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Homogeneous Models of Thiophene Hds Reactions HOMOGENEOUS MODELS OF THIOPHENE HDS REACTIONS. SELECTIVITY IN THIOPHENE C-S CLEAVAGE AND THIOPHENE REACTIONS WITH DINUCLEAR METAL COMPLEXES. William D. Jones,* David A. Vicic, R. Martin Chin, James H. Roache, and Andy W. Myers. Department of Chemistry, University of Rochester, Rochester, NY 14627 Received August 1, 1996 - Abstract: The reactive 16 e metal fragment [(C5Me5)Rh(PMe3)] inserts into a wide variety of thiophene C-S bonds. The structures of the thiophene, benzothiophene, and dibenzothiophene insertion complexes have been determined. While the thiophene complex adopts a planar 6- membered ring structure the other metallacycles are bent, and all molecules possess localized diene structures. The mechanism of C-S cleavage was found to proceed by way of initial sulfur coordination. 2-Methylbenzothiophene gives a kinetic product resulting from cleavage of the sulfur-vinyl bond, but then rearranges to cleave the sulfur-aryl bond. A number of substituted dibenzothiophenes were examined, showing little electronic effect of substituents, but showing a large steric effect of substituents at the 4 and 6 positions. 4,6-Dimethyldibenzothiophene does not undergo cleavage, but instead forms an S-bound complex. Reactions of a cobalt analog, (C5Me5)Co(C2H4)2 with thiophenes also lead to C-S cleaved products, and the use of a dinuclear iridium system produces a butadiene complex in which both C-S bonds have been cleaved. Introduction of these sulfur containing compounds prior to The hydrodesulfurization of petroleum is one treatment. Figure 2 shows how this original mixture of several steps in the hydrotreating of oil in which of compounds is changed upon HDS treatment at sulfur is removed from thiols and thiophenes as temperatures of 350 - 390 °C. While all of the 1 benzothiophenes are removed, it can be observed H2S. The process uses a supported molybdenum sulfide catalyst containing cobalt, and operates under that specific dibenzothiophenes are difficult to pressures of 150-600 psi hydrogen at 300-450 °C. desulfurize. These include The sulfur content in oil of 1-5% is reduced to 0.1% 4-methyldibenzothiophene, 4,6-dimethyldibenzo- in gasoline, and future sulfur limits may be reduced thiophene, and other polyalkylated dibenzo- to as little as 0.003-0.04%. thiophenes. Important studies performed by Kabe, Work in our laboratory has focussed upon Ishihara, and Tajima using a mixture of polyaromatic homogeneous models for the desulfurization of sulfur-containing compounds in light oil with a thiophenes.3-12 Mechanistic studies might provide commercial HDS catalyst under typical operating conditions show the limitations of the current 3 2 W. D. Jones and L. Dong, J. Am. Chem. Soc. 1991, 113, generation of catalysts. Figure 1 shows a GC trace 559-564. 4 L. Dong, S. B. Duckett, K. F. Ohman, and W. D. Jones, J. * Author to whom correspondence should be addressed. Am. Chem. Soc. 1992, 114, 151-160. 1 H. Topsøe, B. S. Clausen, and F. E. Massoth, 5 R. M. Chin and W. D. Jones, Angew. Chem. Int. Ed. Engl. “Hydrotreating Catalysis”, Springer-Verlag, Berlin (1996). 1992, 31, 357-358. 2 T. Kabe, A. Ishihara, and H. Tajima, Ind. Eng. Chem. 6 R. M. Chin and W. D. Jones, Organometallics 1992, 11, Res. 1992, 31, 1577-1580. 2698-2700. 2 insights into the workings of the heterogeneous insertion product displays a planar catalyst systems that could lead to new catalyst metallathiabenzene ring, although once again bond developments with improved desulfurization length alternation is seen in the diene portion of the characteristics. In this manuscript, results with ring.13 homogeneous systems that cleave the C-S bonds of thiophenes are summarized. Figure 1. GC trace of polyaromatic sulfur containing compounds in light oil. (Reproduced with permission from ref. 2 ) Results and Discussion The reactive fragment [(C5Me5)Rh(PMe3)] Figure 2. GC trace of desulfurized oil at each produced by heating solutions of temperature. (Reproduced with permission from (C5Me5)Rh(PMe3)(Ph)H has been found to insert ref. 2 ) into a wide variety of thiophene C-S bonds (Scheme I).3 One of the first products to be structurally Scheme I: characterized was the adduct formed with 2,5- Rh dimethylthiophene. This C-S insertion product Me3P H shows a bent 6-membered ring in which the sulfur Rh and butadiene portion of the ring form a plane that is Me3P Rh Me3P 60°C - C6H6 oriented at an angle of 26° to the rhodium-sulfur-Ca S S S S plane. The distances around the ring are consistent S with a localized bonding structure (Scheme II). In S Rh Me3P contrast, the structure of the parent thiophene Rh Me3P Rh S S Me3P S S 7 W. D. Jones and R. M. Chin, J. Am. Chem. Soc. 1992, 114, 9851-9858. 8 Rh G. P. Rosini and W. D. Jones, J. Am. Chem. Soc. 1992, Rh + Rh Me3P Me3P Me3P S 114, 10767-10775. S S 9 W. D. Jones and R. M. Chin, J. Am. Chem. Soc. 1994, 1 : 1 116, 198-203. 10 W. D. Jones and R. M. Chin, J. Organomet. Chem. 1994, 472, 311-316. 11 W. D. Jones, R. M. Chin, T. W. Crane, and David M. 13 C. Blonski, A. W. Myers, M. Palmer, S. Harris, and W. Baruch, Organometallics 1994, 13, 4448-4452. D. Jones, “The Structure of Metallathiabenzenes: Planar vs 12 A. W. Myers, W. D. Jones, S. M. McClements, J. Am. Nonplanar Geometries. An Experimental and Theoretical Chem. Soc. 1995, 117, 11704-11709. Investigation,” manuscript in preparation. 3 Scheme II: has been investigated using a variety of labelling 4 Jones: 1.34 techniques. Attempts to directly observe an 1.44 Me 1.45 1.31 1.35 1.34 intermediate in the reaction at low temperature S Me S 2.067 failed. Irradiation of (C Me )Rh(PMe )H in the Rh 2.01 Rh 5 5 3 2 Cp*(PMe ) 3 Cp*(PMe3) presence of thiophene at -40 °C results in the 26° pucker planar 2 Angelici: formation of a 3:1 ratio of (C5Me5)Rh(PMe3)(h - 1.458 1.330 1.41 1.375 Me Me Me PMe2Ph 1.347 C,S-C4H4S) and (C5Me5)Rh(PMe3)(2-thienyl)H. 1.394 S S S Me Me Me Ir 2.074 Ir 2.348 Upon warming the sample, the 2-thienyl hydride 1.986 2.203 Ir Cp* Cp*(PMe2Ph) Cp* planar 32° pucker complex converts to the C-S insertion complex. If this rearrangement is carried out in the presence of Bianchini: Merola: Maitlis: an excess of thiophene-d , no incorporation of the 1.44 4 1.461 Et3P S 1.36 planar Pt deuterated thiophene into the product is observed, 1.323 planar S Et3P 1.483 S 2.07 2.243 2.035 1.301 2.009 Ir indicating that the rearrangement takes place Ir 2.374 Ph2P Cl planar Me3P Cl Ph2P PPh2 without dissociation of the thiophene from the metal Me P PMe3 3 center. Field: Jones: The rearrangement of the 2-thienyl deuteride PMe2 Me W S 1.32 Me2P S 1.351 Fe complex was examined by the reaction of 2.20 1.47 Me P 1.439 2 1.318 1.33 PMe2 (C5Me5)Rh(PMe3)Cl2 with 2-thienyllithium followed 23° pucker planar by sec-butylborodeuteride. Rearrangement to the A number of other mononuclear transition C-S insertion complex takes place in a non- metal complexes have been found to be capable of regiospecific fashion, producing a 1:1 mixture of the inserting into the C-S bonds of thiophenes, two deuterated isomers. This observation could be benzothiophenes, and dibenzothiophenes.14-20 As interpreted in terms of occurring either through a shown in Scheme II, the products formed have been symmetrical S-bound intermediate or by way of an characterized as containing in some cases a h2-thiophene complex that rapidly equilibrates delocalized metallathiabenzene ring and in others as between the double bonds of the thiophene (Scheme an isolated thiobutadiene fragment. In most III). examples, the 6-membered ring formed upon Scheme III: insertion is planar. Delocalization of the p-electrons into an electron deficient metal has been cited as a Rh D reason for the molecule adopting a planar geometry, Me3P S although steric factors seem to be more important in Rh 14,16 Me3P D D Rh + Rh OR determination of the planarity. S PMe3 PMe3 S S The mechanism of insertion of D 1 : 1 [(C5Me5)Rh(PMe3)] into the C-S bond of thiophene Rh Rh D S Me3P S Me3P D 14 J. Chen, L. M. Daniels, R. J. Angelici, J. Am. Chem. Soc. rapid equilibration 1990, 112, 199-204. 15 A second labelling experiment proceeded J. Chen, L. M. Daniels, R. J. Angelici, Polyhedron 1990, similarly using 3-thienyllithium followed by sec- 9, 1883-1891. 16 H. E. Selnau and J. S. Merola, Organometallics 1993, butylborodeuteride to generate the 3-thienyl 12, 1583-1591. deuteride complex. Rearrangement of the 3-thienyl 17 C. Bianchini, A. Meli, M. Peruzzini, F. Vizza, P. deuteride complex to the 2-thienyl hydride complex Frediani, V. Herrera, and R. A. Sanchez-Delgado, J. Am. proceeds regiospecifically, i.e., only the 3-deutero-2- Chem. Soc. 1993, 115, 7505-7506. thienyl product is formed (Scheme IV). A second, 18 I. E. Buys, L. D. Field, R. W. Hambley, A. E. D. McQueen, J. Chem. Soc., Chem. Commun. 1994, 557-558. slower rearrangement to the C-S insertion product 19 J. J. Garcia, B. E. Mann, H. Adams, N. A. Bailey, and P. then proceeds non-regiospecifically, as observed in M. Maitlis, J. Am. Chem. Soc. 1995, 117, 2179-2186.
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