Oxygen nucleophiles catalysis

Aside from alcohols, other oxygen nucleophiles have also participated in hydroalkoxylation reactions with alkynes. The most common of these are 1,3-dicarbonyl compounds, whose enol oxygens are readily available to add to alkynes. Cyclization reactions of this type have been carried out under Pd(0) catalysis with various aryl or vinyl iodides or triflates, often in the presence of CO, affording the corresponding furan derivatives (Equation (95)).337-340 A similar approach employing cyclic 1,3-diketones has also been reported to prepare THFs and dihydropyrans under Pd, Pt, or W catalysis.341 Simple l-alkyn-5-ones have also been isomerized to furans under the influence of Hg(OTf)2.342... 

This conclusion was verified by a C NMR experiment carried out in 2h2 8o which gave a 0.05 ppm upfield shift in the resonance for the C-3 carbon relative to the carbon resonance in 2H2 0. The upfield shift in the carbon resonance establishes that the oxygen nucleophile that adds to the C-3 carbonyl group when 6 binds to pepsin must come from water. These labeling results are not consistent with the addition of the Asp-32 carboxyl group to the carbonyl group to form a covalent tetrahedral species as would occur during nucleophilic catalysis. 

The most intense interest in the addition of oxygen nucleophiles to Tr-allylpalladium complexes has centered on the delivery of OAc. For example, in allylpalladium-OAc complexes, acetate can be induced to migrate to the allyl ligand by the addition of CO (equation 46).162-164 Rearrangements and isomerizations of allyl acetates can also be readily accomplished via Pd catalysis (equations 47 and... 

In the Au(I) catalysis of electron-poor alkynes such as 4, the catalytically active species is likely to be a cationic ligand-stabilized gold(I) Jt-complex, as in previously reported additions of oxygen nucleophiles to alkynes [5], Gold catalysts are very soft and thus carbophilic rather than oxophilic. On the basis of this assumption a plausible mechanism can be formulated as shown in Scheme 6. The cationic or strongly polarized neutral Au(I)-catalyst coordinates to the alkyne, and nucleophilic attack of the electron-rich arene from the opposite side leads to the formation of a vinyl-gold intermediate 7 which is stereospecifically protonated with final formation of the Z-olefm 8 [2, 4]. Regioselectivity is dominated by elec-... 

Hydrolysis of letraa I koxy silane (TMOS or TEOS) is generally performed in the presence of a catalyst which can be an acid, a base or a nucleophile. This is also the case for the hydrolysis of R/Si(OMe)350. In the case of TMOS and TEOS, the acid catalysis is due to the reversible protonation of the alkoxy group which converts it to a better leaving group. However, the nucleophilic attack of the oxygen atom of water is still a key step (equation 17). In the case of basic catalysis, nucleophilic attack of the OH- anion at the silicon centre leads to a penta-coordinated intermediate, followed by the elimination of the RO group (equation 18). For nucleophilic catalysis (promoted by F, HMPA, imidazole, 7V,7V-dimethylaminopyridine as well as OH ) the formation of a penta-coordinated species (equation 19) increases the reactivity of the silicon atom towards the nucleophilic attack of water that leads to an hexa-coordinated intermediate, which finally leads to the product of hydrolysis or condensation. 

As a general introduction, the first chapter deals with homogeneous catalysis of organic reactions, mainly acid—base catalysis, but also with nucleophilic catalysis and catalysis by metal ions. In Chapter 2, proton transfer to and from carbon is discussed and in Chapter 3 proton transfer to and from other atoms, mainly oxygen and nitrogen. 

Key Words Ethylene oxide, Ethylene, Epoxidation, Silver, Cl promotion, Cs promotion. Promotion, Selectivity, Oxametallacycle, Adsorption, Desorption, Chemisorption, Activation energy, Ag-O bond. Reaction mechanism, Oxidation, Cyclisation, Heterogeneous catalysis, Selective oxidation, Eletrophilic oxygen. Nucleophilic oxygen. Subsurface O atoms, Ag/a-A Oj catalyst. 2008 Elsevier B.V. 

Keywords Alkenes Alkynes Catalysis Heterofunctionalization Oxygen nucleophiles... 

The conjugate addition of oxygen nucleophiles to acceptor-substituted olefins is the oxa-Michael reaction (Scheme 15). The term is derived from heteroatom replacement nomenclature, meaning that oxygen takes the place of a CH2 unit (RCH2 RO ). Oxa-Michael reactions have been known for many years and are often catalyzed by bases or acids [7]. Catalysis by metals has been reported sporadically in the older literature, e.g. for the case of alcohol addition to vinyl ketones with a Nieuwland catalyst (HgO, BF3-OEt2, ROH) [75-77]. A patent describes a PdCl2-catalyzed addition of alcohols to acrolein or methacrolein [78]. 

Experimental evidence for such nucleophilic catalysis is sununarized in Figure 7.27. Hydrolysis of 2,4-dinitrophenyl benzoate catalyzed by acetate ion labeled with on both oxygen atoms led to product in which both benzoic acid and acetic acid were labeled with 0, but about 75% of the label derived from one 0 in the reactant was found in the benzoate ion. The data were interpreted as evidence for the intermediacy of the mixed anhydride CH3CO2COC6H5. 

The cycloisomerization of a-allenyl ketones to the corresponding substituted furans was the first example of a gold-catalyzed addition of an oxygen nucleophile to an allene (Scheme 4-86). Traditionally, silver or palladium catalysts were employed for cyclizations of this type advantages of gold catalysis incluiie shorter reaction times, milder conditions, an or lower catalyst loadings. Variable amounts of... 

Oxygen nucleophiles The enantioselective addition of water to enones in an aqueous environment, catalysed by a copper complex with an achiral ligand, non-covalently bound to DNA has been reported to produce the corresponding )8-hydroxy ketones with <82% ee. Deuterium labelling demonstrated that the reaction is diastereospecific, with only the S yn-hydration product formed, for which outcome, there is no equivalent in conventional homogeneous catalysis ... 

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