Carbon-oxygen bonds selectivity

The mode of chemisorption of CO is a key-factor concerning selectivity to various products. Hydrocarbons can only be produced if the carbon-oxygen bond is broken, whereas this bond must stay intact for the formation of oxygenates. It is obvious that catalysts favoring the production of hydrocarbons must chemisorb carbon monoxide dissociatively (e.g. Fe) while those favoring the formation of oxygenates must be able to chemisorb carbon monoxide molecularly (e.g. Rh). 

A rather more complex tertracyclic indole based compound lowers blood pressure by selective blockade of a 1-adrenergic receptors. Reaction of the anion from indole (72-1) with butyrolactone (72-2) leads to the scission of the carbon-oxygen bond in the reagent and the formation of the alkylated product (72-3). The acid is then cyclized onto the adjacent 2 position to give the ketone (72-4) by treatment with a Lewis acid such as polyphosphoric acid. Reaction with bromine then leads to the brominated ketone (72-5). This is subjected to reductive alkylation with ethylene... 

The palladium catalysed conversion of alkenes to enols, also known as the Wacker reaction, has also been used in the formation of oxygen heterocycles. In the example shown in 3.68. the subsequent formation of two carbon-oxygen bonds leads to the desired dioxabicyclo[3.2.1]octane derivative. The first Wacker reaction gives selectively a six membered ring formation (other possible routes would lead to even larger rings), while in the second Wacker reaction the selective formation of the five membered ring is observed.86... 

J. Hartung, T. Gottwald, K. Spehar, Selectivity in the Chemistry of Oxygen-Centered Radicals—The Formation of Carbon-Oxygen Bonds, Synthesis 2002, 1469—1498. 

Based on the above discussion it was thought that the trifluoro-methyl ketones would be more polarized and thus create a great electrophilicity on the carbonyl carbon which facilitates -OH attack by the serine residue. Yet there is no carbon-oxygen bond to be cleaved In the ketone moiety, and therefore the enzyme-trifluoromethyl ketone transition state complex does not undergo catalytic conversion. The above rationale seems reasonable as trifluoromethyl ketones were found to be extraordinary selective and potent inhibitors of cholinesterases (56) of JHE from T. ni (57) and of meperidine carboxylesterases from mouse and human livers (58). Since JH homologs are alpha-beta unsaturated esters, a sulfide bond was placed beta to the carbonyl in hopes that it would mimic the 2,3-olefln of JHs and yield more powerful inhibitors (54). This empirical approach was extremely successful since it resulted in compounds that were extremely potent inhibitors of JHEs from different species (51,54,59). 

The cleavage of mixed dialkyl ethers occurs at the more substituted carbon-oxygen bond. Methyl ethers of secondary or tertiary alcohols give methanol and secondary or tertiary alkyl bromides selectively by reacting with BBra [17], although the addition of Nal and 15-crown-5 ether can change this selectivity (Eq. 9) [18]. In contrast, methyl ethers of primary alcohols are generally cleaved at the Me-O bond [19]. 

Hydrosilylation of carbon-oxygen bonds is a mild method for selective reduction of carbonyl functions. Parks and Piers have found that aromatic aldehydes, ketones, and esters are hydrosilylated at room temperature in the presence of 1-4 mol % B(CgF5)3 and 1 equiv. PhsSiFl [154]. On the basis of kinetic experiments the authors suggested that the reduction takes place by an unusual nucleophilic/electrophilic mechanism— the substrate itself serves to nucleophilically activate the Si-H bond, and hydride transfer is facilitated by the borane Lewis aeid (Eq. 99). 

The systematic name for carbonic anhydrase is carbonate hydro-lyase, and its numerical code is EC. 4.2.1.1. The first number identifies it as a lyase the second as an enzyme that catalyzes the breakage of a carbon-oxygen bond, leading to unsaturated products and the third as a hydro-lyase, participating in a reaction involving the elimination of water. The last number is the specific serial number assigned to this enzyme. In this text, the trivial names are used. Names of a selected list of clinically useful enzymes with their EC codes, systematic names, other common names, and abbreviations are given in Appendix V. 

On the other hand, ethylene-TPD on p UO3 indicated the desorption of acetaldehyde (490 K). In addition, an unexpected product was also observed. This product was identified as furan (C4H4O, m/e 68, 39) which desorbed at ca. 550 K with a carbon selectivity of ca. 40 %. Furan formation from ethylene on UO3 requiring the formation of one carbon-carbon bond and of one carbon-oxygen bond, is most likely accompanied by oxygen depletion from the UO3 surfaces and subsequent reduction of U cations into lower oxidation states. The observation of furan from ethylene shows that one may obtain oxygenated products with a high carbon number from ethylene (a relatively abundant feed stock) via one single step. 

From these dai a Gray selected a set of best values for the heats of formation of alkoxy radicals, which are shown in Table 6. These heats of formation, when combined with the heats of formation of the alkyl radicals may be used to calculate the carbon-oxygen bond dissociation in the alkoxy radicals. Values are shown in Table 6. In addition, these heats of formation of alkoxy radicals may be combined with similar data for the alcohols and ethers to obtain bond dissociation energies i)(R—OH), i>(RO—H) and D(R—OR ). These derived energies are given in Table 7. 

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