The liquid-phase oxidation of cyclohexene or tetraline with N2O was studied in various solvents in the presence of onium salts or without them. The onium salts exerted significant promoting effect on the reactions. The activation of the oxidant was studied by IR spectroscopy. It was found that the interactions of the ions in the onium salts and the polarised N–O bond further enhanced polarisation leading to an activation of the oxidant, thus, increasing the rate of oxidation

Csányi, László J.

Kiss T., János

Pálinkó, István

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         ONIUM SALTS AS CATALYSTS IN THE LIQUID-PHASE OXIDATION OF CYCLOHEXENE OR TETRALINE BY N2O  János T. Kissa, László J. Csányib, István Pálinkóa,*  aDepartment of Organic Chemistry, University of Szeged, Dóm tér 8, Szeged, H-6720 Hungary  bDepartment of Inorganic and Analytical Chemistry, University of Szeged, Dóm tér 7, Szeged, H-6720 Hungary  Abstract  The liquid-phase oxidation of cyclohexene or tetraline with N2O was studied in various solvents in the presence of onium salts or without them. The onium salts exerted significant promoting effect on the reactions. The activation of the oxidant was studied by IR spectroscopy. It was found that the interactions of the ions in the onium salts and the polarised N–O bond further enhanced polarisation leading to an activation of the oxidant, thus, increasing the rate of oxidation.  Keywords: liquid-phase oxidation; N2O activation; onium salts; cyclohexene; tetraline; promoting effect.  Introduction  N2O could be an environmentally benign oxidation agent, since upon providing the useful product(s) after oxygen transfer N2 is only formed. However, it is a sluggish reactant in both gas-phase and liquid-phase oxidations: high pressures (53-120 bar) and high temperatures (150-250 °C) are required, for a recent review, see ref. [1]. Nevertheless, it is an oxidant of interest for the chemical industry as well [2, 3]. Research in this area is going on, recently, a chemoselective catalyst (RuIV-CoIII oxides supported on γ-Al2O3) was developed for the oxidative preparation of aldehydes and ketones from saturated and unsaturated primary and secondary alcohols using N2O. Still, the reactions only proceed under pressure (10 bar) and at relatively high temperature (110 ˚C) [4]. Under forcing conditions (200 ˚C or higher, 25 bar total pressure) cyclic alkenes could be oxidised to cyclic ketones [5] without promoters. In our hands, however, the liquid-phase oxidation of cyclohexene or tetraline with N2O took place under mild conditions (70 °C, 1 bar) with appreciable rate in the presence of onium salts. The onium salts were proved to be necessary and choice of solvent was found to be critical. Kinetic investigations are detailed elsewhere [6], here we report on results acquired during studying the interactions between the oxidant, the onium salt and the solvent by IR spectroscopy.   Experimental  The onium salts (Hex4NCl and Me(Oct)3NCl), the solvents (chlorobenzene, acetonitrile, diethyl carbonate and CCl4), cyclohexene and tetraline were the products of                                                  *Corresponding author, palinko@chem.u-szeged.hu  381Aldrich Co and were used without further purification. The oxidant N2O (Ph. EUR 4 grade) was supplied by Linde Co. The O2 content was < 0.03% by gas chromatography (5A molecular sieve column). The presence of traces of O2 in N2O is an indication that not even traces of NO can be present, because NO would react with O2 instantly. The reactions were performed in the liquid phase at 70 °C at atmospheric pressure. Samples were withdrawn after 2 hours and the concentrations of the reactants and the products were determined by gas chromatography. Identification was performed using authentic samples of the compounds (both the products and the reactants). For studying the possible interactions between the onium salt and N2O in various solvents a BIORAD Digilab FTS-65/896 spectrometer equipped with a DTGS detector was used. The resolution was 4 cm-1 and 128 scans were collected for a spectrum. Changes in the positions and band areas of N-O stretching (ν1), the ν2 deformation and the N-N stretching (ν3) vibrations were monitored. Spectral evaluations were done with the WinIR package. Before measurements the solvents were purged with N2 for 5 min to remove traces of O2. Then, the N2 atmosphere was fully replaced by N2O, the gas burette was connected, the oxidant was allowed to dissolve at atmospheric pressure under intense stirring for 5-10 hours, and the gas uptake was noted. The N2O stock solutions could be stored without notable change under a protecting N2O blanket (at atmospheric pressure) in a closed vessel for at least 1-2 days. However, the N2O concentration dropped immediately when the vessel was opened. To avoid N2O loss when these solutions were diluted or mixed with onium salt solutions and also when the IR cell was filled, the stock solution in the reactor was passed via a siphon fitted with a 3-way tap either to the IR cell, or to a 4 cm3 mixing syringe. The syringe initially contained 2.00 cm3 solvent or onium salt solution, to which 2.00 cm3 N2O stock solution was added at atmospheric pressure. After thorough mixing, the IR cell was filled from the mixing syringe.   Results and discussion  As far as reaction rates are concerned chlorobenzene and CCl4 proved to be the best solvents (Table 1). The reactions were negligible or very slow in acetonitrile and diethyl carbonate. Moreover, these solvents were not transparent in frequency ranges important in studying N2O vibrations (see later), therefore, these systems were excluded from further considerations. The products in the remaining two solvents were cyclohexene oxide (chex-ox) and 2-cyclohexene-1-ol (chex-ol) from cyclohexene, α-tetralone (tetr-one) and α-tetralol (tetr-ol) from tetraline. Cyclohexene could be oxidised without onium salts in chlorobenzene and the products were the same as in the their presence. However, the reaction was much faster when onium salts were added. In CCl4 only the alcohol could be detected and only in very minute amount without onium salts. Tetraline could not be oxidised in any of the solvents without onium salts. Oxidation of this compound was found to be selective. The oxo (tetr-one) compound was only formed in chlorobenzene, while exclusively alcohol (tetr-ol) was identified in CCl4. It was also observed that oxidation was accelerated considerably in CCl4 containing trace amounts of water.  382Table 1 Product distribution (in mmol) in the liquid-phase oxidation of cyclohexene and tetraline with N2O after 120 min reaction time quantities of cyclohexene (chex) and tetraline  (tetr) are 19.744 and 22.078 mmol, respectively; amount of the onium promoter is  0.024 mmol  at the beginning of the reaction Hydrocarbon  Catalyst Chex-ox Chex-ol Tetr-one Tetr-ol Chlorobenzene Chex – 0.053 0.068 – – Chex Hex4NCl 0.084 0.130 – – Chex Me(Oct)3NCl 0.064 0.410 – – Tetr – – – 0.000 0.000 Tetr Hex4NCl – – 0.821 0.000 Tetr Me(Oct)3NCl – – 0.499 0.000 CCl4 Chex – 0.000 0.020 – – Chex Hex4NCl 0.019 0.302 – – Chex Me(Oct)3NCl 0.009 0.164 – – Tetr – – – 0.000 0.000 Tetr Hex4NCl – – 0.000 2.585 Tetr Me(Oct)3NCl – – 0.000 1.076   chex-ox – cyclohexene oxide, chex-ol – 2-cyclohexene-1-ol, tetr-one – α-tetralone, tetr-ol – α-tetralol  During the reaction the bonding system of N2O should change. Since the reaction rate is only appreciable in the presence of onium salts, the interaction between the ion pair and the N2O molecule should initiate this change. Infrared spectroscopy is an appropriate tool to detect it. The fundamental frequencies of N2O are collected in Table 2 in the gas phase as well as in the solvents used in this study. Fortunately, the solvents where the reaction rates were appreciable (chlorobenzene and CCl4) are transparent in the frequency ranges of fundamental vibrations.  Table 2 Wavenumbers for the fundamental peaks of N2O Fundamental frequencies (cm-1) N–N- stretch. (ν3) N–O stretch. (ν1) deform. (ν2)   ←N—N→–←O ←N–←NO→ ↑ NNO↓               ↓ Gas phase 2224 1285 589 Chlorobenzene 2218 1281 583 Acetonitrile –a 1284 586 Diethyl carbonate 2224 –b 585 CCl4 2218 1281 586 a – severely disturbed by the strong self-absorption of the solvent b – could not be detected  It is to be seen that the frequencies do not change significantly on going from the gas phase to any of the solvents used. It was also observed that their positions remained the same when onium salts were present, however, the intensities of the ν1 fundamental  383vibration were very much altered with the addition of the onium salts, while the other two hardly changed. Figure 1 allows the comparison of the IR spectra of the N2O solution with and without the onium salts and Table 3 summarises the ratios of the band areas at various onium salt concentrations for both promoters. Data reveal that in chlorobenzene the ν1 band disappears and in CCl4 greatly diminishes. At the same time, apparently nothing happens with the two other fundamental vibrations. These observations indicate that the N–O bond disruption is promoted by ion-pair formation between the cation of the onium salt and its counter ion at the O-end and the N-end of the N–O bond, respectively.  aν3 ν1 ν2 bν3 ν1 ν2 Fig. 1. IR spectra for N2O with (upper curve) and without onium salts (lower curve) in (a) chlorobenzene and (b) CCl4  384Table 3 Ratios of fundamental N2O IR band areas in the presence of promoter onium salts or without them using various solvents  Ratio of band areas Promoter Concentration (mol/dm3) Aν3/Aν1 Aν3/Aν2 in chlorobenzene – 0 9.6 40.0 Hex4NCl 0.153 –a 46.6 Me(Oct)3NCl 0.153 –a 48.9 in CCl4 – 0 6.0 40.3 Hex4NCl 0.155 20.3 48.1  0.197 15.0 50.0 Me(Oct)3NCl 0.0214 24.0 48.0  0.143 19.1 51.0 a – the ν1 band disappeared  The resonance theory representation of N2O (Scheme 1) places charges even Scheme 1. Limiting sonto the undisturbed molecule.  tructures for N2O according to resonance theory The inner nitrogen holds the positive charge in either limiting structure, while the neg the reaction mixture (where the hydrocarbon, the onium salt, the solvent as well asonclusions The experimental results indicated that the interactions between the cation and the coucknowledgement. This work was supported by the National Science Fund of Hungary N N O N N O ative charge can be located either on the terminal nitrogen or the oxygen. Thus, the anionic part of the onium salt surely will interact with the inner nitrogen, while both ends might interact with the cation. Since change was only found with the ν1 fundamental vibration, therefore it is safe to state that the cation interacts with the oxygen. In N2O are present) the terminal nitrogen somehow interacts with the double bond or one of the double bonds of these molecules. This interaction ceases by the end of the reaction since N2 evolution occurs and at the same time or close to it the products are formed.   C nter anion of the onium salts with the N–O bond in N2O significantly contribute to the activation of the molecule. The activated moiety reacts in the oxidation reactions of cyclohexene as well as tetraline under mild conditions.  Athrough grant K62288. The financial help is highly appreciated.  385References [1] V.N. Parmon, G. I. Panov, A. Uriarte, A.S. Noskov: Catal. Today 100 (2005), 115. [2] G.I. Panov, C.A. Dubkov, E.V. Starokon, L.V. Pirutko: PCT Int. Appl. (2003) CODEN: PIXXD2 WO 2003078372. [3] G. Bub, R. Fornika, B. Fischer, D. Guschin, T. Meyer, W. Scheldrick: PCT Int. Appl. (2003) CODEN: PIXXD2 WO 2003059856. [4] T.L. Stuchinskaya, M. Mushawir, E.F. Kozhevnikova, I.V. Kozhevnikov: J. Catal. 231 (2005), 41. [5] E.V. Starokon, K.A. Dubkov, D.E. Babushkin, V.N. Parmon, G.I. Panov: Adv. Synth. Catal. 346 (2004), 268. [6] L.J. Csányi, K. Jáky, J.T. Kiss, I. Ilisz, P. Forgo, Gy. Dombi: J. Mol. Catal. A 263 (2007), 48.     386

Onium salts as catalysts in the liquid-phase oxidation of cyclohexene or tetraline by N2O

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