Selenophene thiophene

The reactivity sequence furan > tellurophene > selenophene > thiophene is thus the same for all three reactions and is in the reverse order of the aromaticities of the ring systems assessed by a number of different criteria. The relative rate for the trifluoroacetylation of pyrrole is 5.3 x lo . It is interesting to note that AT-methylpyrrole is approximately twice as reactive to trifluoroacetylation as pyrrole itself. The enhanced reactivity of pyrrole compared with the other monocyclic systems is also demonstrated by the relative rates of bromination of the 2-methoxycarbonyl derivatives, which gave the reactivity sequence pyrrole>furan > selenophene > thiophene, and by the rate data on the reaction of the iron tricarbonyl-complexed carbocation [C6H7Fe(CO)3] (35) with a further selection of heteroaromatic substrates (Scheme 5). The comparative rates of reaction from this substitution were 2-methylindole == AT-methylindole>indole > pyrrole > furan > thiophene (73CC540). 

The effect of substituents on the reactivity of heterocyclic nuclei is broadly similar to that on benzene. Thus mem-directing groups such as methoxycarbonyl and nitro are deactivating. The effects of strongly activating groups such as amino and hydroxy are difficult to assess since simple amino compounds are unstable and hydroxy compounds exist in an alternative tautomeric form. Comparison of the rates of formylation and trifiuoroacetylation of the parent heterocycle and its 2-methyl derivative indicate the following order of sensitivity to substituent effects furan > tellurophene > selenophene = thiophene... 

A quantitative study has been made on the effect of a methyl group in the 2-position of five-membered heteroaromatic compounds on the reactivity of position 5 in the formylation and trifluoroacetylation reaction. The order of sensitivity to the activating effect of the substituent is furan > tellurophene >selenophene = thiophene (77AHC(2l)ll9). 

Rate data are also available for the solvolysis of l-(2-heteroaryl)ethyl acetates in aqueous ethanol. Side-chain reactions such as this, in which a delocalizable positive charge is developed in the transition state, are frequently regarded as analogous to electrophilic aromatic substitution reactions. In solvolysis the relative order of reactivity is tellurienyl> furyl > selenienyl > thienyl whereas in electrophilic substitutions the reactivity sequence is furan > tellurophene > selenophene > thiophene. This discrepancy has been explained in terms of different charge distributions in the transition states of these two classes of reaction (77AHC(21)119>. 

Electrophilic substitution in furan, thiophene, selenophene and pyrrole has, up to 1970, been comprehensively reviewed by Marino.66 Italian workers have determined the relative reactivities of selenophene and thiophene as well67 relative rates are given in Table I. Including furan, the order of reactivity is furan > selenophene > thiophene. 

A recent study of proton transfer from rhenium Fisher-type carbine complexes (13) shows that the reactions lead to the formation of an aromatic product (14), following the same rules as reactions that lead to the formation of products stabilized by simple resonance. The conjugate bases of these carbine complexes represent aromatic heterocycles, i.e., substituted furan, selenophene, and thiophene derivatives, respectively. The aromatic stabilization of these heterocycles is known to follow the order furan < selenophene < thiophene (Scheme 1) [43],... 

The reactivity sequence furan > selenophene > thiophene > benzene has also been observed in the nucleophilic substitutions of the halogenonitro derivatives of these rings.21,22 This shows that the observed trend does not depend on the effectiveness of lone-pair conjugation of the heteroatoms NH, O, Se, and S and the 77-electron density at the carbon atoms. It is interesting to note that a good correlation is observed between molecular ionization potentials (determined from electron impact measurements) and reactivity data in electrophilic substitution, in that higher reactivities correspond to lower ionization potentials182 pyrrole furan < selenophene < thiophene benzene (see Table VII). This is expected in view of a... 

The order of reactivities of the 2-positions appear to be pyrrole > furan > tellurophene > selenophene > thiophene. The order for furan and tellurophene was also obtained in trifluoroacetylation (for which no cr+ values are obtainable). However, in solvolysis of 1-arylethyl acetates, tellurophene is more reactive than furan. 

Relative rates. The order of reactivities at position 2 is pyrrole > furan > tellurophene > selenophene > thiophene. Where data are available for both 2- and 3-positions, the following order is seen 2-furan > 2-thiophene > 3-furan > 3-thiophene. Reactivity parameters (+) for the 2- and 3-positions of thiophene for reactions of varying -values (ranging from 0.66 to 12.0) have been established. 

The reactivity of five-membered tings with one heteroatom to electrophilic reagents has been quantitatively compared. Table 1 shows that the rates of substitution for (1) formylation by phosgene and A A -dimethylformamide, (2) acetylation by acetic anhydride and tin(IV) chloride, and (3) trifluoroacetylation with trifluoroacetic anhydride are all in the sequence furan > tellurophene > selenophene > thiophene. Pyrrole is still more reactive as shown by the rate of trifluoroacetylation, the relative rates of bromination of the 2-methoxycarbonyl derivatives (pyrrole > furan > selenophene > thiophene), and the rate data for the reaction of the iron tricarbonyl-complexed carbocation [C6H7Fe(CO)3]+ (Scheme 18) (2-methylindole iV-methylindole > indole > pyrrole > furan > thiophene). 

Differences between the length of the C-2-C-3 double bond and that of C-3-C-4 are 0.06,0.05, and 0.07 A for selenophene, thiophene,19 and furan,19 respectively. If bond equivalence is taken as a measure of aromaticity, then the compounds form the following series furan < selenophene < thiophene. 

Isotope exchange is a technique which gives reliable comparative data on the electrophilic and protophilic reactivity of the five-membered heterocycles furan, thiophene, and selenophene, revealing how the activity of their hydrogen atoms varies with the position of the hydrogen in the ring and with the number and position of substituents. The results from kinetic data on isotopic exchange in deuterated selenophenes, thiophenes, and furans, may be summarized as follows. 

The structures below summarize hydrogen activities of selenophene, thiophene, and furan in protophilic isotope exchange the rate constant of deuterium exchange at the a position of furan is arbitrarily given the value of unity. 

Partial rate factors (/) obtained from deuterium exchange in 2-deuteriated 3- and 5-methylselenophenes and thiophenes were compared with the / values of deuterium exchange in o-, m-, and p-deuteriated toluenes87,88 the comparison shows that the acid- and base-catalyzed exchange of deuterium is similarly affected by methyl substituents in the selenophene, thiophene, and benzene series. Hence, the heterocycles behave here as normal aromatic systems. 

Various copolymers of selenophene have also been reported. These include polymers of selenophene-ethene <1998MM1221>, selenophene-thiophene <1996SM(82)111>, selenophene-pyridine 180 <1996CM2444,... 

The data considered confirm the reactivity sequence pyrrole furan > selenophene > thiophene for substrate selectivity on electrophilic substitution (71 AHC(13)235) and show that the positional selectivity is reduced in the series furan > selenophene > thiophene > pyrrole, which correlate with that for the relative stability of the onium states of the elements (O < Se < S " < N" ") in agreement with the hypothesis proposed previously (79MI2,80KGS1587), not including selenophene and its derivatives. 

Extension166-166 to pyrrole- and selenophene-2-carboxylic acids gives a sequence of sensitivity in the order pyrrole, furan, selenophene, thiophene, benzene, the same order as that observed by Tirouflet et al.ul in the polaro-graphic reduction of nitro derivatives of these rings. A different sequence, however, which correlates better with ground state aromatic character , viz., furan, pyrrole, thiophene, benzene, has been observed in the gas-phase ionization process,168 but results for electrophilic substitution appear to be anomalous.169... 

In the first reaction, the relative rates were obtained by a kinetic approach and are the ratios of the second-order rate constants in the last two reactions, the relative rates were determined using a competitive procedure. The relevant data are summarized in Table XVII. In all three reactions examined, the reactivity sequence is furan > tellurophene > selenophene > thiophene. The reactivity data are consonant, as evidenced by the linear plots obtained when log (k/kn) values for one reaction are plotted against the log (fc/fcnJ values for another (Fig. 4). 

J. Hollinger, A. A. Jahnke, N. Coombs, D. S. Seferos, Controlling Phase Separation and Optical Properties in Conjugated Polymers through Selenophene-Thiophene Copolymerization. /. Am. Chem. Soc. 2010, 132, 8546-8547. 

HoUinger J, Jahnke AA, Coombs N, Seferos DS. Controlling phase separation and optical properties in conjugated polymers through selenophene-thiophene copolymerization. J Am Chem Soc 2010 132 8546-8554. 

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