A New Polyaniline-Based Catalyst for the Oxidation of Alkenes

Abstract

Polymer-supported heterogeneous catalysts based on cobalt acetate and Co(II)Salen were synthesized. As a support, conjugated polymers were used: polyaniline, poly-o-toluidine or poly-o-anisidine. Oxidation reactions of alkenes were carried out on synthesized polymer-supported catalysts at atmospheric pressure in the presence of molecular oxygen. Epoxides or ketones were obtained as main products. Oxidation reactions were carried out under very mild conditions, at room temperature, and the desired products were obtained in very high selectivity and relatively high yields.

oxidation - polyaniline based-catalysts - conjugated-polymers - catalysts - alkenes

References

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6

Method of Synthesis of Co(II)Salen: In 50 mL of 95% boiling EtOH 4.9 g (40 mmol) of salicylaldehyde was dissolved. To this solution 1.2 g (20 mmol) of ethylenediamine was added dropwise. The reaction was stirred for 3.5 h. After completion, the reaction mixture was cooled. N,N′-Bis(salicylidene)ethylenediimine yellow crystals were precipitated, filtered out, washed with 20 mL of EtOH and finally dried. Subsequently, 2 g (7.45 mmol) of Schiff base was dissolved in 120 mL of EtOH. The reaction was carried out under argon atmosphere. The cobalt(II) acetate 1.85 g (8.37 mmol) solution in 15 mL of warm water (60 °C) was added dropwise. A brown-red gelatinous product was formed, which next changed to red crystals. The reaction was stirred for 1 h. After cooling, the crystals of Co(II)Salen complex were filtered and dried at 60 °C (under vacuum) overnight.

7

The Preparation of Polyaniline is Representative for the General Procedure of Monomer Polymerization:
Before polymerization aniline was distilled under vacuum in the presence of Zn powder.
In a typical procedure, freshly distilled aniline (0.1 mol,
9.3 g) was dissolved in 150 mL of 1.5 M aq HCl. The solution was then cooled to about -5 °C in a salt-ice mixture. Ammonium persulfate (0.125 mol, 28.5 g), dissolved in water, was added dropwise to a vigorously stirred mixture. The rate of addition of the oxidizing agent was used to control the temperature of the reaction mixture, which varied between -5 °C and -3 °C. The reaction was stirred for 4 h as recommended in literature. The greenish-black precipitate was filtered off, washed with water and then with methanol until the filtrate became colorless. Oligomers were extracted from polyaniline with THF in a Soxhlet apparatus until the extract became colorless. Poly-o-toluidine and poly-o-anisidine were washed with water and MeOH only. The polymer was then dried at 60 °C for 24 h and deprotonated with 250 mL of 12% aq NH₃ solution for 6 h. Deprotonated polymer was dried at 60 °C for 24 h and washed again with water and MeOH. All these purification steps are important for the entire removal of all oligomeric species accompanying the polymer.

8a

Preparation of Polyaniline Supported Cobalt(II) Acetate is Representative for the Procedure Employed for Cobalt(II) Acetate Based Catalysts: A mixture of polyaniline (500 mg) and cobalt acetate (500 mg) was stirred in an MeCN (25 mL) and HOAc (25 mL) mixture for 72 h
at r.t. Then the reaction mixture was filtered and the solid catalyst was washed with MeCN (5 × 5 mL). The catalyst was dried at 110 °C for 24 h.

8b

Preparation of Polyaniline Supported Co(II)Salen is Representative for the Procedure Employed for Co(II)Salen Based Catalysts: A mixture of polyaniline (500 mg) and Co(II)Salen (500 mg) was stirred in 50 mL of MeCN for
48 h at r.t. The reaction mixture was filtered and the solid catalyst was stirred again with 50 mL of HOAc for 1 h at r.t. The reaction mixture was filtered and the solid filtrate was first washed with HOAc (3 × 10 mL) and then MeCN (5 × 5 mL). The catalyst was dried at 110 °C for 24 h.

9

The Oxidation of 1-Dodecene is Representative for the General Procedure Employed for the Oxidation of Alkenes: The mixture of 2-methylpropanal (1.08 g, 15 mmol) and polymer-supported catalyst (30 mg) was dissolved in MeCN (30 mL). The mixture was bubbled with oxygen for 15 min at r.t. Then, 1-dodecene (0.84 g, 5 mmol) was added. The reaction was carried out under oxygen atmosphere for the time period indicated in Table 1. After completion of the reaction the catalyst was filtered and the solvent was evaporated to yield a residue, which was dissolved in ethyl acetate and washed with NaHCO₃ solution and water. The organic phase was dried over MgSO₄ and the evaporation of solvent yielded the desired product, which was purified by Kugelrohr distillation.

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11 Schuster C. Hölderich WF. Catal. Today  2000,  60:  193