3-alkylthiophenes

Reaction of thiophene or of 2-alkylthiophenes with butyllithium in the presence of HMPT is suggested to lead to the generation of dilithio intermediates (12), which are then cleaved in the usual way (entry 2 in Table I) the use of HMPT is critical in bringing about ring cleavage. 3,4-Dilithio species (13) give conjugated diacetylenes as products (see entries 17 and 26). 

For alkyl substituents, electrophilic substitution (chlorination, bromination, acetylation, trifluoroacetylation, formylation, hydrogen exchange) of 2-alkylthiophenes happens mainly... 

Figure 4. Consideration of Wheland intermediates for electrophilic substitution of 2-alkylthiophenes at (i) C-3, (ii) C-4, (iii) C-5.

Figure 6. Preparation of 2-alkylthiophenes by acylation and reduction of the resulting ketone.

Authentic standards of both series were prepared using combinations of the techniques outlined earlier. Members of series 1 were obtained by acylation of the necessary 2-alkylthiophene with 3,7,11,15-tetramethylhexadecanoic acid and reduction of the resulting ketone (Figure 25). 

Figure 7. Depth profiles of (a) total 2,5-alkylthiophenes and (b) 2-alkylthiophenes (V), (c) 2-alkyl-5-methylthiophenes (VI), (d) 2-alkyl-5-ethylthiophenes (VII), (e) mid-chain 2,5-dialkylthiophenes (VIII) relative to the total 2,5-dialkylthiophenes for the Jurf ed Darawish oil shale.

Alkylthiophenes. We have shown in Figure 2 that 2-tricosylthiophene (7) is very abundant in several samples studied. In a few cases (Tables I and II) this is accompanied by 2-henicosylthiophene (6) in high relative concentration. A similarly high abundance of these compounds has been observed previously in a sample of Cenozoic age (173 mbsf) from Hole 532 (2). The distribution pattern... 

Figure 4. Histograms, based on mass chromatograms of m/z 97+98, showing distribution of 2-alkylthiophenes and 3-(4,8,12-trimethyltridecyl)thiophene.

Alkylthiophene(s), distribution in deep-sea sediments, 627-628,629f Alkylthiophene(s) as sensitive indicators of palaeoenvironmental changes carbon number distribution patterns of isoprenoid carbon skeleton alkylthiophenes, 465,47Qf carbon number distribution patterns of linear carbon skeleton alkylthiophenes, 461,463/ composition of subfractions, 449,450 depth profiles of alkylthiophenes,... 

The photoisomerization of 2-substituted thiophenes into the corresponding 3-substituted derivatives is a common pathway observed for both 2-aryl and 2-alkylthiophenes [4]. However, for synthetic applications, a significant example can be recognized in the photoisomerization of 2-arylthiophenes 33 into the corresponding 3-aryl derivatives 34 by the large-scale irradiation in Et20 with a high-pressure Hg lamp (Scheme 12.10) [28]. 

The ratio fa /p(Me)//0 /P(r-Bu) is 15,700 for the alkylbenzenes but only 69 for the 2-alkylthiophenes. Likewise the fa /m(Me)//0 /m(t-Bu) ratios are 44,700 (alkylbenzenes), 76.4 (2-alkylthiophenes), and 17.6 (3-alkylthi-ophenes). These results are free from any bond-order effects (which would equally affect the activation by both methyl and r-butyl substituents). They indicate that the 2-t-butyl group produces more hindrance than the 3-/-butyl group, possibly through buttressing between it and the sulfur d orbitals. This view is reinforced by results for electrophilic substitution of benzo[b]thiophene (Chapter 8, Section 2.A.b.i). 

The synthesis of 3,4-diaminothiophene has been standardized and optimum conditions established for its condensation with a-diones to produce thieno[3,4- ]pyrazines <2002JOC9073>. Selective monocarbamoylation of 3,4-diaminothiophene has given access to several 2-alkyl-3,4-diaminothiophenes <1997T10331>. Acid-catalyzed reductive alkylation of the monocarbamate by treatment with an aldehyde and selenophenol in the presence of -toluenesulfonic acid gave the 2-alkylthiophene the carbamate could then be cleaved by means of HBr to give the diamine (Scheme 51). 

In what might be viewed as a variation on the ionic hydrogenation theme, this same Russian group has found that the combination of zinc/aluminum halide/p-toluenesulfonic acid reduces 2-alkylthiophenes mainly to the tetrahydro derivatives (equation 30). The minor product, 2-alkyl-2,5-dihydrothiophene, is... 

Thiophene is metalated by butyllithium in the 2-position. 2-Alkylthiophenes are prepared from 2-lithiothiophenes by alkylation with haloalkanes ... 

Bis(methoxycarbonyl)(phenyliodinio)methanide (778), the most common iodonium ylide derived from malonate methyl ester, has found synthetic applications in the C—H insertion reactions [1044-1048] and the cyclopropanation of alkenes [1049-1055], including enantioselective cyclopropanations in the presence of chiral rhodium complexes [1056-1058], Representative examples of these reactions are shown in Scheme 3.306 and include the BFs-catalyzed bis(carbonyl)alkylation of 2-alkylthiophenes 777 [1045] and the optimized procedure for rhodium-catalyzed cyclopropanation of styrene 779 [1052]. 

---