Heterocyclics benzo selenophenes

The major internal comparisons to be made within this chapter are between (13) pyrrole (1), furan (2), thiophene (3), selenophene (4) and tellurophene (5) b) pyrrole (1) and indole (6) (c) indole (6), benzo[6 Jfuran (7) and benzo[6]thiophene (8) d) indole (6), isoindole (9) and indolizine (10) and (e) benzo[6] and benzo[c] fused systems. The names of relevant heterocyclic radicals are given with the structures of the parent heterocycle. 

Benzo[Z)]furans and indoles do not take part in Diels-Alder reactions but 2-vinyl-benzo[Z)]furan and 2- and 3-vinylindoles give adducts involving the exocyclic double bond. In contrast, the benzo[c]-fused heterocycles function as highly reactive dienes in [4 + 2] cycloaddition reactions. Thus benzo[c]furan, isoindole (benzo[c]pyrrole) and benzo[c]thiophene all yield Diels-Alder adducts (137) with maleic anhydride. Adducts of this type are used to characterize these unstable molecules and in a similar way benzo[c]selenophene, which polymerizes on attempted isolation, was characterized by formation of an adduct with tetracyanoethylene (76JA867). 

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). 

Furan and its benzo derivatives 97PF1C117, 98PF1C129, 99PF1C144. Thiophenes, selenophenes, andtellurophenes 97PF1C77,98PF1C87,99PF1C102. Five-membered heterocycles with more than one N atom 97PF1C148,... 

The fusion of a furan (thiophene, selenophene) nucleus (2) with a six-membered heterocyclic ring gives rise to two types of heterocycles (e.g. 1, 3). Whereas the former ones are derivatives of benzo[Z>]furans (thiophenes, selenophenes) (Chapters 3.10-3.16) the latter class of compound are derived from benzo[c]furans (78H(9)865, 80AHC(26)135, 81MI31700) (thiophenes, selenophenes). There are three main reasons for the interest in these compounds. Firstly, the overwhelming majority of compounds discussed in this chapter belong to a class of bicyclic heterocycles which arise from the fusion of a ir-excessive nucleus... 

Bird CW, Cheeseman GWW, Homfeldt AB (1984) Selenophenes, tellurophenes and their benzo derivatives. In Katritzky AR, Rees CW (eds) Comprehensive heterocyclic chemistry I. Pergamon, Oxford, chap 3.16, p 935... 

The benzo[c] derivatives (8.2) of the five-membered heterocycles do not contain a true Kekule benzenoid ring and so tend to undergo other reactions under the conditions for electrophilic substitution. Like indolizine, however, they are very reactive toward electrophilic substitution for a reason that does not appear to have been noted previously, namely that in passing to the transition state, a true benzenoid ring is generated (e.g., 8.60 and 8.61 for benzoic]selenophene and indolizine, respectively) [87JCS(P2)591]. 

Selenophenes, tellurophenes, and their benzo derivatives, in "Comprehensive Heterocyclic Chemistry" (A. R. Katritzky and... 

Useful reviews on redox transformations of thiophen derivatives (179 references), on the stereochemistry of carbonyl derivatives of five-membered heterocycles (257 references), on synthetic approaches to dihydrothiophens (135 references), and on biosteric thiophens" have been published. Aspects of thiophen chemistry have been treated in reviews on the synthesis of heterocycles by thermal [2 + 2] cycloaddition reactions of acetylenes and on aspects and perspectives of organic heterocyclic chemistry. A review comparing the chemistry of thieno[2,3-h]- and thieno [3,2-i ]-thiophen with that of benzo[ft] thiophen and quinoline has been published. In Advances in Heterocyclic Chemistry, the development of benzo[6] thiophen from 1968 to 1980 and of selenophen from 1970 to 1980 was presented. Other aspects of thiophen chemistry are treated in chapters on Dewar Heterocycles," on Cyclizations under Vilsmeier Conditions, on Polyfluoroheteroaromatic Compounds, and on Reactions of Benzyne with Heterocyclic Compounds. " Several dissertations treating various aspects of thiophen chemistry have appeared. " ... 

It should be noted that a number of SPC polymers which contain other heterocycles have been prepared, motivated by their promising optical or electrical properties. Examples include pyridine (65) [123], pyrrole (66) [124], oxadiazole (67) [125], selenophene (68) [126], benzo[2,l,3]thiadiazole (69) [127], benzo[2,l,3]selenadiazole (70) [126], perylene bisimide (71) [128], 1,4-diketo- pyrrolo[3,4-c]pyrrole-l,4-dione (72 and 73) [127,129], and triphenyleamine (74) [127] as part of the polymer backbone by SPC (Figure 19c). Specifically for metal complexation, porphyrin [130], difluoroboraindacene [131], bipyridine [132], phenanthroHne [113], terpyridine [133, 134], and the like [123] were embedded in the backbone. In this context, an interesting report was submitted by Rehahn et al., in which l,l -ferrocenyl units were incorporated into a PPP (Figure 22.20). Due to a low-energy barrier for rotation around the Cp-Fe-Cp axes (Cp = cyclopentadienyl), the obtained polymer 75 was assumed to take randomly coiled conformations [135]. 

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