Dibenzothiophene metallation

Competitive metallation experiments with IV-methylpyrrole and thiophene and with IV-methylindole and benzo[6]thiophene indicate that the sulfur-containing heterocycles react more rapidly with H-butyllithium in ether. The comparative reactivity of thiophene and furan with butyllithium depends on the metallation conditions. In hexane, furan reacts more rapidly than thiophene but in ether, in the presence of tetramethylethylenediamine (TMEDA), the order of reactivity is reversed (77JCS(P1)887). Competitive metallation experiments have established that dibenzofuran is more easily lithiated than dibenzothiophene, which in turn is more easily lithiated than A-ethylcarbazole. These compounds lose the proton bound to carbon 4 in dibenzofuran and dibenzothiophene and the equivalent proton (bound to carbon 1) in the carbazole (64JOM(2)304). 

Although single-electron-transfer (SET) processes would be expected to be important in reactions that use metals as reagents, this type of process has also been recognized in the reduction of carbonyl groups that involve 1,4-dihydronicotinamide derivatives . Recent work by Oae and coworkers" has shown that an SET process is operative in the reduction of dibenzothiophene S-oxide by l-benzyl-l,4-dihydronicotinamide when the reaction is catalyzed by metalloporphins. The reaction is outlined in equation (18), but the study gave results of much more mechanistic than synthetic value. This type of study is relevant to understanding biochemical mechanisms since it is known that methionine sulphoxide is reduced to methionine by NADPH when the reaction is catalyzed by an enzyme isolated from certain yeasts . 

There is still no efficient synthesis of 3-dibenzothiophenecarboxylic acid. The only recorded preparation is via metallation of dibenzothiophene with phenylcalcium iodide followed by carbonation, but the yield was poor. The successful use of 3-lithiodibenzothiophene to give the 3-aldehyde indicates that 3-bromodibenzothiophene would probably be the best precursor of the 3-acid. Both l,4,4a,9b-tetrahydrodibenzo-... 

Metallation of dibenzothiophene 5-oxide with three equivalents of butyllithium followed by carbonation gave a mixture of 4-dibenzo-thiophene carboxylic acid (36 /o) and dibenzothiophene (10%). The reduction or even elimination of sulfoxide groups in the presence of... 

Phenylcalcium iodide metallates dibenzothiophene in the 3-position however, it has no effect on the corresponding sulfone. Mercuration of dibenzothiophene has been accomplished by adding mercuric acetate to a melt of dibenzothiophene, but the position of mercuration was not established. Mercuric nitrate or bisulfite give no identifiable products. An unsuccessful attempt to metallate dibenzothiophene with cross-linked poly(p-lithiost5Tene) in ether has been recorded. ... 

Thianthrene was inert to (COD)2Ni(0) alone, but treatment with 2 mol equivalents of the nickel species in the presence of 2 mol equivalents of bpy converted it into dibenzothiophen (60%) and diphenyl (10%). The active desulfurizing agent was considered to be (bpy)(COD)Ni(0), the requirement for two metal equivalents being that one coordinates a sulfur the other effects rupture of the ring Scheme 8 illustrates this (77JOM51). 

Figure 8.19 Catalytic activity of transition-metal sulphides for hydrodesulphurization of dibenzothiophene. (After Whittingham Chianelli,...

As described in Section IV.B, dibenzothiophenes, when substituted in positions adjacent to the sulfur atom, have reduced activity for direct sulfur extraction. As a result, catalysts that promote aromatic ring hydrogenation offer another route to desulfurization, as the partially hydrogenated ring presents much less steric restrictions to adsorption via r -S type bonding (17,21) or to oxidative addition to form a metallathiabenzene intermediate, as discussed in Section IV.E.3. In addition, the metal-S coordination bond strength is increased by increasing the electron density on sulfur, and the C-S bonds in hydrothiophenes are much weaker. 

Alkyllithium compounds metallate dibenzofuran, dibenzothiophene and IV-alkylcarbazoles (in increasing order of difficulty) to form compounds of type (297) substitution occurs ortho to the heteroatom as expected from benzene chemistry. 

Reaction of 5-bromobenzo[6]thiophene with one mole of n -butyllithium leads to 5-bromo-2-benzo[6]thienyllithium with two moles of n-butyllithium, the 2,5-dilithio derivative is formed (70AHC( 11)177). Monolithio derivatives of dibenzothiophene have been prepared by halogen-metal exchange from the 1-, 2- and 3-bromo compounds. Dilithio-dibenzothiophenes have also been prepared by the same procedure (74AHC(i6)l8i). 

J-Bonded metal complexes, hydrodesulfurization models with benzothiophene, 1, 769 with dibenzothiophene, 1, 769 Bonding studies energetics, 1, 285 overview, 1, 573—603 ring size effects, 1, 396 strength, 1, 609... 

Figure 2.15 Examples of volcano plots, describing the reaction rate as a function ofthe heat of adsorption (left), and the activity of the second-row and third-row transition metal sulfides in the hydrodesulfurization of dibenzothiophene (right).

Yamada ( 10) reported the isolation of a bacteria capable of utilizing petroleum sulfur compounds like dibenzothiophene. Sagadia (11) isolated a bacteria on benzothiophenes, but due to the toxicity of the compound and the oil components and metals the culture could not reduce the sulfur content of the oil. 

Figure 1. Volcano plot showing the effect of the heat of formation of metal sulfides on the dibenzothiophene hydrodesulfurization activity of various mono- and bimetallic catalysts. Adapted from Chianelli et al. [6] and reprinted with permission of John Wiley Sons from L. L. Hegedus, ed., Catalyst Design— Progess and Perspectives, p. 1. Wiley, New York (1987) [1]. Copyright 1987, John Wiley Sons.

In industrial practice, catalytic surfaces are often very complex, not only structurally but also chemically. An example is shown in Fig. 1 from Chianelli et al. [6] for hydrodesulfurization catalysts. The data indicate that maximum dibenzothiophene hydrodesulfurization activity is achieved at intermediate heats of formation of metal sulfides, i.e., at intermediate metal-sulfur bond strengths. Again, while such surface energetic considerations do not have ab initio predictive ability, they are valuable tools for catalyst synthesis and prescreening. 

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