Dehydrogenation with selenium reagents

The direct conversion of a chromanone to a chromone by dehydrogenation has been achieved using palladium on charcoal <74CR(C)(279)l5l> and with selenium dioxide (65JCS2743), but both reagents appear to be somewhat fickle. 

The dehydrogenation of steroidal and other ketones can be accomplished with (43) at elevated temperatures. Similar reactions with lactones and lactams have also been reported. Other interesting oxidations include the angular hydroxylation of ketones and the oxidation of benzylic hydrocarbons, as shown in Scheme 14. In some cases, the selenium reagent may be employed catalytically in the presence of a less expensive cooxidant that reoxidizes Se by-products back to the required Se oxidation state. 

Review. Oxidations with selenium dioxide have been reviewed (634 references). The reagent is useful for oxidation of carbonyl compounds to 1,2-dicarbonyl compounds, for oxidation of olefins to allylic alcohols, and for benzylic oxidations. It has been used for dehydrogenations, but high-potential quinones are probably more useful for this reaction. ... 

Early applications of Se02 as a dehydrogenation reagent were limited to compounds like 1,2-diarylethanes and 1,4-diketones which afforded conjugated unsaturated products [28]. Recently, the first aromatization of cyclohexenes 67 and cyclohexadienes 68 by selenium dioxide activated with trimethylsilylpolyphos-phate (PPSE) has been reported [29]. It was found that the reaction described by Eq. (12) resulted in the formation of a variety of aromatics 69 in yields between 80 and 100 % when substrates were refluxed in carbon tetrachloride for 16 - 22 h. 

Selenium dioxide (SeOa) in tert-alcohols has been widely used for the dehydrogenation of 3-0x0 steroids, and particularly in the conversion of A -3-ketones into A dienones [240]. Other steroidal ketones suffer similar a,jS -dehydrogena-tion e.g. i2-ketones [241] and a 4,4-dimethyl-7-ketone [242]), but the same reagent in different solvents will also effect oxidation of certain steroidal ketones with adjacent methylene groups ( C0-CH2 ) into a -diketones (-CO CO-), a reaction which is common outside the steroid field. In ethanolic solution, for example, 5a-cholestan-3 one was oxidised by SeOa to give 5a-cholestane-2,3-dione [243], isolated in its enoHc forms. 

Chemical Synthesis. If cholesterol is heated with elemental sulfur at 150°C for several days, a mixture of PCAH results (J2), the detailed composition of which is not known. Other dehydrogenation reagents, such as selenium, catalyze the production of alkyl naphthalenes and phenanthrenes from naturally occurring terpenes (J3). Thus, in principle, it is possible that the PCAH in an anoxic sediment could result from the chemical dehydrogenation of various naturally occurring compounds. These reactions may take place slowly over the years and may be catalyzed by trace elements, elemental sulfur, and clay minerals. 

Upon selenium dehydrogenation sabine yielded a product with UV-absorption identical with that of cevanthridine (CCVIII-A), a selenium dehydrogenation product of cevine and related alkaloids (109). On this basis it was concluded that sabine possesses the cevane (CCVIII-B) ring system. The failure of sabine to give the characteristic cevine color reaction with sulfuric acid and to form a potassium salt were interpreted to indicate the absence of a masked a-ketol system (109). The latter conclusion was advanced independently on the basis of the failure of the alkamine to isomerize under alkaline hydrolytic conditions or to reduce triphenyltetrazolium chloride reagent (63). 

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