Thiophene, metallation

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). 

Thiophene metal poisoning as well as hydrogenation of ethylbenzene on metal catalysts require, as a first step, the chemisorption of both organic molecules on the metal active sites. Afterwards, catalyst deactivation can simply take place by the blocking of these sites or by further hydrogenolysis of thiophene and subsequent formation of an inactive surface metal sulfide. We believe that, in our conditions, this last mechanism is probably operating. This hypothesis is supported by the fact that butane was detected in our experiments and, furthermore, XPS analysis showed the formation of metal sulfides (S ) on the deactivated catalysts. 

To summarize, electrophilic substitutions and metalations of thiophenes take place preferably at the a-positions due to the electronegativity of the sulfur atom. This is the consequence of the more effective incorporation of lone pair electrons on the sulfur into the aromatic system. In contrast, electrophilic substitution of benzo[fe]thiophenes occurs at the P-position however, the preference is not strong and substitution can result in other positions. Like thiophene, metalation and metal-halogen exchange are favored at the a-position. Also, oxidative couplings of thiophenes and benzothiophenes are not very... 

Metal complexes containing q -bonded thiophenes, where the ring formally donates 6 electrons and occupies three coordination sites (Lj-type ligand) are the most numerous and stable of the transition metal-thiophene derivatives, examples being available for Cr, Mn, Re, Fe, Ru, Rh, and Ir. Curiously, the synthesis of the first n-thiophene metal complex, viz. Cr(CO)3(q -T) reported by Fischer as early as 1958 [67] represents still today the only example available for a Group 6 metal it-bonded to thiophene its X-ray structure was solved by Dahl in 1965 albeit with a strong rotational disorder for the... 

The majorify of sulfur compounds (thioles and sulfides) have been successfully removed from liquid fuel using a hydrodesulfurization process where high temperature and high pressure are required [7-9, 159]. As mentioned in section 2.5 some sulfur species are very resistant to hydrodesulfurization and those include thiophenic compounds, especially dibenzothiophene and 4,6 dimethyldibenzothiophene [148]. Various methods based on extraction and adsorption have been proposed to remove these compounds [7, 8, 13,145, 147-149, 151-158]. In the extraction route, sulfiir species are first oxidized and then extracted using organic solvents as, for instance acetonitrille [13, 149]. On the other hand, an adsorption process is usually tailored to enhance either adsorption forces, selectivity, or to impose a chemical reaction. So fer the enhancement in the removal of thiophenic compounds was reported on materials where n-complexation can occur as on Cu-Y zeolites [151, 153], or on alumina with highly dispersed sodium [147]. In the latter case, mono- and disodium thiophene metallates are formed. Another desulfurization methods use formation and subsequent precipitation of S-alkylsulfonium salts [148]. 

The study of hydrodesulfurization (HDS) has had tremendous impact on the petroleum industry <88ACR387, 88ACR394). The mechanism of hydrodesulfurization has theoretical and commercial importance and has been the subject of numerous reports investigating mechanism, variation of catalyst and substrate, and product composition. In general, the role of the metal catalyst as well as the structures of thiophene-metal coordination complexes will be discussed in Section 2.12.11.2. In this section, we focus on specific desulfurization to produce isolable products. 

Ziemels, K. E., Hussain, A. T Bradley, D. D. C., Friend, R. H., Ruhe, J., and Wegner, G., Optical spectroscopy of field-induced charge in poly(3-hexyl-thiophene) metal-insulator-semiconductor structure evidence for polarons, Phys. Rev. Lett., 66, 2231-2234 (1991). 

Schatz J, Hoffmann I (2014) Thiophene metallation and cross-coupling chemistry. Top Heterocycl Chem. doi 10.1007/7081 2014 127... 

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