Benzo thiophene, complexes

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. 

Pathway III of Fig. 26 has been demonstrated for thiophene and benzo-thiophene with Ir complexes (4) and for all thiophenes, including dibenzo-thiophene, with Rh complexes (94, 95). These oxidative additions appear to be influenced by substituents present on the carbon atoms adjacent to the sulfur atom. Insertion between sulfur and the unsubstituted carbon is highly preferred. For 2-methylthiophene the exclusive product is the 1-5 bond insertion product, whereas for 3-methylthiophene, no preference for insertion was observed (1-2 and 1-5 bond insertion products were equal). In competitive studies, thiophene was found to be about twice as reactive as 2,5-dimethylthiophene. This behavior is similar to that observed for relative reaction rates of substituted thiophenes observed with conventional HDS catalysts. Thus steric limitations can occur, even with monomeric, homogeneous catalysts. 

Silica gel is an effective catalyst for the t-butylation of thiophene and benzo[/ ]thiophene using t-butyl bromide. 2,5-Di-r-butylthiophene and 3-f-butylbenzo[b]thiophene can be prepared easily by this procedure (84JOC4161). Alkylation of thiophene with f-butyl chloride, isopropyl chloride or ethyl chloride at - 70°C in the presence of A1C13 produced a-complexes under kinetic control. On thermal equilibration, migration of alkyl from position 3 to position 2, as well as disproportionation to dialkyl-and trialkyl thiophenes can occur (86T759). 

Dihydrobenzo[6]thiophene 426 and its 2-methyl derivative182 are readily dehydrogenated with sulfur to the corresponding benzo[ ]-thiophene. 2,3-Dihydrobenzo[6]thiophenes are oxidized by peracetic acid to the corresponding sulfones (Section VI, P, 2), and give crystalline complexes with mercuric chloride. 

Benzo(6]thiophene halogenation, 57, 294 iodination, 59, 254 resonance energy, 56, 352 Ru, Ir complexes, 58, 150 Benzo[6]thiophene, 3-benzoyl-, reaction with hydrazine, 56, 128 Benzo[l)]thiophene, 3-bromo-, chlorination, 57, 293 Benzo[6]thiophene, 4-fluoro-, 60, 17 Benzo[/>)thiophene, 4,5,6,7-tetrafluoro-2,3-dihydro-2-methyl-, 60, 28 Benzo(e]thiophenes, resonance energy,... 

Ab initio MO calculations have been carried out on benzo[. ]thiophene (BT), 2-methylbenzo[/ ]thiophene (2-MeBT), 3-methylbenzo[ ]thiophene (3-MeBT), and a number of organometallic complexes containing these thiophenic moieties in an attempt to understand the usual preference for insertion of metal fragments into the sulfur-carbon(vinyl) bond of BT and the recent observations of insertion into the sulfur-carbon(aryl) bond <19980M3798>. 

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

In the 7] -(S)-coordinated thiophenes, the sulfur appears to be sp -hybridized the metal does not lie in the plane of the thiophene. They are all weakly coordinating compared to THT (a dialkyl sulfide type of molecule) consequently, such complexes can be isolated only in some special cases. The tendency to coordinate to metal ions in the 77-(S) mode generally increases in the order T < BT < DBT. (As per the normal convention, the unstarred symbols T, BT, and DBT refer to the unsubstituted heterocycles thiophene, benzo[. ]thiophene, and diben-zo[i, . thiophene, respectively, while a star indicates the inclusion of substituted thiophenes as well. Cp and Cp refer to the 77 - bound CsHs and CsMcs, respectively.)... 

Several excellent reviews are available on the reactivity of organotransition metal complexes of thiophenes <20010M1259, 2000CCR63, 1998ACR109>. In addition, Sadimenko has provided a compendium of all the known syntheses and reactions of the organometallic derivatives of thiophene and benzo[ ] thiophene <2001AHC(78)1>. 

Catalytic hydrogenolysis of thiophenes, benzo[, ]thiophene, and dibenzothiophene can be brought about with the help of the 16-electron species [(triphos)MH] (M = Rh or Ir triphos = MeC (CH2PPh2)3). These are generated in situ by thermolysis of the appropriate precursors. The rhodium complex 246 effectively catalyzes such hydrogenolyses. The required experimental conditions are rather drastic (Scheme 71) <1998ACR109>. 

The 77 -bound benzo[/ ]thiophene and dibenzothiophene complexes of iridium undergo reduction to the corresponding 77-complexes by hydride reaction with 2 mol of Red-Al results in addition of two H to give the cyclohexadiene complexes (Scheme 78) <2000CCR63>. 

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. 

Figure 75. Redox induced conversion pathways in rhodium and iridium supported by benzo[i>]thiophene complexes.

The rhodium hydride, [(Iriphos)RhH], will react with thiophene and benzo[(>] thiophene to yield complexes 129 and 130, respectively, which yield other Rh-containing complexes when they were subjected to electrophilic reagents [7]. The catalytic transformation of BT into 2-ethylthiophenol can be achieved with this soluble rhodium complex (Scheme 141) [157]. 

A series of intermediates in the catalytic transformation of benzo[3]thiophene to 2-ethylthiophene were characterized. The 16-electron Rh(i) fragment (triphos)RhH [triphos = MeC(CH2PH2)3] reacted with benzo[/ ]thiophene by bond scission to yield the 2-vinylthiophenolate complex (triphos)Rh 77 -S(C6H4)CH=CH2 221. The complex (triphos)RhH j -S(C6H4)CH(CH3) 222 was also obtained. The complex (triphos)RhH3 reacted with dinaphtho[2,l"Al, 2 - f thiophene (DNT) in a similar way as benzo[ ]thiophene, although the heterometallacyclic species were not detected. ... 

Benzo[6]thiophene, methoxynitro-Meisenheimer complexes, 4, 816 Benzo[6]thiophene, 2-methyl-cycloaddition reactions, 4, 793 protonation, 4, 47 sulfonation, 4, 764 synthesis, 4, 879, 915 Benzo[6]thiophene, 3-methyl-cycloaddition reactions, 4, 793 1-oxide... 

Gold catalyzed reactions are currently enjoying considerable interest, and have also found applications in thiophene ring synthesis. A series of (a-alkoxyalkyl)(o-alkynylphenyl) sulfides 14 was subjected to treatment with catalytic amounts of AuCl, giving the benzo[fe]thiophenes 15 in excellent yields. Some additional, more complex examples were also provided <06AG(E)4473>. 

The electron-rich thiophene ring system can be elaborated into complex, fused thiophenes by acid-mediated intramolecular annelation reactions. For example, treatment of alcohol 96 with trimethylsilyl triflate promoted a Friedel-Crafts acylation and subsequent dehydration giving benzo[b]thiophene 97, a potential analgesic <00JMC765>. Treatment of ketone 98 with p-toluenesulfonic acid resulted in the formation of fused benzo[b]thiophene 99 <00T8153>. Another variant involved the cyclization of epoxide 100 to fused benzo[f>]thiophene 101 mediated by boron trifluoride-etherate . 

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