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Iron catalysed nitrosation of olefins to oximes† Cite this: Dalton Trans., 2014, 43, 38 Received 1st July 2013, Accepted 14th October 2013
Ritwika Ray, Abhishek Dutta Chowdhury, Debabrata Maiti* and Goutam Kumar Lahiri*
DOI: 10.1039/c3dt51764k www.rsc.org/dalton
Fe(BF4)2·6H2O/2,6-pyridinedicarboxylic acid catalysed nitrosation of a wide variety of substituted styrenes has been developed in the presence of t-BuONO/NaBH4 under H2 pressure (10 bar) in MeOH–H2O (5 : 1) to afford corresponding oximes in good to excellent yields.
The Beckmann rearrangement of cyclohexanone oxime to ε-caprolactum is an industrially important reaction for the synthesis of Nylon-6.1 Moreover, oximes can be easily reduced to amines,2 which are further used in the manufacturing of dyes, plastics, synthetic fibres and pharmaceuticals. Oximes are used as anti-skinning agents in paint and blocking agents in the polymer industry.3 Biological activities including anti-allergic and anti-inflammatory behaviours of oximes have also been demonstrated.4 Recently, Ley and Bertram have further revealed that oximes and oxime-ethers of hydroxylated benzaldehydes and acetophenones are powerful anti-oxidants.5 Thus, owing to the extensive utility of oximes, both in industry and biology, development of sustainable and novel methods for their synthesis continues to be an active area of research.6 Nitrosation of olefin feedstock in the presence of nitric oxide or alkyl nitrites is one of the convenient approaches for producing oximes. However, catalytic nitrosation of olefins in the presence of various reductants is rare. Only a few reports involving cobalt catalysed oxime formation are known in the literature so far.7 Environmentally benign iron mediated oxime synthesis is limited to the only example reported by Beller et al. where iron(II)-phthalocyanine was used as the active catalytic species.8 Herein we demonstrate a simple and convenient method of Fe(BF4)2·6H2O/2,6-pyridinedicarboxylic acid (dipic) catalysed synthesis of oximes (2) from styrene derivatives (1) in the presence of cheap and readily available tertiary-butylnitrite (t-BuONO) and sodium borohydride (NaBH4) (Scheme 1).9
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: General procedure, characterization data and 1H and 13C-NMR spectra of 2a–2u. See DOI: 10.1039/ c3dt51764k
38 | Dalton Trans., 2014, 43, 38–41
Scheme 1
An outline for the catalytic transformation of 1 to 2.
During our initial investigation, the reaction of styrene with t-BuONO/NaBH4 in the presence of Fe(BF4)2·6H2O/dipic was chosen as the model system. Optimisation of the critical reaction parameters by employing an array of solvents establishes MeOH–H2O (5 : 1) as the best combination (Table 1, entry 3). The use of only MeOH as the solvent results in appreciably lower yield of the desired product (Table 1, entry 5). Notably, addition of water (1 mL) along with MeOH (5 mL) leads to the product formation in excellent yield (Table 1, entry 3). On the other hand, EtOH–H2O (5 : 1) as a solvent system produces acetophenone oxime in 65% yield (GC yield) under the identical reaction conditions (Table 1, entry 8). However, the selective use of EtOH improves the yield to a small extent (Table 1, entry 10), but it is reasonably low compared to the MeOH–H2O (5 : 1) solvent system, which indeed has prompted us to restrict the present investigations in MeOH–H2O (5 : 1). It should also be noted that the yield of the oxime decreased considerably when the same reaction was carried out for longer time and with higher catalyst loading. Therefore, it is presumed that oximes being slowly reduced by BH4−, a prolonged reaction time led to the decomposition of the products.7d Lowering of the yield can further be attributed to the probable side reactions leading to the formation of nitrosoalkanes, nitrosohydroxylamines, azo compounds and N-nitrosoamine oxides.10 However, these complex nitrogen compounds could not be detected since such compounds get adsorbed on the silica gel column during the isolation of the products.7d Other hydrogenation side-products, resulting from the reduction
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Table 1
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Iron catalysed nitrosation of olefins in various solventsa
Entry
Solvent
Conversionb (%)
Yieldc (%)
1 2d 3 4d 5 6 7d 8 9d 10
Toluene MeOH–H2O (5 : 1) MeOH–H2O (5 : 1) MeOH MeOH THF EtOHe–H2O (5 : 1) EtOHe–H2O (5 : 1) EtOHe EtOHe
0 >90 >99 80 90 10 85 90 80 85
0 82 (75) 99 (90) 60 65 5 60 65 70 75
a
Reaction conditions: 3 mmol of styrene, 1 mol% Fe(BF4)2·6H2O, 1 mol% dipic, 1.5 equiv. NaBH4 and 1.5 equiv. t-BuONO, solvent, 6 h at room temperature under 10 bar of H2. b Determined by GC after the reaction. c Determined by GC after workup. Isolated yield is given in parenthesis. d The reaction was performed under normal atmospheric conditions without additional H2 pressure. e Absolute grade EtOH used.
of styrene, such as 2,3-diphenylbutane, ethyl benzene and 1-phenylethyl amine were however detected by GC in trace quantities.11,12 Although under normal atmospheric conditions >90% conversion of styrene was observed in MeOH–H2O (5 : 1) and that resulted in 82% desired product (Table 1, entry 2 and Table S1†), the best yield of 2 is obtained under dihydrogen pressure (Table 1, entry 3 and Table S1†). All the reactions are therefore reported under an additional 10 bar H2 pressure. The precise role of the extra H2 is not clear at present, however, it may be reasonable to assume that the additional H2 pressure extends a better reducing environment towards the formation of the desired products in good to excellent yields. Various iron, ruthenium and cobalt salts have also been tested in the presence of different commercially available biand tridentate ligands (Table 2). It is found that though Fe(BF4)2·6H2O/dipic and Fe(Pc) catalytic species under the present reaction protocol (Table 2, entries 11–12) show comparable activity, lower yield is obtained in the latter case. Hence, optimisation of the suitable reaction conditions reveals that the in situ generated catalyst derived from 1 mol% Fe(BF4)2·6H2O and 1 mol% dipic extends maximum activity and results in acetophenone oxime formation in 90% yield. The optimised reaction conditions indeed allow the reduction of a wide variety of styrene derivatives to generate the corresponding oximes (Table 3). Excellent yields of oximes are obtained with different ortho-, meta- and para-alkyl/alkoxide substituted styrene derivatives (2a–2d, 2l–2m). Halogens at different positions of the styrene ring afforded the corresponding oximes in high yields without any dehalogenation (2e–2k). Remarkably, styrene with a formyl group is found to be completely unaffected under the present reaction protocol,
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Table 2
Catalytic nitrosation of olefinsa
Entry
Catalystb
Conversion (%)
Yieldc (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
FeCl3·6H2O/pic FeCl3·6H2O/dipic FeSO4·7H2O/pic FeSO4·7H2O/dipic Fe(ClO4)2·6H2O/pic Fe(ClO4)2·6H2O/dipic Fe(BF4)2·6H2O/pic Fe(BF4)2·6H2O/bipy Fe(BF4)2·6H2O/dppf Fe(BF4)2·6H2O/trpy Fe(BF4)2·6H2O/dipic Fe(Pc) Fe(acac)3 Ru(trpy)Cl3 RuCl3·3H2O/dipic CoCl2·6H2O/dipic
75 60 65 75 25 80 90 50 85 88 >99 >90 70 75 10 75
45 40 45 40 14 45 65 35 70 69 99 (90) 80 20 25 2 15
a Reaction conditions: 3 mmol of styrene, 1 mol% catalyst, 1 mol% ligand, 1.5 equiv. NaBH4 and 1.5 equiv. t-BuONO in 5 mL MeOH and 1 mL H2O, 6 h at room temperature under 10 bar of H2. b dipic = 2,6pyridine dicarboxylic acid, trpy = 2,2′:6′,2″-terpyridine, dppf = 1,1′-bis(diphenylphosphino)ferrocene, pic = pyridine-2-carboxylic acid. c Determined by GC using n-dodecane as an internal standard. Isolated yield is given in parenthesis.
and selective oxime formation occurs, albeit in low yield (2q). Although the present method is not applicable for 1,2-diaryl substituted olefins, β-methyl substituted styrene derivatives give good yields of the desired oxime products (2t and 2u). Other reducible groups such as –NO2 (2o) and –CN (2p) are also tolerated. These examples further highlight the beneficial features of the present method since such groups can easily be converted to the corresponding amines. Even 4-vinylaniline gives the desired 1-(4-aminophenyl)ethanone oxime in preparatively useful yield (2r). The present reaction protocol thus proceeds in a highly chemo- and regioselective manner, exclusively producing acetophenone oxime derivatives. A tentative mechanism for the catalytic cycle has been outlined in Scheme 2, primarily based on our experimental observations. Under the present reaction conditions, exclusive formation of acetophenone oxime derivatives indeed rules out the possibility of the involvement of NO+ in the initial stage of the reaction, which would otherwise lead to the formation of 2-phenylacetaldehyde oxime and its derivatives on addition to substituted ethylenes.13 Hydrogen evolution has also been observed in the initial stage of the reaction on addition of NaBH4 in the presence of MeOH–H2O (5 : 1). The determining step in the mechanism is the addition of olefin to the metal centre of the iron–dipic complex (A) that leads to the formation of σ-alkyliron(III) complex (B). The formation of iron-dipic complex (A) as the active catalytic species during the reaction has also been evidenced by the ESI(+)-MS (Fig. S37†). The reaction of B with t-BuONO subsequently, results in the generation
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Table 3
Substrate scope for iron catalysed nitrosation of olefinsa
a
Reaction conditions: 3 mmol of styrene, 1 mol% Fe(BF4)2·6H2O, 1 mol% dipic, 1.5 equiv. NaBH4 and 1.5 equiv. t-BuONO in 5 mL MeOH and 1 mL H2O, 6 h at room temperature under 10 bar of H2. b Yields determined by GC after the reaction, isolated yields are in parentheses. c Yields based on 1H NMR with PhTMS as an internal standard. d Substrate: trans-β-methyl-styrene. e Substrate: cis-β-methylstyrene. f Yield for the reaction done on a 1 g scale.
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of an alkyl nitrite species (C), which is finally converted to D, the desired oxime. A set of preliminary mechanistic investigations reveal the formation of black particles (may be due to Fe(0) or FexBy formation), in the absence of alkene under identical reaction conditions. However, no such black particle formation was observed in the presence of styrene under the same reaction conditions. This observation may provide an indirect justification in favour of the proposed formation of the iron-alkyl species as the intermediate which eventually suppresses the formation of black Fe particles in the presence of the dianionic dipicolinic acid as the ligand. This also suggests that the possibility of Fe-nanoparticles acting as the active catalytic species for the hydrogenation process14 in the present case is unlikely. The homogeneous catalytic route has, however, been established by the Hg(0) poisoning experiment as well as by the TEM and XPS data (see ESI† for details).15 The addition of Hg(0) in the catalytic process failed to show any poisoning effect and TEM/XPS did not identify any iron nano-particles/clusters and iron(0) state, respectively. It may also be logical to presume that the conversion of C to D is being facilitated by water since the present catalytic reaction with styrene using MeOH–D2O (5 : 1) as the solvent system exhibits a HRMS (ESI+) peak at m/z = 137.0813 (Fig. S40†) corresponding to PhC(Me)NOD (d1-D) (where calcd m/z (M + H+) = 137.0820). Additionally, water might also play a role in the hydrolysis of NaBH4 to liberate hydrogen16 in the presence of the Fe(II) catalyst system and thereby facilitates the catalytic hydrogenation process under the present reaction conditions. In conclusion, an efficient and selective iron-catalysed reduction protocol has been developed for the conversion of styrene derivatives to their respective oximes. Good to excellent yields are obtained for most of the chosen substrates under mild and benign reaction conditions. This activity was supported by DST (GKL) and BRNS (DM/ 2011/20/37C/13). Financial support received from UGC (fellowship to R.R.) and CSIR (fellowship to A.D.C.), New Delhi, India is gratefully acknowledged.
Notes and references
Scheme 2
An outline of the tentative mechanistic pathway.
40 | Dalton Trans., 2014, 43, 38–41
1 (a) S. Guo, Z. Y. Du, S. G. Zhang, D. M. Li, Z. P. Li and Y. Q. Deng, Green Chem., 2006, 8, 296; (b) J. J. Peng and Y. Q. Deng, Tetrahedron Lett., 2001, 42, 403; (c) W. C. Li, A. H. Lu, R. Palkovits, W. Schmidt, B. Spliethoff and F. Schuth, J. Am. Chem. Soc., 2005, 127, 12595; (d) M. Boero, T. Ikeshoji, C. C. Liew, K. Terakura and M. Parrinello, J. Am. Chem. Soc., 2004, 126, 6280. 2 (a) R. C. Walter, J. Am. Chem. Soc., 1952, 74, 5185; (b) X. Huang, M. Ortiz-Marciales, K. Huang, V. Stepanenko, F. G. Merced, A. M. Ayala, W. Correa and M. De Jesus, Org. Lett., 2007, 9, 1793; (c) Y. K. Choi, M. J. Kim, Y. Ahn and M.-J. Kim, Org. Lett., 2001, 3, 4099.
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3 (a) S. Tanase, J.-C. Hierso, E. Bouwman, J. Reedijk, J. ter Borg, J. H. Bieleman and A. Schut, New J. Chem., 2003, 27, 854; (b) N. Bodor, A. El Koussi, M. Kano and T. Nakamura, J. Med. Chem., 1988, 31, 100; (c) D. Kirsch, W. Hauser and N. Weger, Fundam. Appl. Toxicol., 1981, 1, 169. 4 (a) T. Katagi, H. Kataoka, K. Takahashi, T. Fujioka, M. Kunitomo, Y. Yamaguchi, M. Fujiwara and T. Inoi, Chem. Pharm. Bull., 1992, 40, 2419; (b) T. Katagi, H. Kataoka, Y. Konishi, Y. Takata, S. Kitano, M. Yamaki, T. Inoi, K. Yamamoto, S. Yamamoto and Y. Yamagata, Chem. Pharm. Bull., 1996, 44, 145; (c) H. Kataoka, S. Horiyama, M. Yamaki, H. Oku, K. Ishiguro, T. Katagi, M. Takayama, M. Semma and Y. Ito, Biol. Pharm. Bull., 2002, 25, 1436; (d) A. M. Nilsson, M. A. Bergstrom, K. Luthman, J. L. G. Nilsson and A. T. Karlberg, Food Chem. Toxicol., 2005, 43, 1627. 5 (a) J. P. Ley and H. J. Bertram, Eur. J. Lipid Sci. Technol., 2002, 104, 319; (b) D. Metodiewa, A. Kochman and S. Karolczak, IUBMB Life, 1997, 41, 1067; (c) G. O. Puntel, P. Gubert, G. L. Peres, L. Bresolin, J. B. Rocha, M. E. Pereira, V. S. Carratu and F. A. Soares, Arch. Toxicol., 2008, 82, 755; (d) R. de Lima Portella, R. P. Barcelos, A. F. de Bem, V. S. Carratu, L. Bresolin, J. B. da Rocha and F. A. Soares, Life Sci., 2008, 83, 878; (e) T. Ozen and M. Tas, J. Enzyme Inhib. Med. Chem., 2009, 24, 1141. 6 (a) E. Abele and E. Lukevics, Org. Prep. Proced. Int., 2000, 32, 235; (b) H. Eshghi and A. Hassankhani, Org. Prep. Proced. Int., 2005, 37, 575; (c) J. F. Knifton, J. Org. Chem., 1973, 38, 3296; (d) M. Maheswara, V. Siddaiah, K. Gopalaiah, V. M. Rao and C. V. Rao, J. Chem. Res., 2006, 362; (e) J. N. Armor, J. Am. Chem. Soc., 1980, 102, 1453; (f ) L. Zhang, H. Chen, Z. G. Zha and Z. Y. Wang, Chem. Commun., 2012, 48, 6574. 7 (a) K. Kato and T. Mukaiyama, Bull. Chem. Soc. Jpn., 1991, 64, 2948; (b) K. Kato and T. Mukaiyama, Chem. Lett., 1990, 1395; (c) K. Kato and T. Mukaiyama, Chem. Lett., 1990, 1917; (d) T. Okamoto, K. Kobayashi, S. Oka and S. Tanimoto, J. Org. Chem., 1987, 52, 5089; (e) T. Okamoto, K. Kobayashi, S. Oka and S. Tanimoto, J. Org. Chem., 1988, 53, 4897; (f ) K. Sugamoto, Y. Hamasuna, Y. Matsushita and T. Matsui, Synlett, 1998, 1270. 8 S. Prateeptongkum, I. Jovel, R. Jackstell, N. Vogl, C. Weckbecker and M. Beller, Chem. Commun., 2009, 1990.
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Communication
9 Some recent iron-catalysed reactions of olefins: (a) R. Ray, A. D. Chowdhury and G. K. Lahiri, ChemCatChem, 2013, 5, 2158; (b) A. D. Chowdhury, R. Ray and G. K. Lahiri, Chem. Commun., 2012, 48, 5497; (c) A. D. Chowdhury and G. K. Lahiri, Chem. Commun., 2012, 48, 3448; (d) I. Iovel, K. Mertins, J. Kischel, A. Zapf and M. Beller, Angew. Chem., Int. Ed., 2005, 44, 3913; (e) G. Anilkumar, B. Bitterlich, F. G. Gelalcha, M. K. Tse and M. Beller, Chem. Commun., 2007, 289; (f ) F. G. Gelalcha, B. Bitterlich, G. Anilkumar, M. K. Tse and M. Beller, Angew. Chem., Int. Ed., 2007, 46, 7293; (g) B. Morandi, A. Dolva and E. M. Carreira, Org. Lett., 2012, 14, 2162; (h) J. Y. Wu, B. Moreau and T. Ritter, J. Am. Chem. Soc., 2009, 131, 12915; (i) J. Y. Wu, B. N. Stanzl and T. Ritter, J. Am. Chem. Soc., 2010, 132, 13214. 10 A. R. Middleton and G. Wilkinson, J. Chem. Soc., Dalton Trans., 1980, 1888. 11 (a) H. Eckert and Y. Kiesel, Angew. Chem., Int. Ed. Engl., 1981, 20, 473; (b) K. Kano, M. Takeuchi, S. Hashimoto and Z. Yoshida, J. Chem. Soc., Chem. Commun., 1991, 1728; (c) M. Takeuchi and K. Kano, Organometallics, 1993, 12, 2059. 12 Some recent examples of iron-catalysed hydrogenation reactions: (a) T. S. Carter, L. Guiet, D. J. Frank, J. West and S. P. Thomas, Adv. Synth. Catal., 2013, 355, 880; (b) M. D. Greenhalgh and S. P. Thomas, Synlett, 2013, 531; (c) M. D. Greenhalgh and S. P. Thomas, J. Am. Chem. Soc., 2012, 134, 11900; (d) B. A. F. Le Bailly, M. D. Greenhalgh and S. P. Thomas, Chem. Commun., 2012, 48, 1580; (e) B. A. F. Le Bailly and S. P. Thomas, RSC Adv., 2011, 1, 1435; (f) W. M. Czaplik, M. Mayer and A. Jacobi von Wangelin, ChemCatChem, 2011, 3, 135. 13 J. Meinwald, Y. G. Meinwald and T. N. Barker, J. Am. Chem. Soc., 1964, 86, 4074. 14 P.-H. Phua, L. Lefort, J. A. F. Boogers, M. Tristany and J. G. de Vries, Chem. Commun., 2009, 3747. 15 J. A. Widegren, M. A. Bennett and R. G. Finke, J. Am. Chem. Soc., 2003, 125, 10301. 16 (a) H. I. Schlesinger, H. C. Brown, A. E. Finholt, J. R. Gilbreath, H. R. Hoekstra and E. K. Hyde, J. Am. Chem. Soc., 1953, 75, 215; (b) G. N. Glavee, K. J. Klabunde, C. M. Sorensen and G. C. Hadjipanayis, Inorg. Chem., 1995, 34, 28; (c) G. D. Forster, L. F. Barquin, R. L. Bilsborrow, Q. A. Pankhurst, I. P. Parkin and W. A. Steer, J. Mater. Chem., 1999, 9, 2537.
Dalton Trans., 2014, 43, 38–41 | 41
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Iron catalysed nitrosation of olefins to oximes† Cite this: Dalton Trans., 2014, 43, 38 Received 1st July 2013, Accepted 14th October 2013
Ritwika Ray, Abhishek Dutta Chowdhury, Debabrata Maiti* and Goutam Kumar Lahiri*
DOI: 10.1039/c3dt51764k www.rsc.org/dalton
Fe(BF4)2·6H2O/2,6-pyridinedicarboxylic acid catalysed nitrosation of a wide variety of substituted styrenes has been developed in the presence of t-BuONO/NaBH4 under H2 pressure (10 bar) in MeOH–H2O (5 : 1) to afford corresponding oximes in good to excellent yields.
The Beckmann rearrangement of cyclohexanone oxime to ε-caprolactum is an industrially important reaction for the synthesis of Nylon-6.1 Moreover, oximes can be easily reduced to amines,2 which are further used in the manufacturing of dyes, plastics, synthetic fibres and pharmaceuticals. Oximes are used as anti-skinning agents in paint and blocking agents in the polymer industry.3 Biological activities including anti-allergic and anti-inflammatory behaviours of oximes have also been demonstrated.4 Recently, Ley and Bertram have further revealed that oximes and oxime-ethers of hydroxylated benzaldehydes and acetophenones are powerful anti-oxidants.5 Thus, owing to the extensive utility of oximes, both in industry and biology, development of sustainable and novel methods for their synthesis continues to be an active area of research.6 Nitrosation of olefin feedstock in the presence of nitric oxide or alkyl nitrites is one of the convenient approaches for producing oximes. However, catalytic nitrosation of olefins in the presence of various reductants is rare. Only a few reports involving cobalt catalysed oxime formation are known in the literature so far.7 Environmentally benign iron mediated oxime synthesis is limited to the only example reported by Beller et al. where iron(II)-phthalocyanine was used as the active catalytic species.8 Herein we demonstrate a simple and convenient method of Fe(BF4)2·6H2O/2,6-pyridinedicarboxylic acid (dipic) catalysed synthesis of oximes (2) from styrene derivatives (1) in the presence of cheap and readily available tertiary-butylnitrite (t-BuONO) and sodium borohydride (NaBH4) (Scheme 1).9
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: General procedure, characterization data and 1H and 13C-NMR spectra of 2a–2u. See DOI: 10.1039/ c3dt51764k
38 | Dalton Trans., 2014, 43, 38–41
Scheme 1
An outline for the catalytic transformation of 1 to 2.
During our initial investigation, the reaction of styrene with t-BuONO/NaBH4 in the presence of Fe(BF4)2·6H2O/dipic was chosen as the model system. Optimisation of the critical reaction parameters by employing an array of solvents establishes MeOH–H2O (5 : 1) as the best combination (Table 1, entry 3). The use of only MeOH as the solvent results in appreciably lower yield of the desired product (Table 1, entry 5). Notably, addition of water (1 mL) along with MeOH (5 mL) leads to the product formation in excellent yield (Table 1, entry 3). On the other hand, EtOH–H2O (5 : 1) as a solvent system produces acetophenone oxime in 65% yield (GC yield) under the identical reaction conditions (Table 1, entry 8). However, the selective use of EtOH improves the yield to a small extent (Table 1, entry 10), but it is reasonably low compared to the MeOH–H2O (5 : 1) solvent system, which indeed has prompted us to restrict the present investigations in MeOH–H2O (5 : 1). It should also be noted that the yield of the oxime decreased considerably when the same reaction was carried out for longer time and with higher catalyst loading. Therefore, it is presumed that oximes being slowly reduced by BH4−, a prolonged reaction time led to the decomposition of the products.7d Lowering of the yield can further be attributed to the probable side reactions leading to the formation of nitrosoalkanes, nitrosohydroxylamines, azo compounds and N-nitrosoamine oxides.10 However, these complex nitrogen compounds could not be detected since such compounds get adsorbed on the silica gel column during the isolation of the products.7d Other hydrogenation side-products, resulting from the reduction
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Table 1
Communication
Iron catalysed nitrosation of olefins in various solventsa
Entry
Solvent
Conversionb (%)
Yieldc (%)
1 2d 3 4d 5 6 7d 8 9d 10
Toluene MeOH–H2O (5 : 1) MeOH–H2O (5 : 1) MeOH MeOH THF EtOHe–H2O (5 : 1) EtOHe–H2O (5 : 1) EtOHe EtOHe
0 >90 >99 80 90 10 85 90 80 85
0 82 (75) 99 (90) 60 65 5 60 65 70 75
a
Reaction conditions: 3 mmol of styrene, 1 mol% Fe(BF4)2·6H2O, 1 mol% dipic, 1.5 equiv. NaBH4 and 1.5 equiv. t-BuONO, solvent, 6 h at room temperature under 10 bar of H2. b Determined by GC after the reaction. c Determined by GC after workup. Isolated yield is given in parenthesis. d The reaction was performed under normal atmospheric conditions without additional H2 pressure. e Absolute grade EtOH used.
of styrene, such as 2,3-diphenylbutane, ethyl benzene and 1-phenylethyl amine were however detected by GC in trace quantities.11,12 Although under normal atmospheric conditions >90% conversion of styrene was observed in MeOH–H2O (5 : 1) and that resulted in 82% desired product (Table 1, entry 2 and Table S1†), the best yield of 2 is obtained under dihydrogen pressure (Table 1, entry 3 and Table S1†). All the reactions are therefore reported under an additional 10 bar H2 pressure. The precise role of the extra H2 is not clear at present, however, it may be reasonable to assume that the additional H2 pressure extends a better reducing environment towards the formation of the desired products in good to excellent yields. Various iron, ruthenium and cobalt salts have also been tested in the presence of different commercially available biand tridentate ligands (Table 2). It is found that though Fe(BF4)2·6H2O/dipic and Fe(Pc) catalytic species under the present reaction protocol (Table 2, entries 11–12) show comparable activity, lower yield is obtained in the latter case. Hence, optimisation of the suitable reaction conditions reveals that the in situ generated catalyst derived from 1 mol% Fe(BF4)2·6H2O and 1 mol% dipic extends maximum activity and results in acetophenone oxime formation in 90% yield. The optimised reaction conditions indeed allow the reduction of a wide variety of styrene derivatives to generate the corresponding oximes (Table 3). Excellent yields of oximes are obtained with different ortho-, meta- and para-alkyl/alkoxide substituted styrene derivatives (2a–2d, 2l–2m). Halogens at different positions of the styrene ring afforded the corresponding oximes in high yields without any dehalogenation (2e–2k). Remarkably, styrene with a formyl group is found to be completely unaffected under the present reaction protocol,
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Table 2
Catalytic nitrosation of olefinsa
Entry
Catalystb
Conversion (%)
Yieldc (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
FeCl3·6H2O/pic FeCl3·6H2O/dipic FeSO4·7H2O/pic FeSO4·7H2O/dipic Fe(ClO4)2·6H2O/pic Fe(ClO4)2·6H2O/dipic Fe(BF4)2·6H2O/pic Fe(BF4)2·6H2O/bipy Fe(BF4)2·6H2O/dppf Fe(BF4)2·6H2O/trpy Fe(BF4)2·6H2O/dipic Fe(Pc) Fe(acac)3 Ru(trpy)Cl3 RuCl3·3H2O/dipic CoCl2·6H2O/dipic
75 60 65 75 25 80 90 50 85 88 >99 >90 70 75 10 75
45 40 45 40 14 45 65 35 70 69 99 (90) 80 20 25 2 15
a Reaction conditions: 3 mmol of styrene, 1 mol% catalyst, 1 mol% ligand, 1.5 equiv. NaBH4 and 1.5 equiv. t-BuONO in 5 mL MeOH and 1 mL H2O, 6 h at room temperature under 10 bar of H2. b dipic = 2,6pyridine dicarboxylic acid, trpy = 2,2′:6′,2″-terpyridine, dppf = 1,1′-bis(diphenylphosphino)ferrocene, pic = pyridine-2-carboxylic acid. c Determined by GC using n-dodecane as an internal standard. Isolated yield is given in parenthesis.
and selective oxime formation occurs, albeit in low yield (2q). Although the present method is not applicable for 1,2-diaryl substituted olefins, β-methyl substituted styrene derivatives give good yields of the desired oxime products (2t and 2u). Other reducible groups such as –NO2 (2o) and –CN (2p) are also tolerated. These examples further highlight the beneficial features of the present method since such groups can easily be converted to the corresponding amines. Even 4-vinylaniline gives the desired 1-(4-aminophenyl)ethanone oxime in preparatively useful yield (2r). The present reaction protocol thus proceeds in a highly chemo- and regioselective manner, exclusively producing acetophenone oxime derivatives. A tentative mechanism for the catalytic cycle has been outlined in Scheme 2, primarily based on our experimental observations. Under the present reaction conditions, exclusive formation of acetophenone oxime derivatives indeed rules out the possibility of the involvement of NO+ in the initial stage of the reaction, which would otherwise lead to the formation of 2-phenylacetaldehyde oxime and its derivatives on addition to substituted ethylenes.13 Hydrogen evolution has also been observed in the initial stage of the reaction on addition of NaBH4 in the presence of MeOH–H2O (5 : 1). The determining step in the mechanism is the addition of olefin to the metal centre of the iron–dipic complex (A) that leads to the formation of σ-alkyliron(III) complex (B). The formation of iron-dipic complex (A) as the active catalytic species during the reaction has also been evidenced by the ESI(+)-MS (Fig. S37†). The reaction of B with t-BuONO subsequently, results in the generation
Dalton Trans., 2014, 43, 38–41 | 39
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Table 3
Substrate scope for iron catalysed nitrosation of olefinsa
a
Reaction conditions: 3 mmol of styrene, 1 mol% Fe(BF4)2·6H2O, 1 mol% dipic, 1.5 equiv. NaBH4 and 1.5 equiv. t-BuONO in 5 mL MeOH and 1 mL H2O, 6 h at room temperature under 10 bar of H2. b Yields determined by GC after the reaction, isolated yields are in parentheses. c Yields based on 1H NMR with PhTMS as an internal standard. d Substrate: trans-β-methyl-styrene. e Substrate: cis-β-methylstyrene. f Yield for the reaction done on a 1 g scale.
Dalton Transactions
of an alkyl nitrite species (C), which is finally converted to D, the desired oxime. A set of preliminary mechanistic investigations reveal the formation of black particles (may be due to Fe(0) or FexBy formation), in the absence of alkene under identical reaction conditions. However, no such black particle formation was observed in the presence of styrene under the same reaction conditions. This observation may provide an indirect justification in favour of the proposed formation of the iron-alkyl species as the intermediate which eventually suppresses the formation of black Fe particles in the presence of the dianionic dipicolinic acid as the ligand. This also suggests that the possibility of Fe-nanoparticles acting as the active catalytic species for the hydrogenation process14 in the present case is unlikely. The homogeneous catalytic route has, however, been established by the Hg(0) poisoning experiment as well as by the TEM and XPS data (see ESI† for details).15 The addition of Hg(0) in the catalytic process failed to show any poisoning effect and TEM/XPS did not identify any iron nano-particles/clusters and iron(0) state, respectively. It may also be logical to presume that the conversion of C to D is being facilitated by water since the present catalytic reaction with styrene using MeOH–D2O (5 : 1) as the solvent system exhibits a HRMS (ESI+) peak at m/z = 137.0813 (Fig. S40†) corresponding to PhC(Me)NOD (d1-D) (where calcd m/z (M + H+) = 137.0820). Additionally, water might also play a role in the hydrolysis of NaBH4 to liberate hydrogen16 in the presence of the Fe(II) catalyst system and thereby facilitates the catalytic hydrogenation process under the present reaction conditions. In conclusion, an efficient and selective iron-catalysed reduction protocol has been developed for the conversion of styrene derivatives to their respective oximes. Good to excellent yields are obtained for most of the chosen substrates under mild and benign reaction conditions. This activity was supported by DST (GKL) and BRNS (DM/ 2011/20/37C/13). Financial support received from UGC (fellowship to R.R.) and CSIR (fellowship to A.D.C.), New Delhi, India is gratefully acknowledged.
Notes and references
Scheme 2
An outline of the tentative mechanistic pathway.
40 | Dalton Trans., 2014, 43, 38–41
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9 Some recent iron-catalysed reactions of olefins: (a) R. Ray, A. D. Chowdhury and G. K. Lahiri, ChemCatChem, 2013, 5, 2158; (b) A. D. Chowdhury, R. Ray and G. K. Lahiri, Chem. Commun., 2012, 48, 5497; (c) A. D. Chowdhury and G. K. Lahiri, Chem. Commun., 2012, 48, 3448; (d) I. Iovel, K. Mertins, J. Kischel, A. Zapf and M. Beller, Angew. Chem., Int. Ed., 2005, 44, 3913; (e) G. Anilkumar, B. Bitterlich, F. G. Gelalcha, M. K. Tse and M. Beller, Chem. Commun., 2007, 289; (f ) F. G. Gelalcha, B. Bitterlich, G. Anilkumar, M. K. Tse and M. Beller, Angew. Chem., Int. Ed., 2007, 46, 7293; (g) B. Morandi, A. Dolva and E. M. Carreira, Org. Lett., 2012, 14, 2162; (h) J. Y. Wu, B. Moreau and T. Ritter, J. Am. Chem. Soc., 2009, 131, 12915; (i) J. Y. Wu, B. N. Stanzl and T. Ritter, J. Am. Chem. Soc., 2010, 132, 13214. 10 A. R. Middleton and G. Wilkinson, J. Chem. Soc., Dalton Trans., 1980, 1888. 11 (a) H. Eckert and Y. Kiesel, Angew. Chem., Int. Ed. Engl., 1981, 20, 473; (b) K. Kano, M. Takeuchi, S. Hashimoto and Z. Yoshida, J. Chem. Soc., Chem. Commun., 1991, 1728; (c) M. Takeuchi and K. Kano, Organometallics, 1993, 12, 2059. 12 Some recent examples of iron-catalysed hydrogenation reactions: (a) T. S. Carter, L. Guiet, D. J. Frank, J. West and S. P. Thomas, Adv. Synth. Catal., 2013, 355, 880; (b) M. D. Greenhalgh and S. P. Thomas, Synlett, 2013, 531; (c) M. D. Greenhalgh and S. P. Thomas, J. Am. Chem. Soc., 2012, 134, 11900; (d) B. A. F. Le Bailly, M. D. Greenhalgh and S. P. Thomas, Chem. Commun., 2012, 48, 1580; (e) B. A. F. Le Bailly and S. P. Thomas, RSC Adv., 2011, 1, 1435; (f) W. M. Czaplik, M. Mayer and A. Jacobi von Wangelin, ChemCatChem, 2011, 3, 135. 13 J. Meinwald, Y. G. Meinwald and T. N. Barker, J. Am. Chem. Soc., 1964, 86, 4074. 14 P.-H. Phua, L. Lefort, J. A. F. Boogers, M. Tristany and J. G. de Vries, Chem. Commun., 2009, 3747. 15 J. A. Widegren, M. A. Bennett and R. G. Finke, J. Am. Chem. Soc., 2003, 125, 10301. 16 (a) H. I. Schlesinger, H. C. Brown, A. E. Finholt, J. R. Gilbreath, H. R. Hoekstra and E. K. Hyde, J. Am. Chem. Soc., 1953, 75, 215; (b) G. N. Glavee, K. J. Klabunde, C. M. Sorensen and G. C. Hadjipanayis, Inorg. Chem., 1995, 34, 28; (c) G. D. Forster, L. F. Barquin, R. L. Bilsborrow, Q. A. Pankhurst, I. P. Parkin and W. A. Steer, J. Mater. Chem., 1999, 9, 2537.
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