Oxidation of conjugated dienes

The anodic oxidation of conjugated dienes is much more easily achieved than the oxidation of monoolefins since the conjugation of the --electron system lowers the oxidation potentials of the dienes. Several peak potentials for dienes are summarized in Table l1. 

The typical pattern of anodic oxidation of conjugated dienes is oxidative 1,2- or 1,4-addition of nucleophiles, though the selectivity usually depends on the structure of the diene and the reaction conditions (equation 1). 

Some typical results are shown in Table 2. The table shows that oxidation of conjugated dienes such as isoprene, piperylene (1,3-pentadiene), cyclopentadiene and 1,3-cyclohexadiene with a carbon anode in methanol or in acetic acid containing tetraethylammonium p-toluenesulfonate (EtjNOTs) as the supporting electrolyte yields mainly 1,4-addition products2. 1,3-Cyclooctadiene yields a considerable amount of the allylically substituted product. 

The products of electrochemical oxidation of conjugated dienes are considerably affected by the reaction conditions such as the material of the electrode, the supporting electrolyte and the solvent. The oxidation of butadiene with a graphite or carbon-cloth anode in 0.5 M methanolic solution of NaClCU mainly yields dimerized products along with small amounts of monomeric and trimeric compounds (equation 5)1. The use of platinum or glassy carbon mainly gives monomeric products. Other dienes such as isoprene, 1,3-cyclohexadiene, 2,4-hexadiene, 1,3-pentadiene and 2,3-dimethyl-l,3-butadiene yield complex mixtures of isomers of monomeric, dimeric and trimeric compounds, in which the dimeric products are the main products. 

Oxidation of conjugated dienes in CH3CN-NaC104 in the presence of 1,3-dimethylurea gives a mixture of the possible 4,5-disubstituted 1,3-di-methylimidazolidin-2-ones in about 40% yield.103 Anodic oxidation of 2,4-hexadiene, 1,3-butadiene, and 1,3-cyclohexadiene in CH3CN-H20-NaC104 yields diols, 2-oxazolines, and 3-pyrrolines.104 The product distribution is influenced by the supporting electrolyte. 

A mild triple catalytic system consisting of Pd(OAc)2, hydroquinone, and a transition metal macrocycle (for example, iron phthalocyanine) was reported [243]. The catalytic effect is carried out by the interaction of Pd(II) with the substrate and the acquisition of two electrons, which are further transferred to the benzoquinone that is reduced to hydroquinone. The hydroquinone is then reorganized to benzoquinone by the 02/metal macrocycle system. The following types of transformations were carried out in mild conditions using the developed system 1,4-oxidation of conjugated dienes, oxidation of terminal olefins to methyl ketones, and allylic oxidation. 

When using a carbon electrode, the anodic oxidation of conjugated dienes (7) such as isoprene, pipe-rylene, cyclopentadiene and 1,3-cyclohexadiene in methanol or acetic acid mainly gives oxidative 1,4-addition products (8 equation 13). For example, 1,3-cyclohexadiene gives l,4-dimethoxycyclohex-2-ene (9) in 47% yield (equation 14). 1,3-Cyclooctadiene, in a similar experiment, yields a considerable amount of the allylically substituted product. 

The oxidation of conjugated dienes has been successfiilly applied to the syndiesis of allethrolone."... 

In the palladium-catalyzed 1,4-oxidations of conjugated dienes described so far, only heteroatom nucleophiles have been employed. There is an intrinsic problem in using free carbanions in an oxidation reaction since the oxidant can readily remove an electron and oxidize the carbanion to a radical. Furthermore, in the procedure associated with the best selectivity, i.e., the benzoquinone-based process, acid is required to regenerate the Pd(0)-benzoquinone complex to Pd(II) and hydroquinone. 

As described above for the palladium(0)-catalyzed reactions, carbon-carbon bonds can be obtained by insertion of an olefin into a palladium-vinyl bond (vinylpalladation). This approach has been applied in palladium(II)-catalyzed exchange reactions of olefins by generating the vinylpalladium species from chloropalladation of an acetylene [115,116]. This technique for generating vinylpalladium was later applied to the palladium-catalyzed 1,4-oxidation of conjugated dienes [117]. Thus, the use of substrate 91 in a palladium(II)-catalyzed oxidation in the presence of LiCl afforded 94 in 65% yield [Eq.(49)]. The... 

Oxidation of conjugated dienes in MeCN-NaC104 in the presence of 1,3-dimethyl-urea gives a mixture of the possible 4,5-disubstituted-1,3-dimethylimidazolidin-2-ones in about 40% yield [139]. 

Topics that have formed the subjects of reviews this year include contemporary issues in electron transport research, dynamics of bimolecular photoelectron transfer reactions, photophysical properties of functionalised fullerene derivatives, carbon-carbon bond formation via radical ions, photoinduced electron transfer processes in ketone, aldehyde, and ester synthesis, photochemical reactions between arenenitriles and benzylic donors, photo-oxidation of conjugated dienes, photoredox reactions of aromatic nitro compounds, electron transfer-mediated photochemistry of some unsaturated nitrogen-containing compounds, reactions of 02( Ag), carbon dioxide activation by aza-macrocyclic complexes, and photochromism of chalcone derivatives. ... 

For the oxidation of conjugated dienes with an aryl substituent at one of the systems and both alkenes being trans, the major product arises from reaction of the alkene not attached to the aryl unit, unless that moiety is naphthyl [259]. 

Oxidation of Conjugated Dienes, Allylic Alcohols, and Alkynes... 

Scheme 8-6 Mechanism of Pd-catalyzed 1,4-oxidation of conjugated dienes.

The 1,4-oxidation of conjugated dienes can easily be extended to the synthesis of heterocycles by the use of an intramolecular nucleophile. So far, both nitrogen and oxygen nucleophiles have successfully been employed. 

A number of intramolecular Pd-catalyzed 1,4-oxidations of conjugated dienes were developed.f In these reactions, two nucleophiles are added across the diene, one of which adds intramolecularly. So far, only heteroatom nucleophiles have been employed. In order to extend these intramolecular 1,4-oxidations to carbon nucleophiles, it was found that a vinylpalladium species can be obtained in situ from an alkyne via a chloropalladation. The approach is particularly attractive since it involves a Pd(II) chloride salt and could be compatible with the rest of the catalytic cycle. Reaction of dienyne with LiCl, and benzoquinone in the presence of palladium acetate as the catalyst, afforded the carbocyclization products. The reaction resulted in an overall stereoselective fltiri-addition of carbon and chlorine across the diene t B (Scheme 23). 

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