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Boletín de la Sociedad Chilena de Química

versión impresa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.46 n.1 Concepción mar. 2001

http://dx.doi.org/10.4067/S0366-16442001000100002 

CATALITIC OXIDATION OF 1-HEXENE WITH MOLECULAR
OXYGEN BY THE Cu(CH3CN)2(NO2)2/Pd(CH3CN)2Cl2
BIMETALLIC SYSTEM

A.Castillo1, P.J. Baricelli1* , A.J. Pardey2, S.A. Moya3.

1Centro de Investigaciones Químicas, Facultad de Ingeniería, Universidad de Carabobo,
Valencia, Venezuela (E-mail: pbaricel@thor.uc.edu.ve).
2Escuela de Química, Facultad de Ciencias, Universidad Central de Venezuela,
Caracas 1020-A, Venezuela.
3Departamento de Química Aplicada, Facultad de Química y Biología,
Universidad de Santiago de Chile, Santiago de Chile, Chile.
(Received: March 22, 2000 - Accepted: September 1º, 2000)
*To whom correspondence should be addressed

ABSTRACT

This work describes the study of the oxidation reaction of 1-hexene to oxygenated compounds homogeneous catalyzed by the bimetallic system, Cu(CH3CN)2(NO2)2/Pd(CH3CN)2Cl2 dissolved in acetonitrile/chloroform under oxygen pressure (1-20 atm) at 55-100 ºC. The results show that 2-hexanone and 3-hexanone are the only oxygenated products formed at 55 ºC. Other oxygenated products such as 2-methyl-3-propyl-oxyrane, 2,3-diethyl-oxyrane and butyl-oxyrane were also observed, beside of the above mentioned at temperatures higher than 55 ºC.

Key Words: 1-Hexene, copper catalyst, oxidation reaction.

RESUMEN

Se reporta en el presente estudio la catálisis homogénea de la reacción de oxidación de 1-hexeno a productos oxigenados por el sistema bimetálico Cu(CH3CN)2(NO2)2/Pd(CH3CN)2Cl2 disuelto en acetonitrilo/cloroformo bajo presión de oxígeno (1-20 atm) a 55-100 ºC. Los resultados muestran que la 2-hexanona y la 3- hexanona son los únicos productos oxigenados formados a 55 ºC. A temperaturas mayores que 55 ºC fueron adicionalmente observados otros productos oxigenados, tales como 2-metil-3-propil-oxirano, 2,3-dietil-oxirano y butiloxirano.

PALABRAS CLAVES: 1-Hexeno, catalizadores de cobre, reacción de oxidación.

INTRODUCTION

The olefin oxidation by transition metal nitrocomplexes1,2), has become a very interesting field in homogeneous catalysis due to its relevance in the modern chemical industry.

Traditionally the free radical method, which is low in efficiency and selectivity, has been extensively used. During the last decades the need to develop a different via for the oxidation of organic substrates by molecular oxygen has been suggested 1).

An alternative approach consists in the utilization of transition metallic nitrocomplexes as agents for the transference of oxygen. In this system, the critical step is the transference of one oxygen atom from the nitro ligand of the metallic nitrocomplex to the organic substrate, S in equation 1.

The regeneration of the oxidant species occurs in the second step of the process, in which each atom of the oxygen molecule becomes chemically equivalent when two nitro groups are produced (equation 2).

Ugo et al. 3), Mares et al. 4-6)), Andrew et al.1-7)), Kenneth et al. 1), Tomi et al. 9, 10), Beck et al. 11,12), Maresca et al. 13)and our group 14-15), have used these two reactions with the purpose of oxidizing catalitically a variety of organic and inorganic substrates by molecular oxygen. In this work we report the synthesis, IR characterization and the reactive properties of Cu(II) dinitro complexes in the catalytic oxidation of 1-hexene to 2-hexanone and epoxide compounds by molecular oxygen in acetonitrile.

EXPERIMENTAL

Materials

Silver nitrite (Aldrich), copper dichloride dihydrated (Aldrich), and palladium dichloride (Merck) were used as received. 1-Hexene (Aldrich) and acetonitrile (Mallinckrodt) were distilled from AlLiH4 and P2O5, respectively under nitrogen atmosphere, prior to use. Chloroform (J. T. Baker) was washed with distilled water and dried on CaCl2. The gases N2, O2, He, and air were obtained from Oxicar C. A. and used as received, with the exception of N2, which was purified through silica gel and CaCl2. The Pd(CH3CN)2Cl2 complex was prepared according to the procedure reported in the literature 16).

Instrumentation

The catalytic reactions were carried out in a high pressure Parr reactor provided with Parr 4841 temperature control. The products were analyzed in a Hewlett-Packard 5890 Series II gas chromatograph using a 25 m x 0.32 mm x 0.52 mm Ultra I column (Cross-Linked Methyl Silicone Gum Phase), which was operated at 35 ºC. N2 was used as carrier (20 mL/min). Calibration curves were prepared periodically for 1-hexene and 2-hexanone using benzene as an internal reference. Mass spectrometry determination was carried out using a GC/MS Hewlett-Packard 5890 System Series II/5971, provided with Ultra I column using He as carrier (20 mL/min). The IR spectra were obtained from solid samples in KBr using a Perkin-Elmer 1760 FT-IR spectrophotometer.

Catalyst preparation and test

The Cu(CH3CN)2(NO2)2 complex was prepared from CuCl2·2H2O (171.2 mg, 1.0 mmol) and AgNO2 (307.6 mg, 2.0 mmol), each one dissolved in 10 mL of acetonitrile. The mixture was stirred for 15 min under nitrogen atmosphere to produce a green solution and a white precipitate (AgCl), which was separated by filtration. The solution was concentrated by evaporation under vacuum to generate a green solid, which was washed with acetonitrile and analyzed by infrared spectroscopy. Yield: (70 %). IR (KBr disk) nCN: 2360 and 2343 cm-1 and nNO2: 1423, 1342 and 812 cm-1.

The catalytic solutions were prepared in a Parr reactor by mixing 16 mL of an acetonitrile solution of Cu(CH3CN)2(NO2)2 (0.50 mmol) with 6.5 mL of 1-hexene (50 mmol) dissolved in 10 mL of chloroform, olefin/Cu = 100. Then the reactor was loaded with oxygen at the desired pressure (typically 10 atm) and heated (typically at 55 °C) with constant stirring (typically by 8 h).

In those reactions related to the effect of Pd(CH3CN)2Cl2 on the catalysis, stoichiometric amounts (typically 0.123 mmol) of this complex were added after and before the addition of the 1-hexene and the copper(II) complex respectively. At the end of a desired reaction time gas samples (0.2 mL) were removed from the reactor vessel and analyzed by GC and GC-MS.

RESULTS AND DISCUSSION

The studies of the reactions showed that 1-hexene is not oxidized by the Cu(CH3CN)2(NO2)2 complex under oxygen pressure (2 - 20 atm) at 55 ºC by 4 h. On the other hand, when the Pd(CH3CN)2Cl2(1:4, Pd:Cu) complex was added to the above described solution under similar reaction conditions, formation of 2-hexanone, 3-hexanone and 2-hexane was observed. In addition, after cooling the reaction solution a green solid is formed. The IR (KBr disk) of the green solid shows a broad band at 1655 cm-1, which is characteristic of the nitrosyl ligand (NO). The presence of 2-hexanone is attributed to the isomerization of 1-hexene catalyzed by the Pd(II) complex 17). A control experiment carried out in the absence of the cooper(II) complex confirms this suggestion. Also other control experiments, ca. 1-hexene/oxygen/solvent system without the Pd(II)/Cu(II) couple and 1-hexene/Pd(CH3CN)2Cl2/solvent system without Cu(II), both systems under O2 atmosphere show no formation of oxidized products. These results indicate that presence of the bimetallic Pd(II)/Cu(II) system is necessary in the catalytic oxidation of 1-hexene by molecular oxygen. For this Pd(II)/Cu(II) bimetallic system the effects of varying the reaction conditions (pressure of O2, Pd(II)/Cu(II) ratio, reaction time and temperature) on the catalytic activity and the selectivity were explored. These behaviors are shown in Fig. 1 - 3 and Table I.

Catalytic runs were carried out for a series of oxygen pressure over the range 0-20 atm (Fig. 1). The experimental results showed a non lineal increase on the conversion of 1-hexene to 2-hexanone by the bimetallic Pd(II)/Cu(II) system as a function of the pressure of O2(P(O2)). This behavior is consistent with O2 activation occurring via a pre-equilibrium formation of the nitro cooper(II) complex followed by slower reaction to give the oxidized products, i.e.

When the molar relationship Pd(II)/Cu(II) is varied from 1:8 to 1:1, P(O2) = 10 atm at 55 ºC by 8 h, a significant increase ( > 80 %) in the catalytic activities was observed. Reaching a maximum at 1:2 molar ratio, then decreases slightly and becomes independent of the amount of Pd(II) added. These results are shown in the Figure 2 and they confirm the importance to have an appropriated concentration (Pd(II)/Cu(II) = 1:1) of both the Pd(II) complex which behave as olefin activator and the Cu(II) nitro complex which acts as oxygen transfer agent. As it can see it necessary to have almost the same concentration of the Cu(II) nitro complex and the Pd(II) olefin activator because those complexes are involved in a 1:1 ratio on the formation of catalytic cycle intermediates via intermolecular mechanism 6, 18).


The reaction time effect was studied over the 1 - 24 h range, 1:4 molar ratio Pd(II)/Cu(II), 1-hexene/Cu = 100, P(O2) = 10 atm at 55 ºC. The results show a non lineal increase in the catalytic activities when the reaction time increases as can it be observed in Figure 3. As It can see the reaction goes through an induction period of about 10 h, after that reaction increase slowly as a function of the time.


When the temperature is varied from 55 to 100 ºC, 1:4 molar ratio Pd(II)/Cu(II) and P(O2) = 10 atm, there is decrease in the formation of 2-hexanone and an increase in the selectivity toward the formation of epoxide products. The mass spectra shows the presence of 2-methyl-3-propyl-oxyrane, 2,3-diethyl-oxyrane and butyl-oxyrane in the catalytic solution heated at 100 ºC. These results are summarized in Table I. The increment of the temperature cause a change in the mechanism of the reaction from b-hydrogen elimination mechanism that favor the formation of the 2-hexanone to concerted fragmentation of metallo-cycle intermediates which favor the formation of the epoxide products 19).


On the basis of these results it is possible to suggest a mechanism for the catalytic oxidation of 1-hexane to 2-hexanone by the bimetallic Pd(II)/Cu(II) system under O2 pressure. Scheme 1 summarizes the probable mechanism. As can be observed 1-hexene coordinates to the palladium(II) complex making this olefin more active to undergoes a nucleophilic attack by the nitro group of the copper(II) complex to produce the bimetallic specie (a), which undergoes a b-elimination to generate the intermediate (b). In this specie the simultaneous migration of the hydride ion from the Pd(II) to the a-carbon of the 1-hexene, occurs with the oxygen transfers to generate 2-hexanone and the nitrosyl complex. Then the nitrosyl complex is reoxidized by molecular oxygen to regenerate the nitro cooper(II) complex that close the catalytic cycle. Further, the mechanism for formation of the epoxide products via activation of 1-hexene by p-coordination to the Pd(II) complex followed by oxygen transfer from the NO2 ligand coordinated to the Cu(II) center, could be similar to the one discussed by Mares et al. 6).

CONCLUSIONS

The following conclusions have been draw from the catalytic oxidation of 1-hexene by Cu(CH3CN)2(NO2)2 under pressure of O2. The presence of the Pd(CH3CN)2Cl2complex is necessary in order to activate the olefin towards the nucleophilic attack of the nitro group of the copper dinitro complex. At 55 ºC the oxidation of 1-hexene by the bimetallic Pd(II)/Cu(II) catalytic system shows low activity and high selectivity toward the formation of 2-hexanone, while at higher temperatures the selectivity toward the formation of epoxide products is favored.

ACKNOWLEDGMENTS

We thank the financial support of the CODECIHT-UC Venezuela (Proy. 94016), DICYT and FONDECYT, Chile (SAM) and CDCH-UV Venezuela (AJP).

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