Amit
Saha
,
John
Leazer
* and
Rajender S.
Varma
*
Sustainable Technology Division, National Risk Management Research Laboratory, U. S. Environmental Protection Agency, MS 443, Cincinnati, Ohio 45268, USA. E-mail: varma.rajender@.epa.gov; Fax: 513-569-7677; Tel: 513-487-2701
First published on 25th October 2011
Allylic ethers were synthesized in water using magnetically recoverable heterogeneous Pd catalystvia O-allylation of phenols with allylic acetates under ambient conditions. The aqueous reaction medium, easy recovery of the catalyst using an external magnet, efficient recycling, and the high stability of the catalyst renders the protocol economic and sustainable.
Phenols undergo unprecedented allylic substitution reactions with various allylic acetates in air under refluxing aqueous media conditions for 3–10 h whilst in the presence of the [Fe3O4–dopamine–Pd] catalyst; a mild base, sodium bicarbonate, is adequate to produce the allyl ethers (Scheme 1) in good yields.
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Scheme 1 O-Allylation of phenols with allylic acetates. |
The catalyst was prepared following the optimized protocol established earlier.8a–c The aqueous suspension of nano-ferrite was sonicated for 2 h with dopamine, which acts as a pseudo-ligand and as a robust anchor preventing the leaching of immobilized Pd. Dopamine functionalized nano-ferrite was then treated with PdCl2 in basic medium to obtain the Pd(II) catalyst supported on amine functionalized magnetic Fe3O4 nanoparticles. The spherical morphology and the size of the nano catalyst (13–38 nm) were confirmed by transmission electron microscopy (TEM) (Fig. 2). The weight percentage of palladium was found to be 10.23% by inductively coupled plasma–atomic emission spectroscopy (ICP–AES) analysis.
To optimize the reaction conditions, a series of experiments were conducted with a representative reaction of cinnamyl acetate and p-cresol, with variation of reaction parameters, such as base, solvent, reaction temperature etc. (Table 1). It was found that a combination of [Fe3O4–dopamine–Pd], 50 mg (4.8 mol%), and NaHCO3 (2 equiv.) in water is ideal for a fast and efficient reaction (Table 1, entry 13). The reaction was observed to be incomplete in the presence of a lower amount of the Pd catalyst (30 mg), even after prolonging the reaction time. Further, no improvement was observed by increasing the amount of Pd catalyst in terms of reaction time and yield of the product. The reaction does not proceed in either the absence of base NaHCO3 (Table 1, entry 1) or the catalyst [Fe3O4–dopamine–Pd] (Table 1, entry 2). Also, no desired allyl ether was obtained in an identical reaction with [Fe3O4] nanoparticles (without Pd) (Table 1, entry 3). Water was found to be the best solvent for this reaction to obtain improved yields and fast reaction times compared to other common organic solvents such as DMF, THF, CH3CN, toluene, PEG (Table 1, entries 4–7, 9).
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Entry | Base | Solvent | Temp (°C) | Time (h) | Yield (%)b |
a A mixture of p-cresol (1 mmol), cinnamyl acetate (1 mmol), base (2 mmol) and Pd catalyst (50 mg, 4.8 mol%) was heated under air in the specified solvent. b Yields refer to those of column purified isolated products. c The reaction was performed without the catalyst [Fe3O4–dopamine–Pd]. d The reaction was performed with Fe3O4 naoparticles (without Pd). | |||||
1 | — | Water | 100 | 15 | 0 |
2c | NaHCO3 (without catalyst) | Water | 100 | 15 | 0 |
3d | NaHCO3 (with Fe3O4) | Water | 100 | 15 | 0 |
4 | K2CO3 | DMF | 120 | 12 | 77 |
5 | K2CO3 | THF | 66 | 10 | 53 |
6 | K2CO3 | CH3CN | 82 | 10 | 55 |
7 | K2CO3 | Toulene | 100 | 12 | 9 |
8 | K2CO3 | Water | 100 | 10 | 80 |
9 | K2CO3 | PEG | 100 | 12 | 63 |
10 | Cs2CO3 | Water | 100 | 10 | 65 |
11 | Na2CO3 | Water | 100 | 10 | 80 |
12 | NaHCO3 | Water | 100 | 10 | 83 |
13 | NaHCO 3 | Water | 100 | 5 | 85 |
14 | NaOAc | Water | 100 | 10 | 18 |
15 | NaH | DMF | 120 | 12 | Trace |
16 | K3PO4 | Water | 100 | 10 | 66 |
In a typical representative experimental procedure, a mixture of allyl acetate, phenol, Pd catalyst and NaHCO3 in water was heated to reflux in an open atmosphere for a sufficient time to complete the reaction (as determined by TLC). After completion of the reaction, the catalyst was separated from the reaction mixture using an external magnet. The product was extracted by ethyl acetate and was purified by column chromatography. The catalyst was washed with acetone, dried under vacuum, and recycled for 5 consecutive reactions without any significant loss in efficiency (Table 2).
|
|
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No. of cycles | Yield (%) |
1 | 85 |
2 | 84 |
3 | 85 |
4 | 83 |
5 | 82 |
6 | 82 |
Metal leaching was studied by ICP–AES analysis of the catalyst before and after the reaction cycles. The Pd concentration in the heterogeneous catalyst was found to be the same before and after the reaction. The TEM image of the catalyst taken after the fifth cycle of the reaction does not show any significant changes in the morphology and the size of the catalyst nanoparticles (15–37 nm) (Fig. 3), which indicates retention of the catalytic activity after recycling. No Pd metal was detected in the reaction solvent (water) after completion of the reaction. It confirms the fact that dopamine-functionalization provides enough amine-binding sites on the surfaces of the Fe3O4 nanoparticles, which serve as pseudo-ligands by coordinating with Pd and thus help to minimize deterioration and metal leaching, whilst aiding in efficient catalyst recycling.
A series of substituted cinnamyl acetates underwent coupling with a variety of substituted phenols by this procedure to produce the corresponding allyl aryl ethers (Table 3). In general, the reactions are very efficient and clean. The ensuing products are obtained in high yields and good purity. Several functional groups such as OMe, NO2, Cl, and Br are compatible with this reaction, which performs consistently with o-, m-, and p-substituted phenols (Table 3, entries 2–4). A branched allylic acetate (Table 3, entry 13) was used with equal efficiency to provide a linear allyl aryl ether, supporting a mechanism that entails the intermediacy of an η3-allyl Pd complex as the key intermediate in the reaction (Scheme 2).
Entry | Phenols | Allylic acetate | Time (h) | Products | Yields (%)b |
---|---|---|---|---|---|
a A mixture of phenol (1 mmol), allylic acetates (1 mmol), sodium bicarbonate (2 mmol) and Pd catalyst (50 mg, 4.8 mol%) in water (2.5 mL) was heated to reflux in air. b Yields refer to those of column purified isolated products. c The reaction mixture in DMF (2 mL) was heated at 90 °C. | |||||
1 |
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6 |
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80 |
2 |
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5 |
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85 |
3 |
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5 |
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87 |
4 |
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5 |
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83 |
5c |
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10 |
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90 |
6 |
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9 |
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79 |
7 |
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10 |
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85 |
8 |
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3 |
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75 |
9 |
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8 |
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87 |
10 |
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8 |
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75 |
11 |
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8 |
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73 |
12 |
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8 |
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76 |
13 |
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8 |
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81 |
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Scheme 2 Formation of η3-allyl Pd complex as reaction intermediate. |
Footnote |
† Electronic supplementary information (ESI) available: Experimental procedures; 1H and 13C NMR spectra of all products. See DOI: 10.1039/c1gc16174a |
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