comparison of oxoperoxophosphatotungstate phase transfer catalysis with methyltrioxorhenium...

C. R. Acad. Sci. Paris, Serie IIc, Chimie / Chemistry 3 (2000) 183–187© 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS. All rights reserved13871609(00)001316/FLA

Surface chemistry and catalysis / Chimie des surfaces et catalyse

Comparison of oxoperoxophosphatotungstate phasetransfer catalysis with methyltrioxorheniumtwo-phase catalysis for epoxidation by hydrogenperoxideLaurent Sallesa,*, Jean-Marie Bregeaulta, Rene Thouvenotb

aLaboratoire des systemes interfaciaux a l’echelle nanometrique, universite Pierre-et-Marie-Curie, unite CNRS 7069,case 196, 4, place Jussieu, 75252 Paris cedex 05, FrancebLaboratoire de chimie inorganique et materiaux moleculaires, 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 Francois 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 PTCconditions 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(m-O2)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 turnovernumbers with two-phase systems involving MTO or its analogues with H2O2�H2O/CH2Cl2 and a proton sponge for thesynthesis of moderately sensitive epoxides. © 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS

epoxidation / hydrogen peroxide / tungsten peroxo complexes / polyoxoperoxometalate / methyltrioxorhenium/ two-phase catalysis / phase transfer catalysis

Resume – Version francaise abregee — Comparaison de deux systemes catalytiques pour l’epoxydation desalcenes en presence de peroxyde d’hydrogene : oxoperoxophosphatotungstate en transfert de phase et methyl-trioxorhenium en systeme biphasique. L’oxydation menagee de substrats organiques par des entites peroxydiquesconnaıt des developpements spectaculaires [1–6]. L’analyse des systemes H2WO4/H2O2�H2O/H3PO4 et H3[PW12O40]·yH2O/H2O2�H2O/(H3PO4) a permis de prouver l’existence de nouveaux complexes oxoperoxophosphatotungstates [PWxOy ]

z–

(x=1–4). Il est maintenant bien etabli par nos travaux que l’heteropolyanion de Keggin, [PW12O40]3–, n’est qu’un precurseur

permettant la synthese des especes catalytiquement actives du second systeme [7, 8]. L’addition de Q+Cl– (un sel d’oniumbien choisi) a ces systemes conduit a l’isolement de complexes bien definis : 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 donnees ont etecompletees par des travaux impliquant parfois des acides phosphoniques [9–12].

La recherche de nouveaux systemes avec CH3ReO3 (MTO) [13] a conduit a considerer un systeme homogene, MTO/H2O2-t-BuOH [14, 15]. Un systeme biphasique, CH3ReO3/H2O2�H2O/CH2Cl2, est beaucoup plus selectif que le precedent [16–18].L’addition d’une « eponge a protons » (bipyridine par exemple) est d’un grand secours pour la synthese catalytique desepoxydes tres fragiles [16, 17].

* Correspondence and reprints: [email protected]

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

Ce travail presente une etude comparative entre, d’une part, un systeme 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), avecH2O2�H2O/CH2Cl2, et, d’autre part, un systeme biphasique, beaucoup plus simple, avec CH3ReO3 comme precurseur et[CH3Re(O)(O2)2] comme entite active principale.

Les resultats sont rassembles dans le tableau. Avec les epoxydes, qui ne sont pas sensibles a l’hydrolyse (experiences 1 et2), les resultats sont tres comparables pour des tests realises a 20 °C, quels que soient les systemes. Par opposition, avec lelimonene (experience 3), le myrcene (experience 6) et le D-3-carene (exp. 7), il faut operer a 4 °C avec les complexesperoxydiques du rhenium (VII) pour eviter une suroxydation et une decomposition des complexes du cycle catalytique. Seulsl’a-pinene et le (−)-b-citronellene conduisent a des rendements tres moyens ; dans le premier cas (experience 4) l’ajout debipyridine au systeme biphasique permet une amelioration par rapport aux resultats de la CTP ; le sobrerol est alors isole etcaracterise comme sous-produit majoritaire. Pour le test n°5, le meilleur rendement correspond a l’utilisation de la catalysepar les complexes du tungstene (VI), le sous-produit principal etant alors le diepoxyde. Les sels PW2 et PW4, avec uneconcentration equivalente en W(VI) a celle utilisee pour la CTP, ne sont pas aussi actifs que le systeme ‘‘PWn ’’ du tableau.

La formation d’entites mixtes [PO4{Mo4−xWxO20}]3– et l’existence de processus d’echange des unites dinucleaires ont pu

etre mis en evidence a partir de PW4 et de PMo4, melanges a 20 °C dans l’acetonitrile [22]. De plus, l’analyse par RMN duphosphore 31P dans CDCl3, realisee sur des solutions cent fois plus concentrees en tungstene (pour des contraintesexperimentales) montre une evolution rapide des precurseurs, impliquant aussi des systemes dynamiques et vraisemblable-ment des processus d’echange. Ces observations ne permettent pas de bien identifier toutes les especes actives et de faire uneetude cinetique approfondie ; toutefois, l’existence de ces reorganisations rapides contribue a la bonne activite catalytique,qui est assez comparable a celle des complexes peroxydiques du rhenium que nous avons etudies. © 2000 Academie dessciences / Editions scientifiques et medicales Elsevier SAS

epoxydation / peroxyde d’hydrogene / complexes peroxydiques du tungstene / polyoxoperoxometallate /methyltrioxorhenium / catalyse biphasique / catalyse par transfert de phase

Oxidation of organic compounds by peroxo com-plexes has been reported for a number of homoge-neous systems, especially with tungsten ormolybdenum precursors [1–4]. Parallel to this, two-phase catalysis is becoming an area of environmen-tally friendly chemistry: the amount of catalyticspecies in the product phase must be negligible toallow its easy separation. Venturello et al. [5] firstreported an efficient epoxidation system usingmonomeric tungstate and phosphate that proceedsunder 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 sub-strates can be oxidized in the homogeneous phaseor more often in a two-phase system (H3[PM12O40],aq, noted ‘PM12’, M=Mo, W/PTA/H2O2�H2O/CHCl3).It was further demonstrated that the Keggin anions‘PW12’ and ‘PMo12’ decompose to form a variety ofperoxo complexes [7a] including Q3[PO4{W2O2(m-O2)2(O2)2}2] [7b], Q3[PO4{Mo2O2(m-O2)2(O2)2}2] [7c],Q2[HPO4{W2O2(m-O2)2(O2)2}] [8], Q2[M2O3(O2)4-(H2O)2] and/or Q2[M2O3(O2)4], etc., which are trans-ferred into the organic phase. The catalytic proper-ties of ‘PM12’�H2O2 mixtures are mainly related to thecollapse of the polyanionic structure of the Kegginanion. Some related systems involving phosphonicacids to generate other assembling anions have been

presented [9–11]. These anions are efficient precur-sors and/or catalysts under PTC conditions butturnover numbers are sometimes low, due to irre-versible catalyst deactivation [12].

An important improvement arose with the discov-ery of a novel catalytic system, ‘methyltrioxorhenium(MTO) [13]/H2O2�t-BuOH’, [14] for alkene epoxida-tion in the homogeneous phase. One of the activespecies is a Mimoun-type oxobisperoxo complex [3],with a methyl group in the basal plane [15]. Very lowcatalyst concentrations (0.1 mol% of MTO) can beused to epoxidize non-functionalized olefins. An in-teresting feature is the possibility of using MTO intwo-phase catalysis [16–18], which gives high selec-tivity, especially when acid-sensitive epoxides areformed [17]. In this work, we report on an associa-tion of some oxoperoxophosphatotungstates [19],which is very active and can compete with thetwo-phase ‘MTO/H2O�H2O2/CH2Cl2’ system.

The catalytic activity of the peroxophosphato-tungstic precursors is compared with that of theMTO-based two-phase system (table). With epoxideswhich are not very sensitive towards hydrolysis (en-tries 1 and 2), both reactions work well at roomtemperature and give nearly the same results interms of conversion and selectivity. For acid-sensi-

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terms of conversion and selectivity. For acid-sensi-tive epoxides synthesized from terpenes such aslimonene (entry 3), myrcene (entry 6) and D-3-carene (entry 7), the reactions with MTO must beperformed at 4 °C to avoid catalyst decompositionand over-oxidation. Under these conditions similarresults are obtained. Epoxides which are more sensi-tive to Brønsted acids lead, even in the case of thetwo-phase system, to mixtures of diols and by-prod-ucts. For a-pinene and (−)-b-citronellene, epoxideyields are lower: a-pinene gives trans-sobrerol afterhydrolysis and rearrangement of the correspondingepoxide. Addition of proton sponges (e.g.bipyridine) [16–19] to either reaction medium leadsto moderate improvements (see table). With (�)-b-citronellene, the major by-products are di-epoxides.It should be noted that the results, not presentedhere, with pure (Arquad)2[HPO4{W2O2(m-O2)2(O2)2}],PW2 [8a], [PO4

3–]/[W]/[Q+]=1:2:2 or (Arquad)3-[PO4{W2O2(m-O2)2(O2)2}2], PW4 [7a], [PO4

3–]/[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 ox-operoxophosphatotungstic species. The latter are

generated with a system such that [PO43–]/[W]/[Q+]

=1:2:0.4 (see reference [20]). These results give anillustration of the [Q+]/[W] effect on the overall yieldof epoxide which was previously demonstrated withoct-1-ene oxidation at 60 °C [21].

31P-NMR analyses of the organic phase at 298 Kcan only be performed with more concentrated solu-tions (figure, a, [W]:0.9 mol·L–1); they give initiallytwo 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 andthree resonance signals appear between +8 and–0.5 ppm (figure, b). At higher temperature, up to323 K (figure, c), the homogeneous system evolvesrapidly and the line of PW2 becomes hardly visibleamong the relatively broad resonances of the newspecies. According to their chemical shifts and thepresence of unresolved tungsten satellites, theselines could be assigned to oxoperoxophosphato-tungstic anions [7a, 8a]. Unfortunately, the poor reso-lution of the signals of the satellites does not allowthe determination of the P/W ratio. Under these NMRexperimental 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

MTOa (two-phase system) PWnb (PTC)

conversionconversion Reaction timeentry substrate product Reaction timeselectivityselectivity temperaturetemperature

98 %1 h 96 % 1 hRT \99 %RT 99 %24 h 97 %95 %29 h

100%RT

\99 %0.5 h98 %2 h98 %4°C 86 % RT

30 %1 h 55 % 3 hRT 98 %RT 98 %

+2 % bipyridine+6 % bipyridine\99 %2 h 72 % 2 h

RT 74 %4°C 50 %

\99 %2 h\99 %2 h97 %4°C 90 % RT

97 %2 h98 %2 h99 % RT \99 %4°C

a Six millimoles of olefin in CH2Cl2 (5 mL) are stirred for a few minutes at the appropriate temperature. MTO (1 mol%/olefin) is addedto 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 monitoredfor 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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Fig. Figure. 121.5 MHz 31P-NMR spectrum of the oxoperox-ophosphatotungstic system [20]; solvent CDCl3. [W]=0.9 mol·L–1.(a) T=298 K recorded immediately after preparation; (b) T=298K, recorded after a few minutes; (c) T=323 K.

These phase transfer catalysis experiments confirmthe assembling anion effect of the phosphate group;they can compete in terms of yields and turnovernumbers with the more simple two-phase system:‘MTO/H2O2�H2O/CH2Cl2’ and a proton sponge if nec-essary, although the PTC system has not yet been fullyoptimized. We proposed that the dynamic systems,with several anionic species and neutral peroxo frag-ments, may be relevant to the high catalytic activity.Mixing solutions of Q3[PO4{W2O2(m-O2)2(O2)2}2] andQ3[PO4{Mo2O2(m-O2)2(O2)2}2] at room temperature re-sults in the fast formation of mixed-addenda speciesQ3[PO4{Mo4–xWxO20] and the whole array of peroxospecies [PWxOy ]

z– (x=1–4) [8, 21, 22] is likely to existin fast equilibrium. This makes it difficult to identifythe active species kinetically [22]. Moreover, it is shownthat the [Q+]/[W] ratio can have a dramatic effect inthe Ishii–Venturello epoxydation and compoundssuch as phosphate-stabilized peroxotungstate specieshaving nearly a W:P atomic ratio of 2:1 are among theversatile catalytic active species [8, 22].

Acknowledgements.

We thank Dr John Lomas for constructive discussionsand for correcting the manuscript.

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results also in the formation of oxo- and oxoperoxocondensed tungstophosphates responsible for 31P sig-nals in the shielded part of the spectrum (−5 to −10ppm). Because of the low concentration in the catalysistests, [W]:8·10–3 mol·L–1 and [P]:4·10–3 mol·L–1, itis rather difficult to investigate the true catalytic system.

Nevertheless, these NMR results demonstrate theexistence of several equilibria between peroxo specieswhich appear different from those previously de-scribed [8a, 22]. The initial 298 K 31P-NMR spectrum(figure (a, b)) may correspond to the phosphate-basedspecies of the primary catalytic medium used in theepoxidation experiments. An increase in temperatureappears to enhance coordination of ‘unsaturated’peroxotungstic moieties ([WO(O2)2] and/or[WO(O2)2(H2O)2] or dimeric fragments: {W2O2(m-O2)2(O2)2}, [7c] etc.) on phosphate or monohy-drogenophosphate anions.

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[20] Tungstic acid (‘H2WO4’, 10 mmol) is added to 10 mL of 30 %H2O2. After 1 h stirring at 60 °C, followed by centrifugation toremove unreacted ‘H2WO4’, 6 M H3PO4 (0.85 mL, 5 mmol) isadded to the supernatant liquid. After 30 min stirring at roomtemperature, the anions are extracted with a CHCl3 solutionof Arquad 2HT® chloride (4 mmol, 20 mL), [{(C18H37) 75 %+(C16H23) 25 %}2N(CH3)2]

+Cl–. After 3 h further stirring, thetwo phases are separated. The organic phase is dried overMgSO4 and the solvent is removed using a rotary evaporator.

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comparison of oxoperoxophosphatotungstate phase transfer catalysis with methyltrioxorhenium...

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