Thiophene Complex

NH2)50s(2,3-T -L)], where L = furan, pyrrole, and thiophene. Although neither the furan nor thiophene complexes react with maleic anhydride over a period of 10 days, the pyrrole complex (15) reacts rapidly at room temperature and 101.3 kPa to form a mixture of endo (17) and exo (16) complexes. An a2omethine ylide intermediate was postulated as the key intermediate through which maleic anhydride added to the 2- and 5-positions of the coordinated pyrrole ring. 

A typical SSIMS spectrum of an organic molecule adsorbed on a surface is that of thiophene on ruthenium at 95 K, shown in Eig. 3.14 (from the study of Cocco and Tatarchuk [3.28]). Exposure was 0.5 Langmuir only (i.e. 5 x 10 torr s = 37 Pa s), and the principal positive ion peaks are those from ruthenium, consisting of a series of seven isotopic peaks around 102 amu. Ruthenium-thiophene complex fragments are, however, found at ca. 186 and 160 amu each has the same complicated isotopic pattern, indicating that interaction between the metal and the thiophene occurred even at 95 K. In addition, thiophene and protonated thiophene peaks are observed at 84 and 85 amu, respectively, with the implication that no dissociation of the thiophene had occurred. The smaller masses are those of hydrocarbon fragments of different chain length. 

Complexes 79 show several types of chemical reactions (87CCR229). Nucleophilic addition may proceed at the C2 and S atoms. In excess potassium cyanide, 79 (R = R = R" = R = H) forms mainly the allyl sulfide complex 82 (R = H, Nu = CN) (84JA2901). The reaction of sodium methylate, phenyl-, and 2-thienyllithium with 79 (R = R = r" = R = H) follows the same route. The fragment consisting of three coplanar carbon atoms is described as the allyl system over which the Tr-electron density is delocalized. The sulfur atom may participate in delocalization to some extent. Complex 82 (R = H, Nu = CN) may be proto-nated by hydrochloric acid to yield the product where the 2-cyanothiophene has been converted into 2,3-dihydro-2-cyanothiophene. The initial thiophene complex 79 (R = R = r" = R = H) reacts reversibly with tri-n-butylphosphine followed by the formation of 82 [R = H, Nu = P(n-Bu)3]. Less basic phosphines, such as methyldiphenylphosphine, add with much greater difficulty. The reaction of 79 (r2 = r3 = r4 = r5 = h) with the hydride anion [BH4, HFe(CO)4, HW(CO)J] followed by the formation of 82 (R = Nu, H) has also been studied in detail. When the hydride anion originates from HFe(CO)4, the process is complicated by the formation of side products 83 and 84. The 2-methylthiophene complex 79... 

Fig. 29 Bis(naphtolate)pyridine (left) and bis(naphtolate)thiophene complexes (right) for syndio-specific ROP of (3-BL...

The synthesis and chemistry of metal complexes of thiophenes have been reported including the electrophilic additions to osmium-thiophene complexes <9902988> and nucleophilic additions to ruthenium-thiophene complexes <99JOMC242>. The selectivity for the insertion of ruthenium into 3-substituted thiophenes was studied <99CC1793>. For example, treatment of 3-acetylthiophene (84) with Ru(cod)(cot) led to a regioselective 1,2-insertion of ruthenium giving thiaruthenacycle 85. 

Several generalizations emerge from an exhaustive study of these complexes the methoxide ion prefers to attack an a-position in the thiophene nucleus the resulting thiophene complexes are in general more stable than the corresponding Meisenheimer complexes in the benzene series and the stability is increased if the reactive centre already carries an alkoxy substituent. 

Several other reactions of thiophene complexes lead to thioaldehyde or thioketone complexes.78 113 114,154-162 Some examples are summarized in Scheme 11. 

Following the course of equation 9, a bimetallic thiophene complex 2,5-[CpCp Hf(SiH2)Cl]2C4H2S can be similarly obtained. The Hf— Cl bonds in the bimetallic complex are trans to one another, producing one set of diastereomeric SiH2 protons as observed in the H NMR spectrum (Table 2). 

Thiophene complexes, containing a(S)-coordination bond, are represented by a high number of structures [8,13,205b,622,623]. Compound 321 [8] is an example of these complexes. (a-S-Coordination is also characteristic for benzothiophene complexes, for instance 322 [8,13,624] ... 

The cleavage of the C—S bond in r/4-thiophene complexes can be achieved by protonation 147... 

Nucleophilic Attack at Benzenoid Carbon in Benzo[b]thiophene Complexes 830... 

An interesting migration of the metal takes place when the 7] (S)-thiophene complex 255 is treated with a base. Deprotonation at C-2 is followed by migration of the metal from sulfur to the adjacent carbon. On further treatment with triflic acid, reprotonation takes place at C-3 in this system (Scheme 81) <20010M1259>. 

The 77 -thiophene complex of pentammineosmium, on protonation with triflic acid gave the 7] -coordinated ZH-thiophenium complex in high yield (Scheme 82). With the benzo[ ]thiophene complex, however, protonation occurred on the sulfur on reacting with excess triflic acid. 

The 7] -osmium complexes of a-unsubstituted thiophenes undergo Lewis acid-promoted addition with acetals at C-2 to give the thiophenium complexes in good yields <19990M2988>. These can be deprotonated to give the 2-substituted thiophene complexes. The electrophile attacks the substrate on the rivn-face (Scheme 83). 

The high cost associated with the use of stoichiometric quantities of osmium has limited the general use of this strategy. It is interesting that a new molybdenum 7] -thiophene complex has now been developed <2003JA2024>. Its usefulness for further reactions has not yet been demonstrated. 

Thiophene complexes are synthesized by two-electron reduction of 77 -thiophene dicationic complexes. They are thus relatively electron rich and tend to react with electrophiles. 

The picture that emerges is that the bonding within the majority of thiophene molecules adsorbed on the catalyst surfaces is hardly perturbed, and this contrasts sharply with the situation in the thiophene complexes. The thiophene molecule parallel to the surface does not correspond to a metal f/ -bound thiophene. Rather, it is suggestive of a weakly chemisorbed precursor state of thiophene that lies parallel to the surface. In this state, the molecule interacts indiscriminately with the alumina, the basal or edge planes, or both. Moreover, the weakness of this binding enhances the surface mobility of thiophene and allows molecules to move across the surface to the catalytic site for reaction with hydrogen atoms. The few sulfur-bound thiophene molecules, no more than 5-10%, would then correspond to thiophene at the coordinatively unsaturated Mo (or Co) atoms. 

An jj (S) equilibrium (equation 6) is observed in the benzo[fc]thiophene (BT) complexes Cp (CO)2Re(BT), where Cp is Cp or Cp. As for selenophene, the Cp ligand favors j -BT, while Cp favors the rj (S) form it should be noted that 2-MeBT and 3-MeBT form only the jj (S) complexes. The formation of the r] form from the is important for understanding the first step in the HDS of BT, which gives dihydrobenzothiophene (equation 7). This alkene hydrogenation reaction is catalyzed by homogeneous catalysts and may occur similarly on heterogeneous HDS catalysts. While there is no evidence that there is an r] (S) equilibrium in thiophene complexes, both thiophene species may exist on a catalyst surface. Low metal oxidation states would favor r] coordination, which could lead to the initially observed alkene hydrogenation products. 

DHT does not eliminate 1,3-butadiene under these conditions. The 2,5-DHT also releases 1,3-butadiene when adsorbed on single crystal Mo(llO) surfaces and when passed over a 5% Re/Al203 catalyst at 300 °C under HDS conditions. The more saturated C4 products (equation 3) observed in thiophene HDS are presumably formed by hydrogenation of the 1,3-butadiene. In step (e), the active metal site is regenerated by removal of the sulfide ligand as H2 S upon reaction with H2. The key steps (a-d) in this HDS mechanism (Figure 3) have all been observed either in reactions of thiophene complexes or on catalyst surfaces. 

The thiophene complex [Mn(CO)3thiophene]+ (496) reacts with hydride ions at the heterocyclic ring to yield 497. Further protonation results in... 

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