Carbanions autoxidation

Barton, Derek H. R. (1918-1998). An English organic chemist who won the Nobel Prize for chemistry in 1969 with Hassel. The field of conformational analysis in organic chemistry was initiated through his research in the terpene and steroid fields. He did extensive research in the area of carbanion autoxidations. He was instrumental in research concerning the relationship of molecular rotation to structure in complex organic molecules. His education took place in London, France, and Ireland. 

This oxidative process has been successful with ketones,244 esters,245 and lactones.246 Hydrogen peroxide can also be used as the oxidant, in which case the alcohol is formed directly.247 The mechanisms for the oxidation of enolates by oxygen is a radical chain autoxidation in which the propagation step involves electron transfer from the carbanion to a hydroperoxy radical.248... 

A better possibility PI1CH2C)- adds to C60 Then autoxidation of a benzylic C-H bond occurs to give the hydroperoxide. Then the C60 carbanion displaces OH- from the hydroperoxide to give the product. 

The autoxidation mechanism by which 9,10-dihydroanthra-cene is converted to anthraquinone and anthracene in a basic medium was studied. Pyridine was the solvent, and benzyl-trimethylammonium hydroxide was the catalyst. The effects of temperature, base concentration, solvent system, and oxygen concentration were determined. A carbanion-initi-ated free-radical chain mechanism that involves a singleelectron transfer from the carbanion to oxygen is outlined. An intramolecular hydrogen abstraction step is proposed that appears to be more consistent with experimental observations than previously reported mechanisms that had postulated anthrone as an intermediate in the oxidation. Oxidations of several other compounds that are structurally related to 9,10-dihydroanthracene are also reported. 

Mechanism for Base-Catalyzed Autoxidation of 9,10-Dihydroanthracene. The autoxidation of 9,10-dihydroanthracene in pyridine as the solvent and in the presence of benzyltrimethylammonium hydroxide, a strong base, is believed to involve the reaction of a carbanion and molecular oxygen. Indirect evidence of the existence of the carbanion of dihydroanthracene in pyridine solution comes from the color that forms in the presence of the base. When dihydroanthracene is added to a pyridine solution of the base, a deep blood-red color develops immediately. This color is not completely attributable to carbanions since a trace of anthra-quinone alone will produce it. However, under an inert atmosphere (nitrogen) in which no anthraquinone can be formed, a deep red color is also formed. 

Background and Possible Intermediates. Accepting the premise of carbanion formation in the basic media, the mode of reaction with molecular oxygen can now be considered. Sprinzak (8) reported that the autoxidation of fluorene in basic media proceeds by direct reaction of the fluorenyl carbanion with oxygen to form initially the hydroperoxide, which decomposes to yield 9-fluorenone, as depicted below. 

The hydroperoxide radical reacts with another molecule of oxygen (Reaction 5) to give the hydroperoxide-peroxy radical. This radical in turn reacts with a molecule of dihydroanthracene (Reaction 6), to give the dihydroperoxide and generate a radical to propagate the chain. However, the hydroperoxide radical formed in Reaction 4 may be decomposed by a carbanion to the anthracene diradical (Reaction 7). [An example of the decomposition of an unstable hydroperoxide by reaction with an anion is found in the basic autoxidation of 2-nitropropane (3).]... 

Table 7-2 summarizes kinetic data for the reaction of O2 with esters, diketones, and carbon dioxide.35,37-39 Esters react with superoxide ion to form diacyl peroxides or the carboxylate and the alcohol. Initial reaction occurs via a reversible addition-elimination reaction at the carbonyl carbon (Scheme 7-9). This conclusion is supported by the products that are observed in the gas-phase reaction of O2 with phenyl acetate and phenyl benzoate, which has been studied by Fourier-transform mass spectrometry.40 in effect, there is a competition between loss of O2 and loss of the leaving group. Carbanions are poor leaving groups, so that simple ketones without acidic a-hydrogen atoms are unreactive. The KC(O)OO- radical should be a reactive intermediate for the initiation of the autoxidation of allylic hydrogens (see Chapter 5). 

CHEMILUMINESCENCE INVOLVING THE PHOSPHORUS CHEMISTRY. PHOSPHA-l,2-DIOXETANES AS THE MOST LIKELY HIGH-ENERGY INTERMEDIATES IN AUTOXIDATION OF PHOSPHONATE CARBANION... 

The autoxidation of other phosphonate carbanions derived from diethyl diphenylmethylphosphonate (9) and diethyl fluorenylphosphonate (10) showed that DBA (9,10-dibromoanthracene, a triplet energy aacceptor) enhanced the chemiluminescence in spite of the lower energy for the excited triplet benzophenone (68-69 kcal/mol) and fluorenone (53 kcal/mol) than that for singlet DBA (71 kcal/mol). The Stem-Volmer plot of the double reciprocal of the DBA concentration and the chemiluminescence quantum yields established a bimolecular process with the fluorophor and the excited species in these chemiluminescence reactions. The emission quantum yields at the infinitive DBA concentration were calculated to be... 

Motoyoshiya J, Ikeda T, Tsuboi S, Kusaura T, Takeuchi Y, Hayashi S, Yoshioka S, Takaguchi Y, Aoyama H. Chemiluminescence in autoxidation of phosphonate carbanions. Phospha-l,2-dioxetanes as the most likely high-energy intermediates. J Org Chem 2003 68 5950-5. 

The autoxidations of certain carbanions, such as fluorenide anion, have been demonstrated to proceed via a chain mechanism involving electron transfer (Russell, 1953 Russell et al., 1965, 1968) as shown in Scheme 8. In MejSO... 

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