催化脱硫

Catalytic Desulfurization of Dibenzothiophene and Its Hindered Analogues with Nickel and Platinum Compounds Jorge Torres-Nieto, Alma Are´valo, and Juventino J. Garcı´a* Facultad de Quı´mica, UniVersidad Nacional Auto´noma de Me´xico, Me´xico, D.F. 04510 ReceiVed January 29, 2007 Catalytic amounts (1-0.1 mol %) of nickel and platinum compounds in 0, I, and II oxidation states containing mono- and diphosphines ligands, in conjunction with alkyl Grignard reagents, promoted the desulfurization of dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MeDBT), and 4,6-dimeth- yldibenzothiophene (4,6-Me2DBT), to produce the corresponding substituted and unsubstituted biphenyls. It was also observed that the use of polar solvents such as THF yielded thiols, while the use of nonpolar solvents allowed the complete desulfurization of these substrates. Introduction The removal of organosulfur compounds from fuels is a mandatory issue in the goal of succeeding in diminishing the extent of atmospheric pollution caused from the emission of sulfur oxides, resulting from combustion processes.1 As such, increasingly more stringent regulations regarding the limits of permissible sulfur content in fuels have been constantly raised in a number of countries.2 Industrially, the process that is used for the removal of sulfur is known as catalytic hydrodesulfur- ization (HDS).3 The commercial HDS process typically uses cobalt- or nickel-doped molybdenum sulfide catalysts over alumina. The latter is noteworthy, given that in fact the platinum group metals are the ones that exhibit the highest HDS activity, as has been shown by model reactor studies, the latter metals not being used commercially, probably because of their higher cost.4 In addition to these facts, a variety of organometallic compoundssmostly in solutionscontaining different transition metals have also been tested in a number of HDS reactions with thiophenes: the latter compounds, particularly the more hindered dibenzothiophene derivatives 4-methyldibenzothiophene (4- MeDBT) and 4,6-dimethyldibenzothiophene (4,6-Me2DBT), are particularly responsible for the poor sulfur removal achieved by commercial HDS processes.5 An equation that exhibits the HDS process assisted by the presence of metal-supported or organometallic catalysts is presented in eq 1. Implicit in eq 1, the metal catalyst undergoes a number of reaction intermediates to yield the corresponding HDS products and a stoichiometric amount of H2S; the oxidative addition reaction of the C-S bond of the thiophene to the metal catalyst has been found to take place particularly in low oxidation state catalysts such as platinum(0) and nickel(0).5 In addition, it has been reported that the use of nucleophiles such as Grignard reagents yields sulfur-free cross-coupling products such as 2,2′- dimethyl-1,1′-biphenyl from DBT in the presence of nickel(II) halide phosphine precursor complexes (10 mol %).6 In this instance, a closely related report has recently appeared that describes the use of several Ni(0) catalysts (3 mol %) that yield chiral 1,1′-binaphthyls in the asymmetric cross-coupling reaction of dinaphtho[2,1-b:1′,2′-d]thiophene and 1,9-disubstituted diben- zothiophenes, in the presence of Grignard reagents (eq 2).7 It is worth noting that the use of organometallic nickel and platinum complexes in desulfurization reactions of DBTs has also been reported to occur in the presence of other nucleophiles besides Grignard reagents under stoichiometric conditions;8 only a small number of compounds have been known to ring open 4,6-Me2DBT to yield the corresponding desulfurization prod- ucts.9 In the case of our group, a preliminary report that describes the reactivity of a number of nickel complexes that yield the catalytic desulfurization reaction of DBT, 4-MeDBT, and 4,6- Me2DBT under homogeneous conditions in the presence of alkyl Grignards has been recently published.10 In all three cases, the * To whom correspondence should be addressed. E-mail: juvent@ servidor.unam.mx. (1) See for instance: Angelici, R. J. In Encyclopedia of Inorganic Chemistry; King, R. B., Ed.; John Wiley & Sons: New York, 1994; p 1433. (2) U.S. Environmental Protection Agency (http://www.epa.gov/otaq/ gasoline.htm). European Union, EU Directive 98/70/EC, 1998. (3) Topsøe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Catalysis: Science and Technology; Springer-Verlag: Berlin, 1996. (b) Kabe, T.; Ishihara, A.; Qian, W. Hydrodesulfurization and Hydrodenitrogenation: Chemistry and Engineering; Kondasa-Wiley-VCH: Tokyo, 1999. (4) Pecoraro, T. A.; Chianelli, R. R. J. Catal. 1981, 67, 430. (5) For recent reviews see: (a) Sanchez-Delgado, R. A. Organometallic Modeling of the Hydrodesulfurization and Hydrodenitrogenation Reactions; Kluwer Academic Publishers: Dordrecht, 2002. (b) Angelici, R. J. Organometallics 2001, 20, 1259. (c) Angelici, R. J. Polyhedron 1997, 16, 3073. (6) Wenkert, E.; Ferreira, T. W.; Michelotti, E. L. J. Chem. Soc., Chem. Commun. 1979, 637. (7) Shimada, T.; Cho, Y.-H.; Hayashi, T. J. Am. Chem. Soc. 2002, 124, 13396. Cho, Y.-H.; Kina, A.; Shimada, T.; Hayashi, T. J. Org. Chem. 2004, 69, 3811. (8) Eisch, J. J.; Hallenbeck, L. E.; Han, K. I. J. Am. Chem. Soc. 1986, 108, 7763. Eisch, J. J.; Hallenbeck, L. E.; Han, K. I. J. Org. Chem. 1983, 8, 2963. Becker, S.; Fort, Y.; Vanderesse, R.; Caube´re, P. J. Org. Chem. 1989, 54, 4848. Garcı´a, J. J.; Mann, B. E.; Adams, H.; Bailey, N. A.; Maitlis, P. M. J. Am. Chem. Soc. 1995, 117, 2179. Iretskii, A.; Garcı´a, J. J.; Picazo, G.; Maitlis, P. M. Catal. Lett. 1998, 51, 129. (9) Vicic, D. A.; Jones, W. D. Organometallics 1998, 17, 3411. Yu, K.; Li, H.; Watson, E. J.; Virkaitis, K. L.; Carpenter, G. B.; Sweigart, D. A. Organometallics 2001, 20, 3550. Garcı´a, J. J.; Mann, B. E.; Adams, H.; Are´valo, A.; Berne´s, S.; Garcı´a, J. J.; Maitlis, P. M. Organometallics 1999, 18, 1680. (10) Torres-Nieto, J.; Are´valo, A.; Garcı´a-Gutie´rrez, P.; Acosta-Ramı´rez, A.; Garcı´a, J. J. Organometallics 2004, 23, 4534. 2228 Organometallics 2007, 26, 2228-2233 10.1021/om070087y CCC: $37.00 © 2007 American Chemical Society Publication on Web 03/23/2007 reaction yielded the corresponding substituted biphenyls: the latter report is the first clear example of a catalytic desulfur- ization process for 4,6-Me2DBT, which is largely unreactive. A mechanistic proposal that provides insight into these reactions, in which the formation of nickel thiametallacycles was proposed to take place after an initial oxidative addition reaction of the respective thiophene, was also addressed in the same work.10 Herein, we report our findings after that initial report, including an extensive study of the catalytic desulfurization process for the same three dibenzothiophenic substrates in the presence of both nickel and platinum complexes, in different oxidation states and bearing different phosphine ligands (see Figure 1). Results and Discussion Catalytic Desulfurization of DBT with Nickel Compounds. The desulfurization reaction of DBT in the presence of two additional equivalents of MeMgBr and complexes 1-8 as catalyst precursors (1 mol %) under toluene reflux yielded in all cases only the cross-coupling product 2,2′-dimethyl-1,1′- biphenyl in 100%. In the case of EtMgBr and i-PrMgCl, both substituted and unsubstituted biphenyls were observed. A complete chart that summarizes the results obtained with all three Grignards with respect to the nickel complex used in each case for the desulfurization of DBT is presented in Figure 2. As shown in Figure 2, the reactivity of all nickel compounds was optimal in the presence of MeMgBr, probably because of the small steric hindrance presented by this Grignard reagent. In the case of EtMgBr and i-PrMgCl, which present increasing steric bulk, the results obtained showed that a drastic decrease of the catalytic desulfurization activity was observed when either of these was used in the presence of the otherwise more impeded catalysts precursors 3 and 6, both of which contain bis- (ditertbutylphosphine)ethane as ancillary ligand. As mentioned above, whenever EtMgBr and i-PrMgCl were used, both substituted and unsubstituted biphenyl products were observed. One possible explanation for this may be that an initial attack of the nucleophile takes place over the metal center bearing the organic moiety, followed by a â-elimination step (Vide infra).11 Such combination might act as an in situ source of nickel hydride intermediates that allow the release of unsubstituted biphenyls via a hydride migration, which can be envisaged as an HDS product, instead of a cross-coupling product. To note, when this reaction was followed by NMR using a sealed tube, the free alkene formed as a result of the â-elimination step was detected. In this instance, when EtMgBr was used, ethylene was detected, while in the case of i-PrMgCl, 1-propene was found. The proportion of substituted and un- substituted biphenyls indicates that the â-elimination step is favored over cross-coupling, provided that the amount of unsubstituted biphenyl was greater in most of the experiments carried out with those reagents (see Figure 3). In addition to the systems mentioned above using 1 mol % of catalyst precursor, some other experiments were assessed using complexes 1, 7, and 8 at the lower concentration of 0.1 mol %. These results are shown in the Table 1. Entries 1-3 show that the desulfurization reaction is highly efficient: a total conversion of DBT has been observed to occur in all cases. As indicated in Table 1, whenever the experiments were carried out using THF as solvent (entries 4 and 5), the general outcome was the production of 2,2′-dimethyl-1,1′-biphenyl and a quantity of the corresponding thiol (MePh-PhSH), which results from the occurrence of only one cross-coupling step. We propose that the production of the thiol is a consequence of the coordination of THF molecules, which disrupt the completion of the overall desulfurization process, provided that the use of a noncoordinating solvent such as toluene already confirmed the formation of disubstituted biphenyls as the only products (see entries 1-3). The use of solvents with a higher boiling point than toluene, such as o-xylene and mesitylene (entries 6 and 7 of Table 1), resulted in a slight decrease of the catalytic activity, which was probably due to catalyst decomposition. The same kind of temperature effect over thiaplatinacycles used in the HDS (11) Trost, B. M.; Ornstain, P. L. Tetrahedron Lett. 1981, 22, 3463. Figure 1. Catalytic precursors of Ni and Pt used to desulfurize DBT and its hindered analogues. Figure 2. Catalytic desulfurization of DBT with nickel compounds. All reactions were carried out under toluene reflux using 1 mol % of the corresponding nickel catalyst. All yields were quantified by GC-MS, after workup. Figure 3. Products obtained when the Grignard reagents contain â-hydrogens. Catalytic Desulfurization of Dibenzothiophene Organometallics, Vol. 26, No. 9, 2007 2229 process of substituted thiophenes was also observed by our group and has already been reported.12 Mechanistic Insights for the Catalytic Desulfurization of DBTs Using Nickel Compounds. As indicated in our former communication,10 the feasibility of the intermediacy of the corresponding thianickelacycles and nickelacycles during de- sulfurization was also addressed. To do so, complexes [(dippe)- Ni(Ł2-C,S-C12H8)] (12) (31P{1H} NMR in toluene-d8 ä 74.39, d, 2JP-P ) 10 Hz; 75.55, d, 2JP-P ) 10 Hz) and [(dippe)Ni- (Ł2-C,C′-C12H8)] (13) (31P{1H} NMR in toluene-d8 ä 70, s) were prepared and tested as catalyst precursors in a 1 mol % catalytic proportion; the outcome of these experiments is the formation of the corresponding cross-coupling products in yields higher than 90% (see entries 8 and 9 of Table 1). Thiametallacycle 12 reacts at room temperature upon addition of MeMgBr and is re-formed under reflux within the reaction time: a persistent concentration of this compound, together with 13 and [(dippe)2Ni] (31P{1H} NMR in toluene-d8 ä 54.5, s), has been detected to take place after 90% conversion of the starting DBT, implying partial decomposition of the catalyst, as has been mentioned above. An improved mechanistic proposal based on the generally accepted catalytic cycle for the nickel-catalyzed cross-coupling reactions,7,13 along with our previous findings on this subject10 and the additional ones concluded from this work, is depicted in Scheme 1. As shown in the scheme, it is likely that the first active intermediate within the catalytic cycle is the thiametal- lacycle 12. These kinds of compounds have already been proposed as important intermediates in HDS reactions, and as mentioned in the case of 12, experiments using this complex as starting material yielded the desulfurization process: the latter complex was found to exhibit a high activity (Vide supra). Intermediate A is produced by the nucleophilic attack of the Grignard reagent to the metal center in 12. This species may evolve by two different routes: the first one is the reductive elimination of the alkyl and biphenyl groups (the latter one still bearing the sulfur moiety), to form the thiolate B; the second route is the â-elimination reaction, which is restricted exclusively to those Grignards that hold protons in the â-position and is, thus, the route for unsubstituted biphenyls that was mentioned earlier (Vide supra). Intermediate B is proposed to be responsible for the formation of the thiol compounds that were also mentioned before (entries 4 and 5, Table 1): the coordination of THF molecules to this intermediate is likely to lead to the release of the corresponding magnesium thiolates, thereof producing thiols once the reaction mixture had been subjected to workup. Under noncoordinating solvents such as toluene (entries 1-3, Table 1), B is likely to evolve into intermediate C, once a second C-S bond activation stepsthis time over the thiolate moietyshad occurred. The second nucleophilic attack over the metal center, promoted by another equivalent of Grignard, would drive the release of the magnesium salts (MgS and MgBr2)14 and the formation of the alkyl-substituted nickel- (II) complex, that is, C. This intermediate can evolve by two routes, as was also the case of A, either by undergoing the cross- coupling reaction promoting the disubstituted biphenyl or by performing a second â-elimination step, in which case the final unsubstituted biphenyl would be formed instead. Finally, after either of these steps had taken place, the resulting 14-electron “[(dippe)Ni]” intermediate would regenerate the thiametallacycle 12 in the presence of additional thiophene and, thus, complete the desulfurization cycle. Catalytic Desulfurization of 4-MeDBT with Nickel Com- pounds. As in the case of DBT, the reactivity of 4-MeDBT, which is one of the most refractory organosulfur compounds in fuels, was also addressed. To do so, complexes 1 and 8, which had shown the highest activity in the desulfurization of DBT, were tried; the results obtained showed that these complexes were also very active with this substrate (Table 2). As can be seen in Table 2, the complete desulfurization of 4-MeDBT using 1 mol % of 1 and 0.1 mol % of 8 was observed. The fact that this reaction was achieved under such conditions constitutes a very important result; it effectively confirms the usefulness of these nickel compounds for the generation of efficient systems for the desulfurization of organosulfur com- pounds as stable as 4-MeDBT: the complete conversion of it into the cross-coupling product 2,2′,3-trimethyl-1,1′-biphenyl has been found to take place in both cases. Catalytic Desulfurization of 4,6-Me2DBT with Nickel Compounds. Compounds 1-8 (2 mol %) were tested in the catalytic desulfurization of the more hindered 4,6-Me2DBT using alkyl Grignards. As in the case of 4-MeDBT, the reaction with 4,6-Me2DBT is a very important one, provided that this substrate is considered to be by far the most refractory organosulfur compound of its family. To the best of our knowledge, the results obtained herein represent the first clear examples of homogeneously catalyzed desulfurization reactions of 4,6-Me2- DBT using organometallic compounds. A chart that summarizes the results obtained thus far is presented in Figure 4. As illustrated in this figure, the reactivity of the nickel complexes used presented an optimal behavior in the desulfurization of 4,6-Me2DBT whenever MeMgBr was used as the Grignard reagent, the selectivity of the process being exclusive to the cross-coupling product 2,2′,3,3′-tetramethyl-1,1′-biphenyl. As in the case of DBT, the reactions performed over 4,6-Me2DBT showed a strong dependence on the overall steric hindrance displayed by the reagents: the monophosphine-containing compounds 7 and 8 were found to exhibit the highest activity (12) Herna´ndez, M.; Miralrio, G.; Are´valo, A.; Berne´s, S.; Garcı´a, J. J.; Lo´pez, C.; Maitlis, P. M.; Del Rı´o, F. Organometallics 2001, 20, 4061. (13) Tamao, K.; Sumitani, K.; Kiso, Y.; Zembayashi, M.; Fujioka, A.; Kodama, S.; Nakajima, I.; Manato, A.; Kumada, M. Bull. Chem. Soc. Jpn. 1976, 49, 1958. (14) MgS was identified by X-ray powder diffraction.10 Table 1. Nickel-Catalyzed Desulfurization of DBT with Grignard Reagentsa entry complex (equiv) thiophene (equiv) grignard (equiv) solvent organics (%) 1 1 (1) DBT (1000) MeMgBr (2000) toluene MePh-PhMe (100) 2 7 (1) DBT (1000) MeMgBr (2000) toluene MePh-PhMe (100) 3 8 (1) DBT (1000) MeMgBr (2000) toluene MePh-PhMe (100) 4 1 (1) DBT (100) MeMgBr (200) THF MePh-PhMe (60), MePh-PhSH (25), DBT (15) 5 8 (1) DBT (100) MeMgBr (200) THF MePh-PhMe (73), MePh-PhSH (27) 6 1 (1) DBT (100) MeMgBr (200) o-xylene MePh-PhMe (95), DBT (5) 7 1 (1) DBT (100) MeMgBr (200) mesitylene MePh-PhMe (90), DBT (10) 8 12 (1) DBT (100) MeMgBr (200) toluene MePh-PhMe (91), DBT (9) 9 13 (1) DBT (100) MeMgBr (200) toluene MePh-PhMe (100) a All reactions were carried out under reflux of their corresponding solvent for 5 days, typically using 0.012 mmol of the corresponding nickel catalyst. All yields were quantified by GC-MS, after workup. 2230 Organometallics, Vol. 26, No. 9, 2007 Torres-Nieto et al. of all. In the case of the diphosphine-substituted nickel compounds, a decreasing trend in reactivity could be established depending on the bulkiness of the ancillary ligands, and as such, complexes 1, 2, 4, and 5, which bear the less bulky diphos- phines, displayed moderate reactivity in the desulfurization reaction, while complexes 3 and 6, with more bulky diphos- phines, exhibited no reactivity at all. Noteworthy, the nature of the Grignard reagent was also found to be important in the final outcome of the catalysis: the use of the more bulky Grignards, EtMgBr or i-PrMgCl vs MeMgBr, was found to inhibit the desulfurization reaction in all cases. Another important observation found for this process is that regarding the nature of the solvent used. Unlike DBT, the desulfurization of 4,6-Me2DBT does not require the reaction to take place in a particular solvent: the reaction in either toluene or THF yielded the same result, every time. A possible explanation for this is that the THF molecules cannot coordinate to the formed metallacycles (and to the rest of the intermediaries) in any of these cases, as a result of the increased steric hindrance opposed by the proximity that the two methyl groups in 4,6- Me2DBT likely hold toward the nickel center in all intermediates within the catalytic cycle. As in the case of DBT, the use of higher boiling point solvents such as o-xylene or mesitylene also diminished the catalytic activity, presumably due to catalyst decomposition (Vide supra). Catalytic Desulfurization of DBT with Platinum Com- pounds. It is well known that several platinum compounds can cleave the C-S bond of DBT, 4Me-DBT, and 4,6-Me2DBT and that in the