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Ruthenium catalyzed selective hydrosilylation of aldehydes† Basujit Chatterjee and Chidambaram Gunanathan*
Received 3rd October 2013, Accepted 15th November 2013 DOI: 10.1039/c3cc47593j www.rsc.org/chemcomm
A chemoselective hydrosilylation method for aldehydes is developed using a ruthenium catalyst [(Ru( p-cymene)Cl2)2] and triethylsilane; a mono hydride bridged dinuclear complex [{(g6-p-cymene)RuCl}2(l-H-l-Cl)] and a Ru(IV) mononuclear dihydride complex [(g6-pcymene)Ru(H)2(SiEt3)2] are identified as potential intermediates in the reaction and the proposed catalytic cycle involves a 1,3-hydride migration.
Hydrosilylation of carbonyl compounds is a valuable transformation in chemical synthesis as a single step operation serves on both reduction of carbonyl motifs and protection of resulting alcohols featuring high atom-economy. Catalytic hydrosilylation, which can be carried out under mild experimental conditions, is often used as a convenient alternative to hydrogenation reaction and is advantageous over those performed using the traditional and harsh reducing agents.1 Alkoxysilanes function as useful synthetic intermediates, used for the synthesis of silicon-containing polymers, ceramic materials and thus produced in both small as well as large scales.2 A number of transition metal complexes were reported to catalyze the hydrosilylation of carbonyl motifs.3–11 However, known catalysts for hydrosilylation reactions are highly reactive and they invariably reduce a range of different functionalities. Thus, a synthetic method that can differentiate and catalyze a selective hydrosilylation of aldehydes using a simple, inexpensive catalyst would be valuable and have potential synthetic applications.12,13 Here, we report a chemoselective hydrosilylation of aldehydes under mild and neutral reaction conditions catalyzed by commercially available [(Ru( p-cymene)Cl2)2] 1. A detailed investigation and optimization studies using the common ruthenium complexes and silanes revealed [Ru( p-cymene)Cl2]2 1 and triethylsilane as suitable catalyst and silane, respectively, for hydrosilylation of aldehydes.14 Interestingly, the reaction School of Chemical Sciences, National Institute of Science Education and Research (NISER), Bhubaneswar 751005, India. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental procedure, spectral data for intermediate complexes 2, 3 and silyl ethers and X-ray data for 2 and 3. CCDC 932852 (2) and 971350 (3). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc47593j
888 | Chem. Commun., 2014, 50, 888--890
can be carried out under neat conditions or in toluene and benzyl(triethylsilyl)ether could be obtained in 95% yield from benzaldehyde using 1 mol% of 1 (Table 1, entry 1, see also Tables S1–S3, ESI†). The substrate scope of hydrosilylation of an assortment of aldehydes using [Ru( p-cymene)Cl2]2 1 is investigated and summarized in Table 1. Silyl ether products are obtained in good yields using a range of substituents on aldehydes. Notably, aldehydes containing electron-releasing groups (entries 2–4) and electron withdrawing groups (entries 6–9), and both (entries 10–12) are tolerated and underwent facile hydrosilylation reactions. The heteroaromatic aldehydes such as furfuraldehyde and 2-thenaldehyde underwent hydrosilylation reactions under neat conditions (entries 13 and 14). The dinuclear [Ru( p-cymene)Cl2]2 1 also catalyzed the hydrosilylation of aliphatic aldehydes at room temperature and under neat conditions providing silyl ether products in good yields (entries 16–19). Furthermore, reaction of benzaldehyde with triethylsilane catalyzed by 1 was carried out in the presence of a stoichiometrically equivalent amount of acetophenone and diphenyl carbonate (Scheme 1a). In both competition experiments hydrosilylation of benzaldehyde was alone observed and benzyl triethylsilyl ether was isolated in 92% and 94% yields, respectively. Both acetophenone and diphenylcarbonate were quantitatively recovered. Similar chemoselectivity is observed in hydrosilylation of substrates containing aldehydes and a range of other functional groups (Scheme 1b).14 No hydrosilylation of ester or amide functionalities was observed under these conditions.11 To understand the reaction mechanism of this fundamental and interesting catalytic transformation, reaction of benzaldehyde with triethylsilane catalysed by 1 was monitored by 1H NMR, which indicated a first order kinetics (see, Fig. S1, ESI†) and formation of metal hydride intermediates during the catalysis. A sharp singlet was observed at d 10.18 ppm within 5 min of heating the reaction mixture. Over the time, the intensity of this signal increased and then began to decrease after 30 min upon appearance of another singlet at d 13.53 ppm (see, Fig. S2, ESI†). Both of these singlet signals remained in the spectra recorded further throughout the reaction. Stoichiometric reactions were performed in order to
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Communication Table 1
Hydrosilylation of aldehydes catalyzed by [Ru( p-cymene)Cl2]2a
Time (h)
Yieldb (%)
1c
3.5
95
2c
4.5
89
3
4.5
84
4
12
74
5
6
90
6c
3.5
74
Entry
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Silyl ethers
Scheme 1 Chemoselective hydrosilylation of aldehydes catalyzed by [Ru( p-cymene)Cl2]2 1.
Scheme 2 Synthesis and X-ray structure of [{(Z6-p-cymene)RuCl}2(m-H-m-Cl)] 2. ORTEP diagram of 2 is drawn with 30% probability.
7c
4.5
93
8
9
54
9
9
86
10
7
75
11
4.5
85
12
12
74
13c
4.5
80
14c
15
53
15
15
77
16d
12
70
17c,d
12
63
18c,d
9
70
19c,d
12
82
a Catalyst [Ru( p-cymene)Cl2]2 (1 mol%), aldehyde (1 mmol), triethylsilane (1.3 mmol) and toluene (2 mL) were heated at 50 1C under an argon atmosphere. b Yields of isolated products obtained after column chromatography. c Reactions carried out under neat conditions. d Reactions carried out at 25 1C. Analyses of reaction mixtures (1H NMR, TLC) indicated the presence of unreacted aldehydes and products and no other side product is observed.
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identify these metal hydride intermediates involved in the hydrosilylation of aldehydes. When [Ru( p-cymene)Cl2]2 1 was reacted with triethylsilane in benzene-d6 at room temperature, a monohydrido bridged complex [{(Z6-p-cymene)RuCl}2(m-H-m-Cl)] 2 was obtained in quantitative yield (Scheme 2). The ‘bridged hydride’ in complex 2 displayed a singlet in the 1H NMR spectrum, which is attributable to the metal hydride signal observed at d 10.18 ppm during the catalysis. Suitable crystals were obtained from a toluene solution of 2 and the structure was unequivocally corroborated by single crystal X-ray analysis (Scheme 2). When a toluene solution of benzaldehyde, triethylsilane and 2 (1 mol%) was heated for 3 h the corresponding benzyl triethylsilyl ether was obtained in 95% yield, confirming the potential intermediacy of 2 in the reactions.15 Further, the metal hydride intermediate 3 that resonates a singlet at d 13.53 ppm was also independently prepared from both 1 and 2 and characterized as a mononuclear Ru(IV) dihydride complex [(Z6-p-cymene)Ru(H)2(SiEt3)2] (Scheme 3).14,16 The structure of 3 was also further confirmed by X-ray analysis of the single crystals obtained from a hexane solution of 3. Isolated complex 3 also exhibited catalysis, indicating it’s possible intermediacy in the catalytic cycle.17 On the basis of these observed data, a catalytic cycle for the hydrosilylation of aldehydes with silanes is proposed (Scheme 3). Either direct oxidative addition of triethylsilane18 on dinuclear Ru(II) 2 or reductive elimination of triethylsilane from intermediate 3 in the presence of aldehydes could produce the transient intermediate I, which undergoes 1,3-hydride migration from the metal centre to the carbonyl carbon leading to the reduction and concomitant formation of Ru(II) alkoxo intermediate II. Further oxidative addition of triethylsilane generates the Ru(IV) intermediate III. Coupling of silyl and alkoxo ligands on III by reductive elimination creates O–Si bonds and liberates silyl ethers providing Ru(II)
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6 7 Scheme 3 X-ray structure of 3 (thermal ellipsoids drawn at 30% probability) and the proposed catalytic cycle for the hydrosilylation of aldehydes catalyzed by [Ru(p-cymene)Cl2]2 1.
intermediate IV. The coordination of aldehydes to IV regenerates I and closes the catalytic cycle. In conclusion, we have demonstrated a facile transformation for hydrosilylation of aldehydes using a commercially available dinuclear ruthenium complex [Ru( p-cymene)Cl2]2 1 as a catalyst. Using this novel catalytic method chemoselective hydrosilylation of aldehydes in the presence of other functionalities is also achieved. Potential intermediates [{(Z6-p-cymene)RuCl}2(m-H-m-Cl)] 2 and [(Z6-p-cymene)Ru(H)2(SiEt3)2] 3 involved in the reaction are identified and independently synthesized and characterized. The structures of 2 and 3 are solved by single crystal X-ray analyses. The proposed mechanism involves Ru(II)–Ru(IV) cycles and a 1,3-hydride migration that reduce the aldehyde at the metal centre. We thank the Department of Science and Technology, New Delhi, and NISER for financial support. B.C. is grateful to UGC for a research fellowship. C.G. is a Ramanujan Fellow.
8 9
10
11
12
13
Notes and references 1 For general reviews on hydrosilylation, see: (a) Hydrosilylation: A Comprehensive Review on Recent Advances, ed. B. Marciniec, Springer, Heidelberg, 2009; (b) O. Riant, N. Mostefaı¨ and J. Courmarcel, Synthesis, 2004, 2943–2958. 2 The Chemistry of Organic Silicon Compounds, ed. Z. Rappoport and Y. Apeloig, Wiley, New York, 2001, vol. 3. 3 (a) B. T. Gregg and A. R. Cutler, J. Am. Chem. Soc., 1996, 118, 10069–10084; (b) M. D. Cavanaugh, B. T. Gregg and A. R. Cutler, Organometallics, 1996, 15, 2764–2769; (c) P. K. Hanna, B. T. Gregg and A. R. Cutler, Organometallics, 1991, 10, 31–33. 4 For iron catalyzed hydrosilylation reactions, see: (a) H. Li, L. C. M. Castro, J. Zheng, T. Roisnel, V. Dorcet, J.-B. Sortais and C. Darcel, Angew. Chem., Int. Ed., 2013, 52, 8045–8049; (b) D. Bøzier, G. T. Venkanna, L. C. Misal Castro, J. Zheng, T. Roisnel, J.-B. Sortais and
890 | Chem. Commun., 2014, 50, 888--890
14 15 16 17
18
C. Darcel, Adv. Synth. Catal., 2012, 354, 1879–1884; (c) J. Zheng, L. C. Misal Castro, T. Roisnel, C. Darcel and J. B. Sortais, Inorg. Chim. Acta, 2012, 380, 301–307; (d) L. C. Misal Castro, D. Bøzier, J. B. Sortais and C. Darcel, Adv. Synth. Catal., 2011, 353, 1279–1284; (e) B. Bhattacharya, J. A. Krause and H. Guan, Organometallics, 2011, 30, 4720–4729; ( f ) J. Yang and T. Don Tilley, Angew. Chem., Int. Ed., 2010, 49, 10186–10188; (g) N. S. Shaikh, K. Junge and M. Beller, Org. Lett., 2007, 9, 5429–5432; for reviews, see: (h) M. Zhang and A. Zhang, Appl. Organomet. Chem., 2010, 24, 751–757; (i) K. Junge, K. Schrçder and M. Beller, Chem. Commun., 2011, 47, 4849–4859; ( j) F. Jiang, D. Bezier, J. B. Sortais and C. Darcel, Adv. Synth. Catal., 2011, 353, 239–244; (k) R. H. Morris, Chem. Soc. Rev., 2009, 38, 2282–2291. For nickel catalyzed hydrosilylation reactions, see: (a) J. Zheng, C. Darcel and J.-B. Sortais, Catal. Sci. Technol., 2013, 3, 81–84; (b) L. Postigo and B. Royo, Adv. Synth. Catal., 2012, 354, 2613–2618; (c) L. P. Bheeter, M. Henrion, L. Brelot, C. Darcel, M. J. Chetcuti, J. Sortais and V. Ritleng, Adv. Synth. Catal., 2012, 354, 2619–2624; (d) S. Kundu, W. W. Brennessel and W. D. Jones, Inorg. Chem., 2011, 50, 9443–9453; (e) S. Chakraborty, J. A. Krause and H. Guan, Organometallics, 2009, 28, 582–586; ( f ) B. L. Tran, M. Pink and D. J. Mindiola, Organometallics, 2009, 28, 2234–2243; (g) F.-G. Fontaine, R.-V. Nguyen and D. Zargarian, Can. J. Chem., 2003, 81, 1299–1306; (h) P. Boudjouk, B.-H. Han, J. R. Jacobsen and B. J. Hauck, J. Chem. Soc., Chem. Commun., 1991, 1424–1425; (i) P. Boudjouk, S.-B. Choi, B. J. Hauck and A. B. Rajkumar, Tetrahedron Lett., 1998, 39, 3951–3952. M. Lakshmi kantam, S. Laha, J. Yadav, P. R. Likhar, B. Sreedhar, S. Jha, S. Bhargava, M. Udayakiran and M. Jagadeesh, Org. Lett., 2008, 10, 2979–2982. (a) E. Peterson, A. Y. Khalimon, R. Simionescu, L. G. Kuzmina, J. A. K. Howard and G. I. Nikonov, J. Am. Chem. Soc., 2009, 131, 908–909; ˜o and B. Royo, Chem. (b) A. C. Fernandes, R. Fernandes, C. C. Roma Commun., 2005, 213–214. V. K. Dioumaev and R. M. Bullock, Nature, 2000, 424, 530–532. (a) J. J. K. Smith, K. A. Nolin, H. P. Gunterman and F. Dean Toste, J. Am. Chem. Soc., 2003, 125, 4056–4057; (b) K. A. Nolin, J. R. Krumper, M. D. Pluth, R. G. Bergman and F. D. Toste, J. Am. Chem. Soc., 2007, 129, 14684–14696; (c) G. Du, P. E. Fanwick and M. M. Abu-omar, J. Am. Chem. Soc., 2007, 129, 5180–5187; (d) H. Dong and H. Berke, Adv. Synth. Catal., 2009, 351, 1783–1788; (e) C. K. Toh, Y. N. Sum, W. K. Fong, S. G. Ang and W. Y. Fan, Organometallics, 2012, 31, 3880–3887. (a) C. Cheng and M. Brookhart, J. Am. Chem. Soc., 2012, 134, 11304–11307; (b) C. Cheng and M. Brookhart, Angew. Chem., Int. Ed., 2012, 51, 9422–9424; (c) S. Park and M. Brookhart, Organometallics, 2010, 29, 6057–6064. (a) Hydrosilylation of amides: B. Li, J.-B. Sortais and C. Darcel, Chem. Commun., 2013, 49, 3691–3693; (b) J. T. Reeves, Z. Tan, M. A. Marsini, Z. S. Han, Y. Xu, D. C. Reeves, H. Lee, B. Z. Lu and C. H. Senanayake, Adv. Synth. Catal., 2013, 355, 47–52; (c) Hydrosilylation of esters and amides: K. Matsubara, T. Iura, T. Maki and H. Nagashima, J. Org. Chem., 2002, 67, 4985–4988; (d) Hydrosilylation of esters: (a) ref. 4a; (b) M. Igarashi and T. Fuchikami, Tetrahedron Lett., 2001, 42, 2149–2151. As far as we know, such a chemoselective process is currently limited to only one reported example of dinuclear ruthenium carbonyl complex, which catalyzed the reaction under photolytic conditions. See: Y. Do, J. Han, Y. H. Rhee and J. Park, Adv. Synth. Catal., 2011, 353, 3363–3366. For catalytic hydrosilylation of aldehydes, see: (a) for Fe: Ref. 4f ; (b) for Au: D. Lantos, M. Contel, S. Sanz, A. Bodor and I. T. Horvath, J. Organomet. Chem., 2007, 692, 1799–1805; (c) for Ag: B. M. Wile and M. Stradiotto, Chem. Commun., 2006, 4104–4106; (d) Grubbs’ 1st generation catalyst is reported to catalyse the hydrosilylation of aldehydes and ketones. See: S. V. Maifeld, R. L. Miller and D. Lee, Tetrahedron Lett., 2002, 43, 6363–6366. See ESI†. The intermediate 2 could also be isolated from the reaction mixture upon completion of the reaction, which displayed identical spectral data to that of prepared complex 2. A similar Ru(IV) complex containing trimethyl silyl ligands is reported. See: P. I. Djurovich, P. J. Carroll and D. H. Berry, Organometallics, 1994, 13, 2551–2553. Similar to 3, another intermediate complex containing bulkier triphenylsilane was also observed in situ by 1H NMR. The mononuclear metal dihydride with triphenylsilane ligands resonated a characteristic hydride signal at 11.71 ppm. ¨hler, M. Hofmann and J. Weis, T. Tuttle, D. Wang, W. Thiel, J. Ko Dalton Trans., 2009, 5894–5901.
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Ruthenium catalyzed selective hydrosilylation of aldehydes† Basujit Chatterjee and Chidambaram Gunanathan*
Received 3rd October 2013, Accepted 15th November 2013 DOI: 10.1039/c3cc47593j www.rsc.org/chemcomm
A chemoselective hydrosilylation method for aldehydes is developed using a ruthenium catalyst [(Ru( p-cymene)Cl2)2] and triethylsilane; a mono hydride bridged dinuclear complex [{(g6-p-cymene)RuCl}2(l-H-l-Cl)] and a Ru(IV) mononuclear dihydride complex [(g6-pcymene)Ru(H)2(SiEt3)2] are identified as potential intermediates in the reaction and the proposed catalytic cycle involves a 1,3-hydride migration.
Hydrosilylation of carbonyl compounds is a valuable transformation in chemical synthesis as a single step operation serves on both reduction of carbonyl motifs and protection of resulting alcohols featuring high atom-economy. Catalytic hydrosilylation, which can be carried out under mild experimental conditions, is often used as a convenient alternative to hydrogenation reaction and is advantageous over those performed using the traditional and harsh reducing agents.1 Alkoxysilanes function as useful synthetic intermediates, used for the synthesis of silicon-containing polymers, ceramic materials and thus produced in both small as well as large scales.2 A number of transition metal complexes were reported to catalyze the hydrosilylation of carbonyl motifs.3–11 However, known catalysts for hydrosilylation reactions are highly reactive and they invariably reduce a range of different functionalities. Thus, a synthetic method that can differentiate and catalyze a selective hydrosilylation of aldehydes using a simple, inexpensive catalyst would be valuable and have potential synthetic applications.12,13 Here, we report a chemoselective hydrosilylation of aldehydes under mild and neutral reaction conditions catalyzed by commercially available [(Ru( p-cymene)Cl2)2] 1. A detailed investigation and optimization studies using the common ruthenium complexes and silanes revealed [Ru( p-cymene)Cl2]2 1 and triethylsilane as suitable catalyst and silane, respectively, for hydrosilylation of aldehydes.14 Interestingly, the reaction School of Chemical Sciences, National Institute of Science Education and Research (NISER), Bhubaneswar 751005, India. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental procedure, spectral data for intermediate complexes 2, 3 and silyl ethers and X-ray data for 2 and 3. CCDC 932852 (2) and 971350 (3). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc47593j
888 | Chem. Commun., 2014, 50, 888--890
can be carried out under neat conditions or in toluene and benzyl(triethylsilyl)ether could be obtained in 95% yield from benzaldehyde using 1 mol% of 1 (Table 1, entry 1, see also Tables S1–S3, ESI†). The substrate scope of hydrosilylation of an assortment of aldehydes using [Ru( p-cymene)Cl2]2 1 is investigated and summarized in Table 1. Silyl ether products are obtained in good yields using a range of substituents on aldehydes. Notably, aldehydes containing electron-releasing groups (entries 2–4) and electron withdrawing groups (entries 6–9), and both (entries 10–12) are tolerated and underwent facile hydrosilylation reactions. The heteroaromatic aldehydes such as furfuraldehyde and 2-thenaldehyde underwent hydrosilylation reactions under neat conditions (entries 13 and 14). The dinuclear [Ru( p-cymene)Cl2]2 1 also catalyzed the hydrosilylation of aliphatic aldehydes at room temperature and under neat conditions providing silyl ether products in good yields (entries 16–19). Furthermore, reaction of benzaldehyde with triethylsilane catalyzed by 1 was carried out in the presence of a stoichiometrically equivalent amount of acetophenone and diphenyl carbonate (Scheme 1a). In both competition experiments hydrosilylation of benzaldehyde was alone observed and benzyl triethylsilyl ether was isolated in 92% and 94% yields, respectively. Both acetophenone and diphenylcarbonate were quantitatively recovered. Similar chemoselectivity is observed in hydrosilylation of substrates containing aldehydes and a range of other functional groups (Scheme 1b).14 No hydrosilylation of ester or amide functionalities was observed under these conditions.11 To understand the reaction mechanism of this fundamental and interesting catalytic transformation, reaction of benzaldehyde with triethylsilane catalysed by 1 was monitored by 1H NMR, which indicated a first order kinetics (see, Fig. S1, ESI†) and formation of metal hydride intermediates during the catalysis. A sharp singlet was observed at d 10.18 ppm within 5 min of heating the reaction mixture. Over the time, the intensity of this signal increased and then began to decrease after 30 min upon appearance of another singlet at d 13.53 ppm (see, Fig. S2, ESI†). Both of these singlet signals remained in the spectra recorded further throughout the reaction. Stoichiometric reactions were performed in order to
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Communication Table 1
Hydrosilylation of aldehydes catalyzed by [Ru( p-cymene)Cl2]2a
Time (h)
Yieldb (%)
1c
3.5
95
2c
4.5
89
3
4.5
84
4
12
74
5
6
90
6c
3.5
74
Entry
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Silyl ethers
Scheme 1 Chemoselective hydrosilylation of aldehydes catalyzed by [Ru( p-cymene)Cl2]2 1.
Scheme 2 Synthesis and X-ray structure of [{(Z6-p-cymene)RuCl}2(m-H-m-Cl)] 2. ORTEP diagram of 2 is drawn with 30% probability.
7c
4.5
93
8
9
54
9
9
86
10
7
75
11
4.5
85
12
12
74
13c
4.5
80
14c
15
53
15
15
77
16d
12
70
17c,d
12
63
18c,d
9
70
19c,d
12
82
a Catalyst [Ru( p-cymene)Cl2]2 (1 mol%), aldehyde (1 mmol), triethylsilane (1.3 mmol) and toluene (2 mL) were heated at 50 1C under an argon atmosphere. b Yields of isolated products obtained after column chromatography. c Reactions carried out under neat conditions. d Reactions carried out at 25 1C. Analyses of reaction mixtures (1H NMR, TLC) indicated the presence of unreacted aldehydes and products and no other side product is observed.
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identify these metal hydride intermediates involved in the hydrosilylation of aldehydes. When [Ru( p-cymene)Cl2]2 1 was reacted with triethylsilane in benzene-d6 at room temperature, a monohydrido bridged complex [{(Z6-p-cymene)RuCl}2(m-H-m-Cl)] 2 was obtained in quantitative yield (Scheme 2). The ‘bridged hydride’ in complex 2 displayed a singlet in the 1H NMR spectrum, which is attributable to the metal hydride signal observed at d 10.18 ppm during the catalysis. Suitable crystals were obtained from a toluene solution of 2 and the structure was unequivocally corroborated by single crystal X-ray analysis (Scheme 2). When a toluene solution of benzaldehyde, triethylsilane and 2 (1 mol%) was heated for 3 h the corresponding benzyl triethylsilyl ether was obtained in 95% yield, confirming the potential intermediacy of 2 in the reactions.15 Further, the metal hydride intermediate 3 that resonates a singlet at d 13.53 ppm was also independently prepared from both 1 and 2 and characterized as a mononuclear Ru(IV) dihydride complex [(Z6-p-cymene)Ru(H)2(SiEt3)2] (Scheme 3).14,16 The structure of 3 was also further confirmed by X-ray analysis of the single crystals obtained from a hexane solution of 3. Isolated complex 3 also exhibited catalysis, indicating it’s possible intermediacy in the catalytic cycle.17 On the basis of these observed data, a catalytic cycle for the hydrosilylation of aldehydes with silanes is proposed (Scheme 3). Either direct oxidative addition of triethylsilane18 on dinuclear Ru(II) 2 or reductive elimination of triethylsilane from intermediate 3 in the presence of aldehydes could produce the transient intermediate I, which undergoes 1,3-hydride migration from the metal centre to the carbonyl carbon leading to the reduction and concomitant formation of Ru(II) alkoxo intermediate II. Further oxidative addition of triethylsilane generates the Ru(IV) intermediate III. Coupling of silyl and alkoxo ligands on III by reductive elimination creates O–Si bonds and liberates silyl ethers providing Ru(II)
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5
6 7 Scheme 3 X-ray structure of 3 (thermal ellipsoids drawn at 30% probability) and the proposed catalytic cycle for the hydrosilylation of aldehydes catalyzed by [Ru(p-cymene)Cl2]2 1.
intermediate IV. The coordination of aldehydes to IV regenerates I and closes the catalytic cycle. In conclusion, we have demonstrated a facile transformation for hydrosilylation of aldehydes using a commercially available dinuclear ruthenium complex [Ru( p-cymene)Cl2]2 1 as a catalyst. Using this novel catalytic method chemoselective hydrosilylation of aldehydes in the presence of other functionalities is also achieved. Potential intermediates [{(Z6-p-cymene)RuCl}2(m-H-m-Cl)] 2 and [(Z6-p-cymene)Ru(H)2(SiEt3)2] 3 involved in the reaction are identified and independently synthesized and characterized. The structures of 2 and 3 are solved by single crystal X-ray analyses. The proposed mechanism involves Ru(II)–Ru(IV) cycles and a 1,3-hydride migration that reduce the aldehyde at the metal centre. We thank the Department of Science and Technology, New Delhi, and NISER for financial support. B.C. is grateful to UGC for a research fellowship. C.G. is a Ramanujan Fellow.
8 9
10
11
12
13
Notes and references 1 For general reviews on hydrosilylation, see: (a) Hydrosilylation: A Comprehensive Review on Recent Advances, ed. B. Marciniec, Springer, Heidelberg, 2009; (b) O. Riant, N. Mostefaı¨ and J. Courmarcel, Synthesis, 2004, 2943–2958. 2 The Chemistry of Organic Silicon Compounds, ed. Z. Rappoport and Y. Apeloig, Wiley, New York, 2001, vol. 3. 3 (a) B. T. Gregg and A. R. Cutler, J. Am. Chem. Soc., 1996, 118, 10069–10084; (b) M. D. Cavanaugh, B. T. Gregg and A. R. Cutler, Organometallics, 1996, 15, 2764–2769; (c) P. K. Hanna, B. T. Gregg and A. R. Cutler, Organometallics, 1991, 10, 31–33. 4 For iron catalyzed hydrosilylation reactions, see: (a) H. Li, L. C. M. Castro, J. Zheng, T. Roisnel, V. Dorcet, J.-B. Sortais and C. Darcel, Angew. Chem., Int. Ed., 2013, 52, 8045–8049; (b) D. Bøzier, G. T. Venkanna, L. C. Misal Castro, J. Zheng, T. Roisnel, J.-B. Sortais and
890 | Chem. Commun., 2014, 50, 888--890
14 15 16 17
18
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