Organometallics 2009, 28, 6379–6382 DOI: 10.1021/om900747b
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Mechanistic Insight into the Reduction of Tertiary Phosphine Oxides by Ti(OiPr)4/TMDS )
Christelle Petit,† Alain Favre-Reguillon,*,‡ Belen Albela,§ Laurent Bonneviot,§ Gerard Mignani, and Marc Lemaire*,† †
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Laboratoire de Catalyse et Synth ese Organique, ICBMS, UMR 5246, Universit e Claude Bernard Lyon 1, 43 bld du 11 nov 1918, 69100 Villeurbanne, France, ‡Laboratoire de Transformations Chimiques et Pharmaceutiques, UMR 7084, Conservatoire National des Arts et M etiers, 2 rue Cont e, 75003 Paris, France, § Ecole Normale Sup erieure de Lyon, Laboratoire de Chimie, Groupe Mat eriaux Hybrides, 46 all ee d’Italie, 69364 Lyon cedex 7, France, and Rhodia Operations, Lyon Research Center, 85, avenue des Fr eres Perret, BP 62, 69192 Saint-Fons Cedex, France Received August 27, 2009 Summary: The reduction mechanism of tertiary phosphine oxides by Ti(OiPr)4/hydrosiloxane was studied. Strong improvement was achieved using a drying agent. ESR spectra of the reaction mixture give evidence for a single electronic transfer (SET) mechanism. Reduction is a fundamental process in chemistry. Furthermore, worldwide demand for environmentally friendly chemical processes and products requires the use of safer reagents that reduce or eliminate the generation of dangerous and toxic byproducts and decrease the amount of waste produced.1 In recent years, the rediscovery of the reductive abilities of hydrosiloxanes2 when associated with some transition metal,3 Lewis acid,4 or fluoride based catalysts5 renewed the interest in using hydrosiloxane as the stoichiometric reductive agent.6 Indeed, hydrosiloxanes, in comparison to most of the conventional reagents, have many advantages, since they are commercially available, air- and moisture-stable, nonpyrophoric, and soluble in most organic solvents. Recently, we have been working on the development of new methods for the reduction of tertiary phosphine oxides
(TPOs), looking for safer and cleaner reagents compatible with sustainable chemistry.7 Lawrence et al. have shown that TPO could be reduced using polymethylhydrosiloxane (PMHS) and a stoichiometric amount of Ti(OiPr)4.8 We recently reported that on switching to a lower molecular weight hydrosiloxane, i.e. tetramethyldisiloxane (TMDS), all classes of TPO were effectively reduced using a catalytic amount of Ti(OiPr)4.7 Despite this effective procedure being used for decades by numerous research teams,9 surprisingly little is known about the mechanism(s) of this reduction, as detailed mechanistic studies have not been undertaken. A catalytic cycle has been proposed by Lawrence et al.6,8 for the reduction of TPO by triethoxysilane. A titanium hydride like complex, which is obtained by a σ-bond metathesis reaction between triethoxysilane and titanium tetraalkoxides, is expected to be the active reductant.10,11 However, for TPO reduction, this mechanism involves the coexistence of cationic and anionic intermediates, i.e. R3PþH and (RO)3TiO-, and implies the generation of a stoichiometric amount of hydrogen. In our hand, the latter has never been observed. Furthermore, no mechanism has been proposed for Ti(OiPr)4/hydrosiloxane (PMHS or TMDS), which are substitutes for the dangerous triethoxysilane.12 At the outset, it seemed unlikely that a single universal mechanism would explain the reduction of a wide range of functional groups. Therefore, we decided to reinvestigate the reduction of TPO by hydrosiloxane.
*To whom correspondence should be addressed. E-mail: alain.
[email protected] (A.F.-R.);
[email protected] (M.L.). (1) Anastas, P. T.; Zimmerman, J. B. Environ. Sci. Technol. 2003, 37, 95A–101A. (2) (a) Hayashi, K.; Iyoda, J.; Shiihara, I. J. Organomet. Chem. 1967, 10, 81–94. (b) Nitzsche, L. S.; Wick, M. Angew. Chem. 1957, 69, 96. (3) Use of Zn catalysts: (a) Bette, V.; Mortreux, A.; Savoia, D.; Carpentier, J.-F. Adv. Synth. Catal. 2005, 347, 289–302 and references therein. Use of Cu catalysts: (b) Lipshutz, B. H. Synlett 2009, 509–524. (c) Hughes, G.; Kimura, M.; Buchwald, S. L. J. Am. Chem. Soc. 2003, 125, 11253–11258. Use of Pd catalysts: (d) Maleczka, R. E.; Rahaim, R. J.; Teixeira, R. R. Tetrahedron Lett. 2002, 43, 7087–7090. (e) Rahaim, R. J.Jr.; Maleczka, R. E.Jr. Tetrahedron Lett. 2002, 43, 8823–8826. (f) Lee, K.; Maleczka, R. E. Org. Lett. 2006, 8, 1887–1888. (g) Chandrasekhar, S.; Reddy, M. V.; Chandraiah, L. Synlett 2000, 1351–1353. (h) Chandrasekhar, S.; Reddy, C. R.; Rao, R. J.; Rao, J. M. Synlett 2002, 349–351. With Pt: (i) Denmark, S. E.; Pan, W. Org. Lett. 2003, 5, 1119–1122. (j) de Vekki, D. A.; Skvortsov, N. K. Russ. J. Gen. Chem. 2004, 74, 197–206. With Ti(OiPr)4: (k) Reding, M. T.; Buchwald, S. L. J. Org. Chem. 1995, 60, 7884. (l) Chandrasekhar, S.; Reddy, C. R.; Ahmed, M. Synlett 2000, 1655–1657. (4) For reduction using TMDS/AlCl3: (a) Nadkarni, D. V.; Hallissey, J. F. Org. Process Res. Dev. 2008, 12, 1142–1145. With FeCl3: (b) Dal Zotto, C.; Virieux, D.; Campagne, J.-M. Synlett 2009, 276–278. With B(C6F5)3: (c) Chandrasekhar, S.; Chandrashekar, G.; Vijeender, K.; Reddy, M. S. Tetrahedron Lett. 2006, 47, 3475–3478. (5) Nadkarni, D.; Hallissey, J.; Mojica, C. J. Org. Chem. 2003, 68, 594–596. (6) For review, see: Lawrence, N. J.; Drew, M. D.; Bushell, S. M. J. Chem. Soc., Perkin Trans. 1 1999, 3381–3391.
(7) (a) Berthod, M.; Favre-Reguillon, A.; Mohamad, J.; Mignani, G.; Docherty, G.; Lemaire, M. Synlett 2007, 1545–1548. For updated references for the reduction see: (b) Busacca, C. A.; Raju, R.; Grinberg, N.; Haddad, N.; James-Jones, P.; Lee, H.; Lorenz, J. C.; Saha, A.; Senanayake, C. H. J. Org. Chem. 2008, 73, 1524–1531and references therein. (8) Coumbe, T.; Lawrence, N. J.; Muhammad, F. Tetrahedron Lett. 1994, 35, 625–628. (9) (a) Vinokurov, N.; Pietrusiewicz, K. M.; Frynas, S.; Wiebcke, M.; Butensch€ on, H. Chem. Commun. 2008, 5408–5410. (b) Ashburn, B. O.; Carter, R. G. Angew. Chem., Int. Ed. 2006, 45, 6737–6741. (c) Gavryushin, A.; Polborn, K.; Knochel, P. Tetrahedron: Asymmetry 2004, 15, 2279– 2288. (d) Wyatt, P.; Warren, S.; McPartlin, M.; Woodroffe, T. J. Chem. Soc., Perkin Trans. 1 2001, 279–297. (e) Russell, M. G.; Warren, S. Tetrahedron Lett. 1998, 39, 7995–7998. (f) Hamada, Y.; Matsuura, F.; Oku, M.; Hatano, K.; Shioiri, T. Tetrahedron Lett. 1997, 38, 8961–8964. (10) Albizzati, E.; Abis, L.; Pettenati, E.; Gianneti, E. Inorg. Chim. Acta 1986, 120, 197–203. (11) (a) Berk, S. C.; Buchwald, S. L. J. Org. Chem. 1992, 57, 3751– 3753. (b) Verdaguer, X.; Lange, U. E. W.; Buchwald, S. L. Angew. Chem., Int. Ed. 1998, 37, 1103–1107. (12) Berk, S. C.; Buchwald, S. L. J. Org. Chem. 1993, 58, 3221.
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Figure 1. ESR spectrum of the colored solution at 298 K with the following measurement conditions: frequency 9.107 GHz, power 1 mW, modulation amplitude 0.4 G. Scheme 1. Proposed Mechanism for the Reduction of TPO with Hydrosiloxane and Catalytic Ti(OiPr)4
Table 1. Reduction of Triphenylphosphine Oxide by TMDS/ Ti(OiPr)4
entry 1 2 3 4 5 6 7 8
Si-H/PdO molar ratio
temp (°C)
2.5 2.5 1.2 1.2 1.2 1.2 1.2 1.2
60 100c 100c 100c 60 60 60 60
drying agenta
conversion (%)b
Na2SO4 Na2SO4 K2CO3 MgSO4 MS 4 A˚e
0 100 86 100 100d 100 100 83
a 10 wt %. b Determined by 31P NMR. c Reaction performed in sealed tube. d Isolated yield. e MS= molecular sieve.
the d-d transition of a d1 Ti(III) species.13 The ESR spectrum observed (Figure 1) does not saturate under high microwave power (150 mW) even at 120 K and therefore could not be fitted to any organic radical. The main signal at g = 1.938 as well as two narrow features at g = 1.964 and 1.960 and a shoulder at 1.956 are too far below the resonance of the free electron ge = 2.0023 to match with any p-type unpaired electron.14 In fact, silicon radicals are always reported with values higher than 2.0050 and carbon and phosphorus are always above 2.000.15 In addition, these signals do not saturate at high microwave power, indicating When Ti(OiPr)4 and TMDS were allowed to react in methylcyclohexane with triphenylphosphine oxide under a dry Ar atmosphere, a blue solution resulted within a few minutes (see the Supporting Information) concomitant with the appearance of ESR-active species. In addition, these species showed very high air sensitivity, resulting in both rapid loss of the coloration and disappearence of the ESR signal. The coloration due to a broad absorption band observed at 508 nm (ε = 5.16 M-1 cm-1) is consistent with
(13) Paradies, J.; Crudass, J.; MacKay, F.; Yellowlees, L. J.; Montgomery, J.; Parsons, S.; Oswald, I.; Robertson, N.; Sadler, P. J. J. Inorg. Biochem. 2006, 100, 1260–1264. (14) Abragan, A.; Bleaney, B. In Electron Paramagnetic Resonance of Transition Ions; Oxford University Press: Oxford, U.K., 1970. (15) (a) Weil, J. A.; Bolton, J. R. Electron Paramagnetic Resonance: Elementary Theory and Practical Applications; Wiley: New York, 2007. (b) Astakhov, O.; Carius, R.; Lambertz, A.; Petrusenko, Y.; Borysenko, V.; Barankov, D.; Finger, F. J. Non-Cryst. Solids 2008, 354, 2329–2332. (c) Ballestri, M.; Chatgilialoglu, C.; Lucarini, M.; Pedulli, G. F. J. Org. Chem. 1992, 57, 948–952.
Communication Table 2. Reduction of Phosphine Oxides by TMDS/Ti(OiPr)4/ Na2SO4
a Determined by 31P NMR; 10 wt %. b Isolated yield. c Toluene used as solvent.
that these resonances belong to “d type” unpaired electrons of a less than half-filled d shell: i.e., to Ti(III) d1 ions.14 The main line looks rather symmetrical, as expected for an isotropic environment. However, an isolated d1 species is known as a textbook case for the Jahn-Teller effect that produces a distorted environment and a g tensor with at least two components.16 Single symmetrical signals for Ti(III) species are observed when the unpaired electron is strongly delocalized on different centers in Ti clusters or in Ti species paired with aluminum centers. This is the case for monotitanocene species reduced using MAO (methylaluminum oxide) known to be active in catalytic alkene polymerization; then, the ESR signal arises at 1.972.17 The actual g value is smaller here, consistent with isopropoxide ligands, which generate a crystal field weaker than that of a cyclopentadienyl ligand. The reductant is also different: a silicon hydride that is not likely to form any direct linkage with an isolated Ti(III) species. We are, rather, dealing with titanium clusters known to form easily in solid-state titanium alkoxide chemistry. Neutral or cationic trimers of [Ti(OiPr)4] have been isolated.18 However, in solution, titanium alkoxides exhibit a dynamic equibrium between monomer and dimer, which is the case for [Ti(OiPr)4], and eventually higher oligomers for ethoxides or methoxides.19 Note that the tetraisoamyloxide (16) Cotton, F. A.; Wilkinson, G.; Gaus, P. L. Basic Inorganic Chemistry, 3rd ed.; Wiley: New York, 1995. (17) Bonoldi, L.; Abis, L.; Fiocca, L.; Fusco, R.; Longo, L.; Simone, F.; Spera, S. J. Mol. Catal. A: Chem. 2004, 219, 47–56. (18) (a) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Kirkpatrick, C. C. J. Am. Chem. Soc. 1981, 103, 5967–5968. (b) Clark, D. L.; Watkin, J. G. Inorg. Chem. 1993, 32, 1766–1772. (c) Reis, D. M.; Nunes, G. G.; Sa, E. L.; Friedermann, G. R.; Mangrich, A. S.; Evans, D. J.; Hitchcock, P. B.; Leigh, G. J.; Soares, J. F. New J. Chem. 2004, 28, 1168–1176. (d) Veith, M.; Mathur, S.; Huch, V. Chem. Commun. 1997, 2197–2198. (19) Babonneau, F.; Doeuff, S.; Leaustic, A.; Sanchez, C.; Cartier, C.; Verdaguer, M. Inorg. Chem. 1988, 27, 3166–3172.
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titanum(IV) species do not form any oligomers and no ESR signal (no reduction) has been observed under the present conditions, while a similar isotropic signal is observed for the tetraethoxytitanium(IV) species. The strong peak here is likely to belong to a dimeric mixed-valence Ti(IV)-O-Ti(III) isopropoxide species.19 There are also other ESR-active species observed during the reduction of titanocene species, among them being a doublet arising at g = 1.990-1984 with a splitting of 7.5 G.17,20 The latter is assigned to a titanium hydride, Ti(III)-H. Here, the small features observed on the righthand side of the main signal may also be assigned to a doublet at g = 1.962 with a splitting of 10 G accounting also for a Ti(III)-H species. This is more likely a hyperfine structure centered on the main signal that could be assigned to the 47Ti and 49Ti isotopes. Indeed, 7.28 and 5.51% natural abundance and nuclear spins of 5/2 and 7/2 for the latter, respectively, would produce six and eight side lines of less than 1% of the main signal intensity. Nonetheless, it is worth noting that the integrated intensity of both narrow features account for only ∼0.1% of all the Ti(III) species present here. Furthermore, in the case of the tetraethoxide titanium, such a doublet was not observed while the reduction and the reaction took place. A catalytic cycle could thus be proposed, taking into account the presence of Ti(III) (Scheme 1). Unlike Lawrence’s mechanism,6,8 an equimolar amount of water is produced as a byproduct instead of hydrogen, while the reduction of TPO only requires 1 Si-H per PdO group, whereas 2 Si-H per PdO are needed in Lawrence’s mechanism.6,8 Thus, the influence of the molar ratio and drying agent on TPO reduction was studied (Table 1). No reduction was observed at 60 °C with a Si-H/PdO molar ratio of 2.5 and 10 mol % Ti(OiPr)4 (Table 1, entry 1). High conversion was obtained at 100 °C. However, the boiling point of TMDS is 71 °C and the reaction needs to be done in a sealed tube (Table 1, entry 2).7 When the Si-H/PdO ratio was decreased to 1.2, we were pleased to see that conversion reamined high (Table 1, entry 3). Therefore, we could expect that the stoichiometry of the reduction occurred according to Scheme 1. Note that clustered titanium alkoxides are likely to contribute to the one-electron-transfer redox reaction, since they are expected to be easier to reduce than the tetrahedral monomer. The nature of such a cluster is not yet fully characterized under the present reaction conditions, and this is why the tetrahedral monomer is the only species mentioned in the cycle. The formation of water as byproduct could have an influence on the catalytic system by hydrolysis of Ti(OiPr)4 or other titanium species acting as catalyst and on the conversion by reoxidation of triphenylphosphine. Thus, a drying agent (10 wt %) was used and complete conversion was obtained (Table 1, entry 4). The reaction temperature was then decreased to 60 °C, i.e., below the boiling point of TMDS, allowing the use of classical glassware to reach the quantitative reduction of triphenylphosphine oxide (Table 1, entry 5). The workup was straightforward, i.e. at the end of the reaction, the mixture was cooled and filtered. The solid was washed with pentane to give the triphenylphosphine (20) (a) Bueschges, U.; W. Chien, J. C. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 1525–1538. (b) Chien, J. C. W.; Salajka, Z.; Dong, S. Macromolecules 1992, 25, 3199–3203. (c) Williams, E. F.; Murray, M. C.; Baird, M. C. Macromolecules 2000, 33, 261–268.
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in quantitative yield. Various dehydrating agents were evaluated (Table 1, entries 6-8), and quantitative conversion was obtained with the exception of molecular sieve (Table 1, entry 8). In the last case, the low conversion could be attributed to the sorption of Ti(OiPr)4 in the sieve. By using a drying agent, this reduction represents a noteworthy refinement of the literature. Moreover, given that TMDS is a mild and safe reagent, these conditions may be attractive as a general way to reduce TPO. To assess this, we reduced a series of tertiary phosphine oxides using the TMDS/Ti(OiPr)4/Na2SO4 conditions (Table 2). We screened a wide range of phosphine oxides in the reduction reaction with the TMDS/Ti(OiPr)4/Na2SO4 system. In these experiments, diphosphines (Table 2, entries 1-3), chiral BINAP (Table 2, entry 4), and trialkylphosphines (Table 2, entries 5 and 6) were obtained in satisfactory yield. The trialkylphosphines have been isolated as borane complexes, and as expected, BINAP was reduced without racemization.
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At the present stage, no complete mechanistic rationale can be provided. However, the presence of Ti(III) suggests that the reduction of tertiary phosphine oxide occurs via a single electron transfer (SET) mechanism rather than a titanium hydride-like complex. Further studies are in progress to fully understand the mechanism of TPO reduction by Ti(OiPr)4 and hydrosiloxane. By utilizing TMDS as a safe stoichiometric hydride source, catalytic amounts of Ti(OiPr)4, and a drying agent, tertiary phosphine oxide could be easily reduced at 60 °C in 24 h. The workup is straightforward and could be applied to reductions of various TPOs.
Acknowledgment. This work was supported by Rhodia Operations through a Ph.D. grant to C.P. Supporting Information Available: Text and figures giving full synthetic details for the preparation of compounds and a UV-vis spectrum. This material is available free of charge via the Internet at http://pubs.acs.org.
