Ruthenium-catalysed ring-closing metathesis (RCM) is a powerful technique for the preparation of medium-to-large rings in organic synthesis, but the details of the intimate mechanism are obscure. The dynamic behaviour of an RCM-relevant ruthenacyclobutane complex and its reactivity with ethene were studied using low-temperature NMR spectroscopy to illuminate the mechanism of this widely used reaction. These kinetic and thermodynamic experiments allowed for mapping the energy surface of the key steps in the RCM reaction as mediated by Grubbs-type catalysts for alkene metathesis. The highest barrier along the RCM path is only 65 kJ mol(-1), which shows that this catalyst has extremely high inherent activity. Furthermore, this transition state corresponds to that connecting the intermediates in this reaction leading to ring opening of the cyclopentene product. This shows that ring closing is kinetically slightly favoured over ring opening, in addition to being driven by the loss of ethene.

The reaction of phosphonium alkylidenes [(H2IMes)RuCl2=CHPR3]+[A]- (R = C6H11, A = OTf or B(C6F5)4, 1-Cy; R = i-C3H7, A = ClB(C6F5)3 or OTf, 1-iPr) with 1 equiv of ethylene at -78 degrees C, in the presence of 2-3 equiv of a trapping olefin substrate, yields intermediates relevant to olefin metathesis catalytic cycles. Dimethyl cyclopent-3-ene-1,1-dicarboxylate gives solutions of a substituted ruthenacyclobutane 3 of relevance to ring closing metathesis catalysis. 1H and 13C NMR data are fully consistent with its assignment as a ruthenacyclobutane, but 1JCC values of 23 Hz for the CalphaH2-Cbeta bond and 8.5 Hz for the CalphaH-Cbeta bond point to an unsymmetrical structure in which the latter bond is more activated than the former. In contrast, trapping with acenaphthylene leads to an olefin carbene complex (6) in which the putative ruthenacyclobutane has opened; this species was also fully characterized by NMR spectroscopy and compared to related species reported previously.

Combined capture of CO2 and subsequent hydrogenation allows for base/methanol-promoted homogeneous hydrogenation of CO2 to methyl formate. The CO2, captured as an amidinium methyl carbonate, reacts with H2 with no applied pressure of CO2 in the presence of a catalyst to produce sequentially amidinium formate, then methyl formate. The production of methyl formate releases the base back into the system, thereby reducing one of the flaws of catalytic hydrogenations of CO2: the notable consumption of one mole of base per mole of formate produced. The reaction proceeds under 20 atm of H2 with selectivity to formate favored by the presence of excess base and lower temperatures (110 °C), while excess alcohol and higher temperatures (140 °C) favor methyl formate. Known CO2 hydrogenation catalysts are active in the ionic liquid medium with turnover numbers as high as 5000. It is unclear as to whether the alkyl carbonate or CO2 is hydrogenated, as we show they are in equilibrium in this system. The availability of both CO2 and the alkyl carbonate as reactive species may result in new catalyst designs and free energy pathways for CO2 that may entail different selectivity or kinetic activity.

Initiation processes in a family of ruthenium phosphonium alkylidene catalysts, some of which are commercially available, are presented. Seven 16-electron zwitterionic catalyst precursors of general formula (H(2)IMes)(Cl)(3)Ru=C(H)P(R(1))(2)R(2) (R(1) = R(2) = C(6)H(11), C(5)H(9), i-C(3)H(7), 1-Cy(3)-Cl, 1-Cyp(3)-Cl, 1-(i)Pr(3)-Cl; R(1) = C(6)H(11), R(2) = CH(2)CH(3), 1-EtCy(2)-Cl; R(1) = C(6)H(11), R(2) = CH(3), 1-MeCy(2)-Cl; R(1) = i-C(3)H(7), R(2) = CH(2)CH(3), 1-Et(i)Pr(2)-Cl; R(1) = i-C(3)H(7), R(2) = CH(3), 1-Me(i)Pr(2)-Cl) were prepared. These compounds can be converted to the metathesis active 14-electron phosphonium alkylidenes by chloride abstraction with B(C(6)F(5))(3). The examples with symmetrically substituted phosphonium groups exist as monomers in solution and are rapid initiators of olefin metathesis reactions. The unsymmetrically substituted phosphonium alkylidenes are observed to undergo reversible dimerization, the extent of which is dependent on the steric bulk of the phosphonium group. Kinetic and thermodynamic parameters of these equilibria are presented, as well as experiments that show that metathesis is only initiated through the monomers; thus dedimerization is required for initiation. In another detailed study, the series of catalysts 1-R(3) were reacted with o-isopropoxystyrene under pseudo-first-order conditions to quantify second-order olefin binding rates. A more complex initiation process was observed in that the rates were accelerated by catalytic amounts of ethylene produced in the reaction with o-isopropoxystyrene. The ability of the catalyst to generate ethylene is related to the nature of the phosphonium group, and initiation rates can be dramatically increased by the intentional addition of a catalytic amount of ethylene.

NMR chemical shifts and 1 J(C R C ) constants of substituted and unsubstituted ruthenacyclobutanes were calculated using DFT and were found to be in good agreement with experiment. Notably, these calculations confirm that the difference between metallacycle 1 J(C R C ) constants correlate to C R -C actiVation. The relatiVe stabilities of the unimolecular ROM and RCM pathways are consistent with literature reports.Olefin metathesis has become a widely used technique in broad ranging fields of chemistry. 1 Ruthenium-based catalysts pioneered by Grubbs 2 have been notably successful for this purpose. The second-generation Grubbs catalyst, 3 which features a ruthenium alkylidene and a 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene N-heterocyclic carbene support ligand, is understood to effect metathesis through a [2 + 2] cycloaddition between the olefin and the metal-alkylidene bond to form a ruthenacyclobutane intermediate. Cycloreversion to form the opposing carbene generates the metathesis product.Observation of the critical cycloaddition and cycloreversion steps of the catalytic cycle have been hindered by the tendency of the catalyst to retain the coordinated PR 3 ligand present in the parent complex. Recently, Piers et al. synthesized the ruthenacyclobutanes 1 and 2 through the reaction of a phosphonium alkylidene with ethylene 4 and dimethyl cyclopent-3-ene-1,1-dicarboxylate, 5 respectively. The direct observation of these stable ruthenacyclobutane metathesis intermediates has allowed some of the critical steps of metathesis to be studied directly. 6 On the basis of the NMR spectra, Romero and Piers concluded that 1 has C 2V symmetry with a kite-shaped metallacycle, 4 which was further supported by Wenzel and Grubbs, 4,7 although the spectra showed several unusual features. The

A series consisting of a tungsten anion, radical, and cation, supported by the N-heterocyclic carbene 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) and spanning formal oxidation states W(0), W(I), and W(II), has been synthesized, isolated, and characterized. Reaction of the hydride CpW(CO)(2)(IMes)H with KH and 18-crown-6 gives the tungsten anion [CpW(CO)(2)(IMes)](-)[K(18-crown-6)](+). Electrochemical oxidation of [CpW(CO)(2)(IMes)](-) in MeCN (0.2 M (n)Bu(4)N(+)PF(6)(-)) is fully reversible (E(1/2) = -1.65 V vs Cp(2)Fe(+•/0)) at all scan rates, indicating that CpW(CO)(2)(IMes)(•) is a persistent radical. Hydride transfer from CpW(CO)(2)(IMes)H to Ph(3)C(+)PF(6)(-) in MeCN affords [cis-CpW(CO)(2)(IMes)(MeCN)](+)PF(6)(-). Comproportionation of [CpW(CO)(2)(IMes)](-) with [CpW(CO)(2)(IMes)(MeCN)](+) gives the 17-electron tungsten radical CpW(CO)(2)(IMes)(•). This complex shows paramagnetically shifted resonances in the (1)H NMR spectrum and has been characterized by IR spectroscopy, low-temperature EPR spectroscopy, and X-ray diffraction. CpW(CO)(2)(IMes)(•) is stable with respect to disproportionation and dimerization. NMR studies of degenerate electron transfer between CpW(CO)(2)(IMes)(•) and [CpW(CO)(2)(IMes)](-) are reported. DFT calculations were carried out on CpW(CO)(2)(IMes)H, as well as on related complexes bearing NHC ligands with N,N' substituents Me (CpW(CO)(2)(IMe)H) or H (CpW(CO)(2)(IH)H) to compare to the experimentally studied IMes complexes with mesityl substituents. These calculations reveal that W-H homolytic bond dissociation energies (BDEs) decrease with increasing steric bulk of the NHC ligand, from 67 to 64 to 63 kcal mol(-1) for CpW(CO)(2)(IH)H, CpW(CO)(2)(IMe)H, and CpW(CO)(2)(IMes)H, respectively. The calculated spin density at W for CpW(CO)(2)(IMes)(•) is 0.63. The W radicals CpW(CO)(2)(IMe)(•) and CpW(CO)(2)(IH)(•) are calculated to form weak W-W bonds. The weakly bonded complexes [CpW(CO)(2)(IMe)](2) and [CpW(CO)(2)(IH)](2) are predicted to have W-W BDEs of 6 and 18 kcal mol(-1), respectively, and to dissociate readily to the W-centered radicals CpW(CO)(2)(IMe)(•) and CpW(CO)(2)(IH)(•).

Cationic tungsten(V) methylidynes [L4W(X)[triple bond]CH]+[B(C6F5)4]- [L = PMe3, 0.5dmpe (dmpe = Me2PCH2CH2PMe2), X = Cl, OSO2CF3] have been prepared in high yield by a one-electron oxidation of the neutral tungsten(IV) methylidynes L4W(X)[triple bond]CH with [Ph3C]+[B(C6F5)4]-. The ease and reversibility of the one-electron oxidation of L4W(X)[triple bond]CH were demonstrated by cyclic voltammetry in tetrahydrofuran (E1/2 is approximately -0.68 to -0.91 V vs Fc). The paramagnetic d1 (S = 1/2) complexes were characterized in solution by electron spin resonance (g = 2.023-2.048, quintets due to coupling to 31P) and NMR spectroscopy and Evans magnetic susceptibility measurements (mu = 2.0-2.1 muB). Single-crystal X-ray diffraction showed that the cationic methylidynes are structurally similar to the neutral precursor methylidynes. In addition, the neutral (PMe3)4W(Cl)[triple bond]CH was deprotonated with a strong base at the trimethylphosphine ligand to afford (PMe3)3(Me2PCH2)W[triple bond]CH, a tungsten(IV) methylidyne complex that features a (dimethylphosphino)methyl ligand.

The thermal W–W bond homolysis in [CpW(CO)2(IMe)]2 (IMe = 1,3-dimethylimidazol-2-ylidene) was investigated and was found to occur to a large extent in comparison to other tungsten dimers such as [CpW(CO)3]2. CpW(CO)2(IMe)H was prepared by heating a solution of [IMeH]+[CpW(CO)2(PMe3)]−, and it exists in solution as a mixture of interconverting cis and trans isomers. The carbene rotation in CpW(CO)2(IMe)H was explored by DFT calculations, and low enthalpic barriers (<3.5 kcal mol–1) are predicted. CpW(CO)2(IMe)H has pK a MeCN = 31.5(3), and deprotonation with KH gives K+[CpW(CO)2(IMe)]− (·MeCN). Hydride abstraction from CpW(CO)2(IMe)H with Ph3C+PF6 – in the presence of a coordinating ligand L (MeCN or THF) gives [CpW(CO)2(IMe)(L)]+PF6 –. Electrochemical measurements on the anion [CpW(CO)2(IMe)]− in MeCN, together with digital simulations, give an E 1/2 value of −1.54(2) V vs Cp2Fe+/0 for the [CpW(CO)2(IMe)]•/– couple. A thermochemical cycle provides the solution bond dissociation free energy of the W–H bond of CpW(CO)2(IMe)H as 61.3(6) kcal mol–1. In the electrochemical oxidation of [CpW(CO)2(IMe)]−, reversible dimerization of the electrogenerated radical CpW(CO)2(IMe)• occurs, and digital simulation provides kinetic and thermodynamic parameters for the monomer–dimer equilibrium: k dimerization ≈ 2.5 × 104 M–1 s–1, k homolysis ≈ 0.5 s–1 (i.e., K dim ≈ 5 × 104 M–1). Reduction of [CpW(CO)2(IMe)(MeCN)]+PF6 – with cobaltocene gives the dimer [CpW(CO)2(IMe)]2, which in solution exists as a mixture of anti and gauche rotamers. As expected from the electrochemical experiments, the dimer is in equilibrium with detectable amounts of CpW(CO)2(IMe)•. This species was observed by IR spectroscopy, and its presence in solution is also in accordance with the observed reactivity toward 2,6-di-tert-butyl-1,4-benzoquinone, chloroform, and dihydrogen.