Selenophenes electrophilic

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]. 

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>. 

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. 

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... 

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... 

Selenophene compounds react faster than the corresponding thiophene derivatives in both electrophilic and nucleophilic substitutions. This may be due to the capacity of selenophene to delocalize both positive and negative charges, since the selenium atom is larger and more polarizable than the sulfur atom and consequently selenophene can release its p-electrons and accept electrons into its free -orbitals more readily than thiophene. 

Individual substitutions may not necessarily be true electrophilic aromatic substitution reactions. Usually it is assumed that they are, however, and with this assumption the furan nucleus can be compared with others. For tri-fluoroacetylation by trifluoroacetic anhydride at 75 C relative rates have been established, by means of competition experiments 149 thiophene, 1 selenophene, 6.5 furan, 1.4 x 102 2-methylfuran, 1.2 x 105 pyrrole, 5.3 x 107. While nitrogen is usually a better source of electrons for an incoming electrophile (as in pyrrole versus furan) there are exceptions. For example, the enamine 63 reacts with Eschenmoser s salt at the 5-position and not at the enamine grouping.150 Also amusing is an attempted Fischer indole synthesis in which a furan ring is near the reaction site and diverted the reaction into a pyrazole synthesis.151... 

At present, the chemisty of selenophenes and tellurophenes is a relatively scantily studied area. Nevertheless, a number of new valuable contributions dealing with their chemistry have emerged. Electrophilic cyclization of l-(l-alkynyl)-2-(methylseleno)arenes provides a route to a variety of 2,3-disubstituted benzo[fe]selenophenes, as illustrated by the preparation of the system 88. Other useful electrophiles for similar reactions are E or NBS <06JOC2307>. Similar chemistry has also been employed in preparation of 2,3-disubstituted benzo[f>]selenophenes on solid phase <06JCC163>. In addition, syntheses of 2,3-dihydroselenolo[2,3- >]pyridines have been achieved using radical chemistry <06OBC466>. 

Counteranion-bound S-, Se-, and re-(trifluoromethyl)dibenzothiophe-nium-, -selenophenium-, and -tellurophenium-3-sulfonates 42-44, 48, and 49, developed as another series of the electrophilic trifluoromethylating agents, are useful because the resultant trifluoromethylated products are easily separated from the by-product dibenzothiophene-, -selenophene-, or -tellurophene-3-sulfonic acids because these by-products are soluble in water (Eq. 17) (95JFC). 

Electrophilic attack at ring heteroatoms is rare for the neutral compounds, although examples are known for thiophenes and selenophenes. However, pyrrole anions undergo easy reaction with electrophiles at both C and N atoms. 

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

The reactivity of five-membered rings with one heteroatom to electrophilic reagents has been quantitatively compared. Table 1 shows that the rates of substitution for (a) formylation by phosgene and V,iV-dimethylformamide, (b) acetylation by acetic anhydride and tin(IV) chloride, and (c) trifluoroacetylation with trifluoroacetic anhydride (71AHC(13)235) are all in the sequence furan > tellurophene > selenophene > thiophene. Pyrrole is still more reactive as shown by the rate for trifluoroacetylation, by the relative rates of bromination of the 2-methoxycarbonyl derivatives (pyrrole > furan > selenophene > thiophene), and by the rate data on the reaction of the iron tricarbonyl-complexed carbocation [C6H7Fe(CO)3]+ (Scheme 5) (2-methylindole ss V-methylindole > indole > pyrrole > furan > thiophene (73CC540)). 

Table 1 Relative Rates of Reaction of Thiophene, Selenophene, Tellurophene and Furan in Selected Electrophilic Substitution Reactions...

The reactions of the lithio derivatives of benzo[ >]-fused systems indole, benzo[6]furan and benzo[h]thiophene are similarly diverse. Since indole and benzo[h]thiophene undergo electrophilic substitution mainly in the 3-position, the ready availability of 2-lithio derivatives by deprotonation with n-butyllithium is particularly significant and makes available a wide range of otherwise inaccessible compounds. The ready availability of 3-iodoselenophene and hence of 3-lithioselenophene (73CHE845) provides a convenient route to 3-substituted selenophenes. 2-Lithiotellurophenes are especially important precursors of tellurophene derivatives because of the restricted range of electrophilic substitution reactions which are possible on tellurophenes (77AHC(2l)ll9). 

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). 

Lithiated selenophenes and tellurophenes have been used as intermediates in the preparation of a wide range of derivatives. A comprehensive tabulation of these transformations is available (790R(26)l) and this illustrates the large number of electrophiles which have been reacted with selenienyl- and, to a lesser extent, tellurienyl-lithiums. An extension of the synthetic potential of these lithiated derivatives is achieved by first converting them into iodonium salts (111), which may then be reacted with nucleophiles (Scheme 12) (74SC63). 

For the trifluoroacetylation of 2-substituted thiophenes, furans, and pyrroles in C2H4C12, 75°, the p values are —7.4, —10.3, and ca. —4.5, respectively.259 The value for substituted benzenes is not known. In the gas phase ionization of substituted furans, thiophenes, selenophenes, and pyrroles,264 a reaction proceeding through a positively charged molecular ion taken to be analogous to the Wheland intermediate for electrophilic substitution, the p values are reported to be —20.2, —16.5,... 

The principal electrophiles to attack ring selenium are either oxidants or alkylating agents. Oxidation of substituted selenophenes and 2-benzoselenophene with an excess of 2,2-dimethyldioxirane leads to 1,1-dioxides, whereas oxidation of... 

Substituted selenophenes were formed in moderate to excellent yields by electrophilic cyclization of (Z)-selenoenynes with different electrophiles such as I2, IC1, PhSeBr, and PhSeCl. The reaction proceeded clearly under mild reaction conditions [125],... 

Systems considered are the parent heterocycles with one heteroatom (furan, thiophene, selenophene, and pyrrole), their monoaza and polyaza derivatives, and the monobenzo derivatives of the above systems. Dibenzo derivatives such as dibenzofuran and carbazole are not taken into account, since they have no position in the five-mem bered ring susceptible to electrophilic attack. Likewise, no consideration is given to more complicated systems, formed by the fusion of two heterocyclic rings such as thienothiophenes or pyrrolopyridines, for which, in any case, no quantitative work is available. 

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... 

Fig. 5. The extended selectivity relationship for electrophilic reactions at the a position of selenophene. The numbers identify the reactions see Table XIV.

The comparison among the reactivities towards electrophiles of the five-membered rings has been extended to tellurophen.318 In both the reactions examined (tin tetrachloride catalyzed acetylation and trifluoroacetylation), tellurophen exhibits a reactivity intermediate between those of selenophen and furan. 

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