Selenophenes substitution

Dechambenait, P., Ferlay, S., Kyrtsakas, N., Hosseini, M.W. (2010). Amidinium based ionic liquids. New Journal of Chemistry, 34,1184-1194. ISSN 1144-0546 Della Rosa, C., Kneeteman, M., Mancini, P.M.E. (2005). 2-Nitrofurans as dienophiles in Diels-Alder reactions. Tetrahedron Letters, 46, 8711-8714, ISSN 0040-4039 Della Rosa, C., Kneeteman, M., Mancini, P.M.E. (2007). Behavior of selenophenes substituted with the electron withdrawing groups in polar Diels-Alder reactions. Tetrahedron Letters, 48, 7075-7078, ISSN 0040-4039... 

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

Table 2 Relative Rates of Reaetion of Thiophene, Selenophene, Tellurophene and Furan in Selected Electrophilie Substitution Reaetions...

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>. 

The direct combination of selenium and acetylene provides the most convenient source of selenophene (76JHC1319). Lesser amounts of many other compounds are formed concurrently and include 2- and 3-alkylselenophenes, benzo[6]selenophene and isomeric selenoloselenophenes (76CS(10)159). The commercial availability of thiophene makes comparable reactions of little interest for the obtention of the parent heterocycle in the laboratory. However, the reaction of substituted acetylenes with morpholinyl disulfide is of some synthetic value. The process, which appears to entail the initial formation of thionitroxyl radicals, converts phenylacetylene into a 3 1 mixture of 2,4- and 2,5-diphenylthiophene, methyl propiolate into dimethyl thiophene-2,5-dicarboxylate, and ethyl phenylpropiolate into diethyl 3,4-diphenylthiophene-2,5-dicarboxylate (Scheme 83a) (77TL3413). Dimethyl thiophene-2,4-dicarboxylate is obtained from methyl propiolate by treatment with dimethyl sulfoxide and thionyl chloride (Scheme 83b) (66CB1558). The rhodium carbonyl catalyzed carbonylation of alkynes in alcohols provides 5-alkoxy-2(5//)-furanones (Scheme 83c) (81CL993). The inclusion of ethylene provides 5-ethyl-2(5//)-furanones instead (82NKK242). The nickel acetate catalyzed addition of r-butyl isocyanide to alkynes provides access to 2-aminopyrroles (Scheme 83d) (70S593). 

Benzo[h]seienophene, 2,3-dihydro-2-methyl-synthesis, 4, 961 Benzo[h]selenophene, 3-halo-2-substituted synthesis, 4, 966... 

Selenolopyrylium salts, 4, 1034—1036 Selenolo[2,3-c]pyrylium salts synthesis, 4, 969 Selenolo[3,2-b]pyrylium salts synthesis, 4, 1035 Selenolo[3,2-c]pyrylium salts synthesis, 4, 969 Selenoloseknophenes electrophilic substitution, 4, 1057 NMR, 4, 13 synthesis, 4, 135 UV spectra, 4, 1044 Selenoloselenophenes, alkyl-synthesis, 4, 967 Selenolo[2,3-b]selenophenes ionization potentials, 4, 1046 Selenolo[3,2- bjselenophenes dipole moments, 4, 1049 ionization potentials, 4, 1046 structure, 4, 1038, 1039 Selenolo 3,4-f)]selenophenes H NMR, 4, 1042 synthesis, 4, 1067 Selenolo[3,4-c]selenophenes non-classical reactions, 4, 1062 synthesis, 4, 1076 Selenolothiophenes electrophilic substitution, 4, 1057 H NMR, 4, 1041 UV spectra, 4, 1044 Selenolo[2,3- bjthiophenes... 

The C-Se and C-Te bonds are formed by an internal homolytic substitution of vinyl or aryl radicals at selenium or tellurium with the preparation of selenophenes and tellurophenes, respectively. An example is shown below, where (TMSIsSiH was used in the cyclization of vinyl iodide 65 that affords... 

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. 

This chapter follows the organization used in the past. A summary of the electronic properties leads into reports of electrocyclic chemistry. Recent reports of studies of HDS processes and catalysts are then summarized. Thiophene ring substitution reactions, ring-forming reactions, the formation of ring-annelated derivatives, and the use of thiophene molecules as intermediates are then reported. Applications of thiophene and its derivatives in polymers and in other small molecules of interest are highlighted. Finally, the few examples of selenophenes and tellurophenes reported in the past year are noted. 

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. 

Data from microwave spectra on the centrifugal effect of rotational transitions of selenophene and its deuterium-substituted derivatives have been determined experimentally and compared with the calculated theoretical values of the centrifugal stretching constants by means of the force constants determined from the solution of the inverse vibrational problem.26 The two sets of values show good agreement, indicating that the system of force constants obtained for selenophene correctly reflects the characteristic features of the force field of the molecule. 

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... 

Simple resonance theory suggests that for 2-substituted selenophenes the selenium atom can be considered to be situated in an ortho position and the influence of mesomeric and inductive effects or chemical shifts should be parallel for the 3-carbon (cf. resonance formula 5-7 for a-I-M substituted derivative). 

Phosphorus pentaselenide prepared from red, amorphous selenium effects cyclization of 1,4-diketones (22) to substituted selenophenes (23) in moderate... 

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. 

Relative Rates of Electrophilic Substitution for Selenophene and Thiophene... 

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