Thiophenes chromium complex

Methylthiophene is metallated in the 5-position whereas 3-methoxy-, 3-methylthio-, 3-carboxy- and 3-bromo-thiophenes are metallated in the 2-position (80TL5051). Lithiation of tricarbonyl(i7 -N-protected indole)chromium complexes occurs initially at C-2. If this position is trimethylsilylated, subsequent lithiation is at C-7 with minor amounts at C-4 (81CC1260). Tricarbonyl(Tj -l-triisopropylsilylindole)chromium(0) is selectively lithiated at C-4 by n-butyllithium-TMEDA. This offers an attractive intermediate for the preparation of 4-substituted indoles by reaction with electrophiles and deprotection by irradiation (82CC467). 

The crystal structures of (3a,4-8,8a-f/)-[5.7,8-trimethyl-8H-cyclohepta-(fe)-thiophen]- and (1—3,3a.8a-f7)-[5,7-dimethyl-4//-cyclohepta-(c)-thiophen]-chromium tricarbonyl complexes, (55) and (56). respectively, have been determined. ... 

In view of the extensive nature of this field, remarkably few complexes of heterocyclic systems have been reported. One of the first to be isolated was tricarbonyl-7r-thiophene chromium 28), and the preparation of tricarbonyl-7r-pyrrolyl manganese has recently been reported (29). 

Arenes and heteroarenes which are particularly easy to metalate are tricarbo-nyl( 76-arene)chromium complexes [380, 381], ferrocenes [13, 382, 383], thiophenes [157, 158, 181, 370, 384], furans [370, 385], and most azoles [386-389]. Meta-lated oxazoles, indoles, or furans can, however, be unstable and undergo ring-opening reactions [179, 181, 388]. Pyridines and other six-membered, nitrogen-containing heterocycles can also be lithiated [59, 370, 390-398] or magnesiated [399], but because nucleophilic organometallic compounds readily add to electron-deficient heteroarenes, dimerization can occur, and alkylations of such metalated heteroarenes often require careful optimization of the reaction conditions [368, 400, 401] (Schemes 5.42 and 5.69). 

Fig. 6. "B-NMR data for 2-diethylboryl-5-methyl thiophene (a) and its pentacarbonyl-chromium complex (b). In the latter, donor-acceptor relationship between boron and the carbonyl group is postulated to account for its higher field nB shift (c).

Anionic ortho-Vries, rearrangement which involves a 1,3-transposition of a carbamoyl group occurred also in the chromium complex 256 on wanning the Uthinm intermediate 257 to —20°C (equation 118). The lithio benzo[i>]thiophene 258 obtained at —78°C was allowed to attain room temperature, and when it was left stirring for 12 hours it gave the salicylamide 259 (equation 119). ... 

Fig. 218. Analysis of a benzene solution of (a) arenetricarbonyl chromium complexes, and (b) arenetricarbonyl chromium thiophen complexes.

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. 

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) ... 

CjHjS, Thiophene, tetrahydro-gold complexes, 26 85-87 C4H,NO, 2-Propenamide, 2-methyl-nickel complex, 26 205 C4H1()02, Ethane, 1,2-dimethoxy-solvates of chromium, molybdenum, and tungsten carbonyl cyclopentadienyl complexes, 6 343 tungsten complex, 26 50 ytterbium complex, 26 22 C4H i02.NaC5H5, Ethane, 1,2-dimethoxy-compd. with cyclopentadienylsodium, 26 341... 

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. 

As in years past, the published thiophene literature dwarfs the selenophene and tellurophene literature. However, there was a significant increase in the number of interesting selenophene and tellurophene references last year. The preparation of T)5-selenophene complexes of chromium, magnesium, ruthenium and iridium were studied <950M332>. These studies took advantage of the NMR-active selenium isotope 77Se (7.58% natural abundance) as a tool to directly study binding of selenophene derivatives to a catalyst surface. 

Removal of thiophene impurities from petroleum feedstocks is accomplished by a process called hydrodesulfurization (HDS) which involves the insertion of metals into the thiophene ring between the C-S bond. In order to better understand the mechanism of this reaction, different groups have utilized selenophene model systems due to the enhanced NMR characteristics of Se. Metal complexes of selenophenes that have been studied include rhodium <19970M2751>, molybdenum <2006POL499>, manganese <20010M3617, 19950M332>, chromium... 

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. 

Mass Spectrometry. The mass spectra of thiophen-2- and -3-carboxanilides, of thiophenic chalcone analogues, of 2,5-diaryl-substituted thiophens, of a terthienyl, and of some chromium tricarbonyl complexes have all been studied. 

Studies in solution suggest specific association of chloroform with the benzo [b] thiophene molecule and interaction of acetone with the sulfur atom.67 Complexation constants of benzo [b]thiophene with polar solvents (e.g., DMF, 2-pyrrolidone) have been determined by an NMR method.77 In the complex of benzo [b] thiophene with Ag+, the metal ion is strongly associated with the lone pair of electrons on the sulfur atom78 in the 1,3,5-trinitrobenzene complex, NMR data suggest that the trinitro compound lies over the phenyl ring of the heterocycle.78,79 Likewise, in the chromium tricarbonyl complex, the Cr(CO)3 group is bonded to the benzenoid ring.80 Studies of the structure of benzo[b]thiophenium salts will be discussed in Section III,A,2. 

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