Thiophenes alkylthiophenes

Process Description. Reactors used in the vapor-phase synthesis of thiophene and aLkylthiophenes are all multitubular, fixed-bed catalytic reactors operating at atmospheric pressure, or up to 10 kPa and with hot-air circulation on the shell, or salt bath heating, maintaining reaction temperatures in the range of 400—500°C. The feedstocks, in the appropriate molar ratio, are vaporized and passed through the catalyst bed. Condensation gives the cmde product mixture noncondensable vapors are vented to the incinerator. 

Acylthiophenes. Manufacturing methods introducing the carboxaldehyde group into the 2- or 5-positions of thiophene and alkylthiophenes utilise the Vilsmeier-Haack reaction. To synthesize 2-thiophenecarboxaldehyde (Table 5), a controlled addition of phosphoms oxychloride to thiophene in /V, /V- dim ethyl form am i de is carried out, causing the temperature to rise. Completion of the reaction is followed by an aqueous quench, neutralization, and solvent extraction to isolate the product. 

Manufacture of 2-acetylthiophenes involves direct reaction of thiophene or alkylthiophene with acetic anhydride or acetyl chloride. Preferred systems use acetic anhydride and have involved iodine or orthophosphoric acid as catalysts. The former catalyst leads to simpler workup, but has the disadvantage of leading to a higher level of 3-isomer in the product. Processes claiming very low levels of 3-isomer operate with catalysts that are proprietary, though levels of less than 0.5% are not easily attained. 

This activation of the ortho position is most strikingly illustrated in the reactivity of 2,5-dimethylthiophene, which competitive experiments have shown to undergo the SnCb-catalyzed Friedel-Crafts reaction more rapidly than thiophene and even 2-methylthiophene. The influence of the reagent on the isomer distribution is evident from the fact that 2-methoxythiophene is formylated and bromi-nated (with A -bromosuccinimide) only in the 5-position. Similarly, although 3-bromo-2-methylthiophene has been detected in the bromi-nation of 2-methylthiophene with bromine, only the 5-isomer (besides some side-chain bromination) is obtained in the bromination of alkylthiophenes with A -bromosuccinimide. ° However, the mechanism of the latter type of bromination is not established. No lines attributable to 2-methyl-3-thiocyanothiophene or 2-methyl-3-chIoro-thiophene could be detected in the NMR spectra of the substitution products (5-isomers) obtained upon thiocyanation with thiocyanogen or chlorination with sulfuryl chloride. 2-Methyl- and 2-ethyl-thiophene give, somewhat unexpectedly, upon alkylation with t-butyl chloride in the presence of Feds, only 5-t-butyl monosubstituted and... 

Fatty acids are prepared by acylating thiophene with acid chlorides and reducing the ketones (218) to alkylthiophenes according to Wolff-Kishner or Clemmensen. The latter are then acetylated and oxidized by hypochlorite to 5-alkyl-2-thiophenecarboxylic acids, > ... 

Thiophenes are protonated at C-2. The NMR speetra of a number of protonated alkylthiophenes have been reported (861759). 

SCHEME 2.62 Synthesis of alkylthiophenes 397-399 through oxidative iodination of the thiophene followed by Ni-catalyzed polymerization. (From Bolognesi, A., Botta, C., Geng, Z., Flores, C., and Denti, L., Synth. Met., 71, 2191, 1995 Bolognesi, A., Porzio, W., Bajo, G., Zannoni, G., and Fanning, L., Acta Polym., 50, 151, 1999.)... 

Poly(3-alkylsulfonate thiophene)s, 23 717 Poly(3-alkylthiophenes) (P3ATs) conducting, 7 517 stability, 7 536... 

Scheme 4 Proposed stacking structure. The plane-to-plane distance d for the polymer shown in No. 19 is about 3.8 A [93-95], which is somewhat shorter than the d value (about 3.85 A) reported for head-to-tail-type poly(3-alkylthiophene) HT-P3RTh [128,129] and somewhat longer than the d values (3.6-3.65 A) observed with head-to-head-type poly(4-alkylthiazole-2,5-diyl) [124,129,130] and the charge-transfer-type copolymer of thiophene and 4-alkylthi-azole [131]. Poly(ethylenedioxythiophene) gives a similar d value (3.8 A) [132]. The existence of S in the aromatic ring seems to make the distance d longer...

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). 

Alkylation of thiophene by alkenes, catalyzed by H2SO4, is not as efficient a reaction as acylation polymerization often occurs, resulting in lower yields of desired products. Moreover, the reagent is not selective, since mixtures of 2- and 3-alkylthiophenes result (63AHC(l)l). Various alcohols have been used as the carbenium ion source, in the presence of SnCl4. Thus 7-butanol gives 2,5-di-f-butylthiophene. 

The metal-ammonia reduction of thiophene and its derivatives has formed part of a review (76H(5)905). The formation of hydrogenation products requires the presence of a proton source otherwise only fission products are observed. In the presence of methanol the major products from thiophene are (196) and (197), the latter undergoing further cleavage as shown in Scheme 40. With Li/NH3 or Na/NH3, alkylthiophenes undergo cleavage, the usual work-up leading to carbonyl compounds as shown in Scheme 41. 

The terpenes used were mainly /3-pinene fractions provided by DRT (Soci6td des Derives Rdsiniques et Terpeniques, Vielle-S Girons) and, for certain experiments a turpentine oil containing the main three terpenes a-pinene, /3-pinene, and A -carene. The /3-pinene fractions contained 80-90% /3-pinene, 2% a-pinene, 4-5% myrcene, 2-3% dipentene and 700-1500 ppm S. GC-MS analyses showed that sulfur impurities were composed of alkyl and alkenyl sulfides (mainly dimethyl sulfide), alkyl and alkenyl disulfides (mainly dimethyl disulfide), trisulfides, thiophene and alkylthiophenes (methyl, dimethyl, acetyl and tertiobutyl). 

Dithienosilole-thiophene alternating copolymer 21 is the important example for the electrical conduction of SCPs.42 When the copolymer is doped with iodine, a high electrical conductivity of 400 S cm-1 can be achieved. This value is the highest among SCPs and is also close to that of well-defined poly(3-alkylthiophene). 

The "intermediate" fraction separated from the pyrolysate of the buried mat is also dominated by the compounds I and II which show the same relative concentration ratio as in the related extract and as in the modem mat extract and pyrolysate. The FPD trace (Figure 5D) displays numerous other sulfur compounds, among them n-alkylthiophenes ranging from C13 to C30, peaking between C14 and C21, and other isoprenoids, branched and mid-chain thiophenes in the C13-C20 range (Table II). 

Saturated and unsaturated hydrocarbons (Figure 6C) of the pyrolysate are largely dominated by straight-chain hydrocarbons ranging from C14 to C32. There is a drastic drop of concentration of saturated and unsaturated hydrocarbons after the C30 monoenes elutes. The C28, C29, C30 olefins are dominant with their diolefinic counterparts. The FPD trace of the "intermediate" fraction of the pyrolysate (Figure 6D) is dominated by a C30 n-alkylthiophene and by a C30 n-alkylthiolane-thiol tentatively identified from mass spectra. The n-alkylthiophenes dominate throughout the fraction with a distribution ranging from C13 to C32. The C28-C30 are the most prominent thiophenes. The presumed C28-C30 thiolane-thiols show the same distribution as the C28-C30 thiophenes. 

The distribution of thiophenes in the microbial mat and in the paleosoil of mangrove are noticeably different. In the microbial mat, isoprenoid, branched, midchain and n-alkylthiophenes occur mainly between C14 and C21 and are dominated by the C20 thiophenic isoprenoids. In the paleosoil of mangrove, if some of the isoprenoids and branched thiophenes are present, the sulfur compounds are dominated by the C28-C30 n-alkylthiophenes and by a presumed C30 n-alkylthiolane thiol, and in the extract by a C29 sterane-thiol. These differences are likely to be a reflection of the type of molecules in which sulfur might have been incorporated. 

These will be exemplified by series of alkyl, branched and isoprenoidal thiophenes. These methodologies have been used for the preparation and subsequent identification of series of alkylthiophenes comprising a linearly extended phytane skeleton. 

This and other older methods, which have been reviewed previously (21, 22), have been largely superceded in the laboratory by more controlled, selective reactions, starting from suitable thiophene precursors. We shall discuss two major reactions which can be used for the preparation of alkylthiophenes electrophilic substitution of thiophenes and the formation and reactions of lithiated thiophenes. For further details and other aspects of thiophene chemistry the reader is referred to previous reviews (21, 22) and references cited therein. Accounts of yearly developments in thiophene chemistry can also be found in the relevant Royal Society of Chemistry publications. 

Electrophilic Substitution of Thiophenes. Electrophilic substitution involves the reaction of electron deficient species, so-called electrophiles (E+), with suitable substrates, in this case thiophene or alkylthiophenes, to give a 2-substituted thiophene (Figure 2). 

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