Oxygen ketene mechanism

Mechanisms of Atmospheric Oxidation of the Oxygenates ketene of the same chemical composition as the starting compound ... 

At the present, the most straightforward mechanism for the formation of J5 from 1 is via insertion of CO into the Th-C(acyl) bond to form a ketene (H, (eq. (4)) which subsequently dimerizes. Presumably, initial CO interaction could involve coordination either to the metal ion as shown or to the electrophilic vacant "carbene p atomic orbital. Considering the affinity of the Th(IV) ion for oxygenated ligands, interaction of the ketene oxygen atom with the metal ion seems reason-... 

Scheme 6.25 shows a plausible mechanism involving ruthenium vinylidene and ruthenium-stabilized ketene intermediates. The ketene intermediate was verified through efficient trapping of this spedes with isobutanol to produce esters [23]. Nucleophilic attack by epoxide oxygen at the Ca-carbon of ruthenium vinylidene produces the seven-membered ether spedes 64, which ultimately forms ruthenium... 

In a general base catalysis, the pyridine forms a hydrogen bond to an alcohol function (56). This causes a polarization and increases the nucleophi-licity of the alcohol oxygen thus accelerating the reaction [46c]. The second mechanism postulates a betain intermediate 57 which is formed by a nucleophilic attack of the pyridine on the ketene 59 [46d]. 

Among the facts supporting this mechanism (which is an A-Se2 mechanism because the substrate is protonated in the rate-determining step) are (1) lsO labeling shows that in ROCH=CH2 it is the vinyl-oxygen bond and not the RO bond that cleaves 497 (2) the reaction is subject to general acid catalysis 498 (3) there is a solvent isotope effect when D2O is used.498 Enamines are also hydrolyzed by acids (see 6-2) the mechanism is similar. Ketene dithioacetals R2C=C(SR )2 also hydrolyze by a similar mechanism, except that the initial protonation step is partially reversible.499 Furans represent a special case of enol ethers that are cleaved by acid to give 1,4 diones. Thus... 

The geometries, vibrational frequencies and energies of all the stationary points for the ketene-oxygen reaction have been calculated. Relationships of the reactants, transition states, intermediates, and products are confirmed by intrinsic reaction coordinate calculations and important aspects of the mechanism have been highlighted.266... 

The proposed mechanism suggested that the A-acyl group in 193 migrated to the oxygen, and then a reaction in situ with DMAD took place at the nitrogen, followed by a loss of ketene to give 194 (Scheme 40) (92T9111). 

Polymerizations of ketenes yield varieties of structures, because monomer placements are possible through either the carbon-to-carbon double bond or through the carbon-to-oxygen double bond. Dimethylketene polymerizes by anionic mechanism to a polymer with the following struc-... 

TURRO, N. J., M. F. Chow, and Y. Ito Autoxidation of Ketenes, Diradicaloid and Zwitterionic Mechanisms of Reactions of Triplet Molecular Oxygen and Ketenes. J. Amer. Chem. Soc. 100, 5580 (1978). 

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