Comparison of oxoperoxophosphatotungstate phase transfer catalysis with methyltrioxorhenium two-phase catalysis for epoxidation by hydrogen peroxide

C. R. Acad. Sci. Paris, Se´rie IIc, Chimie / Chemistry 3 (2000) 183 – 187 © 2000 Acade´mie des sciences / E´ditions scientifiques et me´dicales Elsevier SAS. All rights reserved 1387 1609(00) 00131 6/FLA

Comparison of oxoperoxophosphatotungstate phase transfer catalysis with methyltrioxorhenium two-phase catalysis for epoxidation by hydrogen peroxide Laurent Sallesa,*, Jean-Marie Bre´geaulta, Rene´ Thouvenotb a

Laboratoire des syste`mes interfaciaux a` l’e´chelle nanome´trique, universite´ Pierre-et-Marie-Curie, unite´ CNRS 7069, case 196, 4, place Jussieu, 75252 Paris cedex 05, France b Laboratoire de chimie inorganique et mate´riaux mole´culaires, universite´ Pierre-et-Marie-Curie, unite´ CNRS 7071, case 42, 4, place Jussieu, 75252 Paris cedex 05, France

Received 25 November 1999, accepted 16 March 2000 Communicated by Franc¸ois Mathey This article is dedicated to Professor Yves Jeannin on the occasion of his retirement.

Abstract – Onium salts such as Q3[PO4{W2O2(m-O2)2(O2)2}2] or Q2[HPO4{W2O2(m-O2)2(O2)2}] (Q+ = [N(n-C6H13)4]+, [{(C18H37) 75 %+ (C16H33) 25 %}2N(CH3)2]+ (Arquad 2HT®), etc.) are effective under phase transfer catalysis (PTC) conditions for selective epoxidation of alkenes. Associations of the corresponding anionic species and of other unidentified salts under PTC conditions with Arquad were found to be as active and selective as the two-phase system CH3ReO3 (MTO)/H2O2 –H2O/ CH2Cl2. A 31P-NMR study shows several species which may imply breaking of the peroxo-bridged dimetallic {W2O2(mO2)2(O2)2} units; they may be the key for understanding the activity in catalytic epoxidation of cyclooctene, oct-1-ene, (R)-(+)-limonene, a-pinene, ( − )-b-citronellene, D-3-carene, etc. These systems can compete in terms of yields and turnover numbers with two-phase systems involving MTO or its analogues with H2O2H2O/CH2Cl2 and a proton sponge for the synthesis of moderately sensitive epoxides. © 2000 Acade´mie des sciences / E´ditions scientifiques et me´dicales Elsevier SAS epoxidation / hydrogen peroxide / tungsten peroxo complexes / polyoxoperoxometalate / methyltrioxorhenium / two-phase catalysis / phase transfer catalysis Re´sume´ – Version franc¸aise abre´ge´e — Comparaison de deux syste`mes catalytiques pour l’e´poxydation des alce`nes en pre´sence de peroxyde d’hydroge`ne : oxoperoxophosphatotungstate en transfert de phase et me´thyltrioxorhenium en syste`me biphasique. L’oxydation me´nage´e de substrats organiques par des entite´s peroxydiques connaıˆt des de´veloppements spectaculaires [1–6]. L’analyse des syste`mes H2WO4/H2O2H2O/H3PO4 et H3[PW12O40]·y H2O/H2O2H2O/(H3PO4) a permis de prouver l’existence de nouveaux complexes oxoperoxophosphatotungstates [PWx Oy ]z – (x=1–4). Il est maintenant bien e´tabli par nos travaux que l’he´te´ropolyanion de Keggin, [PW12O40]3 – , n’est qu’un pre´curseur permettant la synthe`se des espe`ces catalytiquement actives du second syste`me [7, 8]. L’addition de Q+Cl – (un sel d’onium bien choisi) a` ces syste`mes conduit a` l’isolement de complexes bien de´finis : Q3[PO4{W2O2(m-O2)2(O2)2}2], note´ « PW4 » [7a, b], Q2[HPO4{W2O2(m-O2)2(O2)2}], note´ « PW2 » [8a], Q2[W2O3(O2)4(H2O)2] et/ou Q2[M2O3(O2)4] [7a], etc. Ces donne´es ont e´te´ comple´te´es par des travaux impliquant parfois des acides phosphoniques [9 – 12]. La recherche de nouveaux syste`mes avec CH3ReO3 (MTO) [13] a conduit a` conside´rer un syste`me homoge`ne, MTO/H2O2-tBuOH [14, 15]. Un syste`me biphasique, CH3ReO3/H2O2H2O/CH2Cl2, est beaucoup plus se´lectif que le pre´ce´dent [16 –18]. L’addition d’une « e´ponge a` protons » (bipyridine par exemple) est d’un grand secours pour la synthe`se catalytique des e´poxydes tre`s fragiles [16, 17].

* Correspondence and reprints: [email protected]

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NOTE / PRELIMINARY COMMUNICATION

Surface chemistry and catalysis / Chimie des surfaces et catalyse

L. Salles et al. / C. R. Acad. Sci. Paris, Se´rie IIc, Chimie / Chemistry 3 (2000) 183 – 187 Ce travail pre´sente une e´tude comparative entre, d’une part, un syste`me impliquant des sels peroxydiques d’Arquad 2HT®, obtenus a` partir de [{(C18H37) 75 % +(C16H33) 25 %}2N(CH3)2]+Cl – , pour la catalyse par transfert de phase (CTP), avec H2O2H2O/CH2Cl2, et, d’autre part, un syste`me biphasique, beaucoup plus simple, avec CH3ReO3 comme pre´curseur et [CH3Re(O)(O2)2] comme entite´ active principale. Les re´sultats sont rassemble´s dans le tableau. Avec les e´poxydes, qui ne sont pas sensibles a` l’hydrolyse (expe´riences 1 et 2), les re´sultats sont tre`s comparables pour des tests re´alise´s a` 20 °C, quels que soient les syste`mes. Par opposition, avec le limone`ne (expe´rience 3), le myrce`ne (expe´rience 6) et le D-3-care`ne (exp. 7), il faut ope´rer a` 4 °C avec les complexes peroxydiques du rhe´nium (VII) pour e´viter une suroxydation et une de´composition des complexes du cycle catalytique. Seuls l’a-pine`ne et le (− )-b-citronelle`ne conduisent a` des rendements tre`s moyens ; dans le premier cas (expe´rience 4) l’ajout de bipyridine au syste`me biphasique permet une ame´lioration par rapport aux re´sultats de la CTP ; le sobre´rol est alors isole´ et caracte´rise´ comme sous-produit majoritaire. Pour le test n°5, le meilleur rendement correspond a` l’utilisation de la catalyse par les complexes du tungste`ne (VI), le sous-produit principal e´tant alors le die´poxyde. Les sels PW2 et PW4, avec une concentration e´quivalente en W(VI) a` celle utilise´e pour la CTP, ne sont pas aussi actifs que le syste`me ‘‘PWn ’’ du tableau. La formation d’entite´s mixtes [PO4{Mo4 − x Wx O20}]3 – et l’existence de processus d’e´change des unite´s dinucle´aires ont pu eˆtre mis en e´vidence a` partir de PW4 et de PMo4, me´lange´s a` 20 °C dans l’ace´tonitrile [22]. De plus, l’analyse par RMN du phosphore 31P dans CDCl3, re´alise´e sur des solutions cent fois plus concentre´es en tungste`ne (pour des contraintes expe´rimentales) montre une e´volution rapide des pre´curseurs, impliquant aussi des syste`mes dynamiques et vraisemblablement des processus d’e´change. Ces observations ne permettent pas de bien identifier toutes les espe`ces actives et de faire une e´tude cine´tique approfondie ; toutefois, l’existence de ces re´organisations rapides contribue a` la bonne activite´ catalytique, qui est assez comparable a` celle des complexes peroxydiques du rhe´nium que nous avons e´tudie´s. © 2000 Acade´mie des sciences / E´ditions scientifiques et me´dicales Elsevier SAS e´poxydation / peroxyde d’hydroge`ne / complexes peroxydiques du tungste`ne / polyoxoperoxome´tallate / me´thyltrioxorhenium / catalyse biphasique / catalyse par transfert de phase

Oxidation of organic compounds by peroxo complexes has been reported for a number of homogeneous systems, especially with tungsten or molybdenum precursors [1 – 4]. Parallel to this, twophase catalysis is becoming an area of environmentally friendly chemistry: the amount of catalytic species in the product phase must be negligible to allow its easy separation. Venturello et al. [5] first reported an efficient epoxidation system using monomeric tungstate and phosphate that proceeds under two-phase conditions with an onium salt, Q+X – , as phase transfer agent (PTA). Ishii et al. [6] demonstrated that a wide variety of organic substrates can be oxidized in the homogeneous phase or more often in a two-phase system (H3[PM12O40], aq, noted ‘PM12’, M =Mo, W/PTA/H2O2H2O/CHCl3). It was further demonstrated that the Keggin anions ‘PW12’ and ‘PMo12’ decompose to form a variety of peroxo complexes [7a] including Q3[PO4{W2O2(mO2)2(O2)2}2] [7b], Q3[PO4{Mo2O2(m-O2)2(O2)2}2] [7c], [8], Q2[M2O3(O2)4Q2[HPO4{W2O2(m-O2)2(O2)2}] (H2O)2] and/or Q2[M2O3(O2)4], etc., which are transferred into the organic phase. The catalytic properties of ‘PM12’H2O2 mixtures are mainly related to the collapse of the polyanionic structure of the Keggin anion. Some related systems involving phosphonic acids to generate other assembling anions have been

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presented [9–11]. These anions are efficient precursors and/or catalysts under PTC conditions but turnover numbers are sometimes low, due to irreversible catalyst deactivation [12]. An important improvement arose with the discovery of a novel catalytic system, ‘methyltrioxorhenium (MTO) [13]/H2O2t-BuOH’, [14] for alkene epoxidation in the homogeneous phase. One of the active species is a Mimoun-type oxobisperoxo complex [3], with a methyl group in the basal plane [15]. Very low catalyst concentrations (0.1 mol% of MTO) can be used to epoxidize non-functionalized olefins. An interesting feature is the possibility of using MTO in two-phase catalysis [16–18], which gives high selectivity, especially when acid-sensitive epoxides are formed [17]. In this work, we report on an association of some oxoperoxophosphatotungstates [19], which is very active and can compete with the two-phase ‘MTO/H2OH2O2/CH2Cl2’ system. The catalytic activity of the peroxophosphatotungstic precursors is compared with that of the MTO-based two-phase system (table). With epoxides which are not very sensitive towards hydrolysis (entries 1 and 2), both reactions work well at room temperature and give nearly the same results in terms of conversion and selectivity. For acid-sensi-

L. Salles et al. / C. R. Acad. Sci. Paris, Se´rie IIc, Chimie / Chemistry 3 (2000) 183 – 187

terms of conversion and selectivity. For acid-sensitive epoxides synthesized from terpenes such as limonene (entry 3), myrcene (entry 6) and D-3carene (entry 7), the reactions with MTO must be performed at 4 °C to avoid catalyst decomposition and over-oxidation. Under these conditions similar results are obtained. Epoxides which are more sensitive to Brønsted acids lead, even in the case of the two-phase system, to mixtures of diols and by-products. For a-pinene and (− )-b-citronellene, epoxide yields are lower: a-pinene gives trans-sobrerol after hydrolysis and rearrangement of the corresponding epoxide. Addition of proton sponges (e.g. bipyridine) [16–19] to either reaction medium leads to moderate improvements (see table). With ()-bcitronellene, the major by-products are di-epoxides. It should be noted that the results, not presented here, with pure (Arquad)2[HPO4{W2O2(m-O2)2(O2)2}], PW2 [8a], [PO34 – ]/[W]/[Q+] =1:2:2 or (Arquad)3[PO4{W2O2(m-O2)2(O2)2}2], PW4 [7a], [PO43 – ]/[W]/[Q+] =1:4:3, and, even with a mixture of the two salts (with an equivalent concentration of tungsten (VI)), are always much poorer than with the present oxoperoxophosphatotungstic species. The latter are

generated with a system such that [PO43 – ]/[W]/[Q+] =1:2:0.4 (see reference [20]). These results give an illustration of the [Q+]/[W] effect on the overall yield of epoxide which was previously demonstrated with oct-1-ene oxidation at 60 °C [21]. 31

P-NMR analyses of the organic phase at 298 K can only be performed with more concentrated solutions (figure, a, [W] : 0.9 mol·L – 1); they give initially two lines easily assigned to PW2 (d= + 0.5 ppm) [8], and PW4 as a minor species (d= + 3.5 ppm) [7a]. The PW2 and PW4 lines progressively decrease and three resonance signals appear between + 8 and –0.5 ppm (figure, b). At higher temperature, up to 323 K (figure, c), the homogeneous system evolves rapidly and the line of PW2 becomes hardly visible among the relatively broad resonances of the new species. According to their chemical shifts and the presence of unresolved tungsten satellites, these lines could be assigned to oxoperoxophosphatotungstic anions [7a, 8a]. Unfortunately, the poor resolution of the signals of the satellites does not allow the determination of the P/W ratio. Under these NMR experimental conditions, i.e. without added H2O2, the catalytic system cannot be regenerated and this

Table. Epoxidation of olefin by diluted H2O2 catalysed by rhenium or tungsten peroxo species

entry

substrate

product

MTOa (two-phase system)

PWnb (PTC)

Reaction time temperature

conversion selectivity

Reaction time temperature

conversion selectivity

1h RT 29 h

96 % 99 % 95 %

1h RT 24 h RT

98 % \99 % 97 % 100%

2h 4°C

98 % 86 %

0.5 h RT

\99 % 98 %

1h RT

55 % 98 % +6 % bipyridine 72 % 50 %

2h 4°C

3h RT 2h RT

30 % 98 % +2 % bipyridine \99 % 74 %

2h 4°C

\99 % 90 %

2h RT

\99 % 97 %

2h 4°C

98 % 99 %

2h RT

97 % \99 %

a

Six millimoles of olefin in CH2Cl2 (5 mL) are stirred for a few minutes at the appropriate temperature. MTO (1 mol%/olefin) is added to the solution; 150 equivalents of 10 % H2O2 are added with vigorous stirring. The mixture turns yellow. b An amount of peroxidie precursor containing nearly 0.06 mmol of tungsten is dissolved in CH2Cl2 (5 mL). Then 30 % H2O2 (1.4 mL, 12 mmol) is added to the solution. After a few minutes stirring, olefin (6 mmol) is added to the two-phase mixture: [W] : 8·10−3 mol·L−1. For both systems, the progress of reaction was monitored by GC and analysed after quenching with MnO2. The reactions were monitored for up to 24 h but, as seen in the table, most of the reactions were almost complete after only 2 h. pH of the aqueous phase: 1.0–2.5. T.O.N.: 100; T.O.N.max : 500.

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L. Salles et al. / C. R. Acad. Sci. Paris, Se´rie IIc, Chimie / Chemistry 3 (2000) 183 – 187

These phase transfer catalysis experiments confirm the assembling anion effect of the phosphate group; they can compete in terms of yields and turnover numbers with the more simple two-phase system: ‘MTO/H2O2H2O/CH2Cl2’ and a proton sponge if necessary, although the PTC system has not yet been fully optimized. We proposed that the dynamic systems, with several anionic species and neutral peroxo fragments, may be relevant to the high catalytic activity. Mixing solutions of Q3[PO4{W2O2(m-O2)2(O2)2}2] and Q3[PO4{Mo2O2(m-O2)2(O2)2}2] at room temperature results in the fast formation of mixed-addenda species Q3[PO4{Mo4 – x Wx O20] and the whole array of peroxo species [PWx Oy ]z – (x= 1–4) [8, 21, 22] is likely to exist in fast equilibrium. This makes it difficult to identify the active species kinetically [22]. Moreover, it is shown that the [Q+]/[W] ratio can have a dramatic effect in the Ishii–Venturello epoxydation and compounds such as phosphate-stabilized peroxotungstate species having nearly a W:P atomic ratio of 2:1 are among the versatile catalytic active species [8, 22].

Acknowledgements. We thank Dr John Lomas for constructive discussions and for correcting the manuscript.

References

Fig. Figure. 121.5 MHz 31P-NMR spectrum of the oxoperoxophosphatotungstic system [20]; solvent CDCl3. [W]= 0.9 mol·L – 1. (a) T = 298 K recorded immediately after preparation; (b) T= 298 K, recorded after a few minutes; (c) T = 323 K.

results also in the formation of oxo- and oxoperoxo condensed tungstophosphates responsible for 31P signals in the shielded part of the spectrum (− 5 to − 10 ppm). Because of the low concentration in the catalysis tests, [W] :8·10 – 3 mol·L – 1 and [P] : 4·10 – 3 mol·L – 1, it is rather difficult to investigate the true catalytic system. Nevertheless, these NMR results demonstrate the existence of several equilibria between peroxo species which appear different from those previously described [8a, 22]. The initial 298 K 31P-NMR spectrum (figure (a, b)) may correspond to the phosphate-based species of the primary catalytic medium used in the epoxidation experiments. An increase in temperature appears to enhance coordination of ‘unsaturated’ peroxotungstic moieties ([WO(O2)2] and/or [WO(O2)2(H2O)2] or dimeric fragments: {W2O2(mO2)2(O2)2}, [7c] etc.) on phosphate or monohydrogenophosphate anions.

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[19] Herrmann W.A., Fischer R.W., Rauch M.V., Scherer W., J. Mol. Catal. 86 (1994) 243. [20] Tungstic acid (‘H2WO4’, 10 mmol) is added to 10 mL of 30 % H2O2. After 1 h stirring at 60 °C, followed by centrifugation to remove unreacted ‘H2WO4’, 6 M H3PO4 (0.85 mL, 5 mmol) is added to the supernatant liquid. After 30 min stirring at room temperature, the anions are extracted with a CHCl3 solution of Arquad 2HT® chloride (4 mmol, 20 mL), [{(C18H37) 75 %+ (C16H23) 25 %}2N(CH3)2]+Cl – . After 3 h further stirring, the two phases are separated. The organic phase is dried over MgSO4 and the solvent is removed using a rotary evaporator. [21] Bre´geault J.-M., Thouvenot R., Zoughe´bi S., Salles L., Atlamsani A., Duprey E., Aubry C., Robert F., Chottard G., Studies in Surface Science and Catalysis 82 (1994) 571. [22] Salles L., Piquemal J.-Y., Thouvenot R., Minot C., Bre´geault J.-M., J. Mol. Catal. 117 (1997) 375.

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