Tellurophenes aromaticity

In summary, most of the presently available criteria point to an order of decreasing aromaticity of benzene > thiophene > selenophene pyrrole > tellurophene > furan. 

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). 

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>. 

Thiophenes continue to play a major role in commercial applications as well as basic research. In addition to its aromatic properties that make it a useful replacement for benzene in small molecule syntheses, thiophene is a key element in superconductors, photochemical switches and polymers. The presence of sulfur-containing components (especially thiophene and benzothiophene) in crude petroleum requires development of new catalysts to promote their removal (hydrodesulfurization, HDS) at refineries. Interspersed with these commercial applications, basic research on thiophene has continued to study its role in electrocyclic reactions, newer routes for its formation and substitution and new derivatives of therapeutic potential. New reports of selenophenes and tellurophenes continue to be modest in number. 

Structures and nomenclature for the most important five-membered monocycles with one or more heteroatoms are depicted in Scheme 1. The aromaticity scale in five-membered heterocycles has been long debated.97-101 The decreasing order of aromaticity based on various criteria is (DRE values in kcal/ mol) benzene (22.6) > thiophene (6.5) > selenophene > pyrrole (5.3) > tellurophene > fur an (4.3). Pyrrole and furan have comparable ring strains (Scheme 38). The aromaticity of furan is still controversial 100 the NMR shielding by ring current estimated it at about 60% of the aromaticity of benzene, and other methods reviewed earlier102 estimated it at less than 20%. 

Tellurophene, the most important member of chalcogenophenes, is a light yellow, bad smelling and toxic oil, rather stable in air. Its aromaticity follows the order benzene>thiophene>selenophene>tellurophene>furan." ... 

According to structural indices AN and I, the aromaticity of non-condensed heterocycles varies in the sequence thiophene > pyrrole selenophene > tellurophene > furan. 

Selenophene and tellurophene must be ranked as late developers relative to the other heterocycles covered in this volume. Some indication of the situation is provided by the fact that selenophene was first described in 1923 and tellurophene in 1966, although a few derivatives had been reported earlier. Undoubtedly an important reason for their earlier neglect is the failure of either ring system to be detected in a naturally occurring compound. Much of the stimulus for contemporary studies of these compounds has been provided by the almost unique opportunity of examining the relationship between the aromaticity of these heterocycles and the position of the heteroatom in the periodic system, a topic which has already been discussed in Section 3.01.5.2. 

Selenophene and tellurophene are both reported to be unstable to acid. Treatment of benzo[6]selenophene with polyphosphoric acid gives a product (58) arising from initial 2-protonation followed by electrophilic attack of the resulting carbocation on a second molecule of benzo[6]selenophene. Rearrangement of the 2,3 -linked product (58) to the 2,2 -linked isomer (59) is followed by aromatization to give 2,2 -bibenzo[6]selenienyl (60) (76CS(9)143). 

With its sextet of 71 electrons, thiophene possesses the typical aromatic character of benzene and other similarly related heterocydes. Decreasing orders of aromaticity have been suggested to reflect the strength of this aromatic character benzene > thiophene > pyrrole > furan (9) and benzene > thiophene > selenophene > tellurophene > furan (10). 

---