Selenophenes structure

The major internal comparisons to be made within this chapter are between (13) pyrrole (1), furan (2), thiophene (3), selenophene (4) and tellurophene (5) b) pyrrole (1) and indole (6) (c) indole (6), benzo[6 Jfuran (7) and benzo[6]thiophene (8) d) indole (6), isoindole (9) and indolizine (10) and (e) benzo[6] and benzo[c] fused systems. The names of relevant heterocyclic radicals are given with the structures of the parent heterocycle. 

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... 

Selenophene, bis(TV-chlorothioimino)-molecular structure, 4, 939 Selenophene, 2-bromo-mercuration, 4, 946 Selenophene, 3-bromo-lithiation, 4, 949 synthesis, 4, 955 Selenophene, 3-bromo-2-lithio-synthesis, 4, 955 Selenophene, bromonitro-debromination, 4, 78 synthesis, 4, 955 Selenophene, 2-chloromethyl-solvolysis, 4, 952 Selenophene, 3-cyano-synthesis, 4, 955 Selenophene, deutero-deuterium exchange, 4, 949 Selenophene, 2,5-diacyl-3,4-dihydroxy-synthesis, 4, 964... 

Selenophene, 2-methylmercapto-conformation, 4, 944 Selenophene, 2-nitro-mercuration, 4, 946 Selenophene, 2-phenyl-irradiation, 4, 42, 946 mass spectra, 4, 942 Selenophene, 3-phenyl-mass spectra, 4, 942 Selenophene, tetrachloro-applications, 4, 971 reactions, 4, 955 synthesis, 4, 963 Selenophene, tetrahydro-conformation, 4, 34, 944 IR spectra, 4, 942 mass spectra, 4, 24, 943 molecular structure, 4, 938 NMR, 4, 10, 13 reactions, 4, 88, 958 ring strain, 4, 28 synthesis, 4, 118, 962, 968 Selenophene, tetraphenyl-synthesis, 4, 118, 962, 964 Selenophene, 2-thienyl-... 

The electronic structures of furan, thiophene, and selenophene, their protonated complexes, and their anions have been calculated by the extended Hiickel method.6 The results of these calculations have been used to determine the influence of the heteroatom on the degree of aromaticity and electron density. 

NMR using liquid crystal solvents is now a well-established tool for the investigation of molecular structure. Selenophene was studied in a liquid crystal composed of sodium sulfate, decanol, deuterium oxide, and sodium decylsulfate.12 The refined direct couplings were obtained iteratively with the help of a computer. The ratios of the interproton distances were calculated from the direct couplings and found to be in good agreement with corresponding values calculated from the microwave data. 

In order to understand the rates of racemization of biphenyls and bihetero-cyclics, an accurate knowledge of their geometric structure is essential. Such knowledge makes it possible to estimate the amount of interference caused by substituents in the vicinity of the pivot bond in an assumed coplanar transition state for rotation. A study of the crystal structure of 1 (X = Se) found that the selenophene rings have a small but significant deviation from planarity and are nearly perpendicular to each other.17 The deviation from 9(T is such that the carboxyl groups are in transoid positions. 

Magnetic circular dichroism (MCD) has now become a tool with valuable applications to analytical and structural studies. The MCD spectrum of thiophene is only slightly perturbed by substituents, and this is also expected to be true of the quite similar MCD spectra of selenophene.18 These molecules can therefore be classified as hard chromophores. 

Selenolo[3,2-h]selenophene (12) and selenolo[2,3-b]selenophene (13) have been synthesized from lithium derivatives of 2-(3-bromo-2-selienienyl) 1,3-dioxane and 2-(3-selenienyl) 1,3-dioxane, respectively, by reaction with selenium and methyl chloroacetate followed by Dieckmann cyclization.46 Even the third classical selenophthene (11) has been synthesized by two different routes, using 2,3-bischloromethyl-5-carbomethoxyselenophene (14) or preferably 4-methylseleno-3-selenophene aldehyde (IS).46 The fourth selenophthene isomer (16), which has a nonclassical structure, has not yet... 

Substrates 47 and 48 have also been used to study the relative reactivities of selenophene and thiophene and of selenophene and benzene.71 The higher reactivity of the selenophene ring was demonstrated by the fact that upon formylation 59% of compound 49a was formed and upon acylation 63% of derivative 49b. Acylation of 48 gave exclusively 2-acetyl-5-benzylselenophene. Structures of the products were determined by H NMR. 

Recently, the hydroxy derivatives of furan, thiophene, and selenophene have been studied with regard to their physical properties and reactions. These compounds are tautomeric and if the oxygen function is placed in the 2-position they exist as unsaturated lactones and undergo carbon-carbon rearrangement, whereas the 3-hydroxy derivatives form oxo-enol tautomeric systems. By NMR the structures of the different tautomeric forms have been determined as well as the position of the tautomeric equilibrium and the rate of isomerization. 

Selenation of the 1,4-dicarbonyl precursor 89 using the reagent (Me2Al)2Se gave the benzo[c]selenophene derivative 90 in good yield <06TL2887>. The synthesis and structural studies of 4,7-dimethoxybenzo[c]tellurophene have also been reported <06AG(E)5666>. 

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%. 

Recent X-ray structural studies of selenopheno[3,2-b]selenophene (12) have shown it to be isostructural with thieno[3,2-b]thiophene (2). The C—C bond lengths in these molecules are nearly the same (Fig. 2). 

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

The structural indices of aromaticity, I, of oxadiazoles (145-148), thiadiazoles (150-153) and selenadiazoles (155, 156) are compared with that of the parent furan (144), thiophene (149) and selenophene (154) (Scheme 11). 1,2,3-Oxadiazole (145) is the least stable among them since all attempts to synthesize this compound were unsuccessful, most likely because of its easy isomerization to the acyclic isomer. At the same time its sulfur analogue (150) possesses good stability and has been synthesized. Its 2,4-diaza- (151), 3,4-diaza- (152) and 2,5-diaza-(153) isomers demonstrate even more the extent of n-electron delocalization. There exists a well-known tendency of decreasing aromaticity depending on the type of pyrrole-like heteroatom S > Se > O. However, there is no uniformity in the change in aromaticity in the horizontal rows, i. e., dependence on heteroatom disposition. 

Coupling constants for selenophene and tellurophene and their benzo[6] fused derivatives are given in Table 4. Heteroatom-proton couplings in 2-monosubstituted selenophenes and tellurophenes are in the order 2/z H(5> 3/z-h(4> > Vz-hoj and for 3-monosubstituted selenophenes 2/se-H(5) 35 2/se-H(2> 3/Se-H(4) (75CS(8)8). In selenophenes and tellurophenes of similar structure, tellurium-proton couplings are greater than selenium-proton couplings by a factor of approximately two. 

Methylselenophene-2-thiol and 2,5-dimethylselenophene-3-thiol have been shown to exist as thiols by NMR and IR spectroscopy as well as by comparison of gas phase ionization potentials with those of appropriate model compounds (77ACS(B)198). Dipole moment studies have supported formulation of selenophene-2- and -3-thiols as such (73BSF1924) rather than the 2-thione structure originally proposed for the former compound (71BSF3547). 

Over the last few decades there has been much interest in the thiophene-based systems and considerable effort has been expended in many laboratories on various aspects of their chemistry, including synthesis, structural studies and theoretical calculations and predictions. X-Ray analysis of the crystal structures of thieno[3,2-6]thiophene (3) (49AX356), selenolo[3,2-6]selenophene (4) (69AX(B)1374) and thieno[3,2-6 jfuran derivative (5) (79CC366) have been reported. 

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