Iron-Catalyzed Epoxidation of Aromatic Olefins and 1,3-Dienes

DOI: 10.1002/adsc.201000091

Iron-Catalyzed Epoxidation of Aromatic Olefins and 1,3-Dienes

Kristin Schrçder,a Stephan Enthaler,b Beno�t Join,a Kathrin Junge,a

and Matthias Bellera,*a Leibniz-Institut f�r Katalyse e.V. an der Universit�t Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany

Fax: (+49)-381-1281-5000; phone: (+ 49)-381-1281-0; e-mail: [email protected] Technische Universit�t Berlin, Department of Chemistry, Cluster of Excellence “Unifying Concepts in Catalysis”, Straße

des 17. Juni 135, 10623 Berlin, Germany

Received: February 3, 2010; Revised: May 22, 2010; Published online: July 7, 2010

Abstract: The combination of iron ACHTUNGTRENNUNG(III) chloride,pyridine-2,6-dicarboxylic acid and formamidine li-gands allows for the epoxidation of styrenes andconjugated dienes in excellent chemoselectivity andyields.

Keywords: epoxidation; hydrogen peroxide; iron;mechanistic studies; olefins

Introduction

Oxiranes constitute important building blocks in or-ganic chemistry.[1] Thus, the development of sustaina-ble, efficient, and selective catalysts for epoxidationreactions continues to be an important goal in oxida-tion chemistry. In this respect, the choice of benignoxidants such as hydrogen peroxide is a key issue.

Known molecular-defined transition metal com-plexes which make use of hydrogen peroxide in cata-lytic epoxidation reactions, are often based on ruthe-nium,[2] rhenium,[3] manganese,[4] and more recentlyiron.[5] Obviously, the use of iron-based catalystsoffers significant advantages compared to preciousmetals. In addition to its ubiquitous availability, ironis involved in manifold enzyme-catalyzed processes.Here, frequently stabilization of the metal centretakes place via coordination to the imidazole part ofthe amino acid histidine, which participates in numer-ous biological structures.[6] For instance, it acts asligand and base for non-heme iron enzymes, and is anemerging motif in the so-called 2-His-1-Carboxylatefacial triad.[7] As known from several crystal struc-tures of the active sides of enzymes, only the “imine”-nitrogen of histidine coordinates to the metal centre.Therefore, it should be possible to remove the a-amino acid chain and still retain catalytic activity.[8]

Indeed, imidazoles and pyrrazoles have beenwidely employed as additives in epoxidation sys-tems.[9,10] More recently, we demonstrated that the

combination of imidazole ligands together with iron-ACHTUNGTRENNUNG(III) chloride provided a convenient epoxidation cata-lyst.[11] Unfortunately, in iron-catalyzed epoxidation ofstyrenes in the presence of substituted imidazoles asligand, with or without pyridine-2,6-dicarboxylic acid(H2-pydic) as a co-ligand, displayed only moderateyields and selectivity.[12]

Based on our formal ligand concept as shown inScheme 1 we became attracted by the possibility totest formamidine ligands instead of imidazole to im-

prove the epoxidation of aromatic olefins. Notably,the formamidine motif allows easy ligand tuning bysubstitution of the “amine” part and variation of sub-stituents on the “imine” nitrogen (Scheme 1). In pre-liminary experiments formamidines were successfullyapplied in the epoxidation of trans-stilbene.[13] Herein,we report an improved protocol for the highly selec-tive epoxidation of styrenes and 1,3-dienes applyingiron catalysts containing formamidine ligands.

Results

At the start of our work the reaction of styrene withhydrogen peroxide as terminal oxidant was investigat-ed in detail. Due to the sensitivity of the resulting sty-rene oxide towards acids, this epoxidation is still achallenging benchmark reaction. In general, catalytictests were run at room temperature under an air at-mosphere in the presence of FeCl3·6 H2O, H2-pydicand various formamidine ligands.

As shown in Table 1 N,N-dimethyl-N’-arylformami-dines gave yields of epoxides in the range of 59–78%

Scheme 1. Evolution of formamidine ligands from histidine.

Adv. Synth. Catal. 2010, 352, 1771 – 1778 � 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1771

UPDATES

(Table 1, entries 1–5, 7, 8). Only, ligand 1f with threefluoro substituents led to a diminished yield of 19%(Table 1, entry 6). To our delight ligand 2 with an N’-cyclohexyl group induced both improved conversionand excellent chemoselectivity. On the other handchanging methyl to phenyl substituents at the“amine” part of formamidines 3 decreased the yieldand the selectivity for styrene oxide.

Thus, the catalytic performance of ligand 2 wasmore closely examined. It is clear that all three com-ponents of the catalyst system are necessary to obtainsignificant activity (Table 2, entries 1–5). Optimizationof the catalyst loading resulted in a minimized cata-lyst concentration of 2.5 mol% FeCl3·6H2O (Table 2,entry 7). In order to obtain full conversion it is impor-tant to maintain the iron:H2-pydic:ligand ratio of

1:1:2.5. For example, increasing the amount of ironcompared to H2-pydic inhibited the reaction com-pletely (Table 2, entry 10). This is also true for a de-crease of the formamidine ligand compared toFeCl3·6H2O or H2-pydic (Table 2, entry 13).

Next, we were interested in the limitations of thisreaction. Hence, we added further amounts of sub-strate and oxidant after 1 h reaction time to achievethe maximal TON for our system. The yield of sty-rene oxide is improved up to 97% (39 TON) after theaddition of an extra portion substrate (0.5 mmol) andoxidant (3 equiv.). Further addition was unsuccessfuland the substrate remained intact in the reaction solu-tion despite an excess of oxidant.

Then, the regeneration of the catalyst system wasexplored. Therefore ligand 2 (6 mol%), H2-pydic(6 mol%), styrene (0.5 mmol), and the oxidant(3 equiv. added over 1 h via syringe pump) wereadded at the end of the reaction to investigate the re-maining catalytic activity. Unfortunately, the additiondid not result in any further yield of the epoxide. In-stead degeneration of the already formed epoxidetook place. In contrast, the addition of FeCl3·6 H2O(2.5 mol%), styrene (0.5 mmol), and oxidant

Table 1. Epoxidation of styrene: Variation of formamidineligands.[a]

No Ligand R1 Conv.[b]

[%]Yield[b]

[%]Sel.[c]

[%]

1 1a C6H5 75 62 832 1b 4-MeC6H4 71 59 843 1c 4-t-BuC6H4 98 78 834 1d 4-CF3C6H4 >99 74 745 1e 3,5-F2C6H3 76 62 816 1f 3,4,5-F3C6H2 58 19 337 1g 2,4,6-Me3C6H2 88 76 878 1h 2,6-i-Pr2C6H3 92 75 829 2 Cy 94 93 >9910 3a C6H5 88 69 7811 3b C6H5 9 2 37

[a] Reaction conditions: 0.5 mmol styrene, 5 mol%FeCl3·6 H2O and 12 mol% formamidine ligand, 5 mol%H2-pydic, tert-amyl alcohol (9 mL), 0.44 mmol dodecane,(100 mL, internal standard) were added in sequence atroom temperature in air. To this mixture a solution of30% H2O2 (170 mL, 1.5 mmol) in tert-amyl alcohol(830 mL) was added over a period of 1 h at room temper-ature by a syringe pump.

[b] Conversion and yield were determined by GC analysisand by comparison with authentic samples; to check forreproducibility the reactions were at least repeated twice.

[c] Selectivity (Sel.) refers to the chemoselectivity of epox-ide from olefin.

Table 2. Epoxidation of styrene in the presence of 2.[a]

No H2-pydic[mol%]

FeCl3·6 H2O[mol%]

2[mol%]

Conv.[b]

[%]Sel.[c]

[%]TON[d]

1 – 5 – 11 (4) 32 12 5 - – 0 0 03 5 5 – 0 0 04 – – 12 0 0 05 – 5 12 2 (1) 23 06 5 5 12 94 (93) >99 197 2.5 2.5 6 97 (87) 90 358 1 1 2.4 71 (65) 91 659 0.5 0.5 1.2 0 0 010 1 2 5 0 0 011 1 1 3 55 (49) 89 4912 1 1 2 50 (43) 88 4313 1 1 1 5 (0) 0 0

[a] Reaction conditions: 0.5 mmol styrene, 5 mol%FeCl3·6 H2O and 12 mol% formamidine ligand, 5 mol%H2-pydic, tert-amyl alcohol (9 mL), 0.44 mmol dodecane,(100 mL, internal standard) were added in sequence atroom temperature in air. To this mixture a solution of30% H2O2 (170 mL, 1.5 mmol) in tert-amyl alcohol(830 mL) was added over a period of 1 h at room temper-ature by a syringe pump.

[b] Conversion and yield were determined by GC analysisby comparison with authentic samples; for reproducibili-ty the reactions were at least repeated twice. Yields aregiven in brackets.


[d] TON= turnover number=mole of product per mole ofcatalyst.

1772 asc.wiley-vch.de � 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Synth. Catal. 2010, 352, 1771 – 1778

UPDATES Kristin Schrçder et al.

http://asc.wiley-vch.de

(3 equiv., 1 h) led to an increase of 26% in yield after2 h compared to the result after 1 h. Another extraportion did not enhance the yield any further.

After that, the optimized conditions (2.5 mol%loading of iron catalyst with a 1:1:2.5 ratio of iron/H2-pydic/formamidine) were applied in the epoxidationof different substituted styrenes and 1,3-dienes(Table 3).

For all styrene derivatives good to very good con-version with excellent selectivity is achieved (Table 3,entries 1–7)! In the case of 3-chlorostyrene the cata-lyst loading was increased to 5 mol% in order toensure full conversion (Table 3, entry 6). To our de-light also conjugated cis,cis-1,3-cyclooctadiene and1,2-dihydronaphthalene resulted in full conversionand excellent chemoselectivity towards the mono-oxy-genated product. However, non-conjugated cis,cis-1,5-cyclooctadiene showed diminished yield comparableto cyclooctene (Table 3, entries 10 and 11). Besidestyrenes and 1,3-dienes, the relatively stable substratetrans-stilbene showed diminished yields compared toformer H2-pydic systems resulting in 63% conversionand 44% yield. Nevertheless, the iron/H2-pydic/forma-midine catalyst system is the most efficient iron catalystfor terminal aromatic olefins.

The formation of side products, such as allylic oxi-dation products, was examined through GC-MS anal-ysis for the epoxidation of cyclooctene and 1,5-cyclo-octadiene. Only in the case of 1,5-cyclooctadienewere the allylic oxidation products, 1,5-cyclooctadien-4-one (~5–10% yield) and 1,5-cyclooctadien-4-ol (~3–5% yield), identified. Therefore a-pinene was chosenas an additional probe.[14] Indeed, formation of the al-lylic products verbenol (4,6,6-trimethylbicyclo-ACHTUNGTRENNUNG[3.1.1]hept-3-en-2-ol) and verbenone (4,6,6-trimethylbicycloACHTUNGTRENNUNG[3.1.1]hept-3-en-2-one) in smallamounts beside the corresponding epoxide was ob-served.

After exploring the scope of this novel iron catalystsystem, we were interested in mechanistic aspects. Ini-tially, the role of water was examined. Water incorpo-ration is of special mechanistic interest because tauto-meric iron oxo species can exchange the oxygen ofthe hydroperoxide with the oxygen of water.[15] Ingeneral, the used tert-amyl alcohol (stirring at air androom temperature) had a water content of 0.3–0.5 wt% as determined by Carl–Fischer titration. Ad-dition of >10 equiv. of water to the reaction mixture(50% H2O2 or the urea-H2O2 adduct as oxidant) ledto a significant induction period. However, after 24 hof stirring a yield of 64% or 31% is detected for 50%H2O2 or for urea-H2O2 adduct, respectively. Hence,an increased amount of water slows down the reac-tion, but does not deactivate the catalyst system.

Next, investigations on the reaction rate of compet-itive epoxidation reactions of p-substituted styrenes inthe presence of the iron-H2-pydic-formamidine

system were carried out. Here, logACHTUNGTRENNUNG(kX/kH) was corre-lated with the Hammett s+ value.

Table 3. Fe-catalyzed epoxidation: Scope and limitations.[a]

No Substrate Conv.[b] [%] Yield[b] [%] Sel.[c] [%]

1 97 87 90

2 >99 94 94

3 >99 92 92

4 >99 95 95

5 >99 >99 (83)[d,e] >99

6 81 (>99)[f] 79 (94)[f] 98 (94)[f]

7 95 92 97

8 >99 82 (60)[d] 82

9 88 86 98

10 57 27 48

11 34 24 70

12 18 9 50

[a] Reaction conditions: 0.5 mmol styrene, 2.5 mol%FeCl3·6 H2O and 6 mol% formamidine ligand, 2.5 mol%H2-pydic, tert-amyl alcohol (9 mL), 0.44 mmol dodecane,(100 mL, internal standard) were added in sequence atroom temperature in air. To this mixture a solution of30% H2O2 (170 mL, 1.5 mmol) in tert-amyl alcohol(830 mL) was added over a period of 1 h at room temper-ature by a syringe pump.

[b] Conversion and yield were determined by GC analysisby comparison with authentic samples.


[d] Isolated yields.[e] Traces of the ring opening product 2-p-tolylacetaldehyde

were detected.[f] The catalyst loading was increased to 5 mol%

FeCl3·6 H2O, 12 mol% formamidine ligand and 5 mol%H2-pydic to achieve complete conversion.

Adv. Synth. Catal. 2010, 352, 1771 – 1778 � 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim asc.wiley-vch.de 1773



Correlation following Eq. (1) (X=p-CF3, p-Me, p-Cl, p-F) gave after a linear regression a reaction con-stant value of 1=�1.00�0.14 (Figure 1) and 1=�0.85�0.31 including styrene. Notably, electron-do-nating substituents at the phenyl moiety increased therate constant of the epoxidation. The relatively smallnegative Hammett values indicated less electron den-sity in the transition state in comparison with the sub-strates. Similar Hammett values are reported for theepoxidation of olefins applying hydrogen peroxide asoxidant in the presence of metal peroxo catalysts, forexample, W or Re complexes.[16] Interestingly, relatedstyrene oxidations involving iron porphyrins like[Fe(IV) ACHTUNGTRENNUNG(TMPC+)O] gave lower 1 values (�1.9).[17]

In order to prove the involvement of a spin delocal-ization resonance effect, which indicates the participa-tion of a carbon radical centre,[18] we also performedexperiments with radical scavengers (see Table 5). Inaddition, the sJJ

. and smb substituent constants wereused for the Hammett correlation as developed byJiang and Ji [Eq. (2)].[19]

Here, values of polar (smb) and spin delocalizationeffects (sJJ

.) were established and separated (Table 4,Figure 2). The negative 1mb value accords with theHammett plot trend and shows the electrophilicity ofthe active iron catalyst. Whereas the positive 1JJ

. sug-gests a spin density delocalization at the carbon radi-cal centre similar to epoxidations performed withdioxo-Ru-porphyrins as catalysts.[20]

To assess whether the spin polarization or the polarsubstitution effect is more important, the magnitudeof j1mb/1JJ

. j was calculated. With a value of 0.86(j1mb/1JJ

. j<1) it implies a slightly higher significanceof the spin polarization effect in contrast to the polar-ization effect.

To tackle the problem of possible radical incorpora-tion, we performed the epoxidation of styrene with

added several radical scavengers such as TEMPO(2,2,6,6-tetramethylpiperidine-1-oxyl), BPN (N-tert-butylphenylnitrone), and duroquinone (2,3,5,6-tetra-methyl-p-benzoquinone) (Table 5). In general, all re-actions led to diminished yields of styrene oxide and

Figure 1. Hammett correlation for the epoxidation of differ-ent styrenes: y=�0.9976x+0.3187 with R2 = 0.96.

Table 4. Competitive epoxidation of different p-substitutedstyrenes.[a]

No X (Y) log krel smb[b] sJJ

.[b]

1 H 0 0 02 Me (H) 0.462 0.15 �0.293 Cl (F) 0.179 0.22 0.114 F (Me) 0.240 �0.02 �0.245 CF3 (Cl) �0.264 �0.01 0.49

[a] Reaction conditions: 2.5 mmol of each substrate, 5 mol%FeCl3·6 H2O and 12 mol% formamidine ligand, 5 mol%H2-pydic, tert-amyl alcohol (9 mL), 0.44 mmol dodecane,(100 mL, internal standard) were added in sequence atroom temperature in air. To this mixture a solution of30% H2O2 (57 mL, 0.5 mmol) in tert-amyl alcohol(443 mL) was added over a period of 1 h at room temper-ature by a syringe pump.

[b] See ref.[17]

Table 5. Epoxidation of styrene in the presence of variousradical trapping agents.[a]

No Radical scav-enger (rs)

nrs

[mol%]Conv. [%][b] ,(%)[d]

Sel. [%][c] ,(%)[d]

1 – – 97 902 TEMPO 100 0 03 TEMPO 5 13 (13) 0 (44)4 BPN 100 32 (54) 55 (74)5 BPN 2.5 15 (86) 5 (67)6 Duroquinone 100 63 (90) 79 (84)7 Duroquinone 5 67 (82) 91 (74)

[a] Reaction conditions: 0.5 mmol styrene, 2.5 mol%FeCl3·6 H2O and 6 mol% formamidine ligand 2,2.5 mol% H2-pydic, tert-amyl alcohol (9 mL), 0.44 mmoldodecane, (100 mL, internal standard) were added in se-quence at room temperature in air. To this mixture, a so-lution of 30% H2O2 (170 mL, 1.5 mmol) in tert-amyl alco-hol (830 mL) was added over a period of 1 h at room tem-perature by a syringe pump.

[b] Conversion and yield were determined by GC analysisand determined by comparison with authentic samples.

[c] Selectivity refers to the chemoselectivity of epoxide fromolefin.

[d] Yields are obtained after additionally stirring for 3 days.




lower chemoselectivity; however only the addition of1 equiv. TEMPO inhibited the reaction completely.

Apparently, the persistent TEMPO radical blocksany reactivity and does not favour the oxidation pro-cess like it is known for alcohol oxidations.[21] BPNand duroquinone diminished also the conversion and,in the case of BPN, selectivity, too.

Next, b-pinene was used as a so-called indicativesubstrate. It is well known that in the case of radicalintermediates rearrangement of the strained four-membered ring occurs to give limonene-type prod-ucts.[22] Under our optimized conditions with ACHTUNGTRENNUNG a cata-lyst loading of 2.5 mol% the iron/H2-pydic/2 systemgave a multitude of products (Figure 3).

Interestingly, epoxide 4a followed by myrtenol 4cand the double hydroxylated product 4d constitutedthe main products. Using perillyl alcohol 4e as sub-strate selective oxidation to the corresponding perillaaldehyde 4f occurred. The significant amount of rear-ranged products suggests at least partial participationof OH radicals in this oxidation process.

The formation of perillyl alcohol 4e and the addi-tional hydroxylated derivative 4d indicate radical in-termediates. In addition, b-pinene epoxide 4a, myrte-nol (6,6-dimethylbicyclo ACHTUNGTRENNUNG[3.1.1]-hept-2-en-2-yl]metha-nol) 4c and the cleavage product norinone (6,6-dimethylbicycloACHTUNGTRENNUNG[3.1.1]heptan-2-one) 4b are observed

by GC-MS analysis, which were identified by compar-ison with authentic samples.

Finally, we explored the stability of the formami-dine ligands under catalytic conditions. Hence, we ex-posed 1a to two hours of stirring with FeCl3·6 H2Oand H2-pydic in non-dry tert-amyl alcohol.[23] Then,hydrogen peroxide and substrate were added. Besidethe formation of the epoxide, no change or decompo-sition of the ligand was observed in both cases. How-ever, investigating the stability of ligand 2, we ob-served partial decomposition to cyclohexylamine 6and dicyclohexylurea 9. In addition, small amounts ofN-cyclohexylformamide 8 are observed (Scheme 2).

Apparently, N,N’-1,3-dicyclohexylurea 9 is pro-duced by the reaction of 6 with N-cyclohexylforma-mide 8, which is known in the presence of rutheniumcomplexes.[24]

Testing the different decomposition products of ourligand 2 did not result in similar selectivity (Table 6).

Figure 2. Dual-parameter Hammett correlation for the ep-oxidation of different styrenes (Table 4). y= (0.72�0.07)x +(0.07�0.02) with R2 =0.96.

Figure 3. Oxidation of b-pinene with hydrogen peroxide inthe presence of Fe/H2-pydic/2.

Scheme 2. Partial decomposition of ligand 2 (products were identified by GC-MS and comparison with authentic samples).




Furthermore, only in the presence of cyclohexylaminewere comparable conversions albeit with lower selec-tivity obtained (Table 6, entry 3). This is also true forother styrene substrates (Table 6, entries 7–10). Theseresults demonstrate the importance of the formami-dine ligand structure.

Conclusions

In summary, we have developed novel bio-inspirediron catalysts for oxidation reactions. The convenientand inexpensive combination of FeCl3·6H2O, H2-pydic, and formamidine ligand 2 allows for the epoxi-dation of substituted styrenes and conjugated 1,3-dienes in good to excellent yield and chemoselectivity(8 examples with selectivities >90%). Improved re-sults are obtained compared to all previously knowniron catalysts for this type of epoxidations. Althoughwe assume an electrophilic peroxide mechanism, par-tial radical formation might take place during the re-

action as shown by the oxidation of b-pinene. Nota-bly, ligand 2 is of limited stability during catalysis anddecomposition products of 2 gave significant lowerconversion and/or selectivity.

Experimental Section

General

All solvents and chemicals were obtained commercially andwere used as received. “30%” aqueous H2O2 from Merckwas used as received. The peroxide content varied from30% to 40% as determined by titration. NMR spectra weremeasured using a Bruker AMF 200 spectrometer at200 MHz (1H), 75.5 MHz (13C). All spectra were recorded inCDCl3 or CD2Cl2 and chemical shifts (d) are reported inppm relative to tetramethylsilane referenced to the residualsolvent peaks. Spectra were measured at room temperatureunless otherwise stated. Mass spectra were in general re-corded on an HP 6890/5973 GC-MS. In each case character-istic fragments with their relative intensities in percentagesare shown. Infrared spectra were recorded on a NicoletMagna-IR-Serie 550 spectrometer using KBr plates. Wavenumbers (n) are reported in cm�1 and relative intensities aregiven (w= weak, m= medium, s= strong, sh= shoulder).

Synthesis of N,N-Dimethylformamidines

Dimethylformamide (25.3 mmol) was dissolved in dry dieth-yl ether (50 mL) and cooled to 10 8C. Under an atmosphereof nitrogen phosphorus oxychloride (25.3 mmol) was addedcarefully maintaining the reaction temperature under 10 8C,while a white precipitate was formed. The mixture wasstirred for two hours at room temperature yielding an in-soluble oily residue. A white precipitate was obtained aftercooling to 10 8C and slow addition of the corresponding pri-mary amine (25.3 mmol). The stirring was continued for12 h at room temperature. The mixture was transferred to aseparation funnel and carefully treated with water (50 mL)and an aqueous sodium hydroxide solution until the aque-ous layer reached a pH >8. The aqueous layer was extract-ed with diethyl ether (2 � 50 mL). The combined organiclayers were washed with water (50 mL) and brine (50 mL).Drying with Na2SO4 and removal of the solvent yield thecrude product, which was purified by distillation.

N,N-Dimethyl-N’-cyclohexylformamidine (2): yield: 8%;white solid, mp 37 8C; bp 50–52 8C (1 mbar); 1H NMR(200 MHz, CDCl3, 25 8C): d=1.03–1.85 (m, 10 H, C6H10),2.82 (s, 6 H, CH3), 7.33 [s, 1 H, N=C(H)N]; 13C NMR(50 MHz, CDCl3, 25 8C): d=25.5, 25.7, 36.2, 37.2, 64.8, 153.3[N= C(H)N]; IR (KBr): n= 2929 (s), 1651 (s), 1600 (m),1376 (m), 1325 (m), 1108 (m), 1076 (m), 844 (w) cm�1; MS(ESI): m/z=155 (25, M+), 73 (100); UV-VIS (EtOH, 25 8C):l=276.5 nm.

General Procedure for the Epoxidation of Olefins

In a test tube, FeCl3·6 H2O (0.0125 mmol), tert-amyl alcohol(9 mL), formamidine ligand (0.030 mmol), pyridine-2,6-di-carboxylic acid (0.0125 mmol), olefin (0.5 mmol) and dodec-ane (GC internal standard, 100 mL) were added in sequence

Table 6. Iron-catalyzed epoxidation of styrenes in the pres-ence of ligand decomposition products.[a]

No Ligand Substrate Conv.[%][b]

Yield[%][b] ,(%)[d]

Sel.[%][c] ,(%)[d]

1 “2” >99 93 932 5 2 0 03 6 97 75 774 8 1 0 05 9 2 0 06 5+ 6 98 76 78

7 6 >99 85 (>99) 85 (>99)

8 6 >99 79 (94) 79 (94)

9 6 >99 76 (92) 76 (92)

10 6 85 71 (92) 83 (97)

[a] Reaction conditions: 0.5 mmol styrene/substrate, 5 mol%FeCl3·6 H2O and 12 mol% ligand, 5 mol% H2-pydic, tert-amyl alcohol (9 mL), 0.44 mmol dodecane, (100 mL, inter-nal standard) were added in sequence at room tempera-ture in air. To this mixture a solution of 30% H2O2

(170 mL, 1.5 mmol) in tert-amyl alcohol (830 mL) wasadded over a period of 1 h at room temperature by a sy-ringe pump.

[b] Conversion and yield were determined by GC analysisby comparison with authentic samples.


[d] Yield and selectivities obtained with ligand 2.




at room temperature in air. To this stirred mixture, a solu-tion of 30% hydrogen peroxide (aqueous, 170 mL, 1.5 mmol)in tert-amyl alcohol (830 mL) was added over a period of 1 hat room temperature by a syringe pump. In some cases thereaction mixture was additionally stirred for 5 h to achievebetter results. Conversion and yield were determined by GCanalysis without further manipulations and compared withauthentic samples. In isolation purposes the reaction mix-ture was quenched with Na2SO3 to destroy the excess of hy-drogen peroxide and diluted with water. The solution wasextracted with ethyl acetate (3� 10 mL) and dried over an-hydrous MgSO4. After filtration and solvent removal undervacuum, the crude product was purified either by distillationor by silica gel chromatography on a short column (hexane:ethyl acetate 20:1, 1% Et3N). For isolation in some casesthe reaction mixture was filtered over a short column(silica) and washed with hexane:Et3N (100:1). Afterwardsthe solvent was removed under vacuum, followed by a silicagel chromatography (hexane: ethyl acetate 20: 1, 1% Et3N).Data of the synthesized epoxides were compared with litera-ture known data.[25]

p-(Trifluoromethyl)phenyloxirane: 1H NMR (300 MHz,CDCl3, 25 8C): d= 7.60 (d, J= 8.1 Hz, 2 H), 7.39 (d, J=8.3 Hz, 2 H), 3.91 (dd, J=2.6 Hz, J= 4.1 Hz, 1 H), 3.18 (dd,J=4.1 Hz, J=5.7 Hz, 1 H), 2.76 (dd, J=2.6 Hz, J= 5.6 Hz,1 H); 13C NMR (100 MHz, CDCl3, 25 8C): d=142.0, 125.9,125.6 (q, J=3.7 Hz), 51.9, 51.6; MS (EI): m/z (rel. int.)= 187(19) [M�1]+, 169 (14), 158 (46), 138 (22), 119 (100), 109(23), 91 (40), 63 (15).

p-Methylphenyloxirane: 1H NMR (300 MHz, CD2Cl2,25 8C): d= 7.18–7.13 (m, 4 H), 3.79 (dd, J=2.6 Hz, J=4.1 Hz, 1 H), 3.09 (dd, J=4.0 Hz, J= 5.5 Hz, 1 H), 2.77 (dd,J=2.6 Hz, J= 5.5 Hz, 1 H), 2.34 (s, 3 H); 13C NMR(100 MHz, CD2Cl2, 25 8C): d=138.4, 135.1, 129.5, 125.8,52.7, 51.3, 21.3; MS (EI): m/z (rel. int.) =134 (30) [M]+, 119(22), 105 (100), 98 (25), 91 (18), 75 (16), 77 (24).

1,2-Dihydronaphthaleneoxirane: 1H NMR (300 MHz,CD2Cl2, 25 8C): d=7.00–7.34 (m, 4 H), 3.77 (d, J= 4.3 Hz,1 H), 3.65 (m, 1 H), 2.70 ACHTUNGTRENNUNG(m, 1 H), 2.48 (m, 1 H), 2.33 (m,1 H), 1.69 ACHTUNGTRENNUNG(m, 1 H); 13C NMR (100 MHz, CDCl3, 25 8C): d=136.7, 132.6, 129.6, 128.5, 128.4, 126.2, 55.2, 52.8, 24.4, 21.8;MS (EI): m/z (rel. int.)=146 (100) [M]+, 145 (27), 131 (35),112 (49), 113 (57), 115 (61), 104 (82), 91 (31), 78 (15), 63(15).

General Procedure for the Competetive Epoxidationof p-Substituted Styrenes

A test tube was charged in sequence with FeCl3·6 H2O(0.025 mmol), tert-amyl alcohol (9 mL), formamidine ligand(0.060 mmol), pyridine-2,6-dicarboxylic acid (0.025 mmol)and dodecane (GC internal standard, 100 mL) at room tem-perature in air. Then two different p-substituted styrenes(each 2.5 mmol) were added and the solution was stirred forfive minutes. To this mixture a solution of 30% hydrogenperoxide (aqueous, 57 mL, 0.5 mmol) in tert-amyl alcohol(443 mL) was added over a period of 1 h at room tempera-ture by a syringe pump. Conversion and yield were deter-mined by GC analysis without further manipulations andcompared with authentic samples. Reactions were carriedout twice to check for reproducibility.

Acknowledgements

We thank the State of Mecklenburg-Western Pommerania, theFederal Ministry of Education and Research (BMBF) andthe Deutsche Forschungsgemeinschaft (SPP 1118 and Leib-niz-prize) for financial support. K. S. appreciates the finan-cial support provided by the Graduiertenkolleg 1213 “NeueMethoden f�r Nachhaltigkeit in Katalyse und Technik” andthe Max-Buchner-Forschungsstiftung (DECHEMA). Dr. S.E. thanks the Cluster of Excellence “Unifying Concepts inCatalysis” (sponsored by the Deutsche Forschungsgemein-schaft and administered by the Technische Universit�tBerlin). Dr. B. J. thanks the Alexander-von-Humboldt-Stif-tung. Dr. habil. Haijun Jiao is acknowledged for critical dis-cussion during the preparation of the manuscript and Mr. S.Peitz for performing the Carl-Fischer titration. Mrs. M.Heyken, Mrs. K. Mçller and Mr. G. Wienhçfer (LIKAT) areacknowledged for their valuable support in the laboratory.

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Iron-Catalyzed Epoxidation of Aromatic Olefins and 1,3-Dienes

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