2-Vinylthiophenes

Dehydration to 2-vinylthiophene is better achieved from 2-(2-thienyl) ethanol with powdered potassium hydroxide in the presence of copper than from 1-(2-thienyl) ethanol. a-Chloro-2-thienylpro-pane undergoes a Wurtz reaction with active iron to give 3,4-di-(2-thienyl) hexane in low yield, which has also been obtained through coupling with n-butyllithium. ... 

Thiophenealdehydes have been condensed with aliphatic aldehydes, methyl ketones,cyclic ketones, " benzyl cyanides,and aliphatic nitro compounds to the corresponding vinylthiophenes. By the use of potassium methylate, 2-thiophenealdehyde has been condensed with the reactive methyl groups of iV-heterocyclic compounds. Thiophenealdehydes have... 

Vinyl- and 3-vinylthiophene (73b and 77) are less reactive than the corresponding furans and show a notable preference for extra-annular addition due to the higher reactivity of the diene system, including the side-chain double bond. 2-Vinylthiophene is less reactive than 3-vinylthiophene. Whereas 2-vinylthiophene (73b) reacted with maleic anhydride and 1,4-benzoquinone to give cycloadducts in reasonable yield, 3-vinylthiophene (77) gave a higher yield of the cycloadduct [76, 77] (Scheme 2.31). 

Vinylpyrazoles fail to undergo cycloaddition reactions under conditions used for vinylfurans and vinylthiophenes. l-Phenyl-4-vinylpyrazole (78) and 1-phenyl-5-vinylpyrazole (79) (Figure 2.10) react only with strong dienophiles under pressure and at high temperatures [78-80]. 

Vinylpyridines react with a variety of ADC compounds. The initial Diels-Alder adducts can be isolated, although in many cases the major product contains two molecules of the ADC compound.186,187 Similarly, 2-vinylthiophenes react to give both 1 1 and 1 2 adducts,188 189 and 3-vinylindoles to give 1 1 adducts.190 ... 

The reaction of a-mercaptocarbonyl compounds [e.g. (148)] with buta-l,3-dien-l-yltriphenylphosphonium salts unexpectedly leads to 3-vinylthiophens,138 whereas reaction with vinylphosphonium salts results in the formation of c/ -2,5-dialkyl-2,5-dihydrothiophens (149).139... 

Fig. 6-4 Dependence of the instantaneous copolymer composition F on the initial comonomer feed composition/i and the percent conversion for styrene (MO-2-vinylthiophene (M2) with r = 0.35 and r2 — 3.10. After Walling [1957] (by permission of Wiley, New York) from plot in Mayo and Walhng [1950] (by permission of American Chemical Society, Washington, DC).

One can show the drift of copolymer composition with conversion for various comonomer feed compositions by a three-dimensional plot such as that in Fig. 6-4 for the radical copolymerization of styrene (M, )-2-vinylthiophene (M2). This is an ideal copolymerization with r — 0.35 and r-L — 3.10. The greater reactivity of the 2-vinylthiophene results in its being incorporated preferentially into the first-formed copolymer. As the reaction proceeds, the feed and therefore the copolymer become progressivley enriched in styrene. This is shown by Fig. 6-5, which describes the distribution of copolymer compositions at 100% conversion for several different initial feeds. 

Finally, in this section of syntheses from thiophenes, a catalytic one-step meftod for the preparation of thieno[3,2-2>]thiophene (2) should be mentioned. Reaction of 2-ethylthiophene with SOj over CrjOj/AljOj at 450° afforded a 13% yield of thienothiophene 2, together with thiophene, 2-methylthiophene, 2-vinylthiophene, and 2-aceto-thienone [Eq. (45)]. Under these conditions formation of the isomeric thieno[2,3-6]thiophene (1) was not observed. 

The orientation of the CH=CH2 in 2-vinylfuran and 2-vinylthiophene is of the X,C-cis type (81RCR336), as expected from conjugative interactions which should largely prevail over dipole composition in these compounds. It is also worthy to note that in the radical anion of trans-1,2-di(2-thienyl)ethylene (71G10), the preferred conformation is the one with both the thienyl rings in the S,C-cis orientation (155). The azomethines of five-membered rings retain the X,N-cis conformation (156), like the... 

Bromotoluquinone, 35, 27 5-Bromo-2-vinylthiophene, 38, 89 Buchner funnel, cooling jacket for,... 

By analogy, 2-vinylthiophene reacts with tetrabromocyclopropene by [4 + 2] cycloaddition subsequent loss of HBr with concominant opening of the cyclopropane ring leads to the benzofbjthiophene derivative (310) in 32% yield (90TL4581). 

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). 

The cyclic cooligomerization of 2-vinylthiophene with butadiene, catalyzed by nickel complexes, has given (312) and (313) in a 1 2 ratio (78IZV1469). 

Wittig reactions on thiophenecarbaldehydes have been used to extend the side chain. Thus thiophene-2-carbaldehyde and its vinylogues (362) react with the alkyltriphenylphos-phonium bromides (363) in presence of NaOMe to form the vinylthiophenes (364) (70T1291). Similarly, thiophene-2,5-dicarbaIdehyde leads to (365). The Wittig reaction has also been... 

Polycyclic Thiophene Derivatives from Diels-Alder Reactions of Vinylthiophenes 908... 

Alkenylthiophenes and alkenylbenzo[6]thiophenes are most readily obtained by dehydration of the corresponding alkanols, obtained by reduction of the appropriate acyl derivatives (equation 12 Sections 3.14.2.4, 3.15.9.2.3). Allyl derivatives can be obtained by coupling a suitable allyl halide with the appropriate thienyllithium derivatives (Section 3.14.2.4). An unusual direct synthesis of 3-vinylthiophenes by ring closure of a-mercapto-aldehydes or -ketones with 1,3-butadienyl-l-triphenylphosphonium salts (386) gives satisfactory yields (equation 13) (75CJC3526). 

Successive hydrolysis and dehydrogenation of the adduct of 2-vinylthiophene with maleic anhydride gives benzo[6]thiophene-4,5-dicarboxylic anhydride.366,367... 

Vinylthiophene and maleic anhydride afford a Diels-Alder adduct which is believed to have the structure 135.366,367 Hydrolysis of 135 and subsequent dehydrogenation of the product with sulfur gives benzo[6]thiophene-4,5-dicarboxylic anhydride,367 whereas dehydrogenation of 135 with sulfur, followed by hydrolysis and decarboxylation, affords benzo[6]thiophene.360... 

The synthesis of indoles from vinylpyrroles in reactions analogous to those of vinylfurans (43BSF163) and vinylthiophenes (53JA6329) was not studied until the 1980s, probably because of the inaccessibility and/or instability of the requisite vinylpyrroles [73JCS(P1)2450]. 

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