Stereospecific and regioselective catalytic epoxidation of alkenes by a novel ruthenium(II) complex under aerobic conditions

摘要

J. Chem. Soc. Perkin Trans. 1 1997 3115 Stereospecific and regioselective catalytic epoxidation of alkenes by a novel ruthenium(II) complex under aerobic conditions Venkitasamy Kesavan and Srinivasan Chandrasekaran * Department of Organic Chemistry Indian Institute of Science Bangalore-560 012 India Epoxidation of alkenes by molecular oxygen is effected in high yields by catalysis of RuCl2(biox)2 using isobutyraldehyde as the co-reductant the reaction is stereospecific and regioselective. There has been considerable interest in recent years in both the homogeneous transition metal-catalysed epoxidation of alkenes 1 and in the use of oxoruthenium complexes as catalysts for organic oxidations.2 With regard to aerobic oxidation catalysed by transition metal complexes several reactions involving the combined use of molecular oxygen with reducing agents have been reviewed.3 The combination of three components that is a transition metal organic ligands and a reductant is considered to create an effective oxygenation system for such aerobic reactions.In 1985 Groves and Quinn reported a successful aerobic epoxidation of olefins with a rutheniumndash;porphyrin catalyst.4 All the other epoxidations catalysed by ruthenium complexes involved the use of NaIO4 or PhIO as the terminal oxidants under biphasic conditions.5 Nishiyama et al.6 used dichlorobis( oxazolinyl)bipyridylruthenium(II) complex for the oxidation of alkenes in the presence of PhIO and found oxidative cleavage as the major pathway while Barf et al. reported the epoxidation of alkenes with a combination of (bipyridyl)RuCl2- (DMSO)4 and tert-butylhydroperoxide.7 C2-symmetric 4,49,5,59-tetrahydro-2,29-bisoxazoles have been used as ligands in catalytic asymmetric transfer hydrogenation of ketones.8 Prior to achieving enantioselectivity in epoxidation with ruthenium complexes it is essential to find conditions in favour of epoxidation rather than oxidative cleavage.Drago has demonstrated that in general trans-dioxoruthenium complexes catalysed oxidation of alkenes to yield epoxides while the cis-isomers mediate oxidative cleavage.9 The aim of the present work is to use bisoxazoles as bidentate ligands so that they form a square planar structure around the ruthenium core analogous to rutheniumndash;porphyrin systems.4 Accordingly rutheniumndash;bisoxazole complex 1 was synthesized 10 and our successful results of an oxo-transfer reaction with 1 are presented in this communication.In the preliminary investigations of oxidation of alkenes with 1 PhIO NaIO4 ureandash;H2O2 NaOCl and TBHP were used as terminal oxidants. In all cases the reaction was incomplete (30ndash;50 conversion) and the product was always a mixture of epoxides and cleavage products. However when the oxidation of alkenes was carried out with 1 (2.5 mol) in CH2Cl2 (25 8C 6ndash;12 h) in the presence of molecular oxygen as the oxidant and isobutyraldehyde as the co-reductant excellent yields of epoxides 2ndash;21 were obtained. The results are summarized in Table 1. N O N O Cl Cl N O N O Ru 1 As can be gauged from Table 1 this catalytic epoxidation with 1 is highly stereospecific. Thus trans-stilbene 2 and cisstilbene 4 under the reaction conditions afford the trans-stilbene oxide 3 and cis-stilbene oxide 5 respectively in high yields.Styrene epoxide 11 which is highly unstable under conditions of peracid epoxidation is quite stable under the present reaction conditions. Another salient feature of the present methodology is the high regioselectivity. Thus in the oxidation of 4-vinylcyclohexene 16 and limonene 18 catalysed by 1 the monoepoxides 17 and 19 respectively were the exclusive products isolated in very good yields. A more dramatic example of stereoselectivity is illustrated in the epoxidation of cholest-5-ene 20. Under conditions of catalytic epoxidation the 5b,6b-epoxide 21A is obtained in excess of 94 selectivity and in excellent yields. Thus we have developed a non-porphyrin ruthenium(II) system which catalyses the epoxidation of alkenes under aerobic conditions with great efficiency.The reaction is stereospecific and regioselective and this kind of high stereospecificity and regioselectivity has not been reported with other catalytic epoxidation methodologies.5ndash;7 Since C2-symmetric bisoxazoles can be easily derived from optically active amino alcohols chiral ruthenium complexes are readily accessible and studies of catalytic asymmetric epoxidation of unfunctionalized alkenes with these systems are under progress. Experimental Synthesis of complex 1 trans-Tetrakis(acetonitrile)dichlororuthenium(II) 11 (0.336 g 1 mmol) was refluxed with 4,49,5,59-tetrahydro-2,29-bisoxazole (0.308 g 2.2 mmol) for 6 h in ethanol (10 ml). The solvent was removed under reduced pressure to give a red coloured solid.The solid was recrystallized from EtOHndash;Et2O (1 3) and stored in a desiccator; mp 280 8C (uncorrected); nmax(thin film)/cm21 2950 1610; dH(90 MHz D2O) 3.4 (m 8H) 3.8 (m 8H); dC(22.5 MHz D2O) 53.61 68.87 155.26 (Calc. for C12H16Cl2- N4O4Ru C 31.86; H 3.56; N 12.38. Found C 31.81; H 3.48; N 12.40). Typical experimental procedure for epoxidation Ruthenium complex 1 (2.5 mol) was added to the alkene (1 mmol) dissolved in dichloromethane (4 ml). To this homogeneous solution NaHCO3 (1.5 equiv.) and isobutyraldehyde (1.5 equiv.) were added. The mixture was stirred under an atmosphere of oxygen at 25 8C and the reaction was monitored by TLC. Once the reaction was over the reaction mixture was diluted with CH2Cl2 and filtered through a pad of Celite and silica gel. Removal of solvent yielded the crude product which was purified by flash chromatography over neutral alumina or distillation under reduced pressure.Acknowledgements The authors thank the Department of Science and Technology New Delhi for financial support of this investigation. One of the authors V. K. thanks UGC for a research fellowship. 3116 J. Chem. Soc. Perkin Trans. 1 1997 Table 1 Epoxidation of alkenes catalysed by RuCl2(biox)2 Entry 1 Substrate Ph Ph 2 Reaction time/h 8 Product Ph Ph O 3 Yield a () 95 2 Ph Ph 4 10 Ph Ph O 5 88 3 Ph 6 9 Ph O 7 85 4 Undec-1-ene 8 12 Undec-1-ene oxide 9 100 b 5 Ph 10 8 O Ph 11 85 6 12 10 O 13 72 7 14 6 O 15 100 8 16 10 O 17 90 9 18 8 O 19 92 10 H H C8 H17 H 20 6 O H O H 21Ac 21Bc 95 a Isolated yields. b Yield based on 75 conversion. c The ratios were determined by 1H NMR spectroscopy.References 1 K. A. Jorgensen Chem. Rev. 1989 89 431. 2 W. P. Griffith Chem. Soc. Rev. 1992 21 179. 3 T. Mukaiyama and T. Yamada Bull. Chem. Soc. Jpn. 1995 68 17. 4 J. T. Groves and R. J. Quinn J. Am. Chem. Soc. 1985 107 5790. 5 (a) A. E. M. Boelrijk A. L. Spek T. X. Neenan M. M. Van Vlzen J. Reedjik and H. J. Koojiman J. Chem. Soc. Chem. Commun. 1995 2465; (b) C. Augier L. Malara V. Lazzaeri and B. Waegell Tetrahedron Lett. 1995 36 8775; (c) C. G. Balavoine C. Eskenazi F. Meunier and H. J. Riviere J. Chem. Soc. Chem. Commun. 1985 1111; (d ) C. G. Balavoine C. Eskenazi F. Meunier and H. Riviere Tetrahedron Lett. 1984 25 3187. 6 H. Nishiyama S.-B. Park M.-A. Haga K. Aoki and K. Itoh Chem. Lett. 1994 1111. 7 G. A. Barf D. Hoek and R. A. Sheldon Tetrahedron 1996 52 12 971.8 G. Helmchen A. Krotz K.-T. Ganz and D. Hansen Synlett 1991 257. 9 (a) T. R. Cundari and R. S. Drago Inorg. Chem. 1990 29 487; (b) T. R. Cundari and R. S. Drago Inorg. Chem. 1990 29 2303. 10 M. Onishi and K. Isagawa Inorg. Chim. Acta 1991 179 155. 11 B. F. G. Johnson J. Lewis and I. E. Ryder J. Chem. Soc. Dalton Trans. 1977 719. Paper 7/06729A Received 16th September 1997 Accepted 16th September 1997 J. Chem. Soc. Perkin Trans. 1 1997 3115 Stereospecific and regioselective catalytic epoxidation of alkenes by a novel ruthenium(II) complex under aerobic conditions Venkitasamy Kesavan and Srinivasan Chandrasekaran * Department of Organic Chemistry Indian Institute of Science Bangalore-560 012 India Epoxidation of alkenes by molecular oxygen is effected in high yields by catalysis of RuCl2(biox)2 using isobutyraldehyde as the co-reductant the reaction is stereospecific and regioselective.There has been considerable interest in recent years in both the homogeneous transition metal-catalysed epoxidation of alkenes 1 and in the use of oxoruthenium complexes as catalysts for organic oxidations.2 With regard to aerobic oxidation catalysed by transition metal complexes several reactions involving the combined use of molecular oxygen with reducing agents have been reviewed.3 The combination of three components that is a transition metal organic ligands and a reductant is considered to create an effective oxygenation system for such aerobic reactions. In 1985 Groves and Quinn reported a successful aerobic epoxidation of olefins with a rutheniumndash;porphyrin catalyst.4 All the other epoxidations catalysed by ruthenium complexes involved the use of NaIO4 or PhIO as the terminal oxidants under biphasic conditions.5 Nishiyama et al.6 used dichlorobis( oxazolinyl)bipyridylruthenium(II) complex for the oxidation of alkenes in the presence of PhIO and found oxidative cleavage as the major pathway while Barf et al.reported the epoxidation of alkenes with a combination of (bipyridyl)RuCl2- (DMSO)4 and tert-butylhydroperoxide.7 C2-symmetric 4,49,5,59-tetrahydro-2,29-bisoxazoles have been used as ligands in catalytic asymmetric transfer hydrogenation of ketones.8 Prior to achieving enantioselectivity in epoxidation with ruthenium complexes it is essential to find conditions in favour of epoxidation rather than oxidative cleavage.Drago has demonstrated that in general trans-dioxoruthenium complexes catalysed oxidation of alkenes to yield epoxides while the cis-isomers mediate oxidative cleavage.9 The aim of the present work is to use bisoxazoles as bidentate ligands so that they form a square planar structure around the ruthenium core analogous to rutheniumndash;porphyrin systems.4 Accordingly rutheniumndash;bisoxazole complex 1 was synthesized 10 and our successful results of an oxo-transfer reaction with 1 are presented in this communication. In the preliminary investigations of oxidation of alkenes with 1 PhIO NaIO4 ureandash;H2O2 NaOCl and TBHP were used as terminal oxidants. In all cases the reaction was incomplete (30ndash;50 conversion) and the product was always a mixture of epoxides and cleavage products.However when the oxidation of alkenes was carried out with 1 (2.5 mol) in CH2Cl2 (25 8C 6ndash;12 h) in the presence of molecular oxygen as the oxidant and isobutyraldehyde as the co-reductant excellent yields of epoxides 2ndash;21 were obtained. The results are summarized in Table 1. N O N O Cl Cl N O N O Ru 1 As can be gauged from Table 1 this catalytic epoxidation with 1 is highly stereospecific. Thus trans-stilbene 2 and cisstilbene 4 under the reaction conditions afford the trans-stilbene oxide 3 and cis-stilbene oxide 5 respectively in high yields. Styrene epoxide 11 which is highly unstable under conditions of peracid epoxidation is quite stable under the present reaction conditions. Another salient feature of the present methodology is the high regioselectivity. Thus in the oxidation of 4-vinylcyclohexene 16 and limonene 18 catalysed by 1 the monoepoxides 17 and 19 respectively were the exclusive products isolated in very good yields.A more dramatic example of stereoselectivity is illustrated in the epoxidation of cholest-5-ene 20. Under conditions of catalytic epoxidation the 5b,6b-epoxide 21A is obtained in excess of 94 selectivity and in excellent yields. Thus we have developed a non-porphyrin ruthenium(II) system which catalyses the epoxidation of alkenes under aerobic conditions with great efficiency. The reaction is stereospecific and regioselective and this kind of high stereospecificity and regioselectivity has not been reported with other catalytic epoxidation methodologies.5ndash;7 Since C2-symmetric bisoxazoles can be easily derived from optically active amino alcohols chiral ruthenium complexes are readily accessible and studies of catalytic asymmetric epoxidation of unfunctionalized alkenes with these systems are under progress.Experimental Synthesis of complex 1 trans-Tetrakis(acetonitrile)dichlororuthenium(II) 11 (0.336 g 1 mmol) was refluxed with 4,49,5,59-tetrahydro-2,29-bisoxazole (0.308 g 2.2 mmol) for 6 h in ethanol (10 ml). The solvent was removed under reduced pressure to give a red coloured solid. The solid was recrystallized from EtOHndash;Et2O (1 3) and stored in a desiccator; mp 280 8C (uncorrected); nmax(thin film)/cm21 2950 1610; dH(90 MHz D2O) 3.4 (m 8H) 3.8 (m 8H); dC(22.5 MHz D2O) 53.61 68.87 155.26 (Calc. for C12H16Cl2- N4O4Ru C 31.86; H 3.56; N 12.38. Found C 31.81; H 3.48; N 12.40). Typical experimental procedure for epoxidation Ruthenium complex 1 (2.5 mol) was added to the alkene (1 mmol) dissolved in dichloromethane (4 ml).To this homogeneous solution NaHCO3 (1.5 equiv.) and isobutyraldehyde (1.5 equiv.) were added. The mixture was stirred under an atmosphere of oxygen at 25 8C and the reaction was monitored by TLC. Once the reaction was over the reaction mixture was diluted with CH2Cl2 and filtered through a pad of Celite and silica gel. Removal of solvent yielded the crude product which was purified by flash chromatography over neutral alumina or distillation under reduced pressure. Acknowledgements The authors thank the Department of Science and Technology New Delhi for financial support of this investigation. One of the authors V. K. thanks UGC for a research fellowship. 3116 J.Chem. Soc. Perkin Trans. 1 1997 Table 1 Epoxidation of alkenes catalysed by RuCl2(biox)2 Entry 1 Substrate Ph Ph 2 Reaction time/h 8 Product Ph Ph O 3 Yield a () 95 2 Ph Ph 4 10 Ph Ph O 5 88 3 Ph 6 9 Ph O 7 85 4 Undec-1-ene 8 12 Undec-1-ene oxide 9 100 b 5 Ph 10 8 O Ph 11 85 6 12 10 O 13 72 7 14 6 O 15 100 8 16 10 O 17 90 9 18 8 O 19 92 10 H H C8 H17 H 20 6 O H O H 21Ac 21Bc 95 a Isolated yields. b Yield based on 75 conversion. c The ratios were determined by 1H NMR spectroscopy. References 1 K. A. Jorgensen Chem. Rev. 1989 89 431. 2 W. P. Griffith Chem. Soc. Rev. 1992 21 179. 3 T. Mukaiyama and T. Yamada Bull. Chem. Soc. Jpn. 1995 68 17. 4 J. T. Groves and R. J. Quinn J. Am. Chem. Soc. 1985 107 5790. 5 (a) A. E. M. Boelrijk A. L. Spek T. X. Neenan M. M. Van Vlzen J. Reedjik and H.J. Koojiman J. Chem. Soc. Chem. Commun. 1995 2465; (b) C. Augier L. Malara V. Lazzaeri and B. Waegell Tetrahedron Lett. 1995 36 8775; (c) C. G. Balavoine C. Eskenazi F. Meunier and H. J. Riviere J. Chem. Soc. Chem. Commun. 1985 1111; (d ) C. G. Balavoine C. Eskenazi F. Meunier and H. Riviere Tetrahedron Lett. 1984 25 3187. 6 H. Nishiyama S.-B. Park M.-A. Haga K. Aoki and K. Itoh Chem. Lett. 1994 1111. 7 G. A. Barf D. Hoek and R. A. Sheldon Tetrahedron 1996 52 12 971. 8 G. Helmchen A. Krotz K.-T. Ganz and D. Hansen Synlett 1991 257. 9 (a) T. R. Cundari and R. S. Drago Inorg. Chem. 1990 29 487; (b) T. R. Cundari and R. S. Drago Inorg. Chem. 1990 29 2303. 10 M. Onishi and K. Isagawa Inorg. Chim. Acta 1991 179 155. 11 B. F. G. Johnson J. Lewis and I. E. Ryder J. Chem. Soc.Dalton Trans. 1977 719. Paper 7/06729A Received 16th September 1997 Accepted 16th September 1997 J. Chem. Soc. Perkin Trans. 1 1997 3115 Stereospecific and regioselective catalytic epoxidation of alkenes by a novel ruthenium(II) complex under aerobic conditions Venkitasamy Kesavan and Srinivasan Chandrasekaran * Department of Organic Chemistry Indian Institute of Science Bangalore-560 012 India Epoxidation of alkenes by molecular oxygen is effected in high yields by catalysis of RuCl2(biox)2 using isobutyraldehyde as the co-reductant the reaction is stereospecific and regioselective. There has been considerable interest in recent years in both the homogeneous transition metal-catalysed epoxidation of alkenes 1 and in the use of oxoruthenium complexes as catalysts for organic oxidations.2 With regard to aerobic oxidation catalysed by transition metal complexes several reactions involving the combined use of molecular oxygen with reducing agents have been reviewed.3 The combination of three components that is a transition metal organic ligands and a reductant is considered to create an effective oxygenation system for such aerobic reactions.In 1985 Groves and Quinn reported a successful aerobic epoxidation of olefins with a rutheniumndash;porphyrin catalyst.4 All the other epoxidations catalysed by ruthenium complexes involved the use of NaIO4 or PhIO as the terminal oxidants under biphasic conditions.5 Nishiyama et al.6 used dichlorobis( oxazolinyl)bipyridylruthenium(II) complex for the oxidation of alkenes in the presence of PhIO and found oxidative cleavage as the major pathway while Barf et al.reported the epoxidation of alkenes with a combination of (bipyridyl)RuCl2- (DMSO)4 and tert-butylhydroperoxide.7 C2-symmetric 4,49,5,59-tetrahydro-2,29-bisoxazoles have been used as ligands in catalytic asymmetric transfer hydrogenation of ketones.8 Prior to achieving enantioselectivity in epoxidation with ruthenium complexes it is essential to find conditions in favour of epoxidation rather than oxidative cleavage. Drago has demonstrated that in general trans-dioxoruthenium complexes catalysed oxidation of alkenes to yield epoxides while the cis-isomers mediate oxidative cleavage.9 The aim of the present work is to use bisoxazoles as bidentate ligands so that they form a square planar structure around the ruthenium core analogous to rutheniumndash;porphyrin systems.4 Accordingly rutheniumndash;bisoxazole complex 1 was synthesized 10 and our successful results of an oxo-transfer reaction with 1 are presented in this communication.In the preliminary investigations of oxidation of alkenes with 1 PhIO NaIO4 ureandash;H2O2 NaOCl and TBHP were used as terminal oxidants. In all cases the reaction was incomplete (30ndash;50 conversion) and the product was always a mixture of epoxides and cleavage products. However when the oxidation of alkenes was carried out with 1 (2.5 mol) in CH2Cl2 (25 8C 6ndash;12 h) in the presence of molecular oxygen as the oxidant and isobutyraldehyde as the co-reductant excellent yields of epoxides 2ndash;21 were obtained. The results are summarized in Table 1. N O N O Cl Cl N O N O Ru 1 As can be gauged from Table 1 this catalytic epoxidation with 1 is highly stereospecific.Thus trans-stilbene 2 and cisstilbene 4 under the reaction conditions afford the trans-stilbene oxide 3 and cis-stilbene oxide 5 respectively in high yields. Styrene epoxide 11 which is highly unstable under conditions of peracid epoxidation is quite stable under the present reaction conditions. Another salient feature of the present methodology is the high regioselectivity. Thus in the oxidation of 4-vinylcyclohexene 16 and limonene 18 catalysed by 1 the monoepoxides 17 and 19 respectively were the exclusive products isolated in very good yields. A more dramatic example of stereoselectivity is illustrated in the epoxidation of cholest-5-ene 20. Under conditions of catalytic epoxidation the 5b,6b-epoxide 21A is obtained in excess of 94 selectivity and in excellent yields.Thus we have developed a non-porphyrin ruthenium(II) system which catalyses the epoxidation of alkenes under aerobic conditions with great efficiency. The reaction is stereospecific and regioselective and this kind of high stereospecificity and regioselectivity has not been reported with other catalytic epoxidation methodologies.5ndash;7 Since C2-symmetric bisoxazoles can be easily derived from optically active amino alcohols chiral ruthenium complexes are readily accessible and studies of catalytic asymmetric epoxidation of unfunctionalized alkenes with these systems are under progress. Experimental Synthesis of complex 1 trans-Tetrakis(acetonitrile)dichlororuthenium(II) 11 (0.336 g 1 mmol) was refluxed with 4,49,5,59-tetrahydro-2,29-bisoxazole (0.308 g 2.2 mmol) for 6 h in ethanol (10 ml).The solvent was removed under reduced pressure to give a red coloured solid. The solid was recrystallized from EtOHndash;Et2O (1 3) and stored in a desiccator; mp 280 8C (uncorrected); nmax(thin film)/cm21 2950 1610; dH(90 MHz D2O) 3.4 (m 8H) 3.8 (m 8H); dC(22.5 MHz D2O) 53.61 68.87 155.26 (Calc. for C12H16Cl2- N4O4Ru C 31.86; H 3.56; N 12.38. Found C 31.81; H 3.48; N 12.40). Typical experimental procedure for epoxidation Ruthenium complex 1 (2.5 mol) was added to the alkene (1 mmol) dissolved in dichloromethane (4 ml). To this homogeneous solution NaHCO3 (1.5 equiv.) and isobutyraldehyde (1.5 equiv.) were added. The mixture was stirred under an atmosphere of oxygen at 25 8C and the reaction was monitored by TLC. Once the reaction was over the reaction mixture was diluted with CH2Cl2 and filtered through a pad of Celite and silica gel.Removal of solvent yielded the crude product which was purified by flash chromatography over neutral alumina or distillation under reduced pressure. Acknowledgements The authors thank the Department of Science and Technology New Delhi for financial support of this investigation. One of the authors V. K. thanks UGC for a research fellowship. 3116 J. Chem. Soc. Perkin Trans. 1 1997 Table 1 Epoxidation of alkenes catalysed by RuCl2(biox)2 Entry 1 Substrate Ph Ph 2 Reaction time/h 8 Product Ph Ph O 3 Yield a () 95 2 Ph Ph 4 10 Ph Ph O 5 88 3 Ph 6 9 Ph O 7 85 4 Undec-1-ene 8 12 Undec-1-ene oxide 9 100 b 5 Ph 10 8 O Ph 11 85 6 12 10 O 13 72 7 14 6 O 15 100 8 16 10 O 17 90 9 18 8 O 19 92 10 H H C8 H17 H 20 6 O H O H 21Ac 21Bc 95 a Isolated yields.b Yield based on 75 conversion. c The ratios were determined by 1H NMR spectroscopy. References 1 K. A. Jorgensen Chem. Rev. 1989 89 431. 2 W. P. Griffith Chem. Soc. Rev. 1992 21 179. 3 T. Mukaiyama and T. Yamada Bull. Chem. Soc. Jpn. 1995 68 17. 4 J. T. Groves and R. J. Quinn J. Am. Chem. Soc. 1985 107 5790. 5 (a) A. E. M. Boelrijk A. L. Spek T. X. Neenan M. M. Van Vlzen J. Reedjik and H. J. Koojiman J. Chem. Soc. Chem. Commun. 1995 2465; (b) C. Augier L. Malara V. Lazzaeri and B. Waegell Tetrahedron Lett. 1995 36 8775; (c) C. G. Balavoine C. Eskenazi F. Meunier and H. J. Riviere J. Chem. Soc. Chem. Commun. 1985 1111; (d ) C. G. Balavoine C. Eskenazi F. Meunier and H. Riviere Tetrahedron Lett. 1984 25 3187. 6 H.Nishiyama S.-B. Park M.-A. Haga K. Aoki and K. Itoh Chem. Lett. 1994 1111. 7 G. A. Barf D. Hoek and R. A. Sheldon Tetrahedron 1996 52 12 971. 8 G. Helmchen A. Krotz K.-T. Ganz and D. Hansen Synlett 1991 257. 9 (a) T. R. Cundari and R. S. Drago Inorg. Chem. 1990 29 487; (b) T. R. Cundari and R. S. Drago Inorg. Chem. 1990 29 2303. 10 M. Onishi and K. Isagawa Inorg. Chim. Acta 1991 179 155. 11 B. F. G. Johnson J. Lewis and I. E. Ryder J. Chem. Soc. Dalton Trans. 1977 719. Paper 7/06729A Received 16th September 1997 Accepted 16th September 1997 J. Chem. Soc. Perkin Trans. 1 1997 3115 Stereospecific and regioselective catalytic epoxidation of alkenes by a novel ruthenium(II) complex under aerobic conditions Venkitasamy Kesavan and Srinivasan Chandrasekaran * Department of Organic Chemistry Indian Institute of Science Bangalore-560 012 India Epoxidation of alkenes by molecular oxygen is effected in high yields by catalysis of RuCl2(biox)2 using isobutyraldehyde as the co-reductant the reaction is stereospecific and regioselective.There has been considerable interest in recent years in both the homogeneous transition metal-catalysed epoxidation of alkenes 1 and in the use of oxoruthenium complexes as catalysts for organic oxidations.2 With regard to aerobic oxidation catalysed by transition metal complexes several reactions involving the combined use of molecular oxygen with reducing agents have been reviewed.3 The combination of three components that is a transition metal organic ligands and a reductant is considered to create an effective oxygenation system for such aerobic reactions.In 1985 Groves and Quinn reported a successful aerobic epoxidation of olefins with a rutheniumndash;porphyrin catalyst.4 All the other epoxidations catalysed by ruthenium complexes involved the use of NaIO4 or PhIO as the terminal oxidants under biphasic conditions.5 Nishiyama et al.6 used dichlorobis( oxazolinyl)bipyridylruthenium(II) complex for the oxidation of alkenes in the presence of PhIO and found oxidative cleavage as the major pathway while Barf et al. reported the epoxidation of alkenes with a combination of (bipyridyl)RuCl2- (DMSO)4 and tert-butylhydroperoxide.7 C2-symmetric 4,49,5,59-tetrahydro-2,29-bisoxazoles have been used as ligands in catalytic asymmetric transfer hydrogenation of ketones.8 Prior to achieving enantioselectivity in epoxidation with ruthenium complexes it is essential to find conditions in favour of epoxidation rather than oxidative cleavage.Drago has demonstrated that in general trans-dioxoruthenium complexes catalysed oxidation of alkenes to yield epoxides while the cis-isomers mediate oxidative cleavage.9 The aim of the present work is to use bisoxazoles as bidentate ligands so that they form a square planar structure around the ruthenium core analogous to rutheniumndash;porphyrin systems.4 Accordingly rutheniumndash;bisoxazole complex 1 was synthesized 10 and our successful results of an oxo-transfer reaction with 1 are presented in this communication. In the preliminary investigations of oxidation of alkenes with 1 PhIO NaIO4 ureandash;H2O2 NaOCl and TBHP were used as terminal oxidants. In all cases the reaction was incomplete (30ndash;50 conversion) and the product was always a mixture of epoxides and cleavage products.However when the oxidation of alkenes was carried out with 1 (2.5 mol) in CH2Cl2 (25 8C 6ndash;12 h) in the presence of molecular oxygen as the oxidant and isobutyraldehyde as the co-reductant excellent yields of epoxides 2ndash;21 were obtained. The results are summarized in Table 1. N O N O Cl Cl N O N O Ru 1 As can be gauged from Table 1 this catalytic epoxidation with 1 is highly stereospecific. Thus trans-stilbene 2 and cisstilbene 4 under the reaction conditions afford the trans-stilbene oxide 3 and cis-stilbene oxide 5 respectively in high yields. Styrene epoxide 11 which is highly unstable under conditions of peracid epoxidation is quite stable under the present reaction conditions.Another salient feature of the present methodology is the high regioselectivity. Thus in the oxidation of 4-vinylcyclohexene 16 and limonene 18 catalysed by 1 the monoepoxides 17 and 19 respectively were the exclusive products isolated in very good yields. A more dramatic example of stereoselectivity is illustrated in the epoxidation of cholest-5-ene 20. Under conditions of catalytic epoxidation the 5b,6b-epoxide 21A is obtained in excess of 94 selectivity and in excellent yields. Thus we have developed a non-porphyrin ruthenium(II) system which catalyses the epoxidation of alkenes under aerobic conditions with great efficiency. The reaction is stereospecific and regioselective and this kind of high stereospecificity and regioselectivity has not been reported with other catalytic epoxidation methodologies.5ndash;7 Since C2-symmetric bisoxazoles can be easily derived from optically active amino alcohols chiral ruthenium complexes are readily accessible and studies of catalytic asymmetric epoxidation of unfunctionalized alkenes with these systems are under progress.Experimental Synthesis of complex 1 trans-Tetrakis(acetonitrile)dichlororuthenium(II) 11 (0.336 g 1 mmol) was refluxed with 4,49,5,59-tetrahydro-2,29-bisoxazole (0.308 g 2.2 mmol) for 6 h in ethanol (10 ml). The solvent was removed under reduced pressure to give a red coloured solid. The solid was recrystallized from EtOHndash;Et2O (1 3) and stored in a desiccator; mp 280 8C (uncorrected); nmax(thin film)/cm21 2950 1610; dH(90 MHz D2O) 3.4 (m 8H) 3.8 (m 8H); dC(22.5 MHz D2O) 53.61 68.87 155.26 (Calc.for C12H16Cl2- N4O4Ru C 31.86; H 3.56; N 12.38. Found C 31.81; H 3.48; N 12.40). Typical experimental procedure for epoxidation Ruthenium complex 1 (2.5 mol) was added to the alkene (1 mmol) dissolved in dichloromethane (4 ml). To this homogeneous solution NaHCO3 (1.5 equiv.) and isobutyraldehyde (1.5 equiv.) were added. The mixture was stirred under an atmosphere of oxygen at 25 8C and the reaction was monitored by TLC. Once the reaction was over the reaction mixture was diluted with CH2Cl2 and filtered through a pad of Celite and silica gel. Removal of solvent yielded the crude product which was purified by flash chromatography over neutral alumina or distillation under reduced pressure. Acknowledgements The authors thank the Department of Science and Technology New Delhi for financial support of this investigation.One of the authors V. K. thanks UGC for a research fellowship. 3116 J. Chem. Soc. Perkin Trans. 1 1997 Table 1 Epoxidation of alkenes catalysed by RuCl2(biox)2 Entry 1 Substrate Ph Ph 2 Reaction time/h 8 Product Ph Ph O 3 Yield a () 95 2 Ph Ph 4 10 Ph Ph O 5 88 3 Ph 6 9 Ph O 7 85 4 Undec-1-ene 8 12 Undec-1-ene oxide 9 100 b 5 Ph 10 8 O Ph 11 85 6 12 10 O 13 72 7 14 6 O 15 100 8 16 10 O 17 90 9 18 8 O 19 92 10 H H C8 H17 H 20 6 O H O H 21Ac 21Bc 95 a Isolated yields. b Yield based on 75 conversion. c The ratios were determined by 1H NMR spectroscopy. References 1 K. A. Jorgensen Chem. Rev. 1989 89 431. 2 W. P. Griffith Chem. Soc. Rev. 1992 21 179. 3 T. Mukaiyama and T. Yamada Bull. Chem. Soc. Jpn. 1995 68 17. 4 J. T. Groves and R.J. Quinn J. Am. Chem. Soc. 1985 107 5790. 5 (a) A. E. M. Boelrijk A. L. Spek T. X. Neenan M. M. Van Vlzen J. Reedjik and H. J. Koojiman J. Chem. Soc. Chem. Commun. 1995 2465; (b) C. Augier L. Malara V. Lazzaeri and B. Waegell Tetrahedron Lett. 1995 36 8775; (c) C. G. Balavoine C. Eskenazi F. Meunier and H. J. Riviere J. Chem. Soc. Chem. 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