Readily prepared heterogeneous molybdenum-based catalysts as highly recoverable, reusable and active catalysts for alkene epoxidation

Catalysis Communications 8 (2007) 839–844 www.elsevier.com/locate/catcom

Readily prepared heterogeneous molybdenum-based catalysts as highly recoverable, reusable and active catalysts for alkene epoxidation Shahram Tangestaninejad a,*, Valiollah Mirkhani a, Majid Moghadam b, Gholamhossein Grivani a a

Chemistry Department, Isfahan University, Isfahan, 81746-73441, Iran b Chemistry Department, Yasouj University, Yasouj, 75914-353, Iran

Received 14 February 2006; received in revised form 17 August 2006; accepted 17 August 2006 Available online 9 September 2006

Abstract Polymer-bound aliphatic amines were prepared readily from Merrifield resin and used as supports immobilization of molybdenum hexacarbonyl. The high activity of these polymer-supported molybdenum catalysts has been demonstrated in epoxidation of various alkenes in the presence of tert-butylhydroperoxide (TBHP). These new heterogenized molybdenum epoxidation catalysts can be recovered and reused several times without significant loss of their activities.  2006 Elsevier B.V. All rights reserved. Keywords: Molybdenum carbonyl; Heterogeneous catalyst; Epoxidation; tert-Butylhydroperoxide

1. Introduction Alkene epoxidation is a very useful reaction in industrial organic synthesis. The resultant epoxides are essential precursors in the synthesis of various important substances like plasticizers, perfumes and epoxy resins. Organic peroxy acids and hydroperoxides in the presence of 5B or 6B transition metals (e.g., V, Mo and W) have been known as useful catalytic systems for performing the epoxidation reactions. An important example is Halcon process for the industrial manufacture of propylene oxide, which is carried out by liquid-phase epoxidation of propylene with alkylhydroperoxide as oxygen source, catalyzed by homogeneous Mo (VI) or a heterogeneous Ti (IV) anchored on a SiO2 support. In recent years the use of polymer-supported catalysts in organic transformations has been receiving extraordinary attention [1–4], and the design of functionalized polymers *

Corresponding author. Tel.: +98 311 7932705; fax: +98 311 6689732. E-mail address: [email protected] (S. Tangestaninejad).

1566-7367/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.08.037

carrying catalytically active metal species has generated considerable interest [4–9]. Over the last decade several, highly active Mo (VI) alkene epoxidation catalysts have been developed in the presence of tert-butylhydroperoxide [10,11]. Tempest employed a boronic acid group-containing resin as a support for Mo (VI) in the epoxidation of alkenes by tert-butylhydroperoxide [12]. Sherrington group designed and synthesized aminated polystyrene, polymethacrylate, polybenzimidazole and polysiloxanes resins for immobilization of Mo (VI) catalysts and used them for epoxidation of alkene in the presence of tert-butylhydroperoxide [13–18]. Among these catalysts, the polysiloxanes based Mo (VI) showed more reactivity than the other previously reported polymer-supported molybdenum systems. Previously, we showed that molybdenum hexacarbonyl immobilized on polystyrene can be employed as active and highly reusable alkene epoxidation catalysts with tert-butylhydroperoxide as an oxidant [19]. Here, we describe the use of less expensive and readily preparable heterogeneous polystyrenes as stable supports, for molybdenum hexacarbonyl.

840

S. Tangestaninejad et al. / Catalysis Communications 8 (2007) 839–844

2. Experimental All materials were of commercial reagent grade and obtained from Merck and Fluka. All alkenes were passed through a column containing active alumina to remove peroxidic impurities. A 400 W Hg lamp was used for activation of metal carbonyl. FT-IR spectra were obtained as potassium bromide pellets in the range 400–4000 cm 1 with a Nicolet Impact 400D. 1H NMR spectra were recorded on a Bruker-Avance AQS 300 MHz. 2.1. Functionalization of polystyrenes Scheme 1 shows the preparation procedure of functionalized polystyrenes 1a and 1b. The aminated polystyrene 1b was prepared by a literature method [20], and 1a was prepared by the following procedures: to a 250 mL round bottom flask equipped with a magnetic bar containing 120 mL acetonitrile were added chloromethylated polystyrene (2 g, 2.5 mmol/Cl), ethylenediamine (25 mmol) and NaI (0.13 mmol) and refluxed (48 h). Then the reaction mixtures were filtered and washed with 5 · 40 mL of CH3CN, 5 · 40 mL of 1:1 CH3OH/1M, aqueous K2CO3, 5 · 40 mL of 1:1 CH3OH/H2O and 3 · 40 mL diethyl ether and were dried in an oven (80 C). 2.2. Preparation of polymer-supported molybdenum catalysts 2a and 2b A 250 mL round bottom flask containing 4.5 g (17 mmol) of Mo(CO)6 in 90 mL THF was placed under UV irradiation (400 W Hg lamp) for 30 min and then 1.5 g of aminated polystyrene was added to this solution and refluxed for 3 h. The reaction mixture was filtered, thoroughly washed with THF and was dried in vacuo for 20 h. 2.3. Characterization of resins Resins 1a and 1b were characterized by elemental analysis. The N content of resins 1a and 1b was obtained in 2.29% (0.82 mmol/g, which indicates that only 58% of total

chlorines was substituted by amine) and 3.27% (0.78 mmol/ g, which indicates that only 55.5% of total chlorines was substituted by amine) for 1a and 1b, respectively. The metal loading of polymer-supported molybdenum carbonyls (2a and 2b), which were determined by neutron activation analysis (NAA), obtained 4.60% (0.48 mmol/g) and 4.87% (0.50 mmol/g) for resins 2a and 2b, respectively. The FT-IR spectrum of these metal carbonyl containing resins showed v(CO) (cm 1) at 2009(w), 1930(s), 1877(s), 1838(s) for 2a and 2012(w), 1980(w), 1932(s), 1872(s), 1834(m) for 2b. Fig. 1 shows the SEMs of chloromethylated polystyrene (cross-linked with 2% divinylbenzene) and polymer-supported molybdenum carbonyl resins 2a and 2b. Comparison of images clearly indicates that the smooth and flat surface of the starting polystyrene (cross-linked with 2% divinylbenzene) (Fig. 1c) distinctly altered upon anchoring of the metal carbonyl (Fig. 1a and b). 2.4. General procedure for epoxidation of alkenes To a 25 mL round bottom flask equipped with a magnetic bar were added 4 mL CCl4, 0.5 mmol alkene, 1 mmol TBHP and 0.01 mmol of catalyst and refluxed. The reaction progress was monitored by GLC. The reaction mixture was diluted with CH2Cl2 (20 mL) and filtered. The resin was thoroughly washed with CH2Cl2 and combined washings and filtrates were purified on a silica-gel plate or a silica-gel column. IR and 1HNMR spectral data confirmed the identities of the products. Blank experiment in the presence of an oxidant and using the same experimental conditions in the absence of a catalyst was also performed. 2.5. Reusability of the catalyst The reusability of the polymer-bound molybdenum carbonyl catalysts was studied in repeated epoxidation reaction of cis-cyclooctene. The reactions were carried out as described above. At the end of each reaction, the mixture was filtered, washed with 4 · 6 mL 1:3 CHCl3/THF, 4 · 6 mL 1:3 CHCl3/(CH3)2CO, then was dried in an oven at 80 C and reused. CH3CN/NaI

H2 N

Cl

NH2

Reflux (48 h)

NH

NH2

NH

NH

1a

+

100 0C (4 h)

H2 N

NH

NH2

NH2 N

NH2 Scheme 1. Preparation procedures of aminated polystyrenes 1a and 1b.

NH2 1b

S. Tangestaninejad et al. / Catalysis Communications 8 (2007) 839–844

841

hv (30 min)

Mo(CO)5THF

Mo(CO)6 + THF

1a, 1b Reflux (3 h)

NH

NH2

2a

Mo(CO)4 N NH

Mo

NH2

CO CO CO 2b

NH2 NH

Mo NH2

CO CO CO

Scheme 2. Preparation procedures of catalysts 2a and 2b.

graft on it via ligands which are attached to the polymer beads. Reacting of these aminated polystyrene resins with a solution of Mo(CO)6 in THF (which was activated by UV irradiation) under reflux conditions resulted in covalent attachment of molybdenum carbonyl to give polymer-supported molybdenum carbonyl catalysts 2a and 2b. The success of immobilization was proved by FT-IR, and the amount of molybdenum incorporated on the polymer was determined by neutron activation analysis (NAA). The catalytic activity of the resulting catalysts was initially investigated in the epoxidation of cis-cyclooctene in the presence of tert-butylhydroperoxide. Among the acetone, THF, chloroform, acetonitrile and carbontetrachloride solvents, CCl4 was chosen as the reaction media because higher epoxide yield was obtained in it (Table 1). We also investigated the ability of different oxygen donors such as tert-BuOOH, NaIO4/tetrabutylphosphonium Table 1 Epoxidation of cis-cyclooctene catalyzed by polymer-supported molybdenum carbonyl catalysts 2a and 2b in the presence of TBHP in different solvents under reflux conditionsa Fig. 1. SEMs of: (a) resin 2a; (b) resin 2b; and (c) chloromethylated polystyrene.

3. Results and discussion Scheme 2 shows the preparation procedures of catalysts 2a and 2b. We used the well known chloromethylated polystyrene cross-linked with 2% divinylbenzene as support because it is flexible enough and allows metallic atoms to

Solvent

(CH3)2CO THF CH3CN CHCl3 CCl4

Catalyst 2a

Catalyst 2b

Cyclooctene epoxide (%)b

Time (h)

Cyclooctene epoxide (%)

Time (h)

No reaction No reaction 5 69 92

2.5 2.5 2.5 2.5 1.25

No reaction No reaction 3 71 95

3 3 3 3 1.5

a Reaction conditions: cyclooctene (0.5 mmol), TBHP (1 mmol), catalyst (0.01 mmol), solvent: 4 mL. b GLC yield based on the starting cyclooctene.

842

S. Tangestaninejad et al. / Catalysis Communications 8 (2007) 839–844

Table 2 Epoxidation of cis-cyclooctene catalyzed by polymer-supported molybdenum carbonyl catalysts 2a and 2b in the presence of different oxidant systems under reflux conditionsa Solvent

Oxidant

Catalyst 2a

Catalyst 2b

Cyclooctene epoxide (%)b

Time (h)

Cyclooctene epoxide (%)b

Time (h)

CCl4/H2O (3:1)

NaIO4 H2O2

c

No reaction 1.5

2.5 2.5

No reaction 4

2.5 2.5

CH3CN/H2O (3:1)

NaIO4c H2O2

No reaction 14

2.5 2.5

No reaction 17

2.5 2.5

a b c

Reaction conditions: cyclooctene (0.5 mmol), oxidant (1 mmol), catalyst (0.01 mmol), solvent: 4 mL. GLC yield based on the starting cyclooctene. Phase transfer reagent: tetrabutylphosphonium bromide (0.01 g).

Table 3 Epoxidation of alkenes with TBHP catalyzed by polymer-supported molybdenum carbonyl catalysts 2a and 2b under reflux conditionsa Entry

Alkene

Catalyst 2a

Catalyst 2b

Conversion (epoxide)b

Time (h)

Conversion (epoxide)b

Time (h)

1

92 (92)

1.25

95 (95)

1.5

2

100 (100)

2.5

100 (100)

2

3

96 (96)

2.5

95 (95)

7

4

80 (80)

1.5

74 (74)

2.5

5

56 (56)

5

49 (49)

4

6

98 (98)

3.5

100 (100)

4

7

99 (99)

4.5

100 (100)

5

8

100 (100)c

5

100 (100)c

5

9

100 (97.5 cis)c (2.5 trans)c

5

100 (97 cis)c (3 trans)c

5

3

100 (100)

2

10

a b c

97 (97)

Reaction conditions: Alkene (0.5 mmol), TBHP (1 mmol), catalyst (0.01 mmol), CCl4: 4 mL. GLC yield based on the starting alkenes. Both H NMR and GLC data approved the reported yields.

S. Tangestaninejad et al. / Catalysis Communications 8 (2007) 839–844

843

O

+ OH

O

Verbenone

α−Pinene

Verbenol

α-Pinene oxide

Scheme 3.

bromide and H2O2in the oxidation of cis-cyclooctene and TBHP was chosen as oxygen an donor (Table 2). Polymer-supported molybdenum carbonyl catalysts 2a and 2b can be applied for the epoxidation of a wide range of substituted alkenes (Table 3). These catalysts efficiently convert both cyclic and linear alkenes. 1-Heptene and 1dodecene as linear alkenes were completely converted to the corresponding epoxides by catalysts 2a and 2b. Stilbenes (both cis and trans) were also completely converted by catalysts 2a and 2b. trans-Stilbene gives only trans epoxide and cis-stilbene gives a mixture with high cis/trans epoxide ratio (Table 3). These catalytic systems show a high selectivity in the case of a-pinene, in which the product is a-pinene oxide, and allylic oxidation products, verbenone and verbenol, were not produced in the reaction mixture (Scheme 3). The reusability of a supporting catalyst can be the most important benefit and as transition metal complexes are often expensive to be purchased or prepared this and other advantages of the supported catalysts make them useful for commercial applications. The reusability of the catalysts was monitored by using multiple sequential epoxidation of cyclooctene with TBHP (Table 4). For each catalyst, after run 1, the reaction times decrease and the catalytic activities of catalysts 2a and 2b increase. Catalysts 2a and 2b were consecutively reused 10 times, without a significant loss of their activities. The nature of the recovered catalysts was studied by FTIR spectra, in which the FT-IR spectra of recovered catalysts showed that the CO bands disappeared and this led Table 4 Reusability of polymer-supported molybdenum carbonyl catalysts 2a and 2b in the epoxidation of cyclooctene in the presence of TBHP under reflux conditionsa Run

1 2 3 4 5 6 7 8 9 10

Catalyst 2a

Catalyst 2b

Cyclooctene epoxide (%)

Time (h)

Cyclooctene epoxide (%)

Time (h)

92 93 93 93 91 89 88 86 80 78

1.25 1 1 1 1 1 (1.25) 1 (1.25) 1 (1.25) 1 (1.25) 1 (1.25)

95 94 95 954 94 94 94 91 93 90 (94)

1.5 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 (1.5)

(94) (94) (92) (92) (89)

a Reaction conditions: alkene (0.5 mmol), TBHP (1 mmol), catalyst (0.01 mmol), CCl4: 4 mL; GLC yield based on starting cyclooctene.

to an increase in the catalytic activity. The mechanism of these reactions was reported previously [21]. No molybdenum was detected in the filtrates after each run. On the other hand, the analysis of catalysts after each run (by neutron activation analysis) showed that the Mo content of the catalysts after each run remains constant. Also, the catalytic behavior of the separated liquid was tested by addition of fresh olefin to the filtrates after each run. Execution of the oxidation reaction under the same reaction conditions, as with the catalyst, showed that the obtained results are the same as blank experiments. In addition, in Table 4, the yields in parenthesis, which have been obtained in 1.25 h, confirm this fact. 4. Conclusion We readily prepared polymer-supported molybdenum carbonyl catalysts by simple reaction of aminated polystyrenes and molybdenum hexacarbonyl. These supported catalysts are highly reactive in epoxidation of a wide range of alkenes such as linear and cyclic alkenes. They also efficiently convert the cis and trans stilbenes with a high selectivity isomer. These catalysts are highly reusable and show 10 times reusability without appreciable decrease in their initial activities. References [1] J. Shuttleworth, S.M. Allin, R.D. Wilson, D. Nasturica, Synthesis (2000) 1035. [2] S.V. Ley, I.R. Baxendale, R.N. Bream, J.A.G. Leach, L.M. Nesi, G.S. Scott, I. Storer, S.J. Taylor, J. Chem. Soc. Perkin Trans. 1 (2000) 3815. [3] A. Akelah, Chem. Rev. 81 (1981) 557. [4] D.C. Bailey, S.H. Langer, Chem. Rev. 81 (1981) 109. [5] U. Blaser, Catal. Today 60 (2000) 161. [6] H.U. Blaser, M. Studer, Apll. Catal A: General 189 (1999) 191. [7] H.U. Blaser, B. Pugin, F. Spinder, J. Mol. Catal. A: Chem. 231 (2005) 1. [8] N.E. Leadbeater, M. Marco, Chem. Rev. 102 (2002) 3217. [9] Y.R. de Minguel, E. Brule, R.G. Margue, J. Chem. Soc., Perkin Trans. 1 (2001) 3085. [10] S. Gil, R. Gonzalez, R. Mestres, V. Sanz, A. Zapater, React. Funct. Polym. 42 (1999) 65. [11] D.C. Sherrington, Catal Today 57 (2000) 87. [12] E. Tempesti, L. Giuffre, F.D. Renzo, C. Mazzachia, G. Modica, J. Mol. Catal. 45 (1988) 255. [13] D.C. Sherrington, S. Simpson, J. Catal. 131 (1991) 115. [14] D.C. Sherrington, S. Simpson, React. Polym. 131 (1993) 13. [15] M.M. Miller, D.C. Sherrington, S. Simpson, J. Chem. Soc., Perkin Trans. 2 (1994) 2091. [16] M.M. Miller, D.C. Sherrington, J. Catal. 152 (1995) 368. [17] M.M. Miller, D.C. Sherrington, J. Catal. 152 (1995) 377.

844

S. Tangestaninejad et al. / Catalysis Communications 8 (2007) 839–844

[18] J.H. Ahn, D.C. Sherrington, J. Chem. Soc., Chem. Commun. (1996) 229. [19] (a) G. Grivani, S. Tangestaninejad, M.H. Habibi, V. Mirkhani, Catal. Commun. 6 (2005) 375; (b) S. Tangestaninejad, M.H. Habibi, V. Mirkhani, M. Moghadam, G. Grivani, Inorg. Chem. Commun. 9 (2006) 575;

(c) G. Grivani, S. Tangestaninejad, M.H. Habibi, V. Mirkhani, Appl. Catal. A: General 299 (2006) 31. [20] J.J. Parlow, D.A. Mischke, S.S. Woodard, J. Org. Chem. 62 (1997) 5908. [21] S. Bahaduri, D. Mukesh, in: Homogeneous Catalysis Mechanism and Industrial Application, John-Wiley and Sons, Inc., Canada, 2000, p. 183.