Thiophene-chromium tricarbonyl

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). 

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. 

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. 

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). 

When treated by a solid base in the presence of 18-C-6, thiophene tricarbonyl chromium undergoes a rapid deprotonation and alkylation by, for example, ethyl bromide (Eq. 44). Sonication helps this reaction, which is unfortunately non-selective since the 2-ethyl and the 2,5-diethyl substitution products are present. 

Refluxing thiophene with chromium hexacarbonyl gives the orange, diamagnetic, crystalline complex, C4H4SCr(CO)3 [104]. The complex is essentially isomorphous with benzenechromium tricarbonyl [105]. 

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