Metal atoms thiophenes

The most favorable coordination sites in thiophenes are the C2C3 and C4C5 double bonds ( -coordination, 38). This type of coordination greatly enhances the nucleophilic power of the sulfur atom, which then gives rise to two new modes of binding the metal atoms, as in the V, S-p.2-, 42, and V, S-p.3-species, 43. 

Interaction of the iron metal atoms with thiophenes (thiophene, 2-methyl-, and 2,5-dimethylthiophene) in the vapor phase at 77 K with subsequent heating in a carbon monoxide atmosphere also leads to the formation of ferrole 83 [76JOM(l 18)37, 77CJC3509]. The iron cyclopentadienyl ring is planar and all the bonds have multiple character. 

Figure 1. Catalytic activities of MoSx/NaY CoSx/NaY(0), and MoSj/NaY (A) for the HDS of thiophene as a function of the metal content (metal atoms/SC).

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. Although regioselective reactions are routinely performed for oc,p-dihalofurans, regioselectivity is not as easily achieved in Pd-mediated chemistry with oc,p-dihalothiophenes. 

The hydrated furan-2-carboxylate, Np02(C4H30C02)-2H20, and the thiophene-2-carboxylate, Np02(C4H3SC02)-2H20, dehydrate at 90 °C and 100 °C respectively it is possible that the ring O and S atoms in these two carboxylates are also bonded to the metal atom. 

Chelating carboxylates. Furan-2- and thiophene-2-carboxylates are included in Table 70 because the heterocyclic oxygen or sulfur atoms may be bonded to the metal atom. The... 

Metallacyclopentadienes undergo a range of synthetically versatile reactions which proceed with extrusion of the metal atom and attendant ligands. Thus, reactions with alkenes and alkynes afford cyclohexa-1,3-dienes and arenes (Scheme 6), and thiophenes, selena-cyclopentadienes, pyrroles and cyclopentadienones (indenones, fluorenones) can be obtained by treatment with sulfur, selenium, nitroso compounds and CO, respectively. The best studied substrates for such reactions are cobaltacyclopentadienes of the type (24a), which have been converted into a wide variety of arenes, cyclohexadienes and five-membered heterocycles, many of which would be very difficult to obtain by conventional organic procedures (74TL4549, 77JOM(139)169, 80JCS(P2)1344). 

The calculations indicate that when Al-atoms approach thiophene rings, they preferentially interact with the carbon atoms located in the a-positions relative to the sulfur atom. In the case of the thiophene molecule interacting with two Al-atoms, each metal atom forms a single Al-C bond at these positions. 

The insertion of a metal atom into the C-S bond might constitute the first stage in the ultimate desulfurization of thiophene derivatives. It is therefore of great interest that several reactions have been discovered from which such metallothiacycles could be isolated. 

One of the modes of coordination of thiophene to catalyst surfaces most frequently suggested is the S or 771, in which the bond is formed directly between the sulphur and a transition metal atom. In examples of the S-bonding mode which have been well-characterized by X-ray diffraction, a pyramidal geometry has been found [75], which is also present in the absorption of thiophene on metallic surfaces such as copper [76]. 

A good number of mononuclear metal complexes are known to activate C-S bonds, and it is clear that a high electron density around the metal atom generally favors the oxidative addition reaction. One commonly encountered case is the interaction of electron rich, coordinatively unsaturated (16-electron) metal fragments with thiophenes which results in the rupture of one of the C-S bonds with the consequent formation of the corresponding saturated 18-electron thiametallacycles. 

Synthesis with addition of 1 C-atom Metalation of thiophenes... 

Although K electron interactions seem to be important in desulfurization, both sulfur-containing and non-sulflir aromatic compounds interact with metals on the catalyst support (MCM alumina, activated carbon, zeolites) via this mechanism. Specific for thiophenic compounds are only two types of interactions, which can be used for separation. They are the t) -S bonding interactions between the sulfur atom and one metal atom, and the S-ps bonding interactions between the sulfur atom and two metal atoms [7], They are illustrated in Fig. 36... 

It is tempting to speculate whether the role of promoter HDS metals (Co, Ni) is the C-S activation of the thiophenes whereas the catalytic metal (group 6, Mo, W) would be responsible for the transfer of hydrogen to the activated substrate. This is a very likely pathway to be envisaged on metal sulfide surfaces where sulfides or thiolates bonded to two or more metal atoms are plentiful. However, this type of interaction is obviously unavailable in single-site complexes, and this could explain why ring opening is frequent but complete desulfurization is rare on mononuclear derivatives. 

---