Oxametallacycle

More recently, reductive elimination of aryl ethers has been reported from complexes that lack the activating substituent on the palladium-bound aryl group (Equation (55)). These complexes contain sterically hindered phosphine ligands, and these results demonstrate how steric effects of the dative ligand can overcome the electronic constraints of the reaction.112,113 Reductive elimination of oxygen heterocycles upon oxidation of nickel oxametallacycles has also been reported, but yields of the organic product were lower than they were for oxidatively induced reductive eliminations of alkylamines from nickel(II) mentioned above 215-217... 

SCHEME 89. Mn-catalyzed epoxidation via oxametallacycles and pathways leading to side products... 

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... 

Oxametallacycles are prepared from unsaturated aldehydes or ketones. Oxidative cyclization of 6-hepten-2-one (312) catalysed by the Ti catalyst Cp2Ti to give cyclopentanol 315 has been developed. The key step is the cleavage of the strong Ti—O bond in the oxametallacycle 313 with oxophilic hydrosilane, and the silyl ether 314 is formed with regeneration of Cp2Ti [129,130], Cyclization of 5-hexenal (316)... 

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. 

Fig. 8. Schematic illustrations (top panel) and real space structure (lower panel) of the oxametallacycle (OMME) intermediate and a weakly adsorbed ethylene epoxide (EO) molecule on the (4 X 4)-oxide overlayer on Agflll. Color codes are the same as Fig. 6. Additional small black circles are added in the ball structures to help correspondence with the schemes above. As revealed by the additional Newman projection, the most favorable structure for the OMME intermediate is when all the C substituents are staggered. Certain optimized DFT distances are given in angstroms [A],...

Fig. 10. Relative energy diagram for the conversion of gas phase ethylene into ethylene epoxide (C2H4O) via an oxametallacycle (OMME) intermediate on the low coverage atomic O phase on Ag lll. States to the right-hand side of the vertical dashed line do not contain epoxide molecules and are simply related to O2 dissociative adsorption on clean Ag lll. (Same color codes as Fig. 9.) Copyright from [52].

Indeed Barteau and co-workers have already made important progress on the issue of epoxidation selectivity (see [60] for example). They have concluded that the branching of the oxametallacycle into ethylene oxide or acetaldehyde is the key factor controlling selectivity, as shown in Scheme IV. 

Oxametallacycle formation from aliphatic alkoxides could occur via C-H scission at the p position, or from epoxides by C-O scission to open the ring. The subsequent C-H and C-C bond activation steps are less clear than those of the aldehydes above. For the oxametallacycle formed from the ethoxide, C-C scission must release CH2 or perhaps CH... 

One species that was proposed by many as an intermediate on the way to epoxide formation is the surface oxametallacycle. It is believed that this species is attached to the metal in a C C O type linkage. Previous density functional theory (DPT) calculations showed that the oxametallacycle is slightly energetically favorable, and the experimental evidence for this species is based only on the reaction of 2-iodoethanol with a silver surface as opposed to an epoxide (43 5). 

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.

Structure, with the benzylic carbon bonded to an oxygen atom on the surface. In this configuration, the benzylic carbon is highly susceptible to attack from a nucleophile, in this case another adsorbed oxygen atom. When attacked, the C—C bond making the styrene breaks, forming surface bound CH2 and benzoate. Further reaction of these species would lead to the formation of benzoic acid and carbon dioxide in a 1 1 ratio, which is observed experimentally. The other case, where the oxametallacycle forms in the linear conformation and a nucleophilic oxygen atom attacks the other carbon, would form phenylacetic acid, which is not observed in their studies (Fig. 12). 

These works are very important in the questions that they raise. All previous studies with styrene only pointed to the liner oxametallacycle being able to make styrene oxide. However, the side products observed show that the branched structure cannot be ignored. If the branched structure does not occur, then more complex mechanisms are taking place in order to make benzoic acid and benzene, and those must be elucidated as well. There is no doubt that the reports from Klust and Madix (56) raised more questions that must be answered. The authors postulate two different pathways after the formation of the oxametallacycle to account for the products produced a C—H bond activation pathway and a nucleophilic attack by adsorbed oxygen atoms. 

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