Oxo

Organometallics 2010, 29, 5517–5525 DOI: 10.1021/om100450a

5517

Highly Efficient Reduction of Sulfoxides with the System Borane/Oxo-rhenium Complexes† Ana C. Fernandes,*,‡ Jose A. Fernandes,§ Carlos C. Rom~ao,§ Luis F. Veiros,‡ and Maria Jose Calhorda*,^ ‡

Centro de Quı´mica Estrutural, Complexo I, Instituto Superior T ecnico, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal, §Instituto de Tecnologia Quı´mica e Biol ogica da Universidade Nova de Lisboa, Avenida da Rep ublica, EAN, 2780-157 Oeiras, Portugal, and ^Departamento de Quı´mica e Bioquı´mica, CQB, Faculdade de Ci^ encias, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal Received May 10, 2010

This work reports the reduction of sulfoxides with boranes, in excellent yields, catalyzed by oxorhenium complexes (1 mol %) at room temperature under air atmosphere. The best results were obtained with catecholborane (HBcat) in the catalytic systems HBcat/ReIO2(PPh3)2, HBcat/Re2O7, and HBcat/MTO. DFT calculations were performed for the system based on ReIO2(PPh3)2, which was studied in more detail. The reaction starts with the formation of ReIO2(R2SO)2, followed by addition of the first molecule of HBcat and loss of R2S. In a second step, a second HBcat molecule attacks the Re(VII) intermediate, reducing the metal back to Re(V), while H2 and catBOBcat are released and the Re catalyst is regenerated, reentering the reaction cycle. The computed reaction barriers are much lower than those for any alternative pathway that could be envisaged.

Introduction The reduction (deoxygenation) of sulfoxides to sulfides is a fundamental reaction in both chemistry and biology. Over

the years, several methods have been developed to reduce sulfoxides.1-29 Among the variety of metal complexes that have been used for the deoxygenation of sulfoxides, oxomolybdenum complexes12,13,16 have attracted considerable interest, because this metal is found in a class of enzymes that are commonly referred to as mononuclear molybdoenzymes or oxotransferases, such as dimethyl sulfoxide reductases,30-32 which catalyze similar reactions.

† Part of the Dietmar Seyferth Festschrift. In honor of Prof. Dietmar Seyferth for his outstanding contributions as editor of Organometallics. *To whom correspondence should be addressed. (A.C.F.) Tel: 351 218419264. Fax: 351 218464457. E-mail: [email protected]. (M.J.C.) Tel: 351 217500196. Fax: 351 217500088. E-mail: [email protected]. (1) Espenson, J. H. Coord. Chem. Rev. 2005, 249, 329-341, and references therein. (2) Kukushkin, V. Y. Coord. Chem. Rev. 1995, 139, 375-407, and references therein. (3) Madesclaire, M. Tetrahedron 1988, 44, 6537-6551, and references therein. (4) Reis, P. M.; Costa, P. J.; Rom~ao, C. C.; Fernandes, J. A.; Calhorda, M. J.; Royo, B. Dalton Trans. 2008, 1727–1733. (5) Bahrami, K.; Khodaei, M. M.; Karimi, A. Synthesis 2008, 2543– 2546. (6) Yoo, B. W.; Park, M. C.; Song, M. S. Synth. Commun. 2007, 37, 4079–4083. (7) Yoo, B. W.; Song, M. S.; Park, M. C. Bull. Korean Chem. Soc. 2007, 28, 171–172. (8) Yoo, B. W.; Song, M. S.; Park, M. C. Synth. Commun. 2007, 37, 3089–3093. (9) Pandey, L. K.; Pathak, U.; Rao, A. N. Synth. Commun. 2007, 37, 4105–4109. (10) Bahrami, K.; Khodaei, M. M.; Khedri, M. Chem. Lett. 2007, 1324–1325. (11) Khurana, J. M.; Sharma, V. S.; Chacko, A. Tetrahedron 2007, 63, 966–969. (12) Fernandes, A. C.; Rom~ao, C. C. Tetrahedron 2006, 62, 9650– 9654. (13) Fernandes, A. C.; Rom~ao, C. C. Tetrahedron Lett. 2007, 48, 9176–79. (14) Roy, C. D.; Brown, H. C. J. Chem. Res. 2006, 10, 642–644. (15) Raju, B. R.; Devi, G.; Nongpluh, Y. S.; Saikia, A. K. Synlett 2005, 358–360. (16) Sanz, R.; Escribano, J.; Fernandez, Y.; Aguado, R.; Pedrosa, M. R.; Arn aiz, F. J. Synthesis 2004, 1629–1632.

(17) Harrison, D. J.; Tam, N. C.; Vogels, C. M.; Langler, R. F.; Baker, R. T.; Decken, A.; Westcott, S. A. Tetrahedron Lett. 2004, 45, 8493–8496. (18) Yoo, B. W.; Choi, K. H.; Kim, D. Y.; Choi, K. I.; Kim, J. H. Synth. Commun. 2003, 33, 53–57. (19) Nicolaou, K. C.; Koumbis, A. E.; Snyder, S. A.; Simonsen, K. B. Angew. Chem., Int. Ed. 2000, 39, 2529–2533. (20) Koshino, N.; Espenson, J. H. Inorg. Chem. 2003, 42, 5735–5742. (21) Arias, J.; Newlands, C. R.; Abu-Omar, M. M. Inorg. Chem. 2001, 40, 2185–2192. (22) Abu-Omar, M. M.; Khan, S. I. Inorg. Chem. 1998, 37, 4979– 4985. (23) Arterburn, J. B.; Perry, M. C. Tetrahedron Lett. 1996, 37, 7941– 7944. (24) Abu-Omar, M. M.; Appelman, E. H.; Espenson, J. H. Inorg. Chem. 1996, 35, 7751–7757. (25) Zhu, Z.; Espenson, J. H. J. Mol. Catal. A: Chem. 1995, 103, 87–94. (26) Bryan, J. C.; Stenkamp, R. E.; Tulip, T. H.; Mayer, J. M. Inorg. Chem. 1987, 26, 2283–2288. (27) Cha, J. S.; Kim, J. E.; Kim, J. D. Tetrahedron Lett. 1985, 26, 6453–6456. (28) Brown, H. C.; Ravindran, N. Synthesis 1973, 42–43. (29) Guidon, Y.; Atkinson, J. G.; Morton, H. E. J. Org. Chem. 1984, 49, 4538–4540. (30) Enemark, J. H.; Cooney, J. J. A.; Wang, J.-J.; Holm, R. H. Chem. Rev. 2004, 104, 1175–1200. (31) Kisker, C.; Schindelin, H.; Rees, D. C. Annu. Rev. Biochem. 1997, 66, 233–267. (32) Hill, R. Chem. Rev. 1996, 96, 2757–2816.

r 2010 American Chemical Society

Published on Web 08/23/2010

pubs.acs.org/Organometallics

5518

Organometallics, Vol. 29, No. 21, 2010

Some rhenium complexes have also been used for the reduction of sulfoxides.20-26 Very recently, we have demonstrated that the catalytic system silane/oxo-rhenium complexes is very efficient for the reduction of sulfoxides with high yields and chemoselectivity.33 As part of our continuous studies on the activation of X-H (X = Si and B) bonds by high-valent oxo-complexes34-36 and the use of these complexes as efficient catalysts in organic reactions,12,13,37-44 we also reported the synthesis of the hydrides (PPh3)2(O)(I)Re(H)OBcat (2) and (PPh3)2(O)(I)Re(H)OBpin (3), from the reaction between catecholborane (HBcat) or pinacolborane (HBpin) and the high-valent oxo-rhenium complex ReIO2(PPh3)2 (1) (eqs 1 and 2).36

Fernandes et al.

of HBcat (eq 3). The deoxygenation of the sulfoxide was completed after 1 h 25 min at room temperature. The same reduction without catalyst required 500 h.17 This result suggests that high-valent oxo-rhenium complexes could be appropriate catalysts for the deoxygenation of sulfoxides.

In this work, we report the reduction of sulfoxides with boranes catalyzed by a series of high-valent oxo-rhenium(V) and -(VII) complexes. A DFT49 study provides a reaction mechanism that is consistent with all the experimental results.

Results and Discussion

The structural characterization by X-ray diffraction of 2 and 3 showed that these hydrides are formed by addition of the B-H bond across the Re-oxo bond without dissociation of any phosphine or substitution of the iodide ligand. The synthesis of the novel hydrides 2 and 3 is a clear demonstration that the high-valent oxo-rhenium complex ReIO2(PPh3)2 activates the B-H bond of boranes. To the best of our knowledge, this is the first example of B-H bond activation by high-valent metal oxo-rhenium complexes and overall the first structurally characterized example of this type of activation reaction. Usually, the activation of boranes reported in the literature involves transition metal complexes in a low oxidation state.45-48 We have also reported36 the use of the hydride 2 as catalyst in the reduction of 4-chlorophenyl sulfoxide with 2 equiv (33) Sousa, S. C. A.; Fernandes, A. C. Tetrahedron Lett. 2009, 50, 6872. (34) Fernandes, A. C.; Fernandes, R.; Rom~ao, C. C.; Royo, B. Chem. Commun. 2005, 213–214. (35) Costa, P. J.; Rom~ao, C. C.; Fernandes, A. C.; Royo, B.; Reis, P. M.; Calhorda, M. J. Chem.;Eur. J. 2007, 13, 3934–3941. (36) Fernandes, A. C.; Fernandes, J. A.; Almeida Paz, F. A.; Rom~ao, C. C. Dalton Trans. 2008, 6686–6688. (37) Fernandes, A. C.; Rom~ao, C. C. Tetrahedron Lett. 2005, 46, 8881–8883. (38) Fernandes, A. C.; Rom~ao, C. C. J. Mol. Catal A: Chem. 2007, 272, 60–63. (39) Fernandes, A. C.; Rom~ao, C. C. J. Mol. Catal A: Chem. 2006, 253, 96–98. (40) Noronha, R. G.; Rom~ao, C. C.; Fernandes, A. C. J. Org. Chem. 2009, 74, 6960–6964. (41) Noronha, R. G.; Rom~ao, C. C.; Fernandes, A. C. Tetrahedron Lett. 2010, 51, 1048–1051. (42) Noronha, R. G.; Rom~ao, C. C.; Fernandes, A. C. Tetrahedron Lett. 2009, 50, 1407–1410. (43) Noronha, R. G.; Costa, P. J.; Rom~ao, C. C.; Calhorda, M. J.; Fernandes, A. C. Organometallics 2009, 28, 6206–6212. (44) Noronha, R. G.; Fernandes, A. C.; Rom~ao, C. C. Catal. Commun. (accepted). (45) Burkhardt, E. R.; Matos, K. Chem. Rev. 2006, 106, 2617–2650. (46) Vogels, C. M.; Westcott, S. A. Curr. Org. Chem. 2005, 9, 687– 699. (47) Beletskaya, I.; Pelter, A. Tetrahedron 1997, 53, 4957–5026. (48) Burgess, K.; Ohlmeyer, M. J. Chem. Rev. 1991, 91, 1179–1191.

Chemical Studies. Initially we investigated the reduction of the test substrate 4-chlorophenyl sulfoxide with catecholborane (HBcat), pinacolborane (HBpin), and borane (BH3 3 THF), catalyzed by the high-valent oxo-rhenium complexes ReIO2(PPh3)2 (1), Re2O7 (4), MTO (5), ReOCl3(PPh3)2 (6), and ReOCl3(dppm) (7), in order to find the best reaction conditions (Table 1). The progress of the reductions was monitored by thin-layer chromatography and by 1H NMR. The results reported in Table 1 show that the most active catalysts are ReIO2(PPh3)2 and Re2O7 and that the monooxorhenium complexes ReOCl3(PPh3)2 and ReOCl3(dppm) are the least effective catalysts. The studies carried out with HBcat showed that the reductions catalyzed by the complexes ReIO2(PPh3)2 and Re2O7 were very fast (5 min) at room temperature under air atmosphere (Table 1, entries 1 and 4). The deoxygenation with MTO was much slower since the total reduction of the sulfoxide was reached only after 25 min at room temperature (Table 1, entry 7). The reaction catalyzed by the monooxo-rhenium complex ReOCl3(PPh3)2 required significantly longer reaction times, 20 h at room temperature and 4 h 30 min in refluxing THF (Table 1, entries 10 and 11). The deoxygenation carried out with ReOCl3(dppm) afforded the sulfide in only 48% yield after 20 h at room temperature (Table 1, entry 13). However at refluxing temperature the sulfoxide was completely reduced after 7 h (Table 1, entry 14). All the reductions performed with HBpin were completed (Table 1, entries 2, 5, 8, 12, and 15). Nevertheless, the reactions of 4-chlorophenyl sulfoxide with HBpin in the presence of 1 mol % of ReIO2(PPh3)2, Re2O7, and MTO (Table 1, entries 2, 5, and 8) were much slower than those carried out with HBcat (Table 1, entries 1, 4, and 7). Interestingly, the deoxygenation of 4-chlorophenyl sulfoxide with BH3 3 THF in the presence of 1 mol % of catalysts ReIO2(PPh3)2, Re2O7, and MTO gave the sulfide in moderate yields (Table 1, entries 3, 6, and 9). In order to study the scope and the limitations of this novel methodology, we tested the catalytic systems HBcat/ ReIO2(PPh3)2, HBcat/Re2O7, and HBcat/MTO with several (49) Parr, R. G.; Yang, W. In Density Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989.

Article


5519

Table 1. Reduction of 4-Chlorophenyl Sulfoxide with Boranes Catalyzed by High-Valent Oxo-rhenium Complexesa

entry

catalyst

borane

time

conversion (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

ReIO2(PPh3)2 ReIO2(PPh3)2 ReIO2(PPh3)2 Re2O7 Re2O7 Re2O7 MTO MTO MTO ReOCl3(PPh3)2 ReOCl3(PPh3)2 ReOCl3(PPh3)2 ReOCl3(dppm) ReOCl3(dppm) ReOCl3(dppm)

HBcat HBpin BH3 HBcat HBpin BH3 HBcat HBpin BH3 HBcat HBcat HBpin HBcat HBcat HBpin

5 min 65 min 20 h 5 min 35 min 20 h 25 min 60 min 20 h 20 h 4.5 h 20 h 20 h 7h 20 h

100 100 43 100 100 43 100 100 48 100 100 100 48 100 100

a All reactions were carried out with 1.0 mmol of sulfoxide and 2.0 mmol of borane at rt, except entries 11 and 14, which were carried out at reflux. Conversion was determined by 1H NMR.

b

Table 2. Reduction of Sulfoxides with the System HBcat/Oxo-rhenium Complexa

a All reactions were carried out with 1.0 mmol of sulfoxide, 2.0 mmol of HBcat, and 1 mol % of catalyst. b Isolated yield.

sulfoxides in THF at room temperature under air (Table 2). The reductions with the system HBcat/ReIO2(PPh3)2 were completed within a few minutes (5-20 min) with excellent yields (Table 2, entries 1, 4, 7, 10, 13, and 16). This system showed good chemoselectivity with tolerance of some functional groups such as halo, ester, and nitro (Table 2, entries 1, 10, and 16). The reusability of the ReIO2(PPh3)2 catalyst was studied using phenyl methyl sulfoxide as substrate. After the first cycle, more fresh substrate and HBcat were added to the reaction mixture, but only 35% conversion was achieved after 7 h.

The catalytic activity of Re2O7 was also explored in the reduction of a series of sulfoxides with HBcat at room temperature under air (Table 2, entries 2, 5, 8, 11, 14, and 17). The system HBcat/Re2O7 (1 mol %) is highly effective for the deoxygenation of sulfoxides, giving the sulfides in a few minutes (5-15 min) with excellent yields and with tolerance of halo, ester, and nitro groups (Table 2, entries 2, 11, and 17). The results obtained with this catalytic system are remarkably similar to those obtained with the system HBcat/ReIO2(PPh3)2 over the whole range of substrates. Finally, we investigated the reduction of sulfoxides with the system HBcat/MTO (1 mol %) (Table 2, entries 3, 6, 9, 12, 15, and 18). From the analysis of these results, we concluded that this catalytic system is also very effective, reaching total conversion of sulfoxides to the corresponding sulfides in excellent yields. However, the reduction of diphenyl sulfoxides (Table 2, entries 3, 6, and 9) required more time then the similar deoxygenations with the systems HBcat/ ReIO2(PPh3)2 and HBcat/Re2O7. The functional groups halo, ester, and nitro were also well tolerated under the reaction conditions of the catalytic system HBcat/MTO (Table 2, entries 3, 12, and 18). The previous data show that several combinations of highvalent oxo-rhenium complexes and boranes are very efficient catalysts for the room-temperature deoxygenation of mono- and diaryl sulfoxides with different functionalizations. Catecholborane is clearly the most efficient borane, in combination with ReVO2 and ReVIIO3 complexes producing catalysts that are rather more active than the one obtained with the mono-oxo complex ReOCl3(PPh3)2. Interestingly, the later complex has been shown to be an efficient catalyst for sulfoxide deoxygenation when combined with PPh3 as oxygen sink.23 In spite of the differences between the catalyst precursors ReIO2(PPh3)2 and Re2O7, both HBcat/ReIO2(PPh3)2 and HBcat/Re2O7 systems are equally reactive toward all substrates examined. The similarity is so high that it suggests the presence of very similar active species. At first glimpse it is not immediately obvious how this can happen, and therefore we tried to gain more insight into the mechanism of this reduction, using the catalytic system HBcat/ReIO2(PPh3)2

5520


(1 mol %) because it provides more handles for spectroscopic monitoring of the reactions. As described in the experimental procedure, the catalytic sulfoxide deoxygenation is carried out by treating the catalyst with sulfoxide in a 1:100 molar ratio and adding the borane to start the catalytic reaction. When 4-chlorophenyl sulfoxide was treated with 2 equiv of HBcat without ReIO2(PPh3)2, only a trace of deoxygenated product was formed. When 4-chlorophenyl sulfoxide was treated with 1 mol % of ReIO2(PPh3)2 without HBcat, at room temperature under air, a color change took place, but only a trace of deoxygenated product was formed. A similar color change is observed during the reduction of 4-methylphenyl sulfoxide with the system HBcat/ReIO2(PPh3)2 (1 mol %). The initially violet solution of ReIO2(PPh3)2 in THF turns to yellow upon addition of the sulfoxide and then changes to green and violet-brown after addition of HBcat. It is obvious that ReIO2(PPh3)2 reacts with the sulfoxide, and the species that is formed reacts with the HBcat to generate the deoxygenated products. This was confirmed by the study of the reaction between ReIO2(PPh3)2 and 2 equiv of 4-methylphenyl sulfoxide using 1H and 31P NMR in C6D6. The resonances of the aromatic protons of the free sulfoxide (δ 7.51 and 6.78 ppm) change to δ 7.30 and 6.80 ppm upon addition of ReIO2(PPh3)2. Likewise, the singlet corresponding to the CH3 group was displaced from δ 1.87 ppm in the free sulfoxide to δ 1.97 ppm in the mixture. The 31P NMR measurements reveal that the resonance corresponding to the PPh3 ligands in ReIO2(PPh3)2 (δ 4.0 ppm) disappears after the addition of 4-methylphenyl sulfoxide. Two new peaks appear at δ -5.2 and 25.4 ppm, and they were assigned to free PPh3 and OPPh3. These results show that both PPh3 ligands in the catalyst precursor are displaced by the large excess of sulfoxide most likely to form ReIO2(R2SO)2 (8). In spite of several attempts, we were unable to isolate any example of a pure complex 8, for a more complete structural characterization, from the reaction between ReIO2(PPh3)2 and 2 equiv of sulfoxide. Concomitantly with PPh3 displacement, some oxidation of the free phosphine is taking place, as indicated by the formation of some free OPPh3. This is not unexpected because the oxygen transfer between R2SO and PPh3 catalyzed by RedO complexes has been reported long ago.23 This could suggest that the deoxygenation of the sulfoxides in our catalytic system was being effected by PPh3 with concomitant formation of OPPh3 as final oxygen sink. However, this would require at least a stoichiometric amount of PPh3 relative to the initial sulfoxide. Since in our catalytic system the sulfoxide is present in ca. 50-fold excess against the phosphine, the deoxygenation of the sulfoxide by PPh3, catalyzed by ReIO2(PPh3)2, would require the regeneration of PPh3 from OPPh3, eventually by the action of the borane. However, neither the reaction of OPPh3 with 2 equiv of HBcat without catalyst nor the reaction of OPPh3 with 2 equiv of HBcat catalyzed by ReIO2(PPh3)2 (1 mol %) led to reduction of OPPh3 after 1 h at room temperature. We can conclude that the regeneration of PPh3 by reduction of OPPh3 with HBcat does not occur in our system and that PPh3 is not the main reducing agent of the sulfoxide. The effect of the amount of borane was also examined in the deoxygenation of 4-chlorophenyl sulfoxide with the catalyst ReIO2(PPh3)2. When this reaction was carried out with 1.2 equiv of HBcat, only 6% of the sulfide was obtained. However, in the presence of 2 equiv of HBcat, the reduction of sulfoxide was complete. Besides the amount, the nature of the borane used has a very pronounced influence on the

Fernandes et al. Chart 1. Competent Catalytic Precursor Sulfoxide Complexes

velocity of the catalytic reduction, which decreases in the order HBcat > HBpin . H3B 3 THF The deoxygenation of 4-chlorophenyl sulfoxide with HBcat catalyzed by the hydride 2, reported in our previous work (eq 3),36 required 1 h 25 min at room temperature under nitrogen atmosphere. This is much longer than the 5 min necessary to complete the same deoxygenation using ReIO2(PPh3)2 as catalyst. In agreement with this, when the catalytic reduction of 4-chlorophenyl sulfoxide is started by the addition of HBcat to ReIO2(PPh3)2, producing the hydride 2, followed by addition of the sulfoxide, complete deoxygenation requires 1 h at room temperature under air. We will specifically address this point later (see Scheme 3). The two preceding examples plainly show that the phosphine hydrides like 2 or 3, which result from the activation of the B-H bond across the RedO bond of ReIO2(PPh3)2, are not, per se, very efficient deoxygenating agents. In agreement with the chemistry of Re2O7 in coordinating solvents, the species existing in THF solution is [cis-(THF)2O3Re-O-ReO3].50 Upon addition of a large excess of sulfoxide, it is quite likely that the similar species [cis-(R2SO)2O3Re-O-ReO3] (9) becomes dominant and reduction of the sulfoxides should start from here upon reaction with the boranes. From the preceding information we propose that the catalytic reactions start from the sulfoxide complexes 8, 9 (90 ), and 10 depicted in Chart 1. We provided evidence for the formation of 8 from the precursor PPh3 analogue ReIO2(PPh3)2 under the reaction conditions, and the other ones are predictable from the known coordination chemistry of the highly electrophilic ReVII trioxides Re2O7 and MeReO3 (MTO).50 The reaction proceeds by attack of the borane on these complexes. Two borane equivalents are required to lead the reaction to completion. On the other hand, the detection of H2 by GC at the end of the reduction suggests further reaction with another molecule of HBcat, affording catBOBcat and hydrogen. DFT Studies. DFT 49 calculations (Gaussian 03; 51 see details in the Experimental Section) were performed on the (50) Rom~ao, C. C.; K€ uhn, F. E.; Herrmann, W. A. Chem. Rev. 1997, 97, 3197–3246. (51) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.

Article

Figure 1. Pathway for the reaction between the catalyst ReIO2(R2SO)2 (8) and HBcat to form the hydride ReHIO(R2SO)2(OBcat) (C). ΔG in kcal mol-1, relative to the pair of reagents, 8 þ HBcat; barriers in italics.

basis of the experimental evidence described above, namely, the formation of ReIO2(R2SO)2 from ReIO2(PPh3)2. This catalyst was selected because more studies were performed using it and it displays very good performance in terms of conversion and speed of reaction. Me2SO was used as a model for the substrate. The fact that the hydride ReHIO(PPh3)2(OBcat) (2) was isolated from the precursor and also exhibited catalytic activity led us to look for a hydride intermediate. Also, hydrides were shown to be formed when high-valent oxo-rhenium or molybdenum complexes activated Si-H or H-H bonds.52 The mechanism should account for the need to use 2 equiv of borane and the formation of both H2 and catBOBcat. The reaction pathway of the catalyst ReIO2(R2SO)2 (8) with the first molecule of HBcat to yield the hydride ReHIO(R2SO)2(OBcat) (C) is shown in Figure 1. The energies shown throughout this work (kcal mol-1) include solvent correction (THF) obtained with the PCM model (see computational details). The reaction takes place in two steps with very small activation barriers (1.8 and 3.3 kcal mol-1, respectively), giving rise to stable species. The reaction is highly exergonic (31.7 kcal mol-1). In the first step, the borane adds to one oxide, forming a new B-O bond. The environment of boron thus changes from planar to tetrahedral. In the transition state TS8B, the B 3 3 3 O distance is still very long (3.387 A˚) and the HBcat molecule is planar. Except for the RedO bond, which disappeared when HBcat approached, B is structurally very close to 8, with very similar bond lengths and angles. In the second step, the H(B) is transferred from boron to rhenium, with the boron environment becoming again planar and the rhenium octahedral. The hydride is trans to iodide, as observed in the X-ray determined structures of the related hydride complexes 2 and (52) (a) Kennedy-Smith, J. J.; Nolin, K. A.; Gunterman, H. P.; Toste, F. D. J. Am. Chem. Soc. 2003, 125, 4056–4057. (b) Nolin, K. A.; Krumper, J. R.; Puth, M. D.; Bergman, R. G.; Toste, F. D. J. Am. Chem. Soc. 2007, 129, 14684–14696. (c) Reis, P. M.; Costa, P. J.; Rom~ao, C. C.; Fernandes, J. A.; Calhorda, M. J.; Royo, B. Dalton Trans. 2008, 1727–1733.


5521

Figure 2. Pathway for the loss of R2S from ReHIO(R2SO)2(OBcat) (C) with formation of ReHIO2(R2SO)(OBcat) (D). ΔG in kcal mol-1, relative to the pair of reagents, 8 þ HBcat; barriers in italics.

3 (eqs 2 and 3 above), while OBcat is trans to the oxo ligand. The Re-O(B) distance is 1.988 A˚, very close to the 1.932 and 1.985 A˚ values observed in the analogous distances in 2 and 3. The loss of thioether from intermediate C takes place easily, with a barrier of 7.2 kcal mol-1, and a new species ReHIO2(R2SO)(OBcat) (D) is formed (Figure 2). In the transition state, the S 3 3 3 O distance has increased from 1.572 A˚ in C to 1.843 A˚. The three steps described above show a low-energy pathway that leads from the catalyst 8 to the formation of the thioether and a new intermediate (D). In D, the metal has been oxidized to Re(VII), owing to the loss of R2S. The former neutral R2SO ligand has given rise to the dianionic oxo ligand, formally changing the metal oxidation state from V to VII. All the previous intermediates (B and C) were Re(V) complexes. A relevant question concerns the order of the reactions. Would it be possible for complex 8 first to lose R2S and then react with HBcat to reach exactly the same intermediate? This alternative was examined, and the corresponding pathway is depicted in Figure 3. The direct loss of R2S from the catalyst can occur with a small barrier (4.6 kcal mol-1), and the species formed (B0 ) can activate the B-H bond in HBcat, although with a relatively large barrier (15.9 kcal mol-1). The S 3 3 3 O distance in the transition state TS8B0 is 1.760 A˚, even shorter than in the corresponding TS of the mechanism shown in Figure 1. Interestingly, B0 can activate the B-H bond in only one step, with simultaneous formation of the new B-O bond and the Re-H bond. In this transition state (TSB0 D), the new B-O bond is almost formed (1.547 A˚), being rather close to the final distance (1.337 A˚). The H atom, on the other hand, is bridging between B and Re (1.268 and 2.227 A˚, for B 3 3 3 H and Re 3 3 3 H, respectively). This pathway is less likely to occur than the previous one (Figures 1 and 2), because the barriers are significantly higher, despite the same initial and final states. The discrepancy of the energy values of intermediate D presented in Figures 2 and 3 arises from a different scale. The energy in Figure 2 (-42.3 kcal mol-1) is relative to

5522


the pair of reactants, 8 and HBcat, interacting weakly, while the energy of D in Figure 3 (-45.9 kcal mol-1) refers to the isolated reagents. Thus, the energy difference (3.6 kcal mol-1) represents the stabilization of the pair of reagents, 8 þ HBcat, when they approach each other, relative to the two separated molecules. At this stage of the reaction, one molecule of R2S has been released by the catalyst, another sulfoxide remains coordinated, and it is known that a second HBcat molecule is involved in the reaction. Therefore, the reaction between D and HBcat was addressed. The pathway shown in Figure 4 depicts the lowest energy route for this reaction, and the most interesting feature is that the new HBcat molecule

Figure 3. Alternative pathway for the reaction between the catalyst ReIO2(R2SO)2 (8) and HBcat losing R2S and forming the intermediate ReHIO2(R2SO)(OBcat) (D). ΔG in kcal mol-1, relative to 8; barriers in italics.

Fernandes et al.

approaches the oxygen in the B-O-Re bond of D. At the end of this reaction, involving intermediates E and F, both H2 and the catBOBcat species have been released, while a new intermediate, G, has been formed. Notice the small barriers involved in the three steps leading from D to G (between 0.9 and 4.3 kcal mol-1). The boron atom of the incoming HBcat becomes tetrahedral in E, but otherwise this species is very similar to D. The coordinated sulfoxide has rotated in order to accommodate the bulky HBcat molecule. Then this HBcat molecule rotates, so that the H(B) approaches the H(Re). In the transition state (TSEF), the B-H bond has elongated to 1.335 A˚ and the Re-H to 1.830 A˚, while the H 3 3 3 H distance of 1.046 A˚ is characteristic of a weak H-H bond. This allows for an easy loss of both H2 and catBOBcat from this transition state. Indeed, H 2 was detected by gas chromatography at the end of the reduction. During this process, Re(VII) has been brought back to Re(V) by the second HBcat molecule, but the catalytic cycle has not yet closed. Intermediate G can either lose another R2S molecule or regenerate the catalyst by reacting with another R2SO substrate molecule. A closer look at D suggests that HBcat might attack another oxygen, since there are two oxo groups bound to Re and they are nonequivalent. Two different products can be obtained, as shown in Figure 5. The immediate conclusion is that, although there is only one transition state, these two alternatives require much higher energies than the first pathway (Figure 4), namely, 25.9 or 23.1 against 4.3 kcal mol-1. The products are very stable, but, considering the unlikeness of overcoming these huge barriers, when competitive pathways are available, we did not even attempt to pursue the study of the reaction from H or H0 . Nevertheless, these two species are interesting. The B-H activation ends with the Bcat fragment attached to the former oxo group and the H transferred to the other O, so that in H the Bcat is stabilized by a hydrogen bond with the neighbor O-H. Even more interesting is the formation of a coordinated dihydrogen molecule in H0 . The H from the

Figure 4. Pathway for the reaction between the intermediate ReHIO2(R2SO)(OBcat) (D) and HBcat affording H2 and catBOBcat and forming the intermediate ReIO2(R2SO) (G). ΔG in kcal mol-1, relative to D þ HBcat; barriers in italics.

Article


5523

Scheme 1. Catalytic Cycle for the Reduction of One R2SdO Molecule

Figure 5. Alternative pathway for the reaction between the intermediate ReHIO2(R2SO)(OBcat) (D) and HBcat forming intermediate H or H0 . ΔG in kcal mol-1, relative to D þ HBcat; barriers in italics. Scheme 2. Entrance of Re(VII) Catalysts MeReO3 (MTO) and Re2O7 in the Catalytic Cycle

Figure 6. Pathways for the reaction between the intermediate ReIO2(R2SO) (G) and R2SO (top) or loss of R2S (bottom). ΔG in kcal mol-1, relative to G þ R2SO (top) or to isolated G (bottom); barriers in italics.

second HBcat migrates to the hydride. The final H-H bond length is 0.824 A˚, only slightly elongated from the 0.744 A˚ calculated for free H2.

Having analyzed these alternative reaction paths, only the recovery of the catalyst remains, although one may still consider the possibility of R2S loss from G. These reaction paths are represented in Figure 6. The reaction with another substrate molecule (top of Figure 6) is accompanied by a lower barrier (almost 10 times smaller than the barrier needed to lose thioether), suggesting that the catalyst will be regenerated. Interestingly, only one molecule of R2S results from a catalyst (8) containing two coordinated sulfoxides, and, thus, one of the initial R2SO ligands in 8 remains coordinated during the entire mechanism. This mechanism, summarized in Scheme 1, requires 2 equiv of HBcat, which will in the end give rise to catBOBcat and H2. The Re(V) complex is oxidized to Re(VII) after losing R2S from the hydride complex C at the end of the first part of the reaction, and the reaction with the second HBcat produces the electrons needed to reduce it back to Re(V).

5524


Scheme 3. Competition between Phosphine Substitution by Sulfoxide and Hydride Formation

When the catalyst is MTO, a Re(VII) complex, addition of R2SdO yields complex 10 (Scheme 2). HBcat adds in a similar way to one RedO bond, forming directly an analogue of D where Me replaces I, and the reaction proceeds. Re2O7 can also add R2SdO to afford complex 90 , where ReO4 plays the same role as Me in 10. Therefore, the proposed catalytic cycle applies to Re(VII) catalysts, although the starting point is a different one (D instead of 8). It should also be noticed that addition of HBcat to 90 or to 10 is similar to the same reaction involving intermediate B0 (the second step of the profile in Figure 3), but with Me (10) or ReO4 (90 ) in place of I (B0 ). Another open question concerns the role of the hydride (PPh3)2(O)(I)Re(H)OBcat, 2. As discussed above, this hydride also catalyzes sulfoxide reduction, but the reaction takes much longer, suggesting that in normal reaction conditions (sulfoxide added before catecholborane) the active species is not 2, but a different one. The energetics of these reactions are shown in Scheme 3. Replacing a phosphine by a sulfoxide is an unfavorable process, but is easier to achieve starting from ReO2I(PPh3)2 (1) (27.5 kcal mol-1) than from the hydride 2 (31.4 kcal mol-1) and probably driven by the high concentration of sulfoxide compared with the concentration of the catalyst precursor. On the other hand, both complexes 1 and 8 react easily with HBcat to yield the hydrides 2 and C. Thus, in the normal course of the reaction, when sulfoxide is added first, followed by addition of HBcat, complex 8 will form preferentially and the hydride C will be the catalyst. Only in the absence of sulfoxide will hydride 2 appear. If substrate is then added, it will also catalyze the reaction.

Conclusions In conclusion, we have developed a facile and very efficient method for the reduction of sulfoxides with high yields and good chemoselectivity, under mild conditions. Other remarkable advantages of this methodology include low catalyst loading (1 mol %), fast reaction times, clean reactions, and stability of the catalysts toward air and moisture, allowing the reaction to be carried out under an air atmosphere. Among the disadvantages, one might mention the

Fernandes et al.

waste of borane, since 2 equiv are needed. The proposed reaction mechanism, based on the results of a detailed DFT computational study, accounts for the formation of catBOBcat and H2 as residues, while one molecule of sulfoxide per complex is reduced, despite the presence of two coordinated sulfoxides in the catalytically active species. The first HBcat reacts with this active species, giving rise to a hydride complex, which loses R2S more easily than the parent species. The remaining intermediate is a Re(VII) complex that is reconverted in the initial Re(V) catalyst, after reacting with the second HBcat molecule and producing catBOBcat and H2. These results extend the scope of the use of oxo-rhenium(V) and -(VII) complexes as effective catalysts for reduction reactions and open a new area of catalysis for these complexes, since reactions with boranes are among the most important synthetic methodologies in organic chemistry. We believe that this procedure will present a useful and attractive alternative to the existing methods for the reduction of sulfoxides to sulfides. Other synthetic applications and further mechanistic studies of these mild and highly efficient catalytic systems are still under investigation in our group.

Experimental Section THF was dried and distilled prior to use over Na/benzophenone under a nitrogen atmosphere. Boranes and oxo-rhenium complexes were obtained from Aldrich. Flash chromatography was performed on MN Kieselgel 60 M 230-400 mesh. 1H NMR and 13C NMR spectra were measured on a Bruker Avance III 400 MHz spectrometer. Chemical shifts are reported in parts per million (ppm) downfield from an internal standard. General Procedure for the Reduction of Sulfoxides with the System HBcat/Oxo-rhenium Complex. To a solution of catalyst (1 mol %) in THF (3 mL) was added the sulfoxide (1.0 mmol) and the solution of HBcat in THF (2.0 mmol). The reaction mixture was stirred under an air atmosphere (the temperatures and the reaction times are indicated in Table 2), and the progress of the reaction was monitored by TLC and 1H NMR. Upon completion, the reaction mixture was evaporated and purified by silica gel column chromatography with the appropriate mixture of n-hexane and ethyl acetate to afford the sulfides, which are all known compounds. 4-Chlorophenyl Sulfide. The spectroscopic data are similar to the commercial sulfide. 1H NMR (400 MHz, CDCl3): δ 7.22 (d, 2H, J = 9.0 Hz), 7.18 (d, 2H, J = 9.0 Hz) ppm. 13C NMR (101 MHz, CDCl3): δ 134.1, 133.6, 132.5, 129.6 ppm. Anal. Calcd for C12H8Cl2S: C, 56.48; H, 3.16; S, 12.57. Found: C, 56.30; H, 3.05; S, 12.36. Phenyl Sulfide. The spectroscopic data are similar to the commercial sulfide. 1H NMR (400 MHz, CDCl3): δ 7.28-7.16 (m, 10 H) ppm. 13C NMR (101 MHz, CDCl3): δ 135.9, 131.1, 129.3, 127.1 ppm. Anal. Calcd for C12H10S: C, 77.37; H, 5.41; S, 17.21. Found: C, 77.28; H, 5.26; S, 17.16. 4-Methylphenyl Sulfide. The spectroscopic data are similar to the commercial sulfide. 1H NMR (400 MHz, CDCl3): δ 7.27 (d, 2H, J = 8.0 Hz), 7.14 (d, 2H, J = 8.4 Hz), 2.36 (s, 3H) ppm. 13 C NMR (101 MHz, CDCl3): δ 137.0, 132.8, 131.2, 130.0, 21.2 ppm. Anal. Calcd for C7H8S: C, 67.69; H, 6.49; S, 25.82. Found: C, 67.50; H, 6.27; S, 25.67. Methyl 2-(Phenylthio)acetate. The spectroscopic data are similar to the commercial sulfide. 1H NMR (400 MHz, CDCl3): δ 7.33 (d, 2 H, J = 9.0 Hz), 7.26-7.16 (m, 3H), 3.65 (s, 3H), 3.58 (s, 2H) ppm. 13C NMR (101 MHz, CDCl3): δ 170.1, 135.1, 129.9, 129.1, 126.9, 52.4, 36.5 ppm. Anal. Calcd for C9H10O2S: C, 59.32; H, 5.53; S, 17.59. Found: C, 59.19; H, 5.40; S, 17.39. Methyl Phenyl Sulfide. The spectroscopic data are similar to the commercial sulfide. 1H NMR (400 MHz, CDCl3): δ

Article 7.25-7.18 (m, 4 H), 7.09-7.07 (m, 1H), 2.41 (s, 3H) ppm. 13C NMR (101 MHz, CDCl3): δ 138.4, 129.0, 126.8, 125.2, 16.0 ppm. Anal. Calcd for C7H8S: C, 67.69; H, 6.49; S, 25.82. Found: C, 67.50; H, 6.35; S, 25.71. 4-Nitrothioanisole. The spectroscopic data are similar to the commercial sulfide. 1H NMR (400 MHz, CDCl3): δ 8.15 (d, 2H, J = 7.7 Hz), 7.31 (d, 2H, J = 8.4 Hz), 2.57 (s, 3H) ppm. 13C NMR (101 MHz, CDCl3): δ 148.9, 144.8, 125.0, 123.9, 14.9 ppm. Anal. Calcd for C7H7NO2S: C, 49.69; H, 4.17; N, 8.28; S, 18.95. Found: C, 49.58; H, 4.03; N, 8.01; S, 18.84. DFT Calculations. All calculations were performed using the Gaussian 03 software package51 and the PBE1PBE functional, without symmetry constraints. That functional uses a hybrid generalized gradient approximation (GGA), including 25% mixture of Hartree-Fock53 exchange with DFT49 exchangecorrelation functional, given by Perdew, Burke, and Ernzerhof (PBE).54 The optimized geometries were obtained with a VDZP basis set (basis b1) consisting of the LanL2DZ basis set55 augmented with an f-polarization function56 for Re and a d-polarization function57 for I, and a standard 6-31G(d,p)58 for the remaining elements. Transition-state optimizations were performed with the synchronous transit-guided quasi-Newton method (STQN) developed by Schlegel et al.59 Frequency calculations were performed to confirm the nature of the stationary points, yielding one imaginary frequency for the transition states and none for the minima. Each transition state was further confirmed by following its vibrational mode downhill on both sides and obtaining the minima presented on the energy profiles. (53) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; John Wiley & Sons: New York, 1986. (54) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78, 1396. (b) Perdew, J. P. Phys. Rev. B 1986, 33, 8822. (55) (a) Dunning, T. H., Jr.; Hay, P. J. Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3, p 1. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (c) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (d) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 2299. (56) Ehlers, A. W.; B€ ohme, M.; Dapprich, S.; Gobbi, A.; H€ ollwarth, A.; Jonas, V.; K€ ohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 111. (57) H€ ollwarth, A.; B€ ohme, M.; Dapprich, S.; Ehlers, A. W.; Gobbi, A.; Jonas, V.; K€ ohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 237. (58) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (c) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209. (d) Gordon, M. S. Chem. Phys. Lett. 1980, 76, 163. (e) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (59) (a) Peng, C.; Ayala, P. Y.; Schlegel, H. B.; Frisch, M. J. J. Comput. Chem. 1996, 17, 49. (b) Peng, C.; Schlegel, H. B. Isr. J. Chem. 1994, 33, 449.


5525

The energy profiles reported result from single-point energy calculations using a VTZP basis set (basis b2) and the geometries optimized at the PBE1PBE/b1 level. Basis b2 consisted of a Stuttgart/Dresden ECP basis set60 with an added f-polarization function56 for Re and a d-polarization function57 for I, and standard 6-311þþG(d,p)61 for the remaining elements. Solvent (dichloromethane) effects were considered in the PBE1PBE/b2//PBE1PBE/b1 energy calculations using the polarizable continuum model (PCM) initially devised by Tomasi and co-workers62 as implemented in Gaussian 03.63 The molecular cavity was based on the united atom topological model applied on UAHF radii, optimized for the HF/ 6-31G(d) level. Structural representations were obtained with Chemcraft.64

Acknowledgment. This research was supported by FCT through project PTDC/QUI/71741/2006. J.A.F. thanks FCT for a postdoctoral grant (SFRH/BPD/ 23461/2005). We wish to acknowledge M. C. Almeida and Dr A. Coelho for providing data from the Elemental Analysis Service at ITQB. The NMR spectrometers are part of the National NMR Network and were purchased in the framework of the National Programme for Scientific Re-equipment, contract REDE/1517/RMN/2005, with funds from POCI 2010 (FEDER) and Fundac- ~ ao para a Ci^encia e a Tecnologia (FCT). Supporting Information Available: Atomic coordinates for all optimized species. This material is available free of charge via the Internet at http://pubs.acs.org. (60) (a) Haeusermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 78, 1211. (b) Kuechle, W.; Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1994, 100, 7535. (c) Leininger, T.; Nicklass, A.; Stoll, H.; Dolg, M.; Schwerdtfeger, P. J. Chem. Phys. 1996, 105, 1052. (61) (a) McClean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639. (b) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (c) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033. (d) Hay, P. J. J. Chem. Phys. 1977, 66, 4377. (e) Raghavachari, K.; Trucks, G. W. J. Chem. Phys. 1989, 91, 1062. (f) Binning, R. C., Jr.; Curtiss, L. A. J. Comput. Chem. 1990, 11, 1206. (g) McGrath, M. P.; Radom, L. J. Chem. Phys. 1991, 94, 511. (62) (a) Cances, M. T.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032. (b) Cossi, M.; Barone, V.; Mennucci, B.; Tomasi, J. Chem. Phys. Lett. 1998, 286, 253. (c) Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151. (63) (a) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999. (b) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. J. Chem. Phys. 2002, 117, 43. (64) http://www.chemcraftprog.com

Oxo

Recommend Documents