Cyclopentadiene anodic oxidation

The product, l,4-diacetoxy-2-allyl-3-methyl-2-cyclopentene, obtained (45% current efficiency) from 2-allyl-3-methyl-l,3-cyclopentadiene through anodic oxidation with carbon rod anode in acetic acid is successfully used as a starting compound in the synthesis of allethrolone as shown in equation 23. 

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. 

Evidence for the operation of cation radical mechanisms for cycloaddition has often been provided by means of a comparison of the results obtained for various methods of generating cation radicals. For example, in the Diels-Alder cycloaddition of phenyl vinyl sulfide to 1,3-cyclopentadiene (Scheme 36) the same adducts are formed whether the cation radicals are generated by chemical ionization (aminium salt), photochemical ionization (the PET method), or electrochemical ionization (anodic oxidation) [65]. 

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. 

Congeneric Nb species are known [4]. A (trimpsi)V(NO)(CO)2 complex (trimpsi = BuSi(CH2PMe2)3) has been reported [5] which undergoes a reversible oxidation to the cationic V(0) complex at —0.74 V versus Cp2Fe/CH2Cl2 and a second, irreversible oxidation at 0.61 V. More highly reduced complexes of vanadium have been prepared, but their electrochemistry has evidently not been explored. The electrochemical synthesis of CpV(CO)4 from a vanadium anode, cyclopentadiene, and 115 bar of CO at 100 °C in DMF has been reported [6]. 

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