Thiophenes iron complexes

Thiophene has been reported to react with Fe(CO)s to produce a material described as thiophene-iron dicarbonyl 52). However, a complex having very similar properties to this substance and also obtained from the reaction of thiophene and Fe(CO)s or Fe3(CO)i2 was found not to contain sulfur (74) and the existence of thiophene-iron dicarbonyl is questionable. The material which does not contain sulfur is found to be identical to one of the products obtained upon reaction of acetylene with iron carbonyl, for which the binuclear structure (LVIII) has been proposed 38, 54, 75). 

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 TT-complex 63 was obtained by the condensation of chromium hexacarbonyl with the thiophene a-derivative of dicarbonylironcyclopentadienyl 64 (76IZV153, 79IZV900). The presence of the electron-donor iron-containing substituent facilitates TT-coordination. 

Cp(CO)2Re(THF) forms the complex 105 upon reaction with thiophene (89JA8753, 910M2436). Similar species are known for 2- and 3-methyl-, 2,5-dimethyl, and tetramethylthiophene (91IC1417). Thiophene in 105 is S-coordi-nated, and the sulfur atom is pyramidal. Treatment of 105 with Fc2(CO)9 produces 106, where the thiophene ligand is bridge-coordinated via the sulfur atom to rhenium and four carbon atoms of the dienic system with iron (the coordination mode). The pyramidal nature of the sulfur atom is preserved. The -coordination of thiophene separates the dienic and sulfur counterparts of the ligand and decreases the TT-electron delocalization, which leads to the enhanced basicity of the sulfur atom. 

Besides complexes of thiosemicarbazones prepared from nitrogen heterocycles, iron(III) complexes of both 2-formylthiophene thiosemicarbazone, 26, and 2-acetylthiophene thiosemicarbazone, 27, have been isolated [155]. Low spin, distorted octahedral complexes of stoichiometry [Fe(26)2A2]A (A = Cl, Br, SCN) were found to be 1 1 electrolytes in nitromethane. Low spin Fe(27)3A3 (A = Cl, Br, SCN) complexes were also isolated, but their insolubility in organic solvents did not allow molar conductivity measurements. Infrared speetra indicate coordination of both via the azomethine nitrogen and thione sulfur, but not the thiophene sulfur. The thiocyanate complexes have spectral bands at 2065, 770 and 470 cm consistent with N-bonded thiocyanato ligands, but v(FeCl) and v(FeBr) were not assigned due to the large number of bands found in the spectra of the two ligands. 

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, seienophene[233,234], and cyclobutadiene iron carbonyl complex[235] react with alkenes to give vinyl heterocycles. The ease of the reaction of styrene with substituted 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]. 

It seems likely that thiophene-metal 7r-complexes are formed when thiophene is condensed with iron vapor, but the products isolated indicate subsequent desulfurization of the thiophene by the metal (18, 41) (see Section IV,B). 

Thiophenes are desulfurized by chromium and iron atoms. The role of complex intermediates is uncertain but with iron the final product appears to be a ferracyclopentadiene derivative (18) ... 

The reactions of a,a -dibromoketones with iron carbonyls generate oxyallyliron complexes (75). These undergo cycloaddition with cyclopentadiene and furan, but with thiophene only products of electrophilic attack are obtained (78JA1765). Thus the oxyallyliron complex (75 R = Me R = H) reacts with thiophene to produce (76) in 37% yield. 

With chromium tricarbonyl 387 and with either triirondodecacar-bonyl 387-389 or iron pentacarbonyl389 benzo[6]thiophene affords a yellow complex, C8H6S-01(00)3, and a red-orange complex, C8H6S-Pe2(C0)e, respectively. 

The antitumour action of the natural antibiotic bleomycin is thought to involve the aerobic degradation of DNA by the Fe2+-bleomycin complex. In order to probe the mechanism of antitumour action of bleomycin, the 4-ethylamido[5,(2 -thienyl)-2-thiophene] imidazole iron(II) complex was synthesized [129]. It was studied in non-aqueous solution using cyclic voltammetry and showed antitumour activity in vitro, its action causing cleavage of the double helical DNA. 

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). 

Most of the listed complexes show strong absorption maxima, probably due to CT-transitions. The transitions in duroquinone-, cyclo-pentadienone-, and thiophene-dioxide-iron tricarbonyl have been assigned on the basis of qualitative MO considerations 408>. MO schemes have been established for Mn(CO)5X 193,195) and arene chromium tricarbonyls 92>. The band at 26670 cm-1 in C8HoCr(CO)3 has been attributed to a Cr- ring CT. We will return to this point in section E6. 

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