Thiophenes, selenophenes and tellurophenes

Alkylating agents capable of forming thiophenium salts include trimethyloxonium tetrafluoroborate (Me30+ BF4 ) and alkyl fluorosulfonates (R0S02F). The salts (e.g. 40) are conveniently isolated as hexafluorophosphates (41). 

Halogens attack the ring heteroatom in selenophene and tellurophene. Thus, the 1-bromoseleno-phene bromide (42) is among the bromination products of benzo[/jjselenophene. Tellurophene reacts with halogens to give 1,1-dihalo derivatives (e.g. 43). 

Thiophenium bis(alkoxycarbonyl)methylides (44) are obtained in high yield by rhodium(II) carboxylate-catalyzed reaction of diazomalonate esters with thiophene derivatives (88JCS(P1)1023). Likewise, ylides from benzo[b]thiophene and dibenzothiophene (e.g. 45) have also been reported by tram-ylidation using phenyliodonium bis(phenylsulfonyl)methylide (88JHC1599). 

It was formerly considered that nitrenes attack only the carbons atom of thiophene (84CHEC-(4)74i). Since 1984, several S,N-ylides formed by the attack of a nitrene on the ring sulfur atom of thiophene have been prepared (89AHC(45)l5l). Thus, ethoxycarbonyl nitrene with polyhalogenothiophenes forms the S,N-ylide (46) in 44% yield (84CC190). The X-ray crystal structure reveals that the sulfur in such ylides is pyramidal. 

Thiophene S,N-ylides are comparable with the 1-mono- and 1,1-dioxides in that they exhibit diene rather than aromatic properties (86JCS(P 1)233). The S,C-ylides appear to be more sluggish in cycloaddition reactions and are therefore considered more aromatic . 

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The much less sophisticated PPP approximation has been shown to well reproduce the electronic spectral features not only of the monocyclic furan, pyrrole, thiophene, selenophene and tellurophene but also many of the benzo fused derivatives as well (79MI30101, 68JPC3975, 68MI30100). 

Table 4 pKa Values of Pyrrole-, Furan-, Thiophene-, Selenophene- and Tellurophene-earboxylie Aeids... 

Ring closures which depend on the conversion of the heteroatom into an electrophile are mostly associated with the formation of thiophene, selenophene and tellurophene rings and some illustrative examples are shown in Scheme 17. The last example which concerns the conversion of reaction with isocyanides is of particular interest since it appears to entail the attack of an electrophilic nitrogen species on the aryl ring. 

Vertical ionization energies of the two highest molecular orbitals and of orbitals mainly localized on the substituent of a-substituted derivatives of furans, thiophenes, selenophenes, and tellurophenes have been determined.25 Assignments of some of the bands other than the first two in the photoelectron spectra of tellurophene and selenophene are proposed, and the effect of the ring on the orbitals mainly localized on the substituent is briefly discussed. 

There are several systematic nuclear magnetic resonance studies of the interaction between the substituents and the protons and ring atoms of five-membered heterocycles. In some 2-substituted furans, thiophenes, selenophenes, and tellurophenes there is a linear correlation between the electronegativity of the chalcogen and several of the NMR parameters.28 As there also is a good correlation between the shifts of the corresponding protons and carbons in the four heterocycles, the shifts of unknown selenophene and tellurophene derivatives can be predicted when those of thiophene are known. This is of special interest for the tellurophene derivatives, since they are difficult to synthesize. In the selenophene series, where a representative set of substituents can be introduced in the 2- as well as in the 3-position, the correlation between the H and 13C shifts and the reactivity parameters according to Swain and Lupton s two-parameter equation... 

However, the rate of substitution of pyrrole is too high and that of benzene too low to be followed by standard techniques, and consequently a kinetic study was limited to furan, thiophene, selenophene, and tellurophene. Activation entropies are constant for all four members of the series, indicating that the arrangement of the atoms around the reaction center is similar, i.e., the transition states of all four rings occur at similar positions along the reaction coordinate. The relative rates for the formylation are thus controlled by the activation enthalpies. At 30UC relative rates are furan (107), thiophene (1), selenophene (3.64), and tellurophene (36.8).68... 

Computer simulation of lanthanide-induced shifts in the 2-formyl and 2-acetyl derivatives of furan, thiophene, selenophene, and tellurophene108 indicate a nearly equipopulated mixture of trans and cis conformers of the furan, and a preponderance of the cis for the thiophene, selenophene, and tellurophene derivatives. This difference is due to an interaction between the ring heteroatom and the carbonyl oxygen lone pair electrons. 

Similar correlations have been observed (75CS(7)2ll) between the 13C NMR spectra of mono-substituted furans, thiophenes, selenophenes and tellurophenes as with their H NMR spectra (cf. Section 2.3.3.3). Thus, for the 2-substituted compounds the A(C-3)/A(C-5) ratios decrease systematically in the series furan (2.58) > thiophene (1.34) = selenophene (1.34) > tellurophene (0.91). Extensive quantitative correlations have been established between the shifts of the corresponding carbon atoms in the different heterocycles (75CS(7)21l). In most cases Tsubstituted heterocycles can be linearly correlated with the electronegativity of the heteroatom, with the couplings being largest for the furans. 

Increasing tendency towards electrophilic attack at the ring heteroatom is shown in thiophenes, selenophenes and tellurophenes. 

Pyrrole-3-carboxylic acid (358) is appreciably weaker than benzoic acid and this is attributed to the stabilization of the undissociated acid by electron release from nitrogen. The 2-carboxylic acids of furan, thiophene, selenophene and tellurophene are all stronger acids than benzoic acid... 

Table 1 1H NMR parameters for Furan, Thiophene, Selenophene and Tellurophene at Infinite Dilution8...

Thiophenes, selenophenes, and tellurophenes 90PHC50 91PHC70 92PHC62 93PHC82. 

Nevertheless, it is approximately true that in reactions of average demand for resonance, cr furan) - crt(thiophene) is 0.1, cr - cr, (furan) is 0.45, and crt - o, (thiophene) is 0.40. Reasonably consistent values of a2 for furan, thiophene, selenophene, and tellurophene are -0.91, -0.81, -0.88, and -0.90, respectively the best 0-3 values for furan and thiophene are -0.45, and -0.43, respectively. No data are available for the relative reactivities of the 3-positions of selenophene or tellurophene. The 3-position of pyrrole is slightly more reactive than the 2-position in hydrogen exchange, but the reactivities are so close that under different demands for resonance, the reactivity order can be inverted. 

High-accuracy molecular dimensions for the parent monocyclic heterocycles have been determined by micro-wave spectroscopy and these can be found in Section 2.3.3.2 (Table 7). The ring dimensions obtained by X-ray diffraction for the 2-carboxylic acid derivatives of furan, thiophene, selenophene, and tellurophene (Table 3) are generally in good agreement with those obtained for the parent heterocycles using microwave spectroscopy (Table 7). 

Neutral pyrroles and indoles are also not generally susceptible to attack at the cyclic nitrogen. However, the pA a of carbazole and 9-methylcarbazole being 6.0 and 8.2, respectively, (in H2S04, EtOH, 4 1) refers to N-protonation. Pyrrole anions undergo easy reaction with various electrophiles. An increasing tendency to electrophilic attack at the ring heteroatom is shown in thiophenes, selenophenes, and tellurophenes. 

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