Aromatization with selenium

JMC1164), and when (148) was treated with sulfur monochloride, the aromatic system (150) was obtained (81JOC4065). The corresponding 1,2,5-selenadiazole was isolated when (148) was heated with selenium dioxide (52% yield). 

Reactions of enamines with selenium dioxide gave low yields of enamino ketones (J8). Aromatization of cyclohexanone derived enamines eould be largely prevented by the use of aeetonitrile as solvent for the reaction. Even then, yields were eonsiderably below the limit of 50%, imposed by the generation of an equivalent of water. 

The reaction mechanism has not been elucidated. Reducing substances presumably release red elementary selenium [1]. Aromatic o-diamines yield highly fluorescent selenodiazoles with selenium dioxide. 

The interaction of polycations with free chalcogens leads, in some cases, to new cluster compounds.185 Reaction of the aromatic tetraatomic selenium dication with diphenyldiselenide affords Ph2Se62+ - compound 159 with a six-membered selenium ring (Scheme 66).186... 

Only a few works on the synthesis of 1,3,4- and 1,2,5-diazoles have been reported, all in older literature [6,14], More attention has been paid to synthesis of benzo-2,l,3-selenadiazoles simply obtained by reaction of aromatic or heteroaromatic 1,2-diamines with selenium dioxide [196-199],... 

Oxidation of fused 1,2,3-selenadiazole 223 with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) at room temperature gave naphtho[l,2-<7][l,2,3]selenadiazole 88 in 55% yield (Equation 26) <2000JHC1325>. Reaction of compound 224 with selenium powder in an equimolar ratio in an inert atmosphere gave aromatized product 225 (Equation 27) <1998JCM784>. 

The reaction of / -butoxycarbonyl hydrazones 263 with selenium oxychloride in CH2CI2 from — 20 °C to room temperature gave 2,3-dihydro-l,2,3-selenadiazoles 264 in 71-77% yields (Equation 33). Under these mild conditions, the aromatization process did not occur, and only 2,3-dihydro compounds were observed. The advantage of this reaction is the ease of workup. Pure compounds 264 were obtained simply by addition to the reaction mixture of an aqueous saturated solution of NaHCC>3 and subsequent evaporation of the organic solvent <2003JOC1947>. 

We used this strategy in chapter 6 under two-group C-X disconnections where bromination of ketones was the usual functionalisation. More relevant here are conversions of carbonyl compounds into 1,2-dicarbonyl compounds by reaction with selenium dioxide SeC>2 or by nitrosation. So acetophenone 57 gives the ketoaldehyde10 58 with SeC>2. These 1,2-dicarbonyl compounds are unstable but the crystalline hydrate 59 is stable and 58 can be reformed on heating. Since aromatic ketones such as 57 would certainly be made by a Friedel-Crafts reaction the disconnection 58a is not between the two carbonyl groups and offers an alternative strategy. 

In a further study of routes to steroids with an aromatic ring C, 18-norandrost-13-enes (273) were found to give 7a-hydroxy-18-norandrosta-8,l 1,13-trienes (274) on reaction with selenium dioxide, while the 18-norandrosta-7,13-dienes reacted with mercury(n) acetate to give the 7a-acetate (276), as well as its 7j8-epimer. Mechanisms, including stepwise dehydrogenations, are discussed.226... 

Pyridazine 3-carbaldehyde can be prepared from 3-hydroxymethylpy-ridazine by oxidation with selenium dioxide (82BSB153). Its 2-oxide was obtained similarly (78JMC1333). Contrary to aromatic aldehydes, py-ridazine-4-carbaldehydes behave differently under the reaction conditions typical for benzoin condensation or cyanhydrin formation. In the first case, a crossed Cannizzaro reaction takes place and, with an equivalent amount of hydrocyanic acid, compound 80 is obtained. This is transformed with acetic anhydride into a mixture of E- and Z-isomers of 81. On attempted chromatographic separation, the mixture is converted into methyl 4-pyridazinecarboxylate (78JHC637). 

Metallaheterocycles with the metal atom capable of supporting a carbon-metal double bond, and with oxygen, nitrogen, sulfur, or selenium as the second heteroatom are the only structures capable of full conjugation. These compounds show furan-like aromaticity with the heteroatom /i-electrons participating. No examples of this type of ring system were uncovered, undoubtedly due to the stability of the metalla-carbon double bonds required for their formation. 

Aromatization has also been accomplished by heating hydroaromatic compounds with selenium, sulfur, or organic disulfides, RSSR. Here hydrogen is eliminated as H2Se, H2S, or RSH. 

In Older to avoid the handling of poisonous hydrogen selenide, two other useful syntheses have been devised. One method uses the reaction of aromatic nitriles with aluminum selenide in the presence of pyridine, triethylamine and water (Scheme 24), while the other is performed by the reaction of nitriles with selenium, carbon monoxide and water in the presence of triethylamine (Scheme 25). These methods are exceedingly convenient in terms of manipulation without the isolation of hydrogen selenide for the preparation of selenoamides. Using these methods, a variety of aromatic and heterocyclic selenoamides can be obtained from the corresponding nitriles in high yields. 

Similarly to 3,3 -diaminobenzidine, other aromatic o-diamines also react with selenium(rV) in HCl medium. The o-phenylenediamine method [30,31] is more sensitive than the DAB method. The following reagents have been proposed for selenium N-methyl-o--phenylenediamine (e = 1.9-10 at 346 nm) [32], 2-aminodiphenylenediamine [33], 4-nitro-1,2-diaminobenzene [34], 4,5-diamino-2,6-dimercaptopyrimidine [35], l,2-diamino-4-chlorobenzene [36], 4,5,6-triaminopyrimidine [37], 3,4-diaminobenzoic acid [38], and 2,3-diaminonaphthalene [39,40]. 

Since PPS is prepared by reaction of p-dichlorobenzene (I) and Na2S in N-methyl-2-pyrrolidone (NMP) at ca, 240° (13), the reaction of p-dlhalobenzenes with Na Se and Na2Te appeared to be at attractive approach. We note that an unsuccessful attempt to synthesize PPSe from p-dibromobenzene (2) and Na2Se has been reported. (14) As we have recently found (la-b) that alkali metals react directly with selenium and tellurium in 1 1 and 2 1 atomic ratios in dipolar aprotic solvents, thus avoiding the use of liquid NH3 commonly used for the preparation of these reagents, and that these reagents react readily with aromatic halides, which are not activated in the usual sense for a nucleophilic... 

Methyl-5,6-dihydropyndo[2,3-J]pyrimidin-7(87/)-onc is obtained by photochemical cyclization of, V-(pyrimidin-4-yl)mcthacrylamidc, which is subsequently aromatized by reaction with selenium dioxide in acetic acid.210-211... 

Thiocarbamates. Thiocarbamates can be prepared in fair to high yield by reaction of amines and disulfides with carbon monoxide with selenium as catalyst. If triethylamine is added as cocatalyst, aromatic amines can also be used. ... 

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