Flash Vacuum Pyrolysis of 2,5-Diphenyloxazole

Abstract

FVP of the title oxazole (1) at 1000°C and 0.5 mm Hg afforded a complex reaction mixture containing benzonitrile, phenylacetonitrile, biphenyl, diphenylmethane, fluorene, o-benzylbenzonitrile (major product, 22%), phenanthrene, anthracene, 9,10-anthraquinone, 2-phenylindole and 3-phenylindole. A radical and carbene mechanism was suggested in order to rationalize the formation of the reaction products.

Introduction

The thermal behaviour of a large variety of heterocyclic compounds has been investigated in connection with mechanistic and/or synthetic studies [1,2,3]. Though flash-vacuum pyrolyses (FVP) of a few substituted and annulated isoxazoles [4,5,6] as well as of different isoxazolones [7,8,9] were investigated (revealing the preferential fission of the N-O bond, and the extrusion of CO₂, respectively) reports on the gas-phase chemistry of oxazoles are very scarce. Thus, it has been proved that unsubstituted oxazole radical cation dissociates by elimination of a hydrogen atom [10] whereas 2-phenyl-1,3-oxazol-5(4H)-one thermally eliminates CO₂ [11]. In order to provide supplementary data concerning FVP of oxazoles and in continuation of our previous investigations of FVP of hydrocarbons [12] and heterocycles [13], we intend to study the thermal behaviour of a series of substituted oxazoles. In the present paper we describe the results of FVP of 2,5-diphenyloxazole (1).

Results and Discussion

The pyrolyses of diphenyloxazole 1 were performed in a flow-system under argon atmosphere at 0.5 mm Hg, the calculated contact times being ≤ 0.2 s. Under these conditions the oxazole 1 showed a high thermal stability. Whereas at 800°C a conversion of only 9% was achieved, the increase of temperature at 1000°C brought the conversion to 91%. The reaction products were examined by GC/MS analysis and by ¹H- and ¹³C-NMR spectroscopy. The product distribution was a very complex one. The pyrolysis products are shown in Scheme 1 in the order of their elution from the GC column.

From the reaction products compounds 2-6 and 9-13 were identified by their characteristic mass spectra which were favourably compared with the mass spectra of authentic specimens. For compounds 2, 3, 4, 6, 9, 10 and 11 mixed GC analysis of mixtures of the pyrolysis product with authentic samples also confirmed the proposed structures. Compound 8, the main pyrolysis product, was separated by column chromatography on silica and identified through its spectral data (see also Experimental) as follows: the infrared spectrum indicated the characteristic absorption of a CN group (2231 cm^-1) along with those of CH₂ groups and aromatic moieties. In the ¹H-NMR spectrum a singlet at δ = 4.21 ppm can be attributed to the CH₂ protons (the deshielding of 0.25 ppm as compared to diphenylmethane being due to the ortho–CN group). Doping the sample of 8 with Eu(fod)3 shift reagent led to a larger molar induced shift (1.9) for H⁶ than for H³ – H⁵. A molar induced shift of 1.4 was observed for CH₂ protons situated in the vicinity of the CN complexing group. The ¹³C-NMR spectrum of 8 indicated the signal of CH₂ at δ = 40.21 ppm and that of CN at δ = 118.11 ppm. The ¹³C chemical shifts of all carbon atoms calculated using known increments agree well with the experimental values. Compound 7, occuring in a small amount (2%), seems to be a steric isomer of 8, giving M=193 and a molecular formula of C₁₄H₁₁N (probably possessing the -CN group in the para position). The increased thermal stability of diphenyloxazole 1 is due to the lack of easily eliminable moieties (like e.g. N₂ or CO₂ in other heterocycles) as well as in the absence of bonds particularly prone to cleavage (like the N-O bond in isoxazoles). The two phenyl substituents conjugated with the oxazole ring also contribute to the stability of 1.

In order to rationalize the formation of the main pyrolysis products a reaction scheme including more competing pathways can be proposed (Scheme 2). Compounds 2, 4-10 can be generated from 1 by bond cleavage routes denoted (a), (b), (c) in Scheme 2. Generation of free radicals seems to be favoured by the reaction conditions (1000°C; quartz apparatus). Phenyl radicals can generate biphenyl (4) and benzene (identified in traces), whereas fission (c) affords benzonitrile (2). On the other hand, in the diradical 16 an usual 1,2-phenyl migration could lead to the intermediate occurrence of phenylketene 17. Thermal decarbonylations of ketenes to carbenes are well-known processes [14]. The phenylcarbene 18, generated from 17 can be the precursor of 5 and 6 (by insertion and hydrogen elimination), of 7 and 8 (by insertion) and of 9 and 10 (by dimerisation respectively double insertion followed by dehydrogenation). Previous work by Hedaya [15] has shown that the reaction products of phenylcarbene - generated from phenyldiazomethane - are strongly determined by the reaction conditions employed, especially by the temperature (the increase of anthracene at 900°C being underlined).

Conclusions

The high thermal stability and the narrow temperature interval of pyrolysis of 2,5-diphenyloxazole were demonstrated. Among the twelve identified pyrolysis products, the most abundant (22%) is o-benzylbenzonitrile. The suggested reaction mechanism will be further tested and/or detailed by investigation of other substituted oxazoles.

Experimental

General

Melting points are uncorrected. The NMR spectra were registered on a Varian Gemini 300 apparatus at 300 MHz for ¹H and at 75 MHz for ¹³C, using TMS as internal standard. The GC/MS analyses were performed on a Varian 3400 gas chromatograph with split/splitless injector coupled with a Varian Saturn II mass spectrometer provided with ion trap. A capillary DB-5 column (30m length, 0.25 mm internal diameter) was used. The analysis conditions were: injector temperature 250°C, split rate 1:50, temperature program 50-250°C at 7°/min, then 20 min. at 250°C, carrier gas helium with flow-rate 1 mL/min., temperature of transfer line 200°C, trap temperature 170°C, electron ionization 70 eV.

2,5-Diphenyloxazole (1)

M.p. 71-72°C, was a commercially available product (Aldrich).

FVP of 2,5-Diphenyloxazole

All FVP were performed in a flow system using a previously described apparatus [16]. The pyrolysis quartz tube (60 cm length, 10 mm diameter) was filled with quartz chips on 30 cm length; this zone was heated with a cylindrical electric oven. The temperature was continously measured by means of a thermocouple and the pressure (0.5mm Hg) with a McLeod manometer. The oxazole sample (usually 30 mg) was sublimed under argon flow (4 mL/min.) in the pyrolysis zone. The reaction products accumulated at the cooled end of the quartz tube were dissolved in chloroform, the solvent was evaporated in vacuo and the residue (about 85% yield) was subjected to GC/MS analysis. Pyrolysis of 1 at 800°C conducted to only 9% conversion. Working at 1000°C a conversion of 91% was achieved. The reaction products in this case were, in order of their elution from GC column: benzene (15) (only traces due to its high volatility); benzonitrile (2) - 8%; phenylacetonitrile (3) - 10%; biphenyl (4) - 3%; diphenylmethane (5) - 4.5%; fluorene (6) - 11%; unknown component with M=193 (7) - 2% (probably the para isomer of 8); o-benzylbenzonitrile (8) - 22%; phenanthrene (9) - 4%; anthracene (10) - 3%, 9,10-anthraquinone (11) - 3%; unreacted oxazole 1 - 9%; 2-phenylindole (12) - 8.5% and 3-phenyl-indole (13) - 4%. Along with these products, representing 92% from the reaction mixture, only minor components (< 2%) were observed in the gas-chromatogram. These minor components were not further investigated. From the above reaction products the compounds 2-6 and 9-13 were identified by their characteristic mass spectra which were favourably compared with the mass spectra of authentic specimens registered in the GC/MS library. For some reaction products (2, 3, 4, 6, 9, 10 and 11) mixed GC samples of pyrolysis product with authentic specimens confirmed the proposed structures. The mass spectra of components 5, 7, 8, 12 and 13 are given below:

Diphenylmethane (5)

Mass spectrum (m/z; relative abundance %): 27 (3); 38 (3); 39 (15); 50 (6); 51 (9); 62 (2); 63 (8); 65 (11); 76 (2); 77 (3); 83 (2); 89 (9); 90 (3); 91 (40); 92 (3); 102 (2); 113 (2); 115 (10); 126 (2); 128 (4); 139 (5); 141 (3); 151 (2); 152 (25); 153 (30); 154 (4); 163 (3); 164 (4); 165 (37); 166 (11); 167 (100; P.I.; M); 168 (87; M+1); 169 (10; M+2).

Benzylbenzonitrile (7)

Mass spectrum (m/z; relative abundance %): 26 (6); 27 (9); 28 (59); 38 (8); 39 (21); 50 (26); 51 (43); 52 (7); 62 (10); 63 (21); 74 (9); 75 (8); 76 (16); 77 (19); 87 (5); 88 (7); 89 (32); 90 (9); 102 (6); 115 (12); 116 (14); 139 (6); 151 (6); 152 (12); 163 (5); 164 (6); 165 (90); 166 (35); 167 (7); 176 (6); 177 (6); 178 (37); 179 (50); 180 (43); 181 (7); 190 (6); 191 (6); 192 (24); 193 (100; P.I.; M); 194 (21; M+1); 195 (0.5; M+2).

o-Benzylbenzonitrile (8)

Mass spectrum (m/z; relative abundance %): 27 (3); 37 (2); 38 (3); 39 (11); 50 (6); 51 (8); 52 (2); 62 (3); 63 (8); 64 (2); 65 (7); 74 (2); 75 (3); 76 (3); 77 (3); 87 (2); 88 (2); 89 (9); 90 (4); 91 (7); 113 (2); 115 (3); 116 (4); 118 (2); 139 (3); 140 (3); 145 (3); 151 (2); 152 (4); 163 (5); 164 (5); 165 (43); 166 (8); 177 (3); 178 (4); 179 (4); 180 (6); 190 (8); 191 (5); 192 (27); 193 (100; P.I.; M); 194 (21; M+1); 195 (2; M+2).

2-Phenylindole (12)

Mass spectrum (m/z; relative abundance %):. 27 (2); 38 (2); 39 (5); 50 (3); 51 (3); 62 (2); 63 (3); 74 (2); 82 (2); 83 (2); 87 (2); 89 (3); 90 (2);115 (2); 139 (3); 163 (5); 164 (6); 165 (27); 166 (5); 167 (2); 190 (2); 191 (4); 192 (15); 193 (100; P.I.; M); 194 (16; M+1); 195 (1; M+2).

3-Phenylindole (13)

Mass spectrum (m/z; relative abundance %):. 39 (4); 50 (2); 51 (2); 63 (3); 89 (7); 90 (3); 139 (2); 163 (2); 164 (2); 165 (17); 166 (3); 167 (2); 190 (2); 191 (4); 192 (14); 193 (100; P.I.; M); 194 (16; M+1); 195 (1; M+2).

For preparative purposes the products of four identical pyrolyses at 1000°C were combined and subjected to liquid chromatography on a silica column (Merck 60; 35-70 mesh) using petroleum ether: diethylether (95:5) as eluent. The separated o-benzylbenzonitrile (thick oil; lit. m.p. [17] = 19°C) gave the following spectral data:

IR Spectrum (CCl₄; cm^-1): 1458 (${\mathsf{\delta }}_{{\text{CH}}_2}$); 2231 (ν_CN); 2853 (${{\mathsf{\delta }}_{{\text{CH}}_2}}^{\text{sim}}$); 2961 (${{\mathsf{\delta }}_{{\text{CH}}_2}}^{\text{asim}}$); 3025 and 3061 (δ_CH).

¹H-NMR Spectrum (CDCl₃, δ _ppm, J Hz;): 4.21 (s;2H; CH₂); 7.20-7.35 (m; 7H; C₆H₅; H⁵; H³); 7.49 (td; 1H; 7.7; 1.5; H⁴); 7.63 (dd; 1H; 1.4; H⁶).

¹³C-NMR Spectrum (CDCl₃, δ _ppm,): 40.21 (CH₂); 112.67 (C¹); 118.11 (CN); 126.71 (CH^para); 126.77 (C⁵); 128.72 (2 CH^meta); 128.98 (2 CH^ortho); 130.05 (C³); 132.83 (C⁶); 132.90 (C⁴); 138.79 (C_quart); 144.98 (C²).