Conjugated dienes oxygenations

Conjugated dienes react with oxygen under the influence of light to give cyclic... 

A number of methods are available for following the oxidative behaviour of food samples. The consumption of oxygen and the ESR detection of radicals, either directly or indirectly by spin trapping, can be used to follow the initial steps during oxidation (Andersen and Skibsted, 2002). The formation of primary oxidation products, such as hydroperoxides and conjugated dienes, and secondary oxidation products (carbohydrides, carbonyl compounds and acids) in the case of lipid oxidation, can be quantified by several standard chemical and physical analytical methods (Armstrong, 1998 Horwitz, 2000). 

Antioxidants may intervene at any of the three major steps of the oxidative process initiation (oxygen consumption), propagation (conjugated dienes and peroxides formation), or termination (lipid peroxidation products). 

Conjugated dienes and polyenes lead to unique cases of conjugated oxidations, such as the formation of endoperoxide by singlet oxygen attack on the endo-diene, e.g. u-terpinene29. 

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer46. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system Pd(II)—BQ—MLm where MLm is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPP)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

In the literature, there are also several examples where the specific oxidation reaction is dependent on the presence of a diene. Often the substrates are conjugated dienes where there is simultaneous oxidation of both double bonds. An important prototype of this reaction is the well-known singlet oxygen oxidation of dienes to endoperoxides, for example the oxidation of a-terpinene (4) to ascaridole (5) (equation 1). 

Oxygenation of 1,3-dienes. Conjugated dienes can undergo an efficient... 

Lipoxygenase (linoleate oxygen oxidoreductase) catalyzes the hydroperoxidation of fatty acids containing a methylene-interrupted conjugated diene system. The degradation of the hydroperoxides results in the formation of numerous secondary products (46-48). 

A selective hydrogenation of conjugated dienes was carried out with a Pd complex which was preactivated with oxygen. Besides the conversion of dienes with good selectivity (98%), diene esters, ketones and nitro compounds were also hydrogenated with fairly good selectivities (equation 9)29. 

Oxidation to CO of biodiesel results in the formation of hydroperoxides. The formation of a hydroperoxide follows a well-known peroxidation chain mechanism. Oxidative lipid modifications occur through lipid peroxidation mechanisms in which free radicals and reactive oxygen species abstract a methylene hydrogen atom from polyunsaturated fatty acids, producing a carbon-centered lipid radical. Spontaneous rearrangement of the 1,4-pentadiene yields a conjugated diene, which reacts with molecular oxygen to form a lipid peroxyl radical. 

The intramolecular addition of carbon nucleophiles to alkenes has received comparatively little attention relative to heterocyclization reactions. The first examples of Pd-catalyzed oxidative carbocyclization reactions were described by Backvall and coworkers [164-166]. Conjugaled dienes with appended al-lyl silane and stabilized carbanion nucleophiles undergo 1,4-carbochlorination (Eq. 36) and carboacetoxylation (Eq. 37), respectively. The former reaction employs BQ as the stoichiometric oxidant, whereas the latter uses O2. The authors do not describe efforts to use molecular oxygen in the reaction with allyl silanes however, BQ was cited as being imsuccessful in the reaction with stabihzed car-banions. Benzoquinone is known to activate Ti-allyl-Pd intermediates toward nucleophilic attack (see below. Sect. 4.4). In the absence of BQ, -hydride eUm-ination occurs to form diene 43 in competition with attack of acetate on the intermediate jr-allyl-Pd" species to form the 1,4-addition product 44. 

The unsaturated molecule Y may be carbon monoxide, an olefin, a conjugated diene, an acetylene, a carbonyl compound, various unsaturated carbon-nitrogen compounds, or probably any of several other unsaturated materials. The reactive part of the covalent metal compound is usually a metal-hydrogen, metal-carbon, metal-oxygen, metal-halogen, metal-nitrogen, or metal-metal group. This reaction... 

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