Unsubstituted thiophenes

Dimethylthiophene reacts photochemically with rran.s-Rh(PMe3)2(CO)Cl to yield the isomeric products 218 (R = Me) and 219 because of C—H activation of thiophene (960M872) as well as species 220 (R = Me). Unsubstituted thiophene in similar conditions gives five products, the 3-thienyl activated isomers 218 (R = H) and 219 (R = H), the 2-thienyl isomers, 221 and 222, and 2-thienyl analogs of 220 (R = H). 

Monocyclic thiophene 1-oxides are very reactive species. There has been only one report on the isolation of such compounds (70JA7610). The formation of unsubstituted thiophene 1-oxide in dilute solution by elimination of methanesulfonate from (508) was reported in 1965 (65CCC1158) evidence for the presence of thiophene 1-oxide in the solution was provided by the UV absorption maxima at 215 and 263 nm, similar to that of the S-methylthiophenium ions (Section 3.14.2.5). However, 2,5-di-f-butylthiophene 1-oxide (144a) prepared from the corresponding thiophene (Section 3.14.2.6) exhibited UV absorption maxima (MeOH) at 245 and 319 nm. There is therefore some doubt as to the species responsible for the UV absorption in the solution mentioned above a possible candidate is (509). 

One of the reasons that polyalkylthiophenes have been investigated is the limited solubilities of unsubstituted thiophenes. The synthetic methods used for the preparation of both types of compounds are very similar. For example, both the parent thiophene and alkylthiophenes can be polymerized by reaction with FeCh in chloroform. Other synthetic approaches involve the reaction of dihalogen precursors in the presence of magnesium, zinc or nickel. Some examples of these approaches are shown in Figure 4.22. In all of these methods, the monomers are linked through the C2 and C5 carbon atoms rather than via 2,4 linkages. 

Unsubstituted thiophene 1,1-dioxide is highly unstable and difficult to isolate. It was prepared for the first time by oxidation of thiophene with dimethyldioxirane at 20C under neutral conditions <1997JA9077, 1999BCJ1919>. The compound has a very short half-life and undergoes [2 + 4] dimerization followed by S02 extrusion to give dihydroben-zothiophene 1,1-dioxide, which reacts further by cycloaddition with thiophene 1,1-dioxide. 

The 7] -osmium complexes of a-unsubstituted thiophenes undergo Lewis acid-promoted addition with acetals at C-2 to give the thiophenium complexes in good yields <19990M2988>. These can be deprotonated to give the 2-substituted thiophene complexes. The electrophile attacks the substrate on the rivn-face (Scheme 83). 

Formal substitution of one of the pyrrole rings of pentaazasapphyrin with a P-unsubstituted thiophene (e.g., 5.71) or selenophene (e.g., 5.73) subunit results in a red-shifted (ca. 10 nm) Soret band ( max = 463 and 464 nm for the free-base forms of 5.71 and 5.73, respectively). By contrast, the mixed heterosapphyrin 5.69 in which the bipyrrolic subunit of monothiasapphyrin 5.71 is formally replaced by a P-unsubsti-tuted bifuran displays a Soret band at ca. 440 nm." ... 

An alternative route to these same two isomeric hexathial36]hexaphyrin- 1.22.2.2.2) macrocycles (7.82a and 7.82b) has also been reported. Using Wittig-type chemistry, 3,4-diethyl-2,5-diformylthiophene 7.81 was reacted with the diethyl-substituted bisphosphonium salt 7.83 to afford macrocycles 7.82a and 7.82b in 0.4% and 0.1% yield, respectively (Scheme 7.7.3). It is of interest to note that in this reaction the formation of cyclotetramers was not observed. This is in direct contrast to the analogous reaction using 3,4-unsubstituted thiophene starting materials wherein only cyclic tetramers were found to form (see Chapter 4). This discrepancy is most likely directly attributable to the steric effects caused by the presence of the thiophene P-ethyl substituents. 

Thiophene 1,1-dioxidcs bearing electron-withdrawing substituents undergo a cycloaddition reaction with benzocyclopropene in tetrahydrofuran followed by sulfur dioxide extrusion to give annulenes 3 in 5-35% yield. The unsubstituted thiophene 1,1-dioxide is, however, not sufficiently reactive and undergoes dimerization rather than cycloaddition. ... 

Much use has been made of conjugated diynes, which are also at the oxidation level of 1,4-dicarbonyl-compounds, which react smoothly with hydrosulhde or sulhde, under mild conditions, to give 3,4-unsubstituted thiophenes. Unsymmetrical 2,5-disubstituted thiophenes can be produced in this way too. ° Since nearly all naturally occurring thiophenes are found in plant genera, and co-occur with polyynes, this laboratory ring synthesis may be mechanistically related to their biosynthesis. 

Vinamidinium salts used as 2-substituted malondialdehyde synthons, produce 3,5-unsubstituted thiophenes. ... 

The cycloaddition/cycloreversion sequence which ensues when thiazoles (the best in this context is 4-phenylthiazole) are heated strongly with an alkyne, generates 2,5-unsubstituted thiophenes. Though the conditions are vigorous, excellent yields can be obtained. ... 

The selective monoarylation of unsubstituted thiophene and furan can be made by using an excess amount of the substrate [ 130]. It has been demonstrated that the use of AgF and DMSO as base and solvent, respectively, enables the reaction at 60 °C [ 135]. Various 2- or 3-substituted thiophenes andbenzothiophenes have been subjected to the catalytic arylation [8,128,130,134-139]. 2,2 -Bithiophene can be arylated at the 5 and 5 positions (Eq. 55) [137]. 

In order for the polymers to have practical application, it is important that they can be processed. The polymerization of unsubstituted thiophene results in a largely intractable polymer of low molecular weight There are two basic approaches to resolving this issue ... 

Furan telechelic PIB macromonomers with well-defined M s and narrow MWD were synthesized by end quenching hving PIB with 2-tributylstannylfuran or 2,2-difurylpropane [137, 161]. Three-arm star, furan functional PlBs were obtained under identical conditions except 1,3,5-tricumyl chloride was used as initiator. The resulting telechelic PI Bs could be efficiently photocured by ultraviolet (U V) radiation in the presence of a cationic photoinitiator and a divinyl ether reactive diluent [162]. Owing to the lower reactivity of thiophene compared to furan the reaction of unsubstituted thiophene with hving PIB resulted in rapid and quantitative monoaddition and the quantitative formation of 2-polyisobutylenyl-thiophene [163]. 

Tuning of the EL was also obtained by preparing copolymers of 3-alkyIthiophene and unsubstituted thiophenes in different molar ratios [149]. 

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