Thiophenium ions

Thiophenium ions were prepared with AICI3/HQ in CD2Q2 ... 

S-Arylbenzo[fi]thiophenium ions (143) undergo ring opening by cleavage of the S - C2 bond when treated with NaOMe in MeOH (92CL1357). With 3-unsubstituted compounds, the primary process is abstraction of the proton attached to C-3 subsequent cleavage of the S-C2 bond results in the formation of acetylenes (144) in quantitative yield. 

Various NMR studies have shown that protonation of thiophene occurs at position 2. Thus the thiophenium ions (48) were prepared (73TL3929) from (47) in quantitative yield by protonation in fluorosulfonic acid at -70 °C protonation occurred exclusively at position 2. 

Thiophenes are best converted to the tetrahydro derivatives by the so-called ionic hydrogenation. This depends on the successive addition of a proton (from trifluoroacetic acid) and a hydride ion (from triethylsilane) (75T311). A subsequent improvement involved the use of HC1/A1C13 to form the thiophenium ion and then reaction with triethylsilane (78T1703) best results are obtained with the substrate/Et3SiH/AlCl3 ratio of 1 3 0.3. The mechanism of the reaction is shown in Scheme 43. Evidence for this has been provided by the use of Et3SiD, when D enters positions 3 and 5 in the product. 

The effect of changing the nucleophile in an SN reaction continues to attract considerable interest. A study of the rates of the XN2 reactions between S-methyldibenzo-thiophenium ion or methyl iodide and many nucleophiles93 has shown that the... 

This structural outcome suggests a lack of aromaticity of the thiophene ring in the l-phenylbenzo[/)]thiophenium ion. A novel [4+2] cycloaddition reaction of 1-phenyl-l-benzothiophenium salts with cyclopentadiene or 1,3-diphe-nylbenzo[r]furan has been reported which experimentally confirmed the olefinic nature of the thiophene ring arising from a lack of aromaticity <2003JOC731>. 

The stability of cr-complexes 2 and their easy formation are substantially governed by the nature of the counter anion. Thus, in HF solution, thiophene and alkylthiophenes form cations stable at temperatures below —40 °C, in HF—BF3 the same cations are stable up to —20 °C and 2,5-dimethyl-2H-thiophenium ion, generated in HF—SbFs is sufficiently stable even at -1-60 °C (66RTC1072). The high stability of 2H-thiophenium ions formed by protonation in the presence of AICI3 in dichloroalkanes as solvents (75ZOR424) is undoubtedly caused by the AICI4 counterions as well as by favorable solvation conditions. The stability of thiophenium ions is mainly caused by their peculiar structure. 

The formation of cr-complexes is reversible, not only for protons but also other electrophilic species able to eliminate from a nodal center, as was observed in the case of 2,5-bis(methylthio)-2H-thiophenium ion (2i)... 

Halogen-substituted thiophenium ions have low stability and can be observed by NMR only at temperatures below -30 °C (86MRC699). Disproportionation of the 2,5-dibromo-2H-thiophenium ion with formation of 5-bromo- and 3,5-dibromo-2H-thiophenium ions was observed on increasing the temperature from —50 to —10 °C (86MRC699). For chlorothiophenium ions, the disproportionation could not be observed so distinctly. Note that 2,4-dichloro-2H-thiophenium ion (5) is stable at room temperature and is the main transformation product of the less stable 2,5-dichloro-2H-thiophenium ion (2j). The latter fact was used for the preparation of difficultly available 2,4-dichlorothiophene (6) from the 2,5-isomer 3,5,4 -trichloro-2,2 -bithiophene (7) was isolated as a byproduct resulted, probably, from the reaction of ion 5 with dichloride 6 (Scheme 7) (90G365). 

The interaction of 2,4-dichlorothiophene (6) with 3,5-dichloro-2H-thiophenium ion (5) to give bithiophene 7 is an electrophilic substitution in which cation 5 is an electrophile. Similarly, the add-induced oligomerization of five-membered heteroaromatics is a resinification. The structure of 3,4-dimethylpyrrole dimer formed in 6N HCl (68JCS(C) 2526) shows that the oligomerization may proceed (Scheme 11) through a 2H-pyrrolium ion, reaction 2 in Scheme 2. 

The key role of 2H-thiophenium ions as electrophilic species was indicated in 1950 by Hartough et al. (50JA1910), who isolated a so-called... 

Preparatively, most interesting results were obtained by Sone et al. (see review (99MI1)) who studied transformations of chlorosubstituted thiophenium ions, the latter playing a role as alkylating agents. Thus, with equimolar amounts of various aromatic compounds and aluminum chloride, 2-chlorothiophene is converted to the corresponding 2-arylthiophenes 10 (Scheme 14) (86BCJ83). 

Stable 2H-thiophenium ions formed on acylation of thiophene and its homologs with aluminum chloride (due to hydrogen chloride originating during the reaction) essentially lower the yields of ketones. The formation of such cr-complexes, a-C-protonation products, does not take place on preparation of ketones under the usual conditions of acylation in thiophene series, namely, in the presence of tin tetrachloride with... 

Thiophenium ions not only play a key role in electrophilic substitution reactions but they also allow some addition reactions to be possible, such as the ionic hydrogenation of thiophene derivatives proceeding by repeated protonations and additions of hydride ion (Scheme 16) (73IZV1918, 75T311, 78T1703, 79MI1). 

Acceleration of ionic hydrogenation can be attained by an increase in the acidity of the medium, by catalysis, for example, with boron trifluoride etherate, or as the result of carrying out the reaction in HSiEts—HCI/AICI3, that is, under conditions providing for the stability of the thiophenium ions (78T1703, 79MI1). 

Thus, a free a-position is kinetically favored for electrophilic attack in all cases, including the most reactive pyrrole. jS-Substituted derivatives are usually the thermodynamically favored products of further transformation primarily formed on replacement of an a-hydrogen atom. These transformations are intermolecular disproportionation processes (see stable thiophenium ions) or intramolecular 1,2-shifts more familiar for unstable cr-complexes formed on acylation or sulfonation. 

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