Initiation steps autooxidation

Formation of an enamine radical cation 45 was proposed as the chain initiation step in the autooxidation of enamines and SchifFs bases of a,/ -unsaturated ketones to give unsaturated 1,4-diones37. Pyrrolidine enamine of 10-methyl-A1(9)-octal-2-one (44) reacts with oxygen at room temperature to produce, after acid hydrolysis, 10-methyl-A1 (9)-octalin-2,8-dione (47) in 20% yield. Addition of a catalytic amount of FeCl3, Cu(OAc)2 or CuCl2 causes a pronounced enhancement in the oxidation rate and increases the yield to 80-85% after 1 h. 

The proposed free radical chain mechanism for this reaction is given in Scheme 3. The striking catalytic effect of the metal ions such as Cu2+ and Fe3+ is attributed to their ability to accept an electron from the enamine in the chain initiation step. The autooxidation of the SchifFs bases of a,/ -unsaturated ketones is thought to proceed similarly via the enamine form of the SchifFs bases. 

In the autooxidation of a polymer such as polystyrene, the initiation step consists of the formation of a benzylic free radical via homolysis of the Ph-C-H bond. Rapid reaction with molecular oxygen normally follows, to give a polymeric ben-zylperoxy radical ... 

Molecular oxygen is the principal oxidant in the ordinary combustion of fuels. Burning is essentially a high-temperature analog of autooxidation, in which the spin barrier to the initiation step (Equation 4.67) is overcome by use of high temperatures. 

The initiation step provides a radical source by thermal or photochemical dissociation of initiators, which then provides bromine radicals by reaction with Br2. Initiators are sometimes present in the alkene as allyl hydroperoxides which may be present due to inadvertent, prior autooxidation. Bromine or HBr may be present in trace amounts in NBS. Reaction of the bromine radical 20 with the substrate 1 proves selective for allylic or benzylic hydrogens due to the near thermoneutral nature of the reaction which breaks the C-H bond and forms the H-Br bond. Reaction of the formed carbon-centered radical 21 with Br2 provides the desired bromide 3 and Br 20. Hydrogen bromide 17 reacts with NBS to form succinimide 4 and resupplies the required low concentration of Br2. Alternatively, reaction of substrate radical 21 with NBS 2 provides product 3 and succinimidyl radical 22 (S ). Due to energy and kinetics considerations, abstraction of the allylic hydrogen by the S should be slower than abstraction of bromine from NBS by an allyl radical. In using solvents in which NBS, succinimide 4 or it s radical 22 are not very soluble, S is not the key chain-carrier. Byproducts and side-reactions can occur with S. ... 

The first step of the accepted mechanism of autooxidation is initiation (Equation 4.6), in which a substrate molecule AB is converted to one or more free radicals by some agent which may be chemical (transition metal ions, ozone, hydroxyl or other pre-existing radicals, etc.) or physical (ultraviolet light, heat, gamma radiation, ultrasonic energy, or mechanical disintegration). 

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