Anodic oxidation unsaturated compounds

The anodic oxidation of trifluoroacetic acid affords the trifluoromethyl radical CFj. Addition of this radical to unsaturated heterocycles can produce bis(trifluoromethylated) compounds (Scheme 119) [225]. 

The observed E0 values in this case indicate the highest stability of rhenium (V) derivatives in this series. The comparison of the preparative data published indicates that these are the +5 and +6 oxidation states that appear to be most stable for the alkoxocomplexes of rhenium. The low-valent (+1 — +3) complexes should either be stabilized by JT-acceptor ligands (CO, PR3, NO, unsaturated hydrocarbons) or contain multiple M M bonds [321, 586, 729, 762,]. The compounds of rhenium (VII) are very unstable and decompose at room temperature in several minutes when isolated. They can be isolated and kept for several days as the complexes with N-donor ligands such as Tmeda or Py [533, 519, 1358]. The decomposition products of rhenium (VI) and (VIT) alkoxides are often described in literature as a black tar. The compound with this kind of appearence turned to be the major product of the anodic oxidation of rhenium in methanol (at high current density) and was shown by the X-ray single crystal study to be Re402(0Me)16 [906]. 

The initiation step of the anodic oxidation involves removal of an electron from the double bond leading to a cation radical as the first reactive intermediate. Depending on the structure of the unsaturated compounds, a variety of reactions will take place after the formation of the tirst intermediate. Thus, t) -cal reactions are addition of nucleophiles (equation 8), allylic substitution (equation 9) and dimerization (equation 10). 

Anodic oxidation of the nitroalkane anion to a radical may lead to dimerization [22], addition to unsaturated systems [23], or substitution in aromatic compounds [24] these reaactions are treated in Chapter 22. 

Nitrates of y-thioacetalated phosphonium salts were conveniently transformed into their corresponding carbonyl compounds by anodic oxidation in MeCN-H20 (9 1 v/v)-0.1 molL LiC104 on Pt or vitreous C anodes using a divided cell [Eq. (90)] [140]. These products are easily transformed into a, jS-unsaturated carbonyl compounds. 

Anodic oxidation in methanol of thiophene and substituted thiophenes at low temperatures (—20 to — 30°C) results in methoxylation and ring opening with loss of sulfur as SO2 [190]. From thiophene is isolated butenedialdehyde tetramethylacetal, some methoxysuc-cinic dialdehyde tetramethylacetal, and a small amount of methyl y6-formylpropionate. In general, oxidation of thiophenes results in the formation of derivatives of a, -unsaturated y-dicarbonyl compounds or y-keto esters. Cyanomethoxylation of 2,5-dimethyl thiophene yields mainly cis- and rm/i5-2-cyano-5-methoxy-2,5-dimethyldihydrothiophene and 3-cyano-2,5-dimethylthiophene [191]. 

Earlier studies had indicated aldehyde and ketone formation in the oxidation of ethylene however, later work using mass spectro-graphic analysis and gas chromatography reported no products other than CO2 and water. The coulombic efficiency determinations for the anodic oxidation of other unsaturated hydrocarbons in acid solution indicate that CO2 is the main reaction product (cf. Table 17). For lower olefins, i.e., C2H4 and oxidation is complete. For larger compounds, other products are formed however, the primary product is CO2. A general equation for hydrocarbon electro-oxidation can thus be written as... 

Yoshida J, Ishichi Y, Isoe S (1992) Intramolecular carbon-carbon bond formation by the anodic oxidation of unsaturated a-stannyl heteroatom compounds. Synthesis of fluorine-containing heterocyclic compounds. J Am Chem Soc 114 7594—7595... 

Silyl-l,3-dienes undergo anodic methoxylation in methanol to give 1,4-addition products with an aUylsilane structure as intermediates. Thra-efoie, they are further oxidized to give l,l,4-trimethoxy-2-butene daivatives as the final products. The products are easily hydrolyzed to provide the corresponding y-methoxy-a, -unsaturated aldehydes. Since 1-trimethylsily 1-1,3-dienes are readily prepared by the reaction of the anion of l,3-bis(trimethylsilyl)propene with aldehydes or ketones, l,3-bis(trimethylsilyl)propene offers a, S-formylvinyl anion equivalent for the reaction with carbonyl compounds (equation 15) . 

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