Dehydrogenation, chloranil selenium

DEHYDROGENATION Chloranil. 2,3-Dichloro-5,5-dicyano-l, 4-benzoquinone (DDQ). Potassium nitrosodisulfonate. Selenium dioxide. Sulfur. 

This ester cannot be dehydrogenated with palladium-charcoal, selenium, or chloranil 16 similar attempts to aromatize 4,5,6,7-tetrahydrobenzo[c]thiophene have failed.616 Diethyl 4,5,6,7-tetra-hydrobenzo[c]thiophene-l,3-dicarboxylate forms an adduct with maleic anhydride.6... 

The elements sulfur and selenium, which combine with the hydrogen evolved to give, respectively, H2S and H2Se. Little is known about this mechanism either. Quinones, which become reduced to the corresponding hydroquinones. Two important quinones often used for aromatizations are chloranil (2,3,5,6-tetrachloro-l,4-benzoquinone) and DDQ (2,3-dichloro-5,6-dicyano-l,4-ben-zoquinone). The latter is more reactive and can be used in cases where the substrate is difficult to dehydrogenate. It is likely that the mechanism involves a transfer of hydride to the quinone oxygen, followed by the transfer of a proton to the phenolate ion ... 

Many years after the classical investigations of the production of minute amounts of aromatic hydrocarbons by selenium dehydrogenation of steroids, Dannenberg found (1956-63) that steroids can be dehydrogenated with chloranil in refluxing xylene or anisole to give optically active aromatized hydrocarbons in yields up to 50%. Cholesterol affords 3-methyl-3 -isooctyl-A -l,2-cyclopentadienophenanthrene (13) and its methyl derivative (14). 

Allylic functionalization of 187 could not be effected under a variety of conditions (e.g., A-bromosuccinimide, lead tetraacetate, selenium dioxide), nor could an additional double bond be introduced under dehydrogenation conditions (e.g., chloranil). Osmium tetroxide was ineffective in hydroxylation of 187 however, potassium permanganate treatment gave the diol 189 although in only 5.5% yield [originally reported as 33% (67)]. Compound 189 could be converted into 190 again in low yield (4.5%). The latter was shown to be identical with yet another degradation product (68, Scheme 11) of securinine by comparison of IR spectra and GLC behavior. 

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