Oxametallacycle intermediates

Modifying the selectivity for a particular product is a more challenging task. To understand why Ag is the most selective catalyst for ethylene epoxidation, an highly important reaction practiced industrially for decades, Linic et al. performed detailed spectroscopic and kinetic isotope experiments and DFT calculations, and they concluded that the selectivity between the partial and total oxidation of ethylene on Ag(l 11) is controlled by the relative stability of two different transition states (TS s) that are both accessible to a common oxametallacycle intermediate One results in the closure of the epoxide ring and ethylene oxide (EO), while the other leads to acetaldehyde (AC) via intra-molecular H shift and eventually combustion. The authors... 

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

Keywords Atomic scale characterization surface structure epoxidation reaction 111 cleaved silver surface oxide STM simulations DFT slab calculations ab initio phase diagram free energy non-stoichiometry oxygen adatoms site specificity epoxidation mechanism catalytic reactivity oxametallacycle intermediate transition state catalytic cycle. 

Computational studies were used also to determine the binding mode of the oxametallacycle. In the previous studies with 2-iodoethanol, the oxametallacycle intermediate was characterized as the five-membered ring containing two metal atoms. However, the case with a single metal center forming a four-membered ring intermediate is also possible, and was considered in these studies. To determine this, both the calculated enthalpies of formation and calculated vibrational spectra of the different oxametallacycles were compared with the HREELS data of the 300 K dosed silver surface. Comparisons showed the optimum structure to be the... 

Figure 8. Two proposed oxametallacycle intermediates, (a) This structure depicts the OME geometry, while ti) depicts the OMME geometry.

The studies on the oxametallacycle intermediate have led to theoretical work with DFT that predicts that Cu-Ag alloys will have enhanced activity [342] (see Chapter 8), and this has been experimentally verified [323,343], validating the initial reports by Lefort [320,321]. 

A. Klust, R. J. Madix, Selectivity limitations in the heterogeneous epoxidation of olefins Branching reactions of the oxametallacycle intermediate in the partial oxidation of styrene, /. Am. Chem. Soc. 128 (2006) 1034. 

The calculations then predicted that the next step along the reaction coordinate was the formation of the stable oxametallacycle, intermediate 2. The final species formed before EO is TS2 in which the Ag-O bond is stretched and the O-C2 bond is beginning to be formed. The authors calculate an activation energy of 67 kJ/mol for ring closure of intermediate 1 in reasonably good agreement with the... 

Linic and Barteau calculated that the heat of adsorption of intermediate 2 was greater than that of intermediate 1 and justified its inclusion in the reaction coordinate even though intermediate 1 was the preferred structure for EO adsorption and decomposition by stating that the energy difference between the adsorbed species is so small as to make the two interchangeable [1]. Even so, when the oxametallacycle (intermediate 2) is synthesised on the surface of Ag(llO) or Ag(lll) by adsorption of 2-iodoethanol, acetaldehyde and no EO is produced on temperature programming. The authors explain that this is because of the... 

Two fundamentally different proposals for the mechanism of oxygen-atom transfer from high-valent metal oxo complexes have been considered. One proposal invokes substrate attack at both the metal and oxo centers to generate an oxametallacycle intermediate (3, Fig. 2). Such a mechanism was first advanced... 

The formation of an oxametallacycle intermediate has also been proposed in couplings of 1,3- or 1,6-enynes with aldehydes [45-48]. In the use of conjugated enynes, this directed process works for both Ni(COD)2/NHC/R3SiH and Ni(COD)2/ PRs/BEts reaction systems. Precoordination of the enyne 28 to form metallacycle 29 can explain the excellent regioselectivities in this process (Scheme 8.35). 1,6-Enynes have also proven to be interesting both synthetically and mechanistically. 

An oxametallacyclic intermediate has been suggested for the oxidation of alkenes to ketones on an oxygenated Rh(l 11) surface.3l3.3l4 Theoretical calculations on the oxidative addition of CH4 to second row metal atoms predict that this will be most favourable for Rh.3l5 The exothermicity of the addition of CH4 to Rh(I) can be improved by the presence of a ligand, but its position (in or out of the RhCH plane) is crucial.316... 

Pyrone synthesis by cycloaddition of CO2 to terminal alkynes (1-hexyne, 1-propyne) has also been investigated. This process can be catalytically promoted, albeit with low yield and selectivity, by Co [74] and Rh [75] complexes. Rh(dppe) (Ti -BPh4), in acetonitrile, at 390 K, catalyzed the formation of 4,6-dimethyl-2-pyrone from 1-propyne and CO2 (1 MPa) with a TON of 50 [75]. The Rh-catalyzed reaction has been proposed to proceed through a mechanism (Scheme 5.15) not involving an oxametallacycle intermediate species. The CO2 insertion into the Rh-C(sp )-o-bond of a Rh-alkenyl intermediate, obtained upon propyne dimerization, affords a linear unsaturated carboxylate which is converted into the pyrone. 

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