHCMe(2)R')(Pyr)(2)(bipy) (1a-1g; R = 2,6-i-Pr(2)C(6)H(3) (Ar), adamantyl (Ad), 2,6-Me(2)C(6)H(3) (Ar'), 2-i-PrC(6)H(4) (Ar(iPr)), 2-ClC(6)H(4) (Ar(Cl)), 2-t-BuC(6)H(4) (Ar(t) (Bu)), and 2-MesitylC(6)H(4) (Ar(M)), respectively; R' = Me, Ph) have been prepared using three different methods. Up to three isomers of the adducts are observed that are proposed to be the trans and two possible cis pyrrolide isomers of syn alkylidenes. Sonication of a mixture containing 1a-1g, HMTOH (2,6-dimesitylphenol), and ZnCl(2)(dioxane) led to the formation of MAP species of the type Mo(NR)(CHCMe(2)R')(Pyr)(OHMT) (3a-3g). DCMNBD (2,3-dicarbomethoxynorbornadiene) is polymerized employing 3a-3g as initiators to yield >98% cis,syndiotactic poly(DCMNBD). Attempts to prepare bipy adducts of bisdimethylpyrrolide complexes led to formation of imido alkylidyne complexes of the type Mo(NR)(CCMe(2)R')(Me(2)Pyr)(bipy) (Me(2)Pyr = 2,5-dimethylpyrrolide; 4a - 4g) through a ligand-induced migration of an alkylidene α proton to a dimethylpyrrolide ligand. X-ray structures of Mo(NAr)(CHCMe(2)Ph)(Pyr)(2)(bipy) (1a), Mo(NAr(iPr))(CHCMe(2)Ph)(Pyr)(OHMT) (3d), Mo(NAr)(CCMe(2)Ph)(Me(2)Pyr)(bipy) (4a), and Mo(NAr(T))(CCMe(3))(Me(2)Pyr)(bipy) (Ar(T) = 2-(2,4,6-i-Pr(3)C(6)H(2))C(6)H(4); 4g) showed structures with the normal bond lengths and angles.

Molybdenum Monoaryloxide Pyrrolide Alkylidene Complexes That Contain Mono-ortho-substituted Phenyl Imido Ligands

Monaryloxide pyrrolide (MAP) molybdenum imido alkylidene complexes of the type Mo(NArX)(CHCMe2R)(Me2Pyr)(OR′) (Me2Pyr = 2,5-dimethylpyrrolide) have been prepared in which NArX is an ortho-substituted phenylimido group (X = Cl (NArCl), CF3 (NArCF3), i-Pr (NAriPr), t-Bu (NArtBu), mesityl (NArM), or TRIP (TRIP = triisopropylphenyl; NArT)) and OR′ = O-2,3,5,6-(C6H5)4C6H (OTPP), O-2,6-(2,4,6-Me3C6H2)2C6H3 (OHMT), or O-2,6-(2,4,6-i-Pr3C6H2)2C6H3 (OHIPT). The object was to explore to what extent relatively “large” NArM or NArT ligands would alter the performance of MAP catalysts in reactions that have been proposed to depend upon the relative size of the imido and OR′ groups. Preliminary studies employing the ring-opening metathesis polymerization of 5,6-dicarbomethoxynorbornadiene as a measure of selectivity suggest that a single phenylimido ortho substituent, even in an NArM or NArT group, does not produce any unique behavior and that the outcome of the ROMP reaction correlates with the overall relative size o...

Terminal Enyne Ring-Closing Metathesis Catalyzed by Ruthenium Carbene Catalysts

Enyne ring-closing metathesis (RCM) has recently emerged as a powerful tool for the formation of 1,3conjugated dienes. Under the influence of its efficacy and reliability, the enyne RCM reaction with ruthenium (Ru)based catalysts has been elegantly applied toward the total synthesis of bioactive compounds or natural products. In spite of its usefulness, the enyne RCM has yet a problem which remains to be solved: endo/exo mode selectivity. The exo/endo-selectivity in the enyne RCM has been studied and the reported results have varied. Mori reported that RCM of enyne having an monoor di-substituted alkene and a terminal alkyne gave only the exo product, while that of enyne having a disubstituted alkene and an internal alkyne gave a mixture of both endo and exo products. Lee reported that a competitive cross metathesis (CM) of the alkyne moiety in enyne substrate with ethylene led to a triene, which then underwent ring closure offering selectively endo product in 9-11 membered macrocycles. In the investigation on the reactivity profile of catalysts, Grela observed that the use of Grubbs’ first-generation complexes containing PCy3 ligand such as A and C led to only an exo product, while the use of second-generation complexes containing N-heterocyclic carbene (NHC) ligand led to a mixture of endo and exo products in the RCM of enyne having a disubstituted alkene and a terminal alkyne. Schrock and Hoveyda reported their elegant results on endoselective enyne RCM reactions promoted by use of in situ generated molybdenum (Mo)-based complexes containing pyrrole ligand and later by both tungsten (W)and Mobased mono-aryloxide pyrrolide (MAP) imido alkylidene complexes. Therefore, we envisioned that the endo/exo selectivity of terminal enyne RCM with Ru carbene catalysts could lead us to the selective synthesis of medium-sized N-containing heterocycles as shown in Scheme 1. We examined the well-known Ru-based complexes A-D as well as pyridine-chelating Ru complex E (Fig. 1). It was expected that the NHC ligand along with pyridine moiety in complex E would have an effect on the endo/exo selectivity in enyne RCM reactions. Our study started from a simple representative substrate 1 bearing a terminal alkyne at one end and a terminal olefin at the other. Each of five catalysts (A-E, 5 mol %) was added to the solution of 1 in benzene and the solution was refluxed for 2 h under an atmosphere of Ar. RCM of enyne 1 mediated by catalysts A-D gave only the 5-membered exocyclized compound 3 (Entry 1-4, Table 1). On the other hand, pyridine chelating catalyst E produced a small but significant amount of 6-membered endo-cyclized product 2 with the endo: exo ratio of 27:73 (45% conversion, 6% isolated yield of combined exo/endo) (Entry 5, Table 1). It is the first example of the endo-cyclized product being formed in the terminal enyne RCM using Ru-based catalyst. Since we were interested in the determination of endo/exo selectivity in terminal enyne RCM with Ru carbene catalysts under various reaction conditions, the task of improving isolated yield in each reaction was beyond the scope of the work described in this paper. Subsequently, we tested enyne RCM of 1 using the catalyst E in several representative solvents (Table 2). Among suitable organic solvents, toluene, benzene, tetrahydrofuran (THF), 1,2-dimethoxyethane (DME) and 1,2-dichloroethane (DCE) were carefully selected and examined because catalyst E showed the activating This paper is dedicated to Professor Eun Lee on the occasion of his honourable retirement. Scheme 1. Terminal enyne RCM with ruthenium carbene catalysts.

Synthesis and structures of bimetallic silicon-containing imido alkylidene complexes of molybdenum Me 2Si[CH [dbnd]Mo(NAr)(OR F3) 2] 2 and PhVinSi[CH [dbnd]Mo(NAr)(OR F3) 2] 2

Bimetallic alkylidene complexes of molybdenum (R F3O) 2(ArN)Mo CH–SiMe 2–CH Mo(NAr)(OR F3) 2 ( 1) and (R F3O) 2(ArN)Mo CH–SiPhVin–CH Mo(NAr)(OR F3) 2 ( 2) (Ar = 2,6- Pr 2 i C 6H 3; R F3 = CMe 2CF 3) have been prepared by the reactions of vinyl silicon reagents Me 2Si(CH CH 2) 2 and PhSi(CH CH 2) 3 with known alkylidene compound PhMe 2C–CH Mo(NAr)(OR F3) 2. Complexes 1 and 2 were structurally characterized. Ring opening metathesis polymerization (ROMP) of cyclooctene using compounds 1 and 2 as initiators led to the formation of high molecular weight polyoctenamers with predominant trans-units content in the case of 1 and predominant cis-units content in the case of 2.

Synthesis and structures of bimetallic silicon-containing imido alkylidene complexes of tungsten (R′O) 2(ArN)W [dbnd]CH–SiR 2–CH [dbnd]W(NAr)(OR′) 2 (R = Me, Ph) and (R′O) 2(ArN)W [dbnd]CH–SiMe 2SiMe 2–CH [dbnd]W(NAr)(OR′) 2

Bimetallic alkylidene complexes of tungsten (R′O) 2(ArN)W CH–SiR 2–CH W(NAr)(OR′) 2 (R = Me ( 1), Ph ( 2)) and (R′O) 2(ArN)W CH–SiMe 2SiMe 2–CH W(NAr)(OR′) 2 ( 3) (Ar = 2 , 6 – Pr 2 i C 6 H 3 ; R′ = CMe 2CF 3) have been prepared by the reactions of divinyl silicon reagents R 2Si(CH CH 2) 2 with known alkylidene compounds R′′–CH Mo(NAr)(OR′) 2. (R′′ = Bu t , PhMe 2C) Complexes 1– 3 were structurally characterized. Ring opening metathesis polymerization (ROMP) of cyclooctene using compounds 1– 3 as initiators led to the formation of high molecular weight polyoctenamers with predominant trans-units content in the case of 1 and 3 and predominant cis-units content in the case of 2.

Dramatic enhancement of the alkene metathesis activity of Mo imido alkylidene complexes upon replacement of one tBuO by a surface siloxy ligand

[(triple bond SiO)Mo(triple bond NAr)(=CHCMe2R)(OtBu)], a well-defined silica supported alkene metathesis catalyst precursor, shows a dramatic enhancement of activity and selectivity compared to [Mo(triple bond NAr)(=CHCMe2R)(OtBu)2] and [(triple bond SiO)Mo(triple bond NAr)(=CHCMe2R)(CH2tBu)], respectively.

Alkane Metathesis Catalyzed by a Well‐Defined Silica‐Supported Mo Imido Alkylidene Complex: [(SiO)Mo(NAr)(CHtBu)(CH2tBu)

Zwei Funktionen zum Preis von einer: Der im Bild gezeigte Alkyl(alkyliden)Mo-Komplex mit einem Imido-Hilfsliganden auf einem Siliciumoxidträger ist nicht nur ein hochaktiver Olefinmetathesekatalysator, sondern er fungiert auch als Katalysatorvorstufe für die Alkanmetathese. Dieses System besteht aus einem einzigen Metall mit dualen Eigenschaften, und es zeigt, dass Hilfsliganden in der Alkanmetathese toleriert werden.