Methyl-substituted selenophenes

Other Elements - Further investigations of the benzophenone sensitised reactions of methyl-substituted selenophenes and tellurophenes with a range of maleic anhydride derivatives have been reported. Cleavage of the acyl-tellurium... 

This gives tautomeric mixtures119 when the tert-butyl group is removed. The methyl ether has been used to obtain 3-hydroxy-2-carbonyl derivatives in the selenophene series.120 The unsubstituted 2-hydroxyselenophene system has been prepared by hydrogen peroxide oxidation of 2-selenophene-boronic acid.121 However, in the 5-methyl-substituted system deboronation became such an important side reaction that 5-methyl-2-hydroxyselenophene had to be prepared by acid-catalyzed dealkylation of 5-methyl-2-fert-butoxy-selenophene. Both 2-hydroxy- and 5-methyl-2-hydroxyselenophene exist mainly as 3-selenolene-2-ones (93) and for the 5-methyl derivative it was possible to isolate the / ,y-unsaturated form (92) and follow the tautomeric isomerization. The activation parameters thus obtained were compared with those for the corresponding furan and thiophene systems. 

Methyl- and methoxy-substituted selenophenes have also been electropolymerized [59] and, as found for other electron-donor substituted monomers, they polymerize at less anodic potentials than the parent compounds. Polymerization of diselenophene has also been reported [323] but no details are given other than the rather low conductivity of the product (10 S cm ). 

The direct combination of selenium and acetylene provides the most convenient source of selenophene (76JHC1319). Lesser amounts of many other compounds are formed concurrently and include 2- and 3-alkylselenophenes, benzo[6]selenophene and isomeric selenoloselenophenes (76CS(10)159). The commercial availability of thiophene makes comparable reactions of little interest for the obtention of the parent heterocycle in the laboratory. However, the reaction of substituted acetylenes with morpholinyl disulfide is of some synthetic value. The process, which appears to entail the initial formation of thionitroxyl radicals, converts phenylacetylene into a 3 1 mixture of 2,4- and 2,5-diphenylthiophene, methyl propiolate into dimethyl thiophene-2,5-dicarboxylate, and ethyl phenylpropiolate into diethyl 3,4-diphenylthiophene-2,5-dicarboxylate (Scheme 83a) (77TL3413). Dimethyl thiophene-2,4-dicarboxylate is obtained from methyl propiolate by treatment with dimethyl sulfoxide and thionyl chloride (Scheme 83b) (66CB1558). The rhodium carbonyl catalyzed carbonylation of alkynes in alcohols provides 5-alkoxy-2(5//)-furanones (Scheme 83c) (81CL993). The inclusion of ethylene provides 5-ethyl-2(5//)-furanones instead (82NKK242). The nickel acetate catalyzed addition of r-butyl isocyanide to alkynes provides access to 2-aminopyrroles (Scheme 83d) (70S593). 

Benzo[h]seienophene, 2,3-dihydro-2-methyl-synthesis, 4, 961 Benzo[h]selenophene, 3-halo-2-substituted synthesis, 4, 966... 

Acetylbenzo[6]tellurophene is also formed in isolable quantities. Benzo[6]selenophene is converted into 2-acyl derivatives by reaction with acid chlorides in the presence of aluminum chloride, whereas similar Friedel-Crafts acylation of benzo[6]thiophene yields 3-substituted products. When the 2-position is blocked, formylation can be directed into the 3-position. Thus treatment of 2-bromobenzo[6]selenophene with dichloromethyl butyl ether (Cl2CHOBu) and titanium tetrachloride yields the 3-formyl derivative (72BSF3955). If the 2- and 3-positions of benzo[6]selenophene are blocked with methyl groups, acylation under Friedel-Crafts conditions occurs in the 6-position (78CR(C)(287)333>. 

The synthesis and chemistry of an 7] -selenophene osmium complex 50 has been studied <19990M1559>. Protonation and electrophilic substitution with acetaldehyde diethyl acetal occurred at C-2. Methylation of complex 50 with methyl triflate gave 51 which upon treatment with tetrabutylammonium borohydride (TBAB) led to the selenophene ring-opened complex 52 (Scheme 5). 

Dian et al. [334] reported the electrochemical polymerization of 3-substituted and 3-,4-disubstituted selenophenes in acetonitrile saturated with lithium perchlorate. Two methods were used to study systematically the influence of the substituents on polymer formation cyclic voltammetry and chronoamperometry. The film formation process is greatly influenced by the electronic and steric effects of the substituents. For example, electron donating groups such as methyl and methoxy substituents appear to stabilize the radical cation intermediates, diminish the oxidation potential, and therefore allow polymer formation. The presence of halogen substituents raises the oxidation potential and does not lead to polymeric films. The conductivities of polyselenophene derivatives are quite low. For example, oxidized poly-3-methyl-selenophene and poly-3,4-dimethyl-selenophene have conductivities around 9 x 10 and 6 x 10" S cm", respectively. 

Proton n.m.r. and i.r. studies on 2-mercapto-5-methyl- and 2,5-dimethyl-3-mercapto-furan, -thiophen, and -selenophen indicate the presence of mainly the thiol tautomers.Carbon-13 n.m.r. has been used to distinguish thione and selenone tautomers in appropriately substituted 1,3,4-thiadiazoles and -selenadiazoles, and tetrazole derivatives. N.m.r., i.r., and u.v. studies indicate that 4-aminoisothiazole exists as the amino- and not the imino-tautomer. ... 

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