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.