N-alkyl ammonium perrhenate salts as catalysts for the epoxidation of olefins under mild conditions

Accepted Manuscript N-alkyl ammonium perrhenate salts as catalysts for the epoxidation of olefins under mild conditions

Mirza Cokoja, Robert M. Reich, Fritz E. Kühn PII: DOI: Reference:

S1566-7367(17)30280-7 doi: 10.1016/j.catcom.2017.06.043 CATCOM 5109

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

11 April 2017 11 June 2017 24 June 2017

Please cite this article as: Mirza Cokoja, Robert M. Reich, Fritz E. Kühn , N-alkyl ammonium perrhenate salts as catalysts for the epoxidation of olefins under mild conditions. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Catcom(2017), doi: 10.1016/j.catcom.2017.06.043

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT N-Alkyl Ammonium Perrhenate Salts as Catalysts for the Epoxidation of Olefins Under Mild Conditions

T

Mirza Cokojaa,b,*, Robert M. Reichb, Fritz E. Kühnb

SC RI P

a Chair of Inorganic and Metal-Organic Chemistry, Department of Chemistry and Catalysis Research Center, Technical University of Munich, Lichtenbergstraße 4, D-85747 Garching bei München (Germany). Tel: +49 89 289 13478. e-mail: [email protected].

NU

b Molecular Catalysis, Department of Chemistry and Catalysis Research Center, Technical

AC

CE

PT

ED

MA

University of Munich, Lichtenbergstraße 4, D-85747 Garching bei München (Germany).

ACCEPTED MANUSCRIPT Abstract A series of N-alkylammonium perrhenate salts [N(R1 )3 R2 ]+[ReO 4 ]- with varying substituents R1 and R2 were synthesized and characterized and used as catalysts for the epoxidation of olefins. The effect of the substitution pattern on the catalytic activity was investigated.

T

Keywords

AC

CE

PT

ED

MA

NU

SC RI P

Homogeneous catalysis; Perrhenate; Ionic liquids; Epoxidation; Olefins; Hydrogen peroxide.

ACCEPTED MANUSCRIPT 1.

Introduction

The epoxidation of olefins is a key process for the synthesis of bulk epoxides as well as for intermediates for a variety of fine chemicals [1,2]. Homogeneous epoxidation catalysis with transition metal complexes are generally highly active for the epoxidation of a broad range of olefins [3–6]. However, application of molecular epoxidation catalysts on larger scales has so

T

far not been realized, mainly due to the intrinsic problem of the long-term reusability and

SC RI P

stability of the catalyst [4]. Biphasic epoxidations using ionic liquids (IL) are an alternative methodology for catalyst/product separation [7]. ILs exhibit tunable hydrophobicity and concomitant high polarity, making them versatile reaction media in catalysis [8–11]. For

NU

biphasic epoxidations in ILs, the catalysts are, however, often at least partially soluble in the same phase as the product, and thus cannot be separated to 100 %. Much research has been

MA

devoted to the transfer of water-soluble oxidation reagents, such as permanganates [12] and polyoxometalates (POM) [13–16] into the organic (substrate-containing) phase using crown

ED

ethers or quaternary ammonium salts as phase transfer agents. The stability of such POM species is often pH dependent and thus not always compatible with the epoxide product.

PT

Furthermore, the extraction of the product remains an issue.

CE

Recently, we have shown that ionic liquids (IL) containing the perrhenate anion ([ReO 4 ] act as effective epoxidation catalysts [17–19]. This was rather unexpected, given that perrhenate has

AC

long been considered as notoriously inactive for olefin epoxidation with aq. H2 O2 as oxidant [17]. However, hydrophobic cations such as imidazolium [18] and amidoammonium [19] switch on the activity of [ReO 4 ]- for epoxidations. The activity of perrhenate-ILs, as well as their toxicity [20,21] (perrhenate does not exhibit any significant toxicity) depend on the nature of the cation. The most active imidazolium perrhenates are insoluble in water, however, in aqueous H2 O2 they exhibit a surprisingly good solubility, giving rise to strong hydrogen bonds between [ReO 4 ]- and H2 O2 leading to its activation. The imidazolium perrhenate catalysts enhance the solubility of olefins into the aqueous phase by a factor of up

ACCEPTED MANUSCRIPT to 100, in dependence of the hydrophobicity of the cation [18]. Hence, the catalysis takes place in the aq. phase, most presumably due to the formation of micellar media, as is reported for other similar ILs [22–24]. A beneficial effect of the solubility of the catalyst in aq. H2 O2 is that at low concentrations of H2 O2 the catalyst forms a third phase, whereby the productcontaining phase can easily be extracted from the catalyst, which can be reused by adding

T

H2 O 2 [18].

SC RI P

Although the activity of these catalysts is no match to state-of-the-art molecular catalysts of titanium, vanadium, molybdenum or manganese (in terms of turnover frequencies) [4], their simple

recyclability in a multiphase reaction system could

by far outperform the

NU

homogeneous counterparts. Especially when used in continuous flow reactors, they could offer a relatively simple recycling route without loss of performance or leaching, thereby

MA

gaining attraction for potential applications beyond the laboratory scale. Despite the two main disadvantages of this ionic model catalyst system – the high price of

ED

rhenium and the rather low activity – the mechanism of the perrhenate-catalyzed reaction, particularly the phase transfer by micelles is unprecedented. Furthermore, it allows a broad

PT

variation of the cation, and one the other hand, it implies that the anion does not necessarily

good model case.

CE

require rhenium. These factors render the system simple and variable, with rhenium as a very

AC

The use of ammonium cations of the type [NR4 ]+ would add to the scope of applicable cations and offer an additional benefit to the simplicity of this system as they are generally cheaper and easier to access than imidazolium cations. Hence, in this work, we have studied the effect of varying the chain length of alkyl substituents in N-alkyl ammonium perrhenates (Scheme 1) on the catalytic activity of perrhenate, in analogy to the role of imidazolium cations.

T

ACCEPTED MANUSCRIPT

2.

SC RI P

Scheme 1. Synthesis of ammonium perrhenate salts 1-6.

Experimental

NU

A detailed description of the synthesis, characterization and catalysis studies is presented in

3.

MA

Supporting Information.

Results and Discussion

13

C–NMR spectroscopy, IR

ED

The compounds 1–6 were characterized by means of 1 H– and

spectroscopy and elemental analysis. The catalytic activity of the compounds 1-3 was

PT

evaluated with cyclooctene as test substrate and aq. H2 O 2 as oxidant (Figure 1). While

CE

compound 1 is soluble in aq. H2 O2 , the catalysts 2-6 form a milky emulsion in this medium. In all cases upon addition of cyclooctene two phases are formed. Even after 24 h the catalysts

AC

show a lower activity in comparison to the previously reported benchmark perrhenate catalyst 1,2-dimethyl-3-n-octylimidazolium perrhenate ([MeMeOctIm][ReO 4 ], 8, Im = imidazolium) which shows nearly complete conversion already after 4 h (green curve in Figure 1) [18]. The compounds [NH4 ][ReO 4 ] (1) and [NBu4 ][ReO 4 ] (2, Bu = n-butyl) exhibit a similar activity (23% and 21% conversion respectively), while [N(Oct)4 ][ReO 4 ] (3) is somewhat more active (39%). A possible reason for the higher activity is the relatively enhanced solubility of the substrate in the emulsion phase due to the n-octyl groups.

ACCEPTED MANUSCRIPT Analogously, (alkyl)trimethylammonium perrhenates 4-6 were tested as catalysts for the epoxidation of cyclooctene (all conversion/selectivity data were determined by

1

H NMR

spectroscopy). For [Me3 NDo][ReO 4 ] (5, Me = methyl, Do = n-dodecyl) within the first hour of reaction the activity just slightly lower than that of the so far most active catalyst [MeMeOctIm][ReO 4 ] (8) [18], while [Me3 NOct][ReO 4 ] (4) exhibits a poor activity (Figure 1).

T

After 24 h the activity of 4 and 5 is very similar (44% and 40% conversion respectively). It is

SC RI P

intriguing that despite the initial activity of catalyst 5 the conversion of cyclooctene abruptly ends at 40-45%. There are two possible reasons for the observed decrease in activity: (i) The poisoning or decomposition of the catalyst, or (ii) the decomposition of H2 O2 , e.g. by bromide

NU

from the catalyst precursor. Catalyst decomposition can be ruled out as IR and NMR spectra recorded after the reaction show that the catalyst is pure and structurally intact. Therefore, the

MA

decomposition of H2 O2 is the most presumable reason, which is supported by the observation

AC

CE

PT

ED

of slight gas evolution during the catalytic reactions.

Figure 1. Kinetic data for the catalytic activities of [Me 3 NOct][ReO 4 ] (4), [Me3 NDo][ReO 4 ] (5, full blue curve and dotted curve), and [MeMeOctIm][ReO 4 ] (8, taken from ref. 13) for the epoxidation

of

cyclooctene.

Reaction

conditions:

ratio

catalyst:cis-cyclooctene:H2 O2

ACCEPTED MANUSCRIPT 5:100:250; for the dotted blue curve the ratio is 5:100:500 and after 5 h additional 500 equiv. H2 O 2 were added. T = 70 °C. Epoxide selectivity > 99%.

Note that from elemental analysis the presence of traces of bromide, which would lead to H2 O 2 decomposition, can be excluded. To further investigate the H2 O2 decomposition, in a epoxidation

experiment using catalyst 5

the amount of H2 O2

T

new

was doubled

SC RI P

(catalyst:cyclooctene:H2 O2 ratio of 5:100:500). With a double amount of oxidant, the activity is increased after 2 h of reaction time (Figure 1, blue dotted curve).

Afterwards, a decrease in activity is observed again. Therefore, after 5 h an additional amount

NU

(500 equiv.) of H2 O 2 was added and the reaction continues. The slope after the addition is not entirely linear due to the dilution of the catalyst in water (as 50 wt.% H2 O2 is used). After 24 h

MA

the conversion of cyclooctene is 90%, proving that the catalyst is still intact and that the decrease is caused by the decomposition of H2 O2 It is known that in absence of substrate, The activity of ammonium

ED

imidazolium perrhenate catalysts slowly decompose H2 O 2 [25].

perrhenate catalysts is likely to originate from two competing reactions: the activation and

PT

decomposition of H2 O2 by [ReO 4 ]-, and the H2 O2 activation followed by oxygen transfer to

CE

the substrate. The rate of the latter reaction is dominated by the solubilization of the substrate in the aq. phase by the catalyst or the cation. Since the ammonium perrhenate catalysts do not

AC

dissolve in aq. H2 O 2 , but rather form emulsions, it is to be expected that the solubilization of cyclooctene is rather low. In contrast to tetraalkylammonium perrhenates, imidazolium perrhenates exhibit a pronounced degree of cation-anion interactions via H-bonds both in solution and in the solid state [26]. Therefore, the ion pairing and the basicity of [ReO 4 ]- is likely to be higher for tetraalkylammonium cations, resulting in faster H2 O2 activation. This may be the main reason why in the early stage of epoxidation catalyst 5 is nearly equally active as the best perrhenate catalyst 8 (Figure 1, c.f. blue dotted curve and green curve) despite the lower solubility of the substrate in the aq. phase.

ACCEPTED MANUSCRIPT For [Me3 NHex][ReO 4 ] (6, Hex = n-hexadecyl) it was not possible to obtain reliable results under standard conditions (5 mol%, 70 °C) due to the formation of foam, rendering extraction of liquid samples and collection of kinetic data impossible. Therefore, the catalyst concentration was reduced to 1 mol% and compared to compounds 4 and 5 as well as to the imidazolium-based

catalysts

1-methyl-3-octylimidazolium perrhenate

([MeHOctIm][ReO 4 ],

T

7) and [MeMeOctIm][ReO 4 ] (8). The comparison of the catalytic activities of compounds 4-8

SC RI P

for the epoxidation of cyclooctene is shown in Figure 2. Very surprisingly, at a catalyst concentration of 1 mol% compounds 5 (30% conversion after 8 h; 53% after 24 h) and 6 (51% conversion after 8 h; 71% after 24 h) considerably outperform compound 4 as well as

NU

the imidazolium perrhenate catalysts 7 and 8 (see the ESI, Table S1). While the imidazolium perrhenate catalysts require a minimal concentration of 5 mol% [18], which corresponds to

and

catalyst 6

MA

the critical micelle concentration [25], the catalyst 5 shows an only slight decrease in activity is significantly more active than 5.

This finding shows that for

ED

tetraalkylammonium perrhenates, the activity depends on both the hydrophobicity of the cation and the H2 O2 decomposition. While longer alkyl chains obviously promote the

PT

solubilization of the substrate in the aq. phase, leading to higher conversions (c.f. catalysts 4,

CE

5 and 6 in Figure 2), lower concentrations of perrhenate are detrimental for H2 O2 decomposition. For imidazolium perrhenates the critical catalyst concentration for the

AC

formation of aggregates/micelles in aq. solution seems to play the decisive role in activity.

SC RI P

T

ACCEPTED MANUSCRIPT

(n-dodecyl)trimethylammonium

perrhenate

(6),

octylimidazolium

perrhenate

1-methyl-3-octylimidazolium perrhenate

(8).

MA

(4),

NU

Figure 2. Kinetic data for the catalytic activities of (n-octyl)trimethylammonium perrhenate

Reaction

(5),

perrhenate

conditions:

(n-dodecyl)trimethylammonium (7), ratio

and

1,2-dimethyl-3-

catalyst:cyclooctene:H2 O2

PT

ED

1:100:250, T = 70 °C (selectivity epoxide: > 99%).

The catalysts 6 and 8 were also applied at room temperature (ESI, Figure S2). For catalyst 6

CE

gas formation is much less pronounced, indicating a lower degree of H2 O 2 decomposition. In comparison to catalyst 8 compound 6 is still quite active under mild reaction conditions. After

(12%).

AC

24 h the conversion of cyclooctene using 6 (54%) is distinctly higher than for catalyst 8

In order to suppress undesired decomposition of H2 O2 , the epoxidation of cyclooctene was carried out using the most active tetraalkylammonium perrhenate catalyst 6 under dropwise addition of H2 O2 at 50 °C, since the conversions were too low at 25 °C. The kinetic data indicate that the reaction is significantly slower compared to addition of the total amount of H2 O 2 prior to the start of the reaction (Figure 3). At low H2 O 2 /H2 O concentrations the reaction mixture consists of three phases, which is detrimental for the catalysis. For the first 5 h the

ACCEPTED MANUSCRIPT catalytic conversion of cis-cyclooctene is expectedly rather low. After 5 h 50% of the total amount of H2 O2 were added and from this point on the conversion rate increases. After 24 h the conversion of cyclooctene reaches 91% (c.f. 71% conversion at 70 °C and full addition of H2 O 2 before start), which is a good indicator for slower H2 O2 decomposition upon slower addition of oxidant.

T

Catalyst 6 was used for the epoxidation of other olefins (Table 1). The conversions are low to

SC RI P

moderate, which is most presumably giving rise to the fact that terminal and open-chain olefins are more difficult to epoxidize than cyclic cis-olefins, such as cyclooctene. Another reason for the low activity could be the low solubility of the substrates in the aq. phase.

NU

Interestingly, the selectivity towards the epoxide also decreases significantly, particularly in

AC

CE

PT

ED

MA

comparison to cyclooctene.

Figure 3. Kinetic data of dropwise addition of hydrogen peroxide to 6 and substrate (300 μl per h), 50 °C, overall ratio catalyst:substrate:H2 O2 1:100:500 (selectivity epoxide: > 99%).

ACCEPTED MANUSCRIPT Table 1. Catalytic conversion and selectivity of different substrates using catalyst 6. Substrate

Conversion [%]

Selectivity [%]

1

1-octene

4

30

2

cis-stilbene

14

20

3

trans-β-methylstyrene

33

22

4

styrene

68

48

SC RI P

T

Entry

Reaction conditions: ratio catalyst:substrate:H2 O2 of 1:100:250, T = 50 °C, after 4 h (selectivity towards epoxide).

NU

Since both cyclooctene and the product, cyclooctene oxide, are not miscible with the catalysts 1-6 and the aqueous phase, at the end of the reaction the mixture slowly forms two phases,

MA

which can be separated. Therefore, a recycling experiment using catalyst 6 was performed (Figure 4). At the end of the reaction the organic phase was separated, and all volatiles were

ED

removed from the aq. phase until only the catalyst 6 remained. Thereafter, 100 equiv. of

PT

substrate were added and the mixture was heated to 50 °C. Then another 250 equiv. of H2 O2 were added to start the next run. Another method of recycling is the distillation of all

CE

components, apart from the non-volatile catalyst in vacuum. Both procedures can be repeated

AC

several times without loss of activity or catalyst leaching.

4.

Recycling

experiments

using

catalyst

6.

Reaction conditions: catalyst:cis-

NU

Figure

SC RI P

T

ACCEPTED MANUSCRIPT

4.

MA

cyclooctene:H2 O2 1:100:250, T = 50 °C, t = 4 h (selectivity epoxide: > 99 %).

Conclusions

ED

In summary, a series of N-alkyl ammonium perrhenates were synthesized and characterized.

PT

The evaluation of their activity in epoxidation catalysis reveals that all catalysts are prone to decompose hydrogen peroxide, and that this has a detrimental influence on the overall

CE

substrate conversion. The decomposition can be suppressed by slow addition of oxidant, which has a negative impact on the reaction time. The catalyst activity also strongly depends

AC

on the length of the alkyl chain. Unexpectedly, the catalysts presented here do not significantly lose activity upon decreasing the catalyst concentration, as is the case for imidazolium perrhenates: a decrease from 5 to 1 mol% leads to a pronounced drop for the otherwise most active catalyst 8, while catalyst 6 reaches 51% after 8 h. Certainly, the results of this work raise a lot of further questions. The ammonium perrhenates shown in this study exhibit weak to moderate activity compared to the related imidazoliumand amidoammonium perrhenate catalysts, showing that there appear to be some limitations and open questions regarding the choice of the ammonium cation: does the strength of ion

ACCEPTED MANUSCRIPT contacts play a role and doe these catalysts principally form micelles (or under which conditions do they form?). A deeper evaluation along these lines is an ongoing object of study in our team. In principle, this catalyst class bears the perspective to adjust the catalyst to the desired substrate by changing the solubility properties (and ion pairing) by the substituents at the nitrogen atom of the cation. This preliminary study shows that this work has to be

T

performed in the future, along with the application of oxo-anions of other metals, or non-

SC RI P

metal elements, which would pave the way to the development of easily accessible, cheap, robust and recyclable catalysts.

NU

Acknowledgements

AC

CE

PT

ED

MA

R.M.R. thanks the TUM Graduate School for financial support.

ACCEPTED MANUSCRIPT References [1]

M.G. Clerici, M. Ricci, G. Strukul, in Metal Catalysis in Industrial Processes (Eds. G. P. Chiusoli, P.M. Maitlis, RSC Publishing, 2006, 23–78. A.K. Yudin (Ed.), Aziridines and Epoxides in Organic Synthesis, Wiley-VCH, 2006.

[3]

S. Huber, M. Cokoja, F.E. Kühn, J. Organomet. Chem. 751 (2014) 25–32.

[4]

S.A. Hauser, M. Cokoja, F.E. Kühn, Catal. Sci. Technol. 3 (2013) 552–561.

[5]

O. Cussó, X. Ribas, M. Costas, Chem. Commun. 51 (2015) 14285–14298.

[6]

G. De Faveri, G. Ilyashenko, M. Watkinson, Chem. Soc. Rev. 40 (2011) 1722–1760.

[7]

D. Betz, P. Altmann, M. Cokoja, W.A. Herrmann, F.E. Kühn, Coord. Chem. Rev. 255

SC RI P

T

[2]

NU

(2011) 1518–1540.

P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. 39 (2000) 3772–3789.

[9]

H.-P. Steinrück, P. Wasserscheid, Catal. Lett. 145 (2015) 380–397.

MA

[8]

[10] T. Welton, Chem. Rev. 99 (1999) 2071–2084.

ED

[11] J.P. Hallett, T. Welton, Chem. Rev. 111 (2011) 3508–3576. [12] F. Montanari, D. Landini, F. Rolla, Top. Curr. Chem. 101 (1982) 147–200.

PT

[13] S.-S. Wang, G.-Y. Yang, Chem. Rev. 115 (2015) 4893–4962.

CE

[14] Y. Zhou, Z. Guo, W. Hou, Q. Wang, J. Wang, Catal. Sci. Technol. 5 (2015) 4324–4335. [15] C. Venturello, R. D'Aloisio, J. Org. Chem. 53 (1988) 1553–1557.

AC

[16] M. Fressancourt-Collinet, B. Hong, L. Leclercq, P.L. Alsters, J.-M. Aubry, V. NardelloRataj, Adv. Synth. Catal. 355 (2013) 409–420. [17] I.I.E. Markovits, W.A. Eger, S. Yue, M. Cokoja, C.J. Münchmeyer, B. Zhang, M.D. Zhou, A. Genest, J. Mink, S.L. Zang, N. Rösch, F.E. Kühn, Chem. Eur. J. 19 (2013) 5972–5979. [18] M. Cokoja, R.M. Reich, M.E. Wilhelm, M. Kaposi, J. Schäffer, D.S. Morris, C.J. Münchmeyer, M.H. Anthofer, I.I.E. Markovits, F.E. Kühn, W.A. Herrmann, A. Jess, J.B. Love, ChemSusChem 9 (2016) 1773–1776.

ACCEPTED MANUSCRIPT [19] M. Cokoja, I.I.E. Markovits, M.H. Anthofer, S. Poplata, A. Pöthig, D.S. Morris, P.A. Tasker, W.A. Herrmann, F.E. Kühn, J.B. Love, Chem. Commun. 51 (2015) 3399–3402. [20] S. Steudte, H. Bui Thi Thu, V. Korinth, J. Arning, A. Białk-Bielińska, U. Bottin-Weber, M. Cokoja, A. Hahlbrock, V. Fetz, R. Stauber, B. Jastorff, C. Hartmann, R. W. Fischer, F. E. Kühn, S. Stolte, Green Chem. 17 (2015) 1136–1144.

SC RI P

Kühn, S. Stolte, New J. Chem. 7 (2015) 5431–5436.

T

[21] H. B. T. Thu, M. Markiewicz, J. Thöming, R. M. Reich, V. Korinth, M. Cokoja, F. E.

[22] K. Bica, P. Gärtner, P.J. Gritsch, A.K. Ressmann, C. Schröder, R. Zirbs, Chem. Commun. 48 (2012) 5013–5015.

NU

[23] I. Goodchild, L. Collier, S. L. Millar, I. Prokes, J.C.D. Lord, C.P. Butts, J. Bowers, J.R.P. Webster, R.K. Heenan, J. Colloid Interface Sci. 307 (2007) 455–468.

MA

[24] S. Dorbritz, W. Ruth, U. Kragl, Adv. Synth. Catal. 347 (2005) 1273–1279. [25] M. Cokoja, J. Schäffer, A. Jess, manuscript in preparation.

ED

[26] R.M. Reich, M. Cokoja, I.I.E. Markovits, C.J. Münchmeyer, M. Kaposi, A. Pöthig,

AC

CE

PT

W.A. Herrmann, F.E. Kühn, Dalton Trans. 44 (2015) 8669–8677.

T

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

MA

NU

SC RI P

Graphical abstract

ACCEPTED MANUSCRIPT Highlights

T SC RI P NU MA ED PT CE



Simple N-alkylammonium perrhenate salts catalyze the epoxidation of olefins The activity depends on the length of the alkyl chains and on the The cations are easy to modify and to synthesize, compared to the more active imidazolium congeners The catalysts are robust and can easily be separated from the product

AC

  