Oxygen Bond Formation Reactions

Carbon- Oxygen Bond Formation Reactions C.l. Cyanoethylation of Alcohols 

The reaction is usually performed with homogeneous basic catalysts such as alkali hydroxides, alkoxides, and tetraalkyl ammonium hydroxide (161,162). The mechanism accepted for this transformation starts with the abstraction by the base catalyst of a proton from the hydroxyl group of the alcohol to generate the alkoxide anion, which reacts with acrylonitrile to form the 3-alkoxypropanenitrile anion. The 3-alkoxypropanenitrile anion abstracts a proton from the catalyst to yield 3-alkoxypropane nitrile. 

Conjugate Addition of Methanol to a, (i-unsatumted Carbonyl Compounds 

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The carbon-oxygen bond formation follows the same pathway. For both nitrogen-carbon and oxygen-carbon bond formation, a competing reaction is 13-hydride elimination (if a hydride is present at the heteroatom fragment), which lowers the yield and the reduced arene is obtained after reductive elimination. Reductive elimination of the C-N or C-0 fragments should be faster than 13-hydride elimination in order to avoid reduction of the aryl moiety. The side-reaction is shown at the bottom of Figure 13.25. 

Carbon-Oxygen Bond Formation Hydroxyl or carboxylate groups can participate in a ring-closure reaction by an intramolecular nucleophilic attack to a generated electrophilic center as already described in Schemes 1 and 3. 

Carbon-Oxygen Bond Formation The cathodic reduction of some nitrocarhonyl compounds in aqueous acidic medium gives the hydroxylamino derivatives that can undergo a ring-closure reaction affording anthrandic compounds or isoxazolones [102-104] (Schemes 70 and 71). 

The anodic oxidation of catechol in the presence of 1,3-dimethylbarbituric acid was carried out in aqueous solution containing sodium acetate in an undivided cell at graphite and nickel hydroxide electrodes [114]. The results did not fit with the expected structure (Scheme 47, path A) but a dis-piropyrimidine was isolated in 35% yield (Scheme 47, path B). It seems that the initial attack of 1,3-dimethylbarbituric acid on the anodically formed o-quinone does not occur through the carbon-oxygen bond formation but rather through the carbon-carbon bond formation, giving rise to the final product via several consecutive reaction steps. 

This review is a summary of the work done and potential opportunities for inexpensive and easily accessible base catalysts, such as alkaline earth metal oxides and hydroxides, as well as alkali metals and oxides supported on alkaline earth metal oxides. Preparation methods of these materials, as well as characterization of basic sites are reported. An extensive review of their catalytic applications for a variety of organic transformations including isomerization, carbon-carbon and carbon-oxygen bond formation, and hydrogen transfer reactions is presented. 

The Tsuji-Trost-type reaction is applicable to bifunctional vinyl epoxide 144 and 1,3-diketone using a palladium catalyst as demonstrated by Koizumi, who obtained polymer 145 (Equation (67)). The reaction proceeds at 0 °C to a reflux temperature of THE. The resulting polymer 145 is isolated in a quantitative yield. The molecular weight of 145 is ca. 3000 (PDI = 2.0-2.7) when 5 mol% of Pd(PPh3)4 is employed as a catalyst. Use of Pd2(dba)3 with several bidentate phosphines such as dppe, dppp, dppb, and dppf is also effective for the polymerization reaction. Propargyl carbonate 146 also reacts with bisphenols in the presence of a palladium catalyst to afford polyethers 147 via carbon-oxygen bond formation at s - and r/) -carbon atoms (Equation (68)). 

These occur readily between electron-rich alkenes and electron-poor carbonyl compounds. The first example, reported in 1959 (64HC(19-2)729), was the formation of 4,4-diaryloxetane-2,2-dicarbonitriles by the room temperature reaction of 1,1-diarylethylenes and carbonyl cyanide. Continued investigation of this reaction shows that a telomerization product is also formed, the tetraphenylpentadienedinitrile (55) from 1,1-diphenylethylene and carbonyl cyanide. This may be interpreted to indicate that carbon-carbon bond formation may commence somewhat ahead of carbon-oxygen bond formation (75MI51302). This... 

The formation of oxygen heterocycles through carbon-oxygen bond formation was also reported. Substituted 2-(o-halophenyl)-ethanols were converted to dihydrobenzofuranes using palladium and Buchwald s bulky biaryl-type ligands (3.43.). The reaction was also efficient in the formation of six and seven membered oxygen heterocycles.53... 

Reactions involving alkyl-oxygen bond formation... 

Promising examples of the catalytic asymmetric Darzens condensation, which yields an epoxide product via carbon-carbon and carbon-oxygen bond formation, have been reported recently by two groups (Scheme 10.11). Toke and co-workers used crown ether 24 in the reaction to form the a,P-unsaturated ketone 78 [38b] with 64% ee, whereas the Shioiri group used the cinchona-derived salt 3a [52], which resulted in 78 with 69% ee. The latter authors propose a catalytic cycle involving generation of a chiral enolate in situ from an achiral inorganic base... 

If we consider reaction of mesylate ion with methyl bromide, we find that this is an endergic reaction thus the transition state lies along the reaction coordinate farther toward the products titan the reactants (Figure 5.11). The activated complex will therefore have a structure more resembling the products. There will be significant carbon-oxygen bond formation between the mesylate group and carbon and only a weak residual bond between carbon and bromine. The bromine... 

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