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Perrhenic acid as catalyst for hydrosilylation of aldehydes and ketones and dehydrogenative silylation of alcohols
Catalysis Communications 8 (2007) 1057–1059 www.elsevier.com/locate/catcom
Perrhenic acid as catalyst for hydrosilylation of aldehydes and ketones and dehydrogenative silylation of alcohols Patrı´cia M. Reis, Beatriz Royo
*
Instituto de Tecnologia Quı´mica e Biolo´gica da Universidade Nova de Lisboa, Quinta do Marqueˆs, EAN, Apartado 127, 2781-901 Oeiras, Portugal Received 19 May 2006; received in revised form 19 October 2006; accepted 19 October 2006 Available online 26 October 2006
Abstract Hydrosilylation of aldehydes and ketones with dimethylphenylsilane was catalyzed by perrhenic acid, HReO4, to give silylated ethers in good yields. These hydrosilylation reactions appear to be radical processes, since they were inhibited in the presence of 5,5-dimethyl4,5-dihydro-3H-pyrrole-N-oxide (DMPO) and Ph2NH, two well-known radical scavengers. Perrhenic acid also catalyzes the dehydrogenative silylation of alcohols in neat conditions at 25 °C. It is selective for the silylation of hydroxyl groups in the presence of alkenes and alkyl halides (RCl). Ó 2006 Elsevier B.V. All rights reserved. Keywords: Perrhenic acid; Hydrosilylation; Dehydrogenative silylation; Rhenium; Oxides
1. Introduction Hydrosilylation is an important reaction in organic synthesis, polymer chemistry, and for the production of organosilicon compounds [1]. Silyl ethers are widely used as protecting groups for the hydroxyl functionality in organic synthesis [2] and also play an important role in inorganic chemistry as precursors in the preparation of sol–gels [3]. The largest group of catalysts for reduction reactions such as hydrogenation, hydroformylations, and hydrosilylation features electron-rich, late transition metals in low oxidation states. Recently, we [4–6] and others [7–12] have reported the use of high valent rhenium and molybdenum oxo complexes as catalysts for the hydrosilylation of aldehydes, ketones and imines. This novel reactivity represents a complete reversal from the traditional role of these complexes as oxidation catalysts [13]. In view of the fact that Re2O7 is a effective catalyst for the hydrosilylation of aldehydes [6], we examined the possibility that perrhenic acid, HReO4, a cheaper rhenium precursor could catalyse this reaction with the additional advantage of no need to han*
Corresponding author. Tel.: +351 21 4469754; fax: +351 21 4411277. E-mail address: [email protected] (B. Royo).
1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.10.023
dle sensitive compounds. In addition, we explored the use of this catalyst in the alcoholysis of silanes, which is an attractive route to silyl ethers because the only-side product formed is hydrogen gas [14]. 2. Experimental 2.1. General procedure for the hydrosilylation of aldehydes and ketones and for the dehydrogenative silylation of alcohols with dimethylphenylsilane All operations were performed under air. To an aqueous solution of HReO4 (75–80%, 0.15 mmol), dimethylphenylsilane (3.6 mmol) and the appropriate substrate (3.03 mmol) were added at room temperature (when the substrates were solids, 5 mL of dichloromethane were added to the mixture). The reaction mixture was stirred at 25 °C and after the desired time, all the volatiles were removed under vacuum. The residue was diluted with hexanes (ca. 2 mL), loaded directly on to a silica gel column and chromatographed with the appropriate mixture of hexanes and diethyl ether to give the corresponding silyl ethers. The silylated ethers were characterized by IR and NMR spectroscopy.
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P.M. Reis, B. Royo / Catalysis Communications 8 (2007) 1057–1059
3. Results and discussion Perrhenic acid, HReO4, can be used as an effective catalyst for the hydrosilylation of aldehydes (Scheme 1) with dimethylphenylsilane. The catalytic reaction proceeded in high yields and very rapidly (30 min) at room temperature (Eq. (1)). The reactions were carried out without special need for inert conditions and when the substrates are liquids reactions were performed in neat conditions. For the solid substrates, the reactions were carried out in dichloromethane. Similar results were obtained when other solvents were used in the catalytic reaction, such as diethyl ether, toluene or hexane. Both aromatic and aliphatic aldehydes were converted to the corresponding silyl ethers. In some cases, the silylated ether was obtained accompanied by a substantial quantity of desilylated alcohol, rising from the acidic hydrolysis of the silyl ether. Under the conditions used, several functional groups such as bromide, trifluoromethyl, cyano, esters were well tolerated even though the yield isolated for the cyano functionality was 48% after 2 h at 25 °C. The results are summarized in Table 1. OSiMe2Ph
O R
HReO4
C H
CHO
H
H
R
ð1Þ
C
R
HSiMe2Ph
CHO
R = CF3, Br, COOMe, CN, NO2 CHO
The catalytic hydrosilylation of ketones was investigated using similar experimental procedure (Eq. 2). Perrhenic acid catalyzes the hydrosilylation of 4-tert-butylcyclohexanone and 4-phenyl-2-butanone affording a 72% (of the trans isomer) and 52% yields of the corresponding alkoxysilanes, respectively. Under similar reaction conditions, Re2O7 showed no conversion [6]. O
OSiMe2Ph HReO4 R
H
R'
The use of more sterically encumbered silanes such as Et3SiH and Ph3SiH was explored. When the reaction of 4(trifluoromethyl)benzaldehyde was performed with Et3SiH a 37% yield of triethylsilyl 4-(trifluoromethyl)benzyl ether was isolated. Reaction of 4-(trifluoromethyl)benzaldehyde with Ph3SiH did not take place. The reaction of 4-(trifluoromethyl)benzaldehyde with triethylsilane-d in the presence of a catalytic amount of HReO4 (5 mol%) at room temperature showed the incorporation of the deuterium only in the carbon of the double bond, as confirmed by 1H NMR. Perrhenic acid is also an efficient catalyst for the dehydrogenative silylation of alcohols (Eq. (3)). The reaction was carried out without need for inert conditions. A mixture of the appropriate alcohol and PhMe2SiH in neat conditions was stirred for 24 h at room temperature yielding quantitative conversions to the corresponding silylated ether. The catalyst showed functional group compability, results are summarized in Table 2. 9-Decen-1-ol was converted to the corresponding silyl ether without hydrosilylation of its unsaturated C–C bonds and 3-chloro-2-propanol proceeded smoothly with the C–X bonds intact (Table 2, entries 4 and 3, respectively).
O
R
HReO4
OH
R
HSiMe2Ph O
ð2Þ
HSiMe2Ph
R'
R
OSiMe2Ph + H2
ð3Þ
R = CH2-CH2-C6H5, CH2-(CH2)8-CH3, CH2-CH2-CH2Cl, CH2-(CH2)7-CH=CH2
Scheme 1.
Table 1 Hydrosilylation of aldehydes and ketones with PhMe2SiH catalyzed by HReO4a Entry
Substrate
Solvent
Time
1 2 3 4 5 6 7 8 9
Hexanal 2,2-Dimethyl-4-pentenal 4-(Trifluoromethyl)benzaldehyde 4-Bromobenzaldehyde Methyl 4-formylbenzoate 4-Nitrobenzaldehyde 4-Cyanobenzaldehyde 4-Phenyl-2-butanone 4-tert-Butylcyclohexanone
Neat Neat Neat CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 Neat CH2Cl2
30 min 30 min 30 min 30 min 30 min 30 min 2h 25 min 90 min
a
Conversionb (%)
78 21 76 16
100 89 (22) 100 (62) (12) (37) 52 72e
Yieldc (%) 94 79 86d 95 87d 96d 48d 43 65
Reaction conditions: substrate:HSiMe2Ph = 1:1.2 with 5 mol% HReO4 (relative to substrate) at 25 °C. Conversions determined by 1H NMR spectroscopy; in some cases the silyl ether is accompanied by desilylated alcohol, the yield is indicated in parenthesis. c Isolated yields. d Isolated yields of the mixture of silylated and desilylated ethers. e Only trans isomer formed. b
P.M. Reis, B. Royo / Catalysis Communications 8 (2007) 1057–1059
1059
Table 2 Dehydrogenative silylation of alcohols with PhMe2SiH catalyzed by HReO4a Entry
Substrate
1
2-Phenylethanol
Product
Conversionb (%)
Yieldc (%)
100
96
100 100 100
94 93 95
OSiMe2Ph
2 3 4
1-Decanol 3-Chloro-2-propanol 9-Decen-1-ol a b c
CH3(CH2)8ACH2OSiMe2 Ph ClCH2ACH2ACH2A OSiMe2Ph CH2@CHA(CH2)7ACH2 OSiMe2Ph
Reaction conditions: catalyst (0.15 mmol), substrate (3.03 mmol) and PhMe2SiH (3.60 mmol) in neat conditions, 24 h at 25 °C. Conversions determined by 1H NMR spectroscopy. Isolated yields.
An insight into the catalytic mechanism of the hydrosilylation reaction came from radical trapping experiments, which implicated the involvement of oxygen-centered radicals in the aldehyde hydrosilylation system. Addition of Ph2NH (1:1 ratio of substrate:Ph2NH), an efficient scavenger of oxygen-centered radicals, inhibited the reaction for 2 h, after which catalysis resumed to give the silylated ether. Furthermore, addition of 5,5-dimethyl-4,5dihydro-3H-pyrrole-N-oxide (DMPO) completely shut down silylated ether formation. This provides evidence that a radical mechanism could be involved in the catalytic reaction. Similar results have been observed when MoO2Cl2 was used as catalysts for the hydrosilylation reaction of aldehydes [5]. 4. Conclusions As a conclusion, HReO4 is a very efficient catalyst for the hydrosilylation of aldehydes and ketones and also for the dehydrogenative silylation of alcohols. The following positive advantages: (a) ease of operation without need to handle sensitive compounds; (b) use of the cheap commercially available HReO4; (c) excellent yields of the corresponding silylated ethers at room temperature and in some cases in neat conditions. Radical scavenging experiments strongly suggest the presence of oxygen-centered radicals.
Acknowledgement This work was supported by FCT through Project POCI/QUI/55586/2004. P.M. Reis thanks FCT for a postdoctoral grant. References [1] G.W. Parshall, S.D. Itell, Homogeneous Catalysts: The Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes, Wiley, New York, 1992. [2] P.J. Kocienski, Protecting Groups, Springer, Stuttgart, 1994, p. 28. [3] U. Schubert, C. Lorenzt, Chem. Ber. 128 (1995) 1267. [4] A.C. Fernandes, R. Fernandes, C.C. Roma˜o, B. Royo, Chem. Commun. (2005) 213. [5] P.M. Reis, C.C. Roma˜o, B. Royo, Dalton Trans. (2006) 1842. [6] B. Royo, C.C. Roma˜o, J. Mol. Catal. A 236 (2005) 107. [7] J.J. Kennedy-Smith, K.A. Nolin, H.P. Gunterman, F.D. Toste, J. Am. Chem. Soc. 125 (2003) 4056. [8] K.A. Nolin, R.W. Ahn, F.D. Toste, J. Am. Chem. Soc. 127 (2005) 12462. [9] E.A. Ison, E.R. Trivedi, R.A. Corbin, M.M. Abu-Omar, J. Am. Chem. Soc. 127 (2005) 15374. [10] G. Du, M.M. Abu-Omar, Organometallics 25 (2006) 4920. [11] E.A. Ison, J.E. Cessarich, G. Du, P.E. Fanwick, M.M. Abu-Omar, Inorg. Chem. 45 (2006) 2385. [12] A.C. Fernandes, C.C. Roma˜o, Tetrahedron Lett. 46 (2005) 8881. [13] W.R. Thiel, Angew. Chem., Int. Ed. 42 (2003) 5390. [14] L.D. Field, B.A. Messerle, M. Rehr, L.P. Soler, T.W. Hambley, Organometallics 22 (2003) 2387.
Perrhenic acid as catalyst for hydrosilylation of aldehydes and ketones and dehydrogenative silylation of alcohols Patrı´cia M. Reis, Beatriz Royo
*
Instituto de Tecnologia Quı´mica e Biolo´gica da Universidade Nova de Lisboa, Quinta do Marqueˆs, EAN, Apartado 127, 2781-901 Oeiras, Portugal Received 19 May 2006; received in revised form 19 October 2006; accepted 19 October 2006 Available online 26 October 2006
Abstract Hydrosilylation of aldehydes and ketones with dimethylphenylsilane was catalyzed by perrhenic acid, HReO4, to give silylated ethers in good yields. These hydrosilylation reactions appear to be radical processes, since they were inhibited in the presence of 5,5-dimethyl4,5-dihydro-3H-pyrrole-N-oxide (DMPO) and Ph2NH, two well-known radical scavengers. Perrhenic acid also catalyzes the dehydrogenative silylation of alcohols in neat conditions at 25 °C. It is selective for the silylation of hydroxyl groups in the presence of alkenes and alkyl halides (RCl). Ó 2006 Elsevier B.V. All rights reserved. Keywords: Perrhenic acid; Hydrosilylation; Dehydrogenative silylation; Rhenium; Oxides
1. Introduction Hydrosilylation is an important reaction in organic synthesis, polymer chemistry, and for the production of organosilicon compounds [1]. Silyl ethers are widely used as protecting groups for the hydroxyl functionality in organic synthesis [2] and also play an important role in inorganic chemistry as precursors in the preparation of sol–gels [3]. The largest group of catalysts for reduction reactions such as hydrogenation, hydroformylations, and hydrosilylation features electron-rich, late transition metals in low oxidation states. Recently, we [4–6] and others [7–12] have reported the use of high valent rhenium and molybdenum oxo complexes as catalysts for the hydrosilylation of aldehydes, ketones and imines. This novel reactivity represents a complete reversal from the traditional role of these complexes as oxidation catalysts [13]. In view of the fact that Re2O7 is a effective catalyst for the hydrosilylation of aldehydes [6], we examined the possibility that perrhenic acid, HReO4, a cheaper rhenium precursor could catalyse this reaction with the additional advantage of no need to han*
Corresponding author. Tel.: +351 21 4469754; fax: +351 21 4411277. E-mail address: [email protected] (B. Royo).
1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.10.023
dle sensitive compounds. In addition, we explored the use of this catalyst in the alcoholysis of silanes, which is an attractive route to silyl ethers because the only-side product formed is hydrogen gas [14]. 2. Experimental 2.1. General procedure for the hydrosilylation of aldehydes and ketones and for the dehydrogenative silylation of alcohols with dimethylphenylsilane All operations were performed under air. To an aqueous solution of HReO4 (75–80%, 0.15 mmol), dimethylphenylsilane (3.6 mmol) and the appropriate substrate (3.03 mmol) were added at room temperature (when the substrates were solids, 5 mL of dichloromethane were added to the mixture). The reaction mixture was stirred at 25 °C and after the desired time, all the volatiles were removed under vacuum. The residue was diluted with hexanes (ca. 2 mL), loaded directly on to a silica gel column and chromatographed with the appropriate mixture of hexanes and diethyl ether to give the corresponding silyl ethers. The silylated ethers were characterized by IR and NMR spectroscopy.
1058
P.M. Reis, B. Royo / Catalysis Communications 8 (2007) 1057–1059
3. Results and discussion Perrhenic acid, HReO4, can be used as an effective catalyst for the hydrosilylation of aldehydes (Scheme 1) with dimethylphenylsilane. The catalytic reaction proceeded in high yields and very rapidly (30 min) at room temperature (Eq. (1)). The reactions were carried out without special need for inert conditions and when the substrates are liquids reactions were performed in neat conditions. For the solid substrates, the reactions were carried out in dichloromethane. Similar results were obtained when other solvents were used in the catalytic reaction, such as diethyl ether, toluene or hexane. Both aromatic and aliphatic aldehydes were converted to the corresponding silyl ethers. In some cases, the silylated ether was obtained accompanied by a substantial quantity of desilylated alcohol, rising from the acidic hydrolysis of the silyl ether. Under the conditions used, several functional groups such as bromide, trifluoromethyl, cyano, esters were well tolerated even though the yield isolated for the cyano functionality was 48% after 2 h at 25 °C. The results are summarized in Table 1. OSiMe2Ph
O R
HReO4
C H
CHO
H
H
R
ð1Þ
C
R
HSiMe2Ph
CHO
R = CF3, Br, COOMe, CN, NO2 CHO
The catalytic hydrosilylation of ketones was investigated using similar experimental procedure (Eq. 2). Perrhenic acid catalyzes the hydrosilylation of 4-tert-butylcyclohexanone and 4-phenyl-2-butanone affording a 72% (of the trans isomer) and 52% yields of the corresponding alkoxysilanes, respectively. Under similar reaction conditions, Re2O7 showed no conversion [6]. O
OSiMe2Ph HReO4 R
H
R'
The use of more sterically encumbered silanes such as Et3SiH and Ph3SiH was explored. When the reaction of 4(trifluoromethyl)benzaldehyde was performed with Et3SiH a 37% yield of triethylsilyl 4-(trifluoromethyl)benzyl ether was isolated. Reaction of 4-(trifluoromethyl)benzaldehyde with Ph3SiH did not take place. The reaction of 4-(trifluoromethyl)benzaldehyde with triethylsilane-d in the presence of a catalytic amount of HReO4 (5 mol%) at room temperature showed the incorporation of the deuterium only in the carbon of the double bond, as confirmed by 1H NMR. Perrhenic acid is also an efficient catalyst for the dehydrogenative silylation of alcohols (Eq. (3)). The reaction was carried out without need for inert conditions. A mixture of the appropriate alcohol and PhMe2SiH in neat conditions was stirred for 24 h at room temperature yielding quantitative conversions to the corresponding silylated ether. The catalyst showed functional group compability, results are summarized in Table 2. 9-Decen-1-ol was converted to the corresponding silyl ether without hydrosilylation of its unsaturated C–C bonds and 3-chloro-2-propanol proceeded smoothly with the C–X bonds intact (Table 2, entries 4 and 3, respectively).
O
R
HReO4
OH
R
HSiMe2Ph O
ð2Þ
HSiMe2Ph
R'
R
OSiMe2Ph + H2
ð3Þ
R = CH2-CH2-C6H5, CH2-(CH2)8-CH3, CH2-CH2-CH2Cl, CH2-(CH2)7-CH=CH2
Scheme 1.
Table 1 Hydrosilylation of aldehydes and ketones with PhMe2SiH catalyzed by HReO4a Entry
Substrate
Solvent
Time
1 2 3 4 5 6 7 8 9
Hexanal 2,2-Dimethyl-4-pentenal 4-(Trifluoromethyl)benzaldehyde 4-Bromobenzaldehyde Methyl 4-formylbenzoate 4-Nitrobenzaldehyde 4-Cyanobenzaldehyde 4-Phenyl-2-butanone 4-tert-Butylcyclohexanone
Neat Neat Neat CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 Neat CH2Cl2
30 min 30 min 30 min 30 min 30 min 30 min 2h 25 min 90 min
a
Conversionb (%)
78 21 76 16
100 89 (22) 100 (62) (12) (37) 52 72e
Yieldc (%) 94 79 86d 95 87d 96d 48d 43 65
Reaction conditions: substrate:HSiMe2Ph = 1:1.2 with 5 mol% HReO4 (relative to substrate) at 25 °C. Conversions determined by 1H NMR spectroscopy; in some cases the silyl ether is accompanied by desilylated alcohol, the yield is indicated in parenthesis. c Isolated yields. d Isolated yields of the mixture of silylated and desilylated ethers. e Only trans isomer formed. b
P.M. Reis, B. Royo / Catalysis Communications 8 (2007) 1057–1059
1059
Table 2 Dehydrogenative silylation of alcohols with PhMe2SiH catalyzed by HReO4a Entry
Substrate
1
2-Phenylethanol
Product
Conversionb (%)
Yieldc (%)
100
96
100 100 100
94 93 95
OSiMe2Ph
2 3 4
1-Decanol 3-Chloro-2-propanol 9-Decen-1-ol a b c
CH3(CH2)8ACH2OSiMe2 Ph ClCH2ACH2ACH2A OSiMe2Ph CH2@CHA(CH2)7ACH2 OSiMe2Ph
Reaction conditions: catalyst (0.15 mmol), substrate (3.03 mmol) and PhMe2SiH (3.60 mmol) in neat conditions, 24 h at 25 °C. Conversions determined by 1H NMR spectroscopy. Isolated yields.
An insight into the catalytic mechanism of the hydrosilylation reaction came from radical trapping experiments, which implicated the involvement of oxygen-centered radicals in the aldehyde hydrosilylation system. Addition of Ph2NH (1:1 ratio of substrate:Ph2NH), an efficient scavenger of oxygen-centered radicals, inhibited the reaction for 2 h, after which catalysis resumed to give the silylated ether. Furthermore, addition of 5,5-dimethyl-4,5dihydro-3H-pyrrole-N-oxide (DMPO) completely shut down silylated ether formation. This provides evidence that a radical mechanism could be involved in the catalytic reaction. Similar results have been observed when MoO2Cl2 was used as catalysts for the hydrosilylation reaction of aldehydes [5]. 4. Conclusions As a conclusion, HReO4 is a very efficient catalyst for the hydrosilylation of aldehydes and ketones and also for the dehydrogenative silylation of alcohols. The following positive advantages: (a) ease of operation without need to handle sensitive compounds; (b) use of the cheap commercially available HReO4; (c) excellent yields of the corresponding silylated ethers at room temperature and in some cases in neat conditions. Radical scavenging experiments strongly suggest the presence of oxygen-centered radicals.
Acknowledgement This work was supported by FCT through Project POCI/QUI/55586/2004. P.M. Reis thanks FCT for a postdoctoral grant. References [1] G.W. Parshall, S.D. Itell, Homogeneous Catalysts: The Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes, Wiley, New York, 1992. [2] P.J. Kocienski, Protecting Groups, Springer, Stuttgart, 1994, p. 28. [3] U. Schubert, C. Lorenzt, Chem. Ber. 128 (1995) 1267. [4] A.C. Fernandes, R. Fernandes, C.C. Roma˜o, B. Royo, Chem. Commun. (2005) 213. [5] P.M. Reis, C.C. Roma˜o, B. Royo, Dalton Trans. (2006) 1842. [6] B. Royo, C.C. Roma˜o, J. Mol. Catal. A 236 (2005) 107. [7] J.J. Kennedy-Smith, K.A. Nolin, H.P. Gunterman, F.D. Toste, J. Am. Chem. Soc. 125 (2003) 4056. [8] K.A. Nolin, R.W. Ahn, F.D. Toste, J. Am. Chem. Soc. 127 (2005) 12462. [9] E.A. Ison, E.R. Trivedi, R.A. Corbin, M.M. Abu-Omar, J. Am. Chem. Soc. 127 (2005) 15374. [10] G. Du, M.M. Abu-Omar, Organometallics 25 (2006) 4920. [11] E.A. Ison, J.E. Cessarich, G. Du, P.E. Fanwick, M.M. Abu-Omar, Inorg. Chem. 45 (2006) 2385. [12] A.C. Fernandes, C.C. Roma˜o, Tetrahedron Lett. 46 (2005) 8881. [13] W.R. Thiel, Angew. Chem., Int. Ed. 42 (2003) 5390. [14] L.D. Field, B.A. Messerle, M. Rehr, L.P. Soler, T.W. Hambley, Organometallics 22 (2003) 2387.