Vinylbenzo thiophenes

One example is the reaction with benzyne of 3-vinylbenzo[/ ]thiophene 102, prepared by dehydration of 2-(3-benzo[b]thienyl)ethanol, to give 11-thiabenzo[a]fiuorene 103 (65AJC1781). 

As vinylbenzofurans allow a large variety of substituted dibenzofurans to be synthesized, 2- and 3-vinylbenzo[b]thiophenes allow an easy entry, by Diels Alder reaction with the appropriate dienophiles, to substituted dibenzo-thiophenes which are not easily accessible by other methods. Vinylbenzo-[bjthiophenes are less reactive than the corresponding vinylbenzo[b]furans. Some cycloaddition reactions of 2-vinylbenzo[b]thiophene (82) with various dienophiles are reported [83] in Scheme 2.34. 

Several cycloaddition reactions of vinylthiophene and vinylbenzo[6]thiophene are known. 2-Vinylthiophene easily adds 4-phenyI-l,2,4-triazoline-3,5-dione to give (308) (74JA5591). 2-Vinyl-thiophenes also undergo stereospecific cycloaddition with singlet oxygen to produce the corresponding endoperoxides (Scheme 87) (75TL4471). 

Vinylbenzo[6]thiophene is reported to form a Diels-Alder adduct with 1,4-naphthoquinone (70AHC(ll)i77>. The 3-vinyl isomer is known to be a reactive diene its cycloaddition products with maleic anhydride, p-benzoquinone, benzyne, etc. have been described. 

Vinylbenzo[6]thiophene undergoes Diels-Alder reactions with benzyne,468 maleic anhydride,469 benzo[6]thiophene-1,1-dioxide,469 p-benzoquinone, and various 1,4-naphthaquinones.469,470 With quinones, the product depends on the reaction conditions. For... 

Benzo[6]thienylethylenes are readily obtained by elimination of water from the appropriate alcohol. For example, 3-vinylbenzo[6]-thiophene is conveniently prepared by heating l-(3-benzo[6]thienyl)-ethanol with potassium hydrogen sulfate469,471 or by heating 2-(3-benzo[6]thienyl)ethanol with molten alkali.470 2-Vinylbenzo[fe]thio-phene is obtained by pyrolyzing the acetate of l-(2-benzo[6]thienyl)-ethanol.466,620 Other benzo[6]thiophene alcohols have been dehydrated by heating them with oxalic acid,483 iodine,358,465,467,528 or potassium hydrogen sulfate,465,467 or alone.349,485... 

Vinylbenzothiophenes also react with acetylenic compounds as diene systems in reactions reported in the literature. One example is the reaction of 2-vinylbenzo[6]thiophene with DMAD and with dibenzoylacetylene, affording the corresponding Diels-Alder cycloadducts [89IJC(B)724]. 

Vinylbenzo[6]thiophene 90 reacted with benzoquinone to give a fully aromatized 1 1 cycloadduct 151 as result of a Diels-Alder reaction followed by dehydrogenation by excess quinone. Isolation of a dihydro derivative 152 was possible using a weaker solution of 3-vinylbenzo[b] thiophene, whereas a further excess of the vinyl compound and a very short reaction time afforded the terahydro derivative 153 (79AJC145). On the other hand, primary Diels-Alder adducts are isolated from 1-(3-benzo[6]thiophene)cyclohex-1 -ene and 1 -(3-benzo[fc]thiophene)-3,4-dihydronaphthalene on reaction with MA (50JA571). 

Benzyne in solution reacts as a dienophile Wittig (1957) first demonstrated this in the reaction of benzyne with furan. Corbett and Porter (1965) obtained further supporting evidence by isolating 1,4-addition products from benzyne with 1-vinylnaphthalene and 3-vinylbenzo[b]-thiophene Dilling (1966) did the same with styrene. 

Pericyclic reactions involving thiophenes have been utilized to prepare a variety of complex heterocycles. The intramolecular Diels-Alder reaction of 2-vinylbenzo[i]thiophene 92 produced a pair of tetracyclic adducts 93 and 94 <02TL3963>. Coupling of Fischer carbene 96 with 3-alkynylthiophene 95 led to the formation of thieno[2,3-c]pyran-3-one 97 in one step <02JOC4177>. An intramolecular cycloaddition of 97 then afforded tetracyclic adduct 98. A ruthenium-catalyzed cyclodimerization reaction involving bis-thienyl acetylene derivatives was... 

Multi-ring heteroaromatic compounds containing furan and thiophene rings have been prepared by pressured cycloaddition of 2-vinylbenzo[ ]furan (40a) and 2-vinylbenzo[Z)]thiophene (40b) with 3-nitro-2-cyclohexen-l-one (41) and 2-inden-l-one (42), respectively, generated in situ. ... 

Vinylbenzo[b]thiophen and benzyne (from o-bromofluorobenzene) afford 62 (13%) dehydrogenation of the 1 1 adduct 61 is possibly induced by attack of benzyne.60 Formation of the 1 1 adduct 45 (14%), from benzyne and l,3-diphenylbenzo[c]thiophen (143) is accompanied by some desulfurization to give 9,10-diphenylanthracene (9%)48 45 also loses sulfur at 250°C. 

Closely related to the vinylbenzo[b]thiophene reactions are the condensations of the bisbenzo[b]thienyls 108 and 110 with maleic anhydride in the presence of the dehydrogenating agent chloranil to form 419 and 420, respectively (55-78%), which are hydrolytically decarboxylated (37-62%) to the parent pentacycles 56 and 54 by means of soda lime at 400°C.146... 

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