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View PDF Version Green Chemistry View Article Online COMMUNICATION View Journal | View Issue Hydrogen peroxide/dimethyl carbonate: a green system for epoxidation of N-alkylimines and Cite this: Green Chem., 2016, 18, 4859 N-sulfonylimines. One-pot synthesis of N N Received 20th May 2016, -alkyloxaziridines from -alkylamines and Accepted 22nd July 2016 (hetero)aromatic aldehydes† DOI: 10.1039/c6gc01394e a,b b a www.rsc.org/greenchem Jamil Kraïem, Donia Ghedira and Thierry Ollevier* A green method for epoxidation of imines using an environ- and oxidation with buffered oxone.24 Perfluoro-oxaziridines mentally benign oxidant system, H2O2/dimethyl carbonate, was have also been prepared by the oxidation of the corresponding N 25 developed. -Alkyloxaziridines were prepared in high yields from perfluoro-imines with H2O2 in the presence of a base. These N 26 11,17 Creative Commons Attribution 3.0 Unported Licence. -alkylamines and (hetero)aromatic aldehydes in one-pot fashion, methods usually use toxic solvents such as CH3CN and 19b whereas N-sulfonyloxaziridines have been prepared by using the CH2Cl2. Furthermore, most of them generate stoichiometric same oxidant system and 5 mol% of Zn(OAc)2·2H2O as catalyst. amounts of undesirable waste, and require tedious work-up and purification. 1 Oxaziridines, first reported by Emmons in 1957, exhibit Recently, such synthetic routes utilizing hazardous solvents unusual and distinctive reactivity, which is linked to the and reagents and generating toxic waste have become discour- nature of their N-substituent. For example, N-sulfonyloxaziri- aged and there have been much efforts to develop safer and dines have been used as electrophilic oxygenating agents for environmentally-benign alternatives. This green chemistry 2 3 4 enolates, alkenes and sulfides. N-Alkyloxaziridines undergo approach was introduced in 1990 with the aim at developing 5,6 This article is licensed under a cycloaddition reactions with a variety of heterocumulenes, cleaner processes through the design of innovative and envi- 7 8 9 10 alkenes, alkynes, nitriles and arynes leading to various ronmentally benign chemical reactions. In recent years, increas- five-membered ring heterocycles. In addition, oxaziridines are ingly stringent environmental requirements have led to a great 1 11 Open Access Article. Published on 25 July 2016. Downloaded 9/24/2021 5:12:46 AM. useful as precursors of nitrones, amides, secondary interest in the application of sustainable and green oxidations. 12 1,13 amines and N,N-disubstituted hydroxylamines. Within The concept of green chemistry has been rapidly expanding the past decade, the chemistry of oxaziridines has been signifi- and important advances have been achieved in this field.27 cantly expanded and now encompasses some new interesting Interest in new synthetic methods has also been extended to reaction types, among which the enantioselective oxyamina- many relevant classes of chemical compounds, such as oxaziri- tion of alkenes with N-sulfonyloxaziridines, catalyzed by dines. In this case, two attempts of eco-friendly synthesis of 14 15 Cu(II) and Fe(II) complexes, and the enantioselective Lewis- N-alkyloxaziridines were described in the literature. The first base catalyzed cycloaddition of N-sulfonyloxaziridines with one consisted in the oxidation of imines with H O in aceto- 16 2 2 ketenes can be mentioned. nitrile, catalyzed by 2 mol% of sodium tungstate,11,27b and the Oxaziridines can be readily prepared from imines, through other one described a chemo-enzymatic synthesis of N-alkylox- 17 a number of methods including oxidation with peracids, par- aziridines using H O /octanoic acid system as oxidant of 18,19 2 2 ticularly m-chloroperbenzoic acid (MCPBA), oxidation with N-alkylimines.17 In both cases, the conversions of imines to 20 O2 in the presence of transition metals, oxidation with H2O2 oxaziridines did not reach completion and tedious work-up 1 21,22 23 in combination with acetic anhydride, nitriles, and urea and further purification were needed. Furthermore, these methods did not avoid the use of toxic solvents, such as CH3CN, and are limited to the synthesis of N-alkyloxaziridines aDépartement de chimie, Université Laval, 1045 avenue de la Médecine, rather than other types of oxaziridines. Québec (Québec) G1V 0A6, Canada. E-mail: [email protected] Among environmentally-friendly oxidation reagents, hydro- b Laboratoire de Développement Chimique, Galénique et Pharmacologique des gen peroxide is particularly attractive, both for its high oxygen Médicaments, Faculté de Pharmacie de Monastir, Université de Monastir, content and the nature of its by-products (water and O ). More- Rue Avicenne, 5000 Monastir, Tunisia 2 †Electronic supplementary information (ESI) available: Experimental procedures over, diluted aqueous solutions of hydrogen peroxide are and copies of 1H NMR and 13C NMR. See DOI: 10.1039/c6gc01394e stable and they can be easily handled and stored. On the other This journal is © The Royal Society of Chemistry 2016 Green Chem.,2016,18,4859–4864 | 4859 View Article Online Communication Green Chemistry hand, dimethyl carbonate (DMC) is an interesting green Table 1 Investigation of solvents on the model reactiona solvent and reagent alternative in organic synthesis due to its high biodegradability and low toxicity defined as the absence of associated irritating or mutagenic effects.28 Furthermore, its industrial production does not require phosgene but only methanol and CO.29 As reported by Rüsch gen. Klaas using Equiv. Equiv. Conversion DMC with large excess of hydrogen peroxide solution (60%) b Entry H2O2 t-BuNH2 Solvent Time (h) 3a (%) under chemo-enzymatic catalysis, alkenes were epoxidized with moderate to high yields.30 These authors assume that the 1 5 1 DMC 24 88 2 3 1.5 DMC 24 85 active oxidant is the monoperoxy carbonic acid methyl ester 3 5 1.5 DMC 15 100 (MeO–CO–OOH) generated by the reaction of hydrogen per- 4 5 1.5 CH3CN 20 98 5 5 1.5 EtOAc 24 61 oxide with DMC, and decomposes to CO2 and methanol after 6 5 1.5 CH3OH 24 50 epoxidation of alkenes. In general, O-alkylmonoperoxycarbonic 7 5 1.5 PCc 18 92 d acids (RO–CO–OOH) are known to be effective epoxidizing 8 5 1.5 H2O100 – d 31 34 9 5 1.5 CH2Cl2 10 0 reagents of olefins. They are generated in situ by the reac- d 31 32 10 5 1.5 Toluene 10 0 tion of ethyl chloroformiate, alkoxycarbonyl-1,2,4-triazoles, 11 5 1.5 DMPUe 10 0d alkoxycarbonylimidazoles33 or dialkyl carbonates using hydro- a 1a 1b gen peroxide solution,30,34 and they could be utilized without Conditions: , 1 mmol of ,H2O2 (30%), solvent (1 mL), rt. b Determined by 1H NMR analysis of crude product. c Propylene isolation because of their instability. In some cases, O-alkyl- carbonate. d The oxaziridine was not detected (TLC, 1H NMR). e 1,3-Di- monoperoxycarbonic acids have been isolated34 or detected.32 methyltetrahydropyrimidin-2(1H)-one. We think that the H2O2/DMC system is still under investigated in synthetic chemistry. Indeed, since the report of Rüsch gen. oxaziridine 3a was achieved when the reaction was performed Creative Commons Attribution 3.0 Unported Licence. Klaas in 1999, no other reactions using this green oxidant system have been described in the literature. by stirring a mixture of t-butylamine (1.5 mmol), benzaldehyde In this paper we describe a simple, efficient and eco-friendly (1 mmol) and aqueous solution of H2O2 (30%, 5 mmol) in 1 mL of DMC for 15 h, furnishing quantitatively oxaziridine 3a synthesis of oxaziridines by use of the green 30% H2O2/DMC system. N-Alkyloxaziridines have been prepared quantitatively (Table 1, entry 3). We assume that the active oxidant is the – – in one-pot synthetic route from the corresponding alkylamines monoperoxycarbonic acid methyl ester (MeO CO OOH) gener- and aldehydes, under mild and neutral conditions, whereas ated by the reaction of H2O2 with DMC for the following N-sulfonyloxaziridines have been prepared with high yields reasons: (i) O-alkylperoxycarbonic acids have already been 31–34 from the corresponding N-sulfonylimines under basic con- reported in the literature, among which O-benzylperoxy- This article is licensed under a carbonic acid, prepared from dibenzyl dicarbonate and H O , ditions, in the presence of Zn(OAc)2·2H2O as a catalyst. 2 2 34 With the aim of synthesizing N-alkyloxaziridines in a one- was isolated and characterized; (ii) H2O2 itself is not ffi pot fashion, we investigated the reaction of t-butylamine 1a su ciently reactive for epoxidation; this hypothesis is sup- – Open Access Article. Published on 25 July 2016. Downloaded 9/24/2021 5:12:46 AM. (1.0 and 1.5 mmol) with benzaldehyde 2a (1 mmol) and 30% ported by the results given in Table 1 (entries 8 11). hydrogen peroxide solution, in the presence of a variety of The optimized conditions found for the one-pot synthesis 3a solvents, affording 2-t-butyl-3-phenyloxaziridine 3a (Table 1). of oxaziridine (Table 1, entry 3) were chosen as model con- ditions for our continuing studies. The substrate scope was We found that, when the one-pot reaction was run in H2O, then investigated, and a variety of N-alkyloxaziridines were pre- CH2Cl2, toluene or DMPU, no transformation into the oxaziri- dine was observed (Table 1, entries 8–11). These solvents are pared from the corresponding aldehydes and alkylamines, using the 30% H O /DMC system (Scheme 1). The conden- indeed known to be inert towards H2O2. Accordingly, these 2 2 1 2 results indicate that hydrogen peroxide itself is not sufficiently sation of alkylamine with aldehyde provided the imine reactive to epoxidize the intermediate imine, whereas, when intermediate, which underwent epoxidation by the oxidizing ff 3 the reaction was run in DMC, acetonitrile, ethyl acetate, agent to a ord oxaziridine .
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