Olefin metathesis featuring ruthenium indenylidene complexes with a sterically demanding NHC ligand

The synthesis and characterization of two new ruthenium indenylidene complexes [RuCl(2)(SIPr)(Py)(Ind)] 6 and [RuCl(2)(SIPr)(3-BrPy)(Ind)] 7 featuring the sterically demanding N-heterocyclic carbene 1,3-bis(2,6-di isopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr) are reported. Remarkable activity was observed with these complexes in ring closing, enyne, and cross metathesis of olefins at low catalyst loadings. The performance of SIPr-bearing complexes 6 and 7 as well as [RuCl(2)(SIPr)(PCy(3))(Ind)] 5 in ring opening metathesis polymerization is also disclosed. This work highlights the enormous influence of the neutral "spectator" ligands on catalyst activity and stability.     

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.     

Synthesis and ring-opening metathesis polymerization of eight-membered unsaturated lactams and related monomers

Novel eight-membered ring unsaturated lactams were synthesized and tested as monomers for the ruthenium-catalyzed ring-opening metathesis polymerization (ROMP). The reaction of a N-protected cyclic alkeneamine was also investigated. The Grubbs’ benzylidene complexes RuCl₂(=CHPh)(PCy₃)₂ or RuCl₂(=CHPh)(PCy₃)(IMesH₂) and selected ruthenium–arene species bearing either phosphine or stable Arduengo-type N-heterocyclic carbene ligands served as catalyst precursors. In most cases, isomerization of the starting materials took place and only 1-benzyl-1-aza-2-ketocyclooct-5-ene afforded a polymeric product. This polyamide was characterized by numerous analytical techniques.     

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.     

Synthesis of 2,6-dideoxysugars via ring-closing olefinic metathesis

[formula: see text] Grubbs' RuCl2 (=CHPh)(PCy3)2 (catalyst 1) and RuCl2(=CHPh)(PCy3)(IMess) (catalyst 2) complexes have been successfully utilized in the construction of beta,gamma-unsaturated delta-lactones containing various substitution patterns of methyl groups. Asymmetric dihydroxylation followed by reduction leads to 3,4-cis-dihydroxy-2,6-dideoxypyranoses, which have proven to play very important biological roles as key components of natural products.     

Tuning the electronic properties of N-heterocyclic carbenes

The electron-donating properties of N-heterocyclic carbenes ([N,N'-bis(2,6-dimethylphenyl)imidazol]-2-ylidene and the respective dihydro ligands) with 4,4'-R-substituted aryl rings (4,4'-R=NEt2, OC(12)H(25), Me, H, Br, S(4-tolyl), SO(4-tolyl), SO2(4-tolyl)) were studied. Twelve new N-heterocyclic carbene (NHC) ligands were synthesized as well as the respective iridium complexes [IrCl(cod)(NHC)] and [IrCl(CO)2(NHC)]. Cyclic voltammetry (DeltaE1/2) and IR (nu (CO)) can be used to measure the electron-donating properties of the carbene ligands. Modifying the 4-positions with electron-withdrawing substituents (4-R=-SO(2)Ar, DeltaE1/2=+0.92 V) results in NHC ligands with virtually the same electron-donating capacity as a trialkylphosphine in [IrCl(cod)(PCy3)] (DeltaE1/2 =+0.95 V), while [IrCl(cod)(NHC)] complexes with 4-R=NEt2 (DeltaE1/2= +0.59 V) show drastically more cathodic redox potentials and significantly enhanced donating properties.     

Stereoselective synthesis of esters with vinylsilicon functionality via ruthenium carbene catalyzed cross-metathesis

A new catalytic route for the synthesis of unsaturated organosilicon compounds with vinylic functionality based on cross-metathesis of vinyltrialkoxy- and vinyltris(trimethylsiloxy)-silanes with allyl esters of carboxylic acids catalyzed by ruthenium carbenes (Grubbs catalysts): [Cl₂(PCy₃)₂Ru(=CHPh)] (I), [Cl₂(PCy)₃(1,3-dimesityl-4,5-dihydroimidazol-2-ylidene)Ru(=CHPh)] (II) is reported. The efficient reaction can be extended to alkyl esters of unsaturated acids, e.g., of methyl 3-butenoate.     

A general method for preparation of metal carbenes via solution- and polymer-based approaches

A new general, synthetically simple, and safe method for the preparation of metal carbene complexes, which is based on diphenyl sulfonium salts as carbenoid precursors, has been developed, and its scope and applications were studied. In general, deprotonation of a sulfonium salt with a base results in a sulfur ylide, which, in turn, reacts with an appropriate metal precursor to give the corresponding metal carbene complex. Thus, starting from benzyldiphenylsulfonium salt, the complexes (PCX)Rh=CHPh (X = P, N) were prepared in quantitative yield. Syntheses of Grubbs' catalyst, (PCy(3))(2)Cl(2)Ru=CHPh, and of Werner's carbene, [Os(=CHPh)HCl(CO)(P(i)Pr(3))(2)], were achieved by this method. Novel trans-bisphosphine Rh and Ir carbenes, ((i)Pr(3)P)(2)(Cl)M=CHPh, which could not be prepared by other known methods, were synthesized by the sulfur ylide approach. The method is not limited to metal benzylidenes, as demonstrated by the preparation of the Ru vinyl-alkylidene, (PCy(3))(2)Cl(2)Ru=CH-CH=CH(2), methoxycarbonyl-alkylidene, (PCy(3))(2)Cl(2)Ru=CH(CO(2)Me), and alkylidene (PCy(3))(2)Cl(2)Ru=CH(CH(3)), (PCy(3))(2)Cl(2)Ru=CH(2) compounds. The problem of recycling of starting materials as well as the issue of facile purification of the product metal carbene complex were addressed by the synthesis of a polymer-supported diarylsulfide, the carrier of the carbenoid unit in the process. Based on the sulfur ylide route, a methodology for the synthesis of metallocarbenes anchored to a polymer via the carbene ligand, using a commercial Merrifield resin, was developed.     

Highly Efficient Ruthenium‐Based Catalytic Systems for the Controlled Free‐Radical Polymerization of Vinyl Monomers

A new set of ruthenium complexes such as 1 and 2 (the Grubbs ruthenium–carbene complex) has been shown to mediate the controlled atom‐transfer radical polymerization (ATRP) of vinyl monomers with exceptional efficiency. Furthermore, there is a striking parallelism between the catalytic activity of these compounds in olefin metathesis and in ATRP.     

Vinyl and carbene ruthenium(II) complexes from hydridoruthenium(II) precursors

The reactions of the hydrido compounds [RuHCl(CO)(L)2][L = PiPr3 (1), PCy3 (2)] with HC(triple bond)CR (R = H, Ph, tBu) afforded by insertion of the alkyne into the Ru-H bond the corresponding vinyl complexes [RuCl(CHCHR)(CO)(L)2], 3-8, which upon protonation with HBF4 gave the cationic five-coordinated ruthenium carbenes [RuCl(CHCH2R)(CO)(L)2]BF4, 9-14. Subsequent reactions of the carbene complexes with PR3(R = Me, iPr) and CH3CN led either to deprotonation and re-generation of the vinyl compounds or to cleavage of the ruthenium-carbene bond and the formation of the six-coordinated complexes [RuCl(CO)(CH3CN)2(PiPr3)2]BF4, 17, and [RuH(CO)(CH3CN)2(PiPr3)2]X, 18a,b. The acetato derivative [RuH(2-O2CCH3)(CO)(PCy3)2], 19, also reacted with acetylene and phenylacetylene by insertion to yield the related vinyl complexes [Ru(CHCHR)(kappa2-O2CCH3)(CO)(PCy3)2], 20, 21, of which that with R = H was protonated with HBF4 to yield the corresponding cationic ruthenium carbene 22. With [RuHCl(H2)(PCy3)2], 25, as the starting material, the five-coordinated chloro(hydrido)ruthenium(II) compounds [RuHCl(PCy3)(dppf)], 26(dppf = [Fe(eta5-C5H4PPh2)2]), [RuHCl[Sb(CH2Ph)3](PCy3)2], 27, and [RuHCl(CH3CN)(PCy3)2], 30, were prepared. The reactions of 27 with HCCR (R = H, Ph) gave the hydrido(vinylidene) complexes [RuHCl(CCHR)(PCy3)2], 28 and 29, whereas treatment of 30 with HC(triple bond)CPh afforded the vinyl compound [RuCl(CHCHPh)(CH3CN)(PCy3)2], 31. The molecular structures of 11(R = tBu, L = PiPr3) and 26 were determined crystallographically.     

(Z)-Selective cross-dimerization of arylacetylenes with silylacetylenes catalyzed by vinylideneruthenium complexes

The vinylideneruthenium(II) complexes bearing bulky and basic tertiary phosphine ligands, RuCl2(=C=CHPh)L2 (L = PPri3, PCy3), serve as good catalyst precursors for (Z)-selective cross-dimerization between arylacetylenes and silylacetylenes in the presence of N-methylpyrrolidine.     

Ruthenium-olefin complexes: effect of ligand variation upon geometry

The development of a model system to study ruthenium-olefin complexes relevant to the mechanism of olefin metathesis has been reported recently. Upon addition of the ligand precursor 1,2-divinylbenzene to [RuCl(2)(Py)(2)(H(2)IMes)(==CHPh)] (H(2)IMes=1,3-dimesityl-4,5-dihydroimidazol-2-ylidene), two ruthenium-olefin adducts are formed. Based on (1)H NMR spectroscopy experiments and X-ray crystallographic analysis, these complexes are assigned as side-bound isomers in which the olefin and H(2)IMes ligands are coordinated cis to each other. Herein is reported an investigation of the generality of these observations through variation of the N-heterocyclic carbene ligand and the ligand precursor.     

Stable N-heterocyclic carbene (NHC)-palladium(0) complexes as active catalysts for olefin cyclopropanation reactions with ethyl diazoacetate

The Pd(0) complexes [(NHC)PdL(n)] (NHC=N-heterocyclic carbene ligand; L=styrene for n=2 or PR(3) for n=1) efficiently catalyse olefin cyclopropanation by using ethyl diazoacetate (EDA) as the carbene source with activities that improve on previously described catalytic systems based on this metal. Mechanistic studies have shown that all of these catalyst precursors deliver the same catalytic species in solution, that is, [(IPr)Pd(sty)], a 14e(-) unsaturated intermediate that further reacts with EDA to afford [(IPr)Pd(=CHCO(2)Et)(sty)], from which the cyclopropane is formed.     

Synthesis and Catalytic Activity of a Two-core Ruthenium Carbene Complex: a Unique Catalyst for Ring Closing Metathesis Reaction

The reaction of a ruthenium carbide complex RuCl2(C:)(PCy3)2 with [H(Et2O)x]+[BF4]– at a molar ratio of 1:2 produced a two-core ruthenium carbene complex, {[RuCl(=CHPCy3)(PCy3)]2(μ-Cl)3}+·[BF4]–, in the form of a yellow-green crystalline solid in a yield of 94%. This two-core ruthenium complex is a selective catalyst for ring closing metathesis of unsubstituted terminal dienes. More importantly, no isomerized byproduct was observed for N-substrates when the two-core ruthenium complex was used as the catalyst at an elevated temperature(137 °C), indicating that the complex is a chemo-selective catalyst for ring closing metathesis reactions.     

Synthesis,structure, and activity of enhanced initiators for olefin metathesis

A series of ruthenium olefin metathesis catalysts of the general structure (H(2)IMes)(PR(3))(Cl)(2)Ru=CHPh (H(2)IMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) have been prepared; these complexes are readily accessible in two steps from commercially available (H(2)IMes)(PCy(3))(Cl)(2)Ru=CHPh. Their phosphine dissociation rate constants (k(1)), relative rates of phosphine reassociation, and relative reaction rates in ring-opening metathesis polymerization (ROMP) and ring-closing metathesis (RCM) have been investigated. The rates of phosphine dissociation (initiation) from these complexes increase with decreasing phosphine donor strength. Complexes containing a triarylphosphine exhibit dramatically improved initiation relative to (H(2)IMes)(PCy(3))(Cl)(2)Ru=CHPh. Conversely, phosphine reassociation shows no direct correlation with phosphine electronics. In general, increased phosphine dissociation leads to faster olefin metathesis reaction rates, which is of direct significance to both organic and polymer metathesis processes.     

Preparation of hydrido(vinylidene)ruthenium(II) complexes and a one-pot synthesis of Grubbs-type ruthenium carbenes

Treatment of the hydrido(dihydrogen) compound [RuHCl(H2)(PCy3)2] 1 with alkynes RC[triple bond, length as m-dash]CH (R=H, Ph) afforded the hydrido(vinylidene) complexes [RuHCl(=C=CHR)(PCy3)2] 2, 3 which react with HCl or [HPCy3]Cl to give the corresponding Grubbs-type ruthenium carbenes [RuCl2(=CHCH2R)(PCy3)2] 4, 5. The reaction of 2 (R=H) with DCl, or D2O in the presence of chloride sources, led to the formation of [RuCl2(=CHCH2D)(PCy3)2] 4-d1. Based on these observations, a one-pot synthesis of compounds 4 and 5 was developed using RuCl3.3H2O as the starting material. The hydrido(vinylidene) derivative 2 reacted with CF3CO2H and HCN at low temperatures to yield the carbene complexes [RuCl(X)(=CHCH3)(PCy3)2] 6, 7, of which 7 (X=CN) was characterized crystallographically. Salt metathesis of 2 with CF3CO2K and KI led to the formation of [RuH(X)(=C=CH2)(PCy3)2] 8, 9. The bis(trifluoracetato) and the diiodo compounds [RuX2(=CHCH3)(PCy3)2] 10, 11 as well as the new phosphine P(thp)3 12 (thp=4-tetrahydropyranyl) and the corresponding complex [RuCl2(=CHCH3){P(thp)3}2] 14 were also prepared. The catalytic activity of the ruthenium carbenes 4-7, 10, 11 and 14 in the olefin cross-metathesis of cyclopentene and allyl alcohol was investigated.     

Carbene stabilization of highly reactive main-group molecules

This article highlights recent efforts of this laboratory in the stabilization of highly reactive, low-oxidation-state, main-group molecules using bulky N-heterocyclic carbene ligands [L: = :C{N(2,6-Pr(i)(2)C(6)H(3))CH}(2); L': = :C{N(2,4,6-Me(3)C(6)H(2))CH}(2); L': = :C{(i-Pr)NC(Me)}(2)]. The syntheses, structures, and computational studies of carbene-stabilized neutral diborenes [L:(H)B═B(H):L and L':(H)B═B(H):L'], a neutral Ga(6) octahedron (L':Ga[Ga(4)Mes(4)]Ga:L'), disilicon (L:Si═Si:L), bis-silylene [L:(Cl)Si-Si(Cl):L], dipnictogens (L:E-E:L, E = P, As; L':P-P:L'), and parent phosphinidene (L:PH) are discussed. Some of the unique challenges associated with this "carbene-stabilization" strategy are also presented.     

Efficiency of a ruthenium catalyst in metathesis reactions of sulfur-containing compounds

[reaction: see text] 1,3-Dimesitylimidazol-2-ylidene ruthenium benzylidene catalyst (RuCl2(=C(H)Ph)(PCy3)(IMes)) has been successfully employed in ring-closing metathesis reactions of acyclic diene sulfides, disulfides, and dithianes and in self-cross metathesis reactions of ene-sulfides, thioethers, and thiols.     

CuX2络合物催化甲基丙烯酸甲酯的氧化聚合

研究了高氧化态过渡金属卤化物络合物催化甲基丙烯酸甲酯(MMA)的氧化聚合.首先在叔胺类聚合物存在条件下以CuBr2/2,2′-联吡啶(bPy)络合物催化MMA在不同溶剂中的氧化聚合,结果在环己酮中得到PMMA均聚物,CuBr2/bPy同叔胺的氧化还原引发可以忽略.随后在环己酮中分别以不同络合物催化MMA的氧化聚合.结果...     

Radical polymerization of styrene controlled by half-sandwich Mo(III)/Mo(IV) couples: all basic mechanisms are possible

Density functional calculations of bond dissociation energies (BDEs) have been used as a guide to the choice of metal system suitable for controlling styrene polymerization by either the stable free radical polymerization (SFRP) or the atom transfer radical polymerization (ATRP) mechanism. In accord with the theoretical prediction, CpMo(eta(4)-C(4)H(6))(CH(2)SiMe(3))(2), 2, is not capable of yielding SFRP of styrene. Still in accord with theoretical prediction, CpMo(eta(4)-C(4)H(6))Cl(2), 1, CpMo(PMe(3))(2)Cl(2), 3, and CpMo(dppe)Cl(2) (dppe = 1,2-bis(diphenylphosphino)ethane), 4, yield controlled styrene polymerization by the SFRP mechanism in the presence of 2,2'-azobisisobutyronitrile (AIBN). This arises from the generation of a putative Mo(IV) alkyl species from the AIBN-generated radical addition to the Mo(III) compound. The controlled nature of the polymerizations is indicated by linear M(n) progression with the conversion in all cases and moderate polydispersity indices (PDIs). Controlled polymerization of styrene is also given by compounds 3 and 4 in combination with alkyl bromides. These complexes then operate by the ATRP mechanism, again in accord with the theoretical predictions. Controlled character is revealed by linear increase of M(n) versus conversion, low PDIs, a stop-and-go experiment, and (1)H NMR and MALDI-TOF analyses of the polymer end groups. The same controlled polymerization is given by a "reverse" ATRP experiment, starting from AIBN and CpMo(PMe(3))(2)Cl(2)Br, 5. On the other hand, when compound 1 or 2 is used in combination with an alkyl bromide (as for an ATRP experiment), the isolated polystyrene shows by M(n), (1)H NMR, and MALDI-TOF analyses that catalytic chain transfer (CCT) radical polymerization takes place in this case. Kinetics simulations underscore the conditions regulating the radical polymerization mechanism and the living character of the polymerization. The complexes herein described are ineffective at controlling the polymerization of methyl methacrylate.