| 1. |
- Brink, A., et al.
(author)
-
Fast and reversible insertion of carbon dioxide into zirconocene-alkoxide bonds. A mechanistic study
- 2014
-
In: Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1477-9226 .- 1477-9234. ; 43:23, s. 8894-8898
-
Journal article (peer-reviewed)abstract
- In two consecutive equilibria the compound (Cp*)(2)Zr(OMe)(2) undergoes insertion of CO2 to form the mono- and bis-hemicarbonates. Both equilibria are exothermic but entropically disfavoured. Magnetisation transfer experiments gave kinetic data for the first equilibrium showing that the rate of insertion is overall second order with a rate constant of 3.20 +/- 0.12 M-1 s(-1), which is substantially higher than those reported for other early transition metal alkoxides, which are currently the best homogeneous catalysts for dimethyl carbonate formation from methanol and CO2. Activation parameters for the insertion reaction point to a highly ordered transition state and we interpret that as there being a substantial interaction between the CO2 and the metal during the C-O bond formation. This is supported by DFT calculations showing the lateral attack by CO2 to have the lowest energy transition state.
|
|
| 2. |
- Kleimark, Jonatan, 1982, et al.
(author)
-
Low Temperature Studies of Iron-Catalyzed Cross-Coupling of Alkyl Grignard Reagents with Aryl Electrophiles
- 2012
-
In: Advanced Synthesis & Catalysis. - 1615-4169. ; 354:2-3, s. 448-456
-
Journal article (peer-reviewed)abstract
- The title reaction has been studied under low temperature conditions. Coupling with active substrates can be done even at dry ice temperature. Initial rate studies at −25°C indicate that high concentrations of any reagent can lead to either complete or partial catalyst deactivation. Under strongly reducing conditions, iron seems to form less active complexes that only slowly re-enter the catalytic cycle, possibly through bimolecular coupling of iron(II) complexes. Computational studies support the experimental observations, and indicate that oxidation states below +I cannot be reached by reductive elimination.
|
|
| 3. |
- Kleimark, Jonatan, 1982, et al.
(author)
-
Mechanistic Investigation of Iron-Catalyzed Coupling Reactions
- 2009
-
In: ChemCatChem. - : Wiley. - 1867-3880 .- 1867-3899. ; 1:1, s. 152-161
-
Journal article (peer-reviewed)abstract
- The mechanism of the iron-catalyzed cross-coupling of aryl electrophiles with alkyl Grignard reagents is studied by a combination of GC monitoring, Hammett competition experiments, and DFT calculations. The reaction follows a pathway where an FeI complex, formed in situ, reacts in a rate-limiting oxidative addition with the aryl electrophile. A rapid thermoneutral transmetalation from a Grignard reagent occurs either before or after the oxidative addition, with little to differentiate between the two pathways. A reductive elimination of the resulting alkyl aryl FeIII complex closes the catalytic cycle. Iron in lower oxidation states can act as a competent precatalyst by oxidation into the FeI-FeIII cycle. FeII complexes can give FeI catalysts through reductive elimination of a bimetallic complex. Added ligands, dilution, and powerful aryl electrophiles all serve to increase the stability of the active catalyst, presumably by counteracting oligomerization of low-valent iron.
|
|
| 4. |
- Larsson, Per-Fredrik, 1983, et al.
(author)
-
Copper-catalyzed cross-couplings with part-per-million catalyst loadings.
- 2009
-
In: Angewandte Chemie (International ed. in English). - : Wiley. - 1521-3773. ; 48:31, s. 5691-3
-
Journal article (peer-reviewed)abstract
- Homeopathic doses: The presence of 0.001 mol % of a copper salt can be sufficient to catalyze arylation reactions of various nucleophiles with phenyl iodide. The two most important factors governing the reaction outcome are the ligand quantity and the temperature.
|
|
| 5. |
|
|
| 6. |
|
|
| 7. |
- Larsson, Per-Fredrik, 1983, et al.
(author)
-
Mechanistic Aspects of Submol% Copper-Catalyzed C-N Cross-Coupling
- 2014
-
In: Chemcatchem. - : Wiley. - 1867-3880. ; 6:5, s. 1277-1282
-
Journal article (peer-reviewed)abstract
- Carbonnitrogen bond formation can be catalyzed by copper in very low concentrations (approximate to 100ppm), with mechanistic features that are distinct from those in the high-concentration regime. The reaction was studied by initial rate kinetics, competitive Hammett studies, and DFT calculations. The deprotonation of the model nucleophile, pyrrole, is limited by mass transfer with the heterogeneous base. The positive reaction order in dimethylethylenediamine was explained by this reagent working not only as a ligand to Cu, but also as a facilitator for mass transfer. The selectivity-determining step in the competitive Hammett study is oxidative addition. Alternative mechanisms for this step, such as single-electron transfer, atom transfer, or sigma-bond metathesis, can be excluded based on the observed Hammett behavior and DFT calculations.
|
|
| 8. |
- Larsson, Per-Fredrik, 1983, et al.
(author)
-
New efficient ligand for sub-mol % copper-catalyzed C-N cross-coupling reactions running under air
- 2012
-
In: Beilstein Journal of Organic Chemistry. - : Beilstein Institut. - 1860-5397. ; 8, s. 1909-1915
-
Journal article (peer-reviewed)abstract
- A new efficient ligand, N,N ''-dimethyldiethylene triamine (DMDETA), has been synthesized and evaluated for sub-mol % copper-catalyzed C-N cross-coupling reactions. The efficiency of the ligand was determined by kinetic methods. DMDETA proved to display efficiency similar to DMEDA and, in addition, the resulting catalyst was tolerant to air.
|
|
| 9. |
- Nilsson Lill, Sten O., 1970, et al.
(author)
-
tBu or not tBu?
- 2012
-
In: Chemistry - A European Journal. - : Wiley. - 0947-6539. ; 18:6, s. 1640-9
-
Journal article (peer-reviewed)abstract
- The regioselectivity in the palladium-catalyzed Heck coupling reaction between an aryl halide and ethyl vinyl ether with four different phosphine ligands: PPh(n)tBu(m) (n=0-3, m=3-n) has been investigated both experimentally and computationally. A zigzag selectivity pattern was experimentally observed upon consecutive replacement of Ph by tBu in the phosphine ligand. Use of a standard DFT method (B3LYP) was shown to give a correct prediction of product preference. However, the trend in relative selectivity among the different ligands could not be correctly described. The use of a more recent DFT functional (M06) parameterized to reproduce dispersion interactions resulted in an improved description. For the sterically most demanding ligands, PtBu(3) and PPhtBu(2), unexpectedly large deviations between experimental and M06 calculated selectivities raised the question of an alternative mechanism for these ligands. In the case of PtBu(3) it was found, in agreement with literature data, that the phosphine ligand could be replaced by a second halide ligand, resulting in an anionic mechanism, with a calculated selectivity in excellent agreement with experimental data. For the PPhtBu(2) ligand, two mechanisms are suggested to operate in parallel, as demonstrated both by computational studies and experimental observation of halide-dependent selectivity. A Halpern effect is observed for all phosphine ligands investigated, that is, the least stable pre-complex results in the most abundant product.
|
|
| 10. |
- Abrahamsson, Maria, et al.
(author)
-
Steric influence on the excited-state lifetimes of ruthenium complexes with bipyridyl-alkanylene-pyridyl ligands.
- 2008
-
In: Inorganic Chemistry. - : ACS. - 0020-1669 .- 1520-510X. ; 47:9, s. 3540-3548
-
Journal article (peer-reviewed)abstract
- The structural effect on the metal-to-ligand charge transfer (MLCT) excited-state lifetime has been investigated in bis-tridentate Ru(II)-polypyridyl complexes based on the terpyridine-like ligands [6-(2,2'-bipyridyl)](2-pyridyl)methane (1) and 2-[6-(2,2'-bipyridyl)]-2-(2-pyridyl)propane (2). A homoleptic ([Ru(2)(2)](2+)) and a heteroleptic complex ([Ru(ttpy)(2)](2+)) based on the new ligand 2 have been prepared and their photophysical and structural properties studied experimentally and theoretically and compared to the results for the previously reported [Ru(1)(2)](2+). The excited-state lifetime of the homoleptic Ru-II complex with the isopropylene-bridged ligand 2 was found to be 50 times shorter than that of the corresponding homoleptic Ru-II complex of ligand 1, containing a methylene bridge. A comparison of the ground-state geometries of the two homoleptic complexes shows that steric interactions involving the isopropylene bridges make the coordination to the central Ru-II ion less octahedral in [Ru(2)(2)](2+) than in [Ru(1)(2))(2+). Calculations indicate that the structural differences in these complexes influence their ligand field splittings as well as the relative stabilities of the triplet metal-to-ligand charge transfer ((MLCT)-M-3) and metal-centered ((MC)-M-3) excited states. The large difference in measured excited-state lifetimes for the two homoleptic Ru-II complexes is attributed to a strong influence of steric interactions on the ligand field strength, which in turn affects the activation barriers for thermal conversion from (MLCT)-M-3 states to short-lived (MC)-M-3 states.
|
|
