otto
(Hive Bee)
04-16-03 01:06
No 427071
      epoxides easy
(Rated as: excellent)
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dear bees,

last week otto searched some journals and came across this:

Advanced Synthesis and Catalysis 2003, 345, No.3, 389 - 392

"A simple and Convenient Method for Epoxidation of Olefins without Metal Catalysts"

They use Bleach to epoxidize various alkenes, styrenes work best. Only drawbacks are the use of acetonitrile and a phosphate buffer.

The experimental procedure:

In a 100mL Schlenk tube , KBr (357 mg, 3.0 mmol or 47 mg, 0,4 mmol respectively), buffer (10 mL, prepared by adjusting a 0.5 molar solution of KH2PO4 to a pH of 10.4 with a 2 molar NaOH solution), acetonitrile (10 mL), substrate (2.0 mmol) and diethylene glycol di-n-butyl ether (100uL, as internal standard for GC) were added. The reaction mixture was warmed to 40°C under 1000 rpm magnetic stirring using a thermostat. Aqueous NaOCl solution (1.1 equivalents) was added and the mixture was extracted with 20 mL of ethyl acetate. The combined organic layers were dried over MgSO4 and analyzed by GC. For isolation of the product, the solvent was removed under vacuum and the crude epoxide was purified by column chromatography (hexane/ethyl acetate 10:1) or destillation.

among the alkenes: time yield epoxide

styrene 1 h 80

propenylbenzene 2 h 87

4-MeOstyrene 20 min 80



This looks perfectly for asarone!

otto
 
 
 
 
    otto
(Hive Bee)
04-16-03 01:10
No 427073
      the next day otto tried it.
(Rated as: excellent)
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the next day otto tried it. After some trials he found, the pH of the buffer isn't that important (at least for the asarone) and it can be replaced by sodium carbonate. The needed amount of bleach was higher than the lit, perhaps due to some impurities in ottos asarone. Otto did:

In a 100mL beaker 2 mmol asarone (416 mg, 385 uL; ottos asarone was vac-destilled from calamus oil), 10 mL acetonitrile, 10 mL tap water, about 400 mg sodium carbonate1 and 350 mg potassium bromide (3 mmol)2 were mixed. Then, 1.5 equivalents of bleach3 was added and the mixture stirred for 15 to 20 minutes4. The mixture was extracted once with 15 mL Dichloromethane5 and the dried organic phase evaporated to give 360 mg (80%) fairly pure asarone epoxide6.

This epoxide was hydrolyzed/rearranged to ketone by the usual H2SO4 - method. 350 mg gave only 75 mg ketone after purification. A key to why that is propably in Post 409525 (Rhodium: "Why asarone cannot be oxidized with peracids", Methods Discourse).
 
[1] This amount worked fine. less may work, too.
[2] This amount can propably be reduced to as little as 50 mg; with higher amounts yields of epoxide are higher according to the literature.
[3] 1.1 eq like in the lit. left unreacted asarone as confirmed by TLC.
[4] Do not exceed this time! the epoxide slowly hydrolyses giving the diol and other stuff.
[5] Any solvent capable of extracting the epoxide should do it. DCM is just easy to remove.
[6] As confirmed by TLC.

TLC Rf values (hexane/ethyl acetate 4:1):

Asarone 0.4
Epoxide 0.1
Diol 0.0
Ketone (TMP2P) 0.2

All visible in UV or by KMnO4-reagent.
 
 
 
 
    sYnThOmAtIc
(Hive Addict)
04-29-03 05:41
No 430122
      Try a different solvent please.  Bookmark   

Hello, acetonitrile is available here but always like to try and avoid it.  Is there anyway you may be able to try the reaction with some sort of easily available alcohol or somehting. Like methanol or something readily available.

Or confirm or deny wether or not it can be used. Does the reference state anywhere why they chose MeCN or whether it can be substituted.

Yes, That pic really is me!
 
 
 
 
    otto
(Hive Bee)
04-29-03 22:24
No 430265
      Hi sYnThOmAtIc, here is what they write about...  Bookmark   

Hi sYnThOmAtIc,

here is what they write about solvents:

The solvent system is of major influence for the outcome of the reaction. Using t-butyl alcohol or water alone, the epoxide is obtained only in 1 - 4%. Biphasic mixtures of organic solvents and water give better results, however the yields with dichloromethane or tetrahydrofuran are still comparably low (ca. 30%). Mixtures of acetonitrile or t-butyl alcohol and buffered water solution (pH=10.4) lead to the best epoxide yield (up to 90%). Variation of the pH from 9.5 - 12.0 and changing the reaction time do not have a significant influence on the reaction. In general, most of the reactions are finished after 0.5 h. Slightly lower yields are obtained at pH 11.6 and 12.

Below, there is a table. epoxidation of a-methylstyrene gives 82% yield in t-BuOH and 90% in CH3CN.

Currently otto has no t-BuOH available.

MeOH was tried but didnt work and caused all inorganic salt to precipitate. TLC shows a bunch of products and starting material.

Looks like either CH3CN or tBuOH...

otto
 
 
 
 
    gruns
(Hive Bee)
09-10-03 08:54
No 458094
      A Simple and Convenient Method for Epoxidation
(Rated as: excellent)
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As referenced in Post 427071 (otto: "epoxides easy", Novel Discourse)

A Simple and Convenient Method for Epoxidation of Olefins without Metal Catalysts
Markus Klawonn, Santosh Bhor, Gerald Mehltretter, Christian Döbler, Christine Fischer, Matthias Beller*
Adv. Synth. Catal. 345, 389-392 (2003) (../rhodium/pdf /non-metal.alkene.epoxidation.pdf)

Institut fur Organische Katalyseforschung an der Universität Rostock e.V., Buchbinderstr. 5 ± 6, 18055 Rostock, Germany
Fax: (+49)-381-4669324, e-mail: matthias.beller@ifok.uni-rostock.de
Received: September 4, 2002; Accepted: November 14, 2002


Abstract

An easy method for epoxidation of olefins using bleach (sodium hypochlorite) and either a stoichiometric or catalytic amount of bromide ion has been developed. Without any transition metal catalyst a variety of non-activated olefins give epoxides in high yields and good selectivity at ambient conditions.

Keywords: epoxidation; epoxides; oxidation; sodium hypochlorite

Introduction

Oxidation reactions of olefins to give epoxides are of major importance for organic synthesis. Nowadays, especially asymmetric epoxidation reactions are in the focus of methodological developments.[1] However, the synthesis of racemic epoxides is still important on laboratory as well as industrial scales. A convenient method for the synthesis of epoxides is the oxidation of olefins with hydrogen peroxide or alkyl peroxides in the presence of transition metal complexes.[2] However, in general the activity of the catalyst is limited and the metal catalyst as well as modifying ligands have to be separated after the reaction. Nevertheless, significant advances have been made in non-asymmetric metalcatalyzed epoxidation reactions in the last decade.[3] Especially noteworthy with respect to simplicity and catalyst productivity was the development of redox active polyoxometalates (POM×s) in combination with phase transfer active agents as catalysts in combination with hydrogen peroxide.

More traditionally, epoxides are synthesized by the reaction of olefins with hydrogen peroxide in the presence of acetic or formic acid.[4] This convenient method involves the in-situ formation of the corresponding peracid, which easily undergoes epoxidation reaction. A drawback of this method are potential side reactions of the acid. Hence, the method is only of limited use for acid-labile olefins or epoxides. An alternative cheap and practical oxidant is bleach (sodium hypochlorite), which might be used either directly or is produced in situ from chlorine under basic conditions. Although in situ generated hypochlorite is still used in the two-step commercial process for propylene epoxide (Scheme 1),[5] comparably few studies described the direct epoxidation of non-activated olefins with hypochlorite without metals being present.[6]



Some time ago we became interested in the improvement of known oxidation reactions of olefins. After having developed a new osmium-catalyzed dihydroxylation reaction using air as terminal oxidant,[7] we studied selective alcohol oxidation reactions[8] and the catalytic dihydroxylation in the presence of sodium hypochlorite[9] as oxidant. Based on this work, we turned interest to epoxidation reactions applying sodium hypochlorite. In this manuscript we describe a novel general method for the epoxidation of olefins using sodium hypochlorite in the presence of a catalytic or stoichiometric amount of bromide ion.

Results and Discussion

While investigating the oxidation of alpha-methylstyrene in the presence of different metal catalysts and sodium hypochlorite, we discovered that epoxidation to 2-phenyl-1-epoxypropane proceeds as a side-reaction independent from the metal catalyst used (Scheme 2).



After studying the available literature we were surprised that the direct epoxidation of non-activate dolefins using sodium hypochlorite as oxidant has not been examined in more detail. Therefore, we decided to take a closer look at this reaction. As shown in Table 1 (entry 1) the reaction proceeds in 15% yield using simple sodium hypochlorite in a biphasic mixture of water and tert-butyl alcohol at room temperature. We thought that the in situ generation of the more active hypobromide will increase the epoxide yield. Indeed, upon addition of 1.5 equiv. of KBr (with respect to sodium hypochlorite) the reaction proceeds smoothly within 2 h giving the corresponding epoxide in 90% yield at 25C. Longer reaction times lead to slight decomposition of the desired product. The solvent system is of major influence for the outcome of the reaction. Using tert-butyl alcohol or water alone, the epoxide is obtained only in 1-4%. Biphasic mixtures of organic solvents and water give better results, however the yields with dichloromethane or tetrahydrofuran are still comparably low (ca. 30%). Mixtures of acetonitrile or tert-butyl alcohol and buffered water solution (pH=10.4) lead to the best epoxide yield (up to 90%). Variation of the pH from 9.5 ± 12.0 and changing the reaction time do not have a significant influence on the reaction. In general, most of the reactions are finished after 0.5 h. Slightly lower yields are obtained at pH 11.6 and 12.

Advantageously, the epoxidation reaction with sodium hypochlorite also proceeds in the presence of catalytic amounts of bromide ions. The reduction of the amount of bromide ions from 1.5 equiv. to 0.2 equiv. leads only to a slight decrease of epoxide. Further reduction of the bromide concentration results in lower epoxide yield (46% at 0.05 equivalents of KBr). Next, we tested whether a combination of hydrogen peroxide and bromide ions is also able to effect epoxidation reactions. However, no conversion of alpha-methylstyrene is observed under these conditions. In order to get more information about the mechanism, we studied the concentration-time dependence of the olefin and reaction products via GC. A >95% conversion of alpha-methylstyrene is observed within the first minute. At the same time 2-phenyl-2-hydroxy-1-propyl bromide is formed in nearly 90% yield. This bromohydrin is converted immediately to the desired epoxide. While nearly 75% of the desired epoxide is obtained within 5 minutes, a maximum yield of epoxide (83%) is seen in between 30 and 60 minutes. Next, we studied the scope and limitations of the procedure. Different types of olefins were tested (Table 2). Aromatic olefins such as styrene, alpha-methylstyrene, beta-methylstyrene, p-chlorostyrene, p-methoxystyrene, and 1-phenylcyclohexene give the corresponding epoxide in 70-93% yield (Table 2, entries 1-12). In general, the reaction is finished within 1 to 2 hours. Much longer reaction times can lead to slightly lower yields due to subsequent decomposition of the epoxide. In case of aromatic olefins apart from the desired epoxidation reaction small amounts of halogenation of the aromatic nucleus can be observed. In addition, the 1,2-dibromo or 1,2-chlorobromo derivatives arising from halogen addition along the double bond are detected. The slow increase in product selectivity with increased reaction time using alpha-methylstyrene as substrate (Table 2, entries 3 and 4) arises from the slow hydrolysis of the byproduct 1,2-dibromo-2-phenylpropane. This subsequent hydrolysis reaction is evident with all substrates forming benzylic bromides as byproducts; e.g., all substituted styrenes. All reactions proceed also well in the presence of catalytic amounts of bromide. However, the addition of 1.5 equiv. of KBr give slightly improved yields. Terminal aliphatic olefins, e.g., 1-octene and butyl allyl ether need longer reaction times for complete conversion (Table 2, entries 13 and 14, 19 and 20). On the otherhand internal aliphatic olefins (5-decene, 2,3-dimethyl-2-butene) show a fast conversion of the olefin, but epoxide formation needs longer times compared to the aromatic olefins.

Conclusion

In summary, we have shown that various non-activated olefins can be converted to epoxides by using simply sodium hypochlorite and bromide salt. It is surprising that this type of non-metal-catalyzed epoxidation has been previously largely overseen. Aromatic olefins furnish the corresponding epoxide with high selectivity at room temperature to 40°C in short time (<1±2 h). Aliphatic olefins react somewhat more sluggishly. It is clear that the method described here is associated with the production of 1 equivalent of NaCl. Nevertheless, the procedure can be performed safely without any additional transition metals at ambient conditions. Further advantages of the procedure remain in the simplicity and the low-priced oxidant.

Table 1 - Epoxidation of alpha-methylstyrene using the NaOCl/KBr system (../rhodium/chemistry /pictures/naocl-epox.tbl1.gif)

Table 2 - Epoxidation of various alkenes using NaOCl (../rhodium/chemistry /pictures/naocl-epox.tbl2.gif)

Experimental Section

General Information

All reactions were carried out without any special precautions under an atmosphere of air. Chemicals and solvents were purchased from Fluka and used as received. 1H and 13C NMR spectra were obtained on a Bruker ARX 400 spectrometer. Gas chromatographic analyses were run on a Hewlett-Packard GC6890 series, HP 5, 5% phenylmethyl siloxane, capillary (30m, 250micromm, 0.25micromm).

General Procedure

In a 100-mL Schlenk tube, KBr (357mg, 3.0 mmol or 47mg, 0.4 mmol, respectively), buffer (10mL, prepared by adjusting a 0.5 molar solution of KH2PO4 to a pH of 10.4 with a 2 molar NaOH solution), acetonitrile (10 mL), substrate (2.0 mmol) and diethylene glycol di-n-butyl ether (100microL, as internal standard for GC) were added. The reaction mixture was warmed to 40C under 1000 rpm magnetic stirring using a thermostat. Aqueous NaOCl solution (Fluka commercial sodium hypochlorite, 1.1 mL of a 12.4% solution, d=1.2 gmL(^-1), 1.1 equivalents) was added at once and stirring and temperature were maintained for 15 minutes to 24 hours depending on the substrate (see Tables above). Then, Na2SO3 (0.5 g) was added and the mixture was extracted with 20 mL of ethyl acetate. The combined organic layers were dried over MgSO4 and analyzed by GC.

For isolation of the product, the solvent was removed under vacuum and the crude epoxide was purified
by column chromatography (hexane/ethyl acetate 10:1) or distillation.

Acknowledgements

This work was supported by the "Fonds der Chemischen Industrie", the "Bundesministerium fur Bildung und Forschung (BMBF)", the "Deutsche Forschungsgemeinschaft", and the State Mecklenburg-West Pomerania.

References and Notes

[1] Selected reviews: a) E. Jacobsen, A. Pfaltz (Eds.), Catalytic Asymmetric Synthesis, Springer, Heidelberg, 1999; b) M. Beller, C. Bolm (Eds.), Transition Metals for Organic Synthesis, Wiley-VCH, Weinheim, 1998; c)K.A. Joergensen, Chem. Rev. 1989, 89, 431.
[2] a) R. A. Sheldon, J. K. Kochi, Metal-Catalyzed Oxidations of Organic Compounds, Academic Press, New York, 1981; b) G. A. Barf, R. A. Sheldon, J. Mol. Catal. 1995, 102, 23; c) I. W. C. E. Arends, R. A. Sheldon, Top. Catal. 2002, 19, 133; d) D. Ostovic, T. C. Bruice, Acc. Chem. Res. 1992, 25, 314.
[3] Selected examples: a) K. Sato, M. Aoki, M. Ogawa, T. Hashimoto, D. Panyella, R. Noyori, Bull. Chem. Soc. Jpn. 1997, 70, 905; b) K. Sato, M. Aoki, M. Ogawa, T. Hashimoto, R. Noyori, J. Org. Chem. 1996, 61, 8310; c) D. C. Duncan, R. C. Chambers, E. Hecht, C. L. Hill, J. Am. Chem. Soc. 1995, 117, 681; d) R. Ben-Daniel, A. M. Khenkin, R. Neumann, Chem. Eur. J. 2000, 6, 3722; e) D. Hoegaerts, B. F. Sels, D. E. de Vos, F. Verpoort, P. A. Jacobs, Cat. Today 2000, 60, 209; f) Y. Ishii, K. Yamawaki, T. Ura, H. Yamada, T. Yoshida, M. Ogawa, J. Org. Chem. 1988, 53, 3587; g) C. Venturello, R. Daloisio, J. Org. Chem. 1988, 53, 1553; h) M. Bosing, A. Noh, I. Loose, B. Krebs, J. Am. Chem. Soc. 1998, 120, 7252; i) D. de Vos, T. Bein, Chem. Commun. 1996, 917; j) R. Neumann, M. Gara, J. Am. Chem. Soc. 1995, 117, 5066; k) R. Neumann, M. Gara, J. Am. Chem. Soc. 1994, 116, 5509.
[4] Recent examples usingperacids for olefin epoxidation without metal catalyst: a) K. Crawford, V. Rautenstrauch, A. Uijttewaal, Synlett 2001, 1127; b) U. Wahren, I. Sprung, K. Schulze, M. Findeisen, G. Buchbauer, Tetrahedron Lett. 1999, 40, 5991; c) D. R. Kelly, J. Nally, Tetrahedron Lett. 1999, 40, 3251.
[5] K. Weissermel, H. J. Arpe, Industrielle Organische Chemie, 5th edn., Wiley-VCH, Weinheim, 1998.
[6] S. Krishnan, D. G.Kuhn, G. A. Hamilton, J. Am. Chem. Soc. 1977, 99, 8121.
[7] a) C. Dˆbler, G. M. Mehltretter, M. Beller, Angew. Chem. Int. Ed. 1999, 38, 3026; b) C. Döbler, G. M. Mehltretter, U. Sundermeier, M. Beller, J. Am. Chem. Soc. 2000, 122, 10289; c) C. Dˆbler, G. M. Mehltretter, U. Sundermeier, M. Beller, J. Organomet. Chem. 2001, 621, 70.
[8] C. Döbler, G. M. Mehltretter, U. Sundermeier, M. Beller, M. Eckert, H.-C. Militzer, Tetrahedron Lett. 2001, 42, 8447.
[9] G. M. Mehltretter, S. Bhor, M. Klawonn, C. Dˆbler, U. Sundermeier, M. Eckert, H.-C. Militzer, M. Beller, Synthesis 2003, 295.

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