Gas phase pyrolysis of heterocyclic compounds, part 3. Flow pyrolysis and annulation reactions of some nitrogen heterocycles. A product oriented study

2000 ARKIVOC  
Continuous flow pyrolysis (CFP) and annulation reactions of pyrrole (1), its benzo-annulated derivatives indole (2) and carbazole (3), pyridine (4), quinoline (5), and isoquinoline (6) have been investigated at 900 o C. While for 1, 2, 4-6 between 15 and 24 products were identified by GC-MS, 3 is rather stable under the reaction conditions and naphthalene (18) was found as the only substantial product. Except for 3, about 10 compounds were always present in the pyrolysate, benzonitrile (14)
more » ... nzonitrile (14) (6-24%) and naphthalene (18) (2-21%) being the most prominent products. Compounds 4-6 have even 16 common pyrolysis products. Among the major products are nitrogen heterocycles like quinoline (5) (4-13%), isoquinoline (6) (0-8%), and indole (2) (0.5-3%), cyano-substituted aromatic hydrocarbons like 14, 1-cyanonaphthalene (17) (0-16%), and 9-cycanoanthracene (19) (0-6%), as well as the corresponding unsubstituted parent compounds like 18, phenanthrene (31) (2-10%), pyrene (38) (1-5%), and fluoranthene (22) (2-8%). In addition to the volatile products, several heavy compounds were identified by direct inlet MS. Possible routes of product formation are outlined. The relative amounts of unreacted starting material clearly indicate that benzo-annulation of pyrrole and pyridine causes considerable increase of thermal stability. Previously, we have reported our results obtained for thiophene, 7 benzo[b]thiophene, and dibenzothiophene. 1 In this communication the results for some nitrogen heterocycles, namely pyrrole (1), indole (2), carbazole (3), pyridine (4), quinoline (5), and isoquinoline (6) are presented. Compounds 1-5 were included in a gas-phase pyrolysis investigation of coal-related aromatic compounds by Bruisma et al. 8 in order to obtain kinetic data on their thermal stability. The relative stability order at temperatures between 1000 and 1220 K was found to be 5 > 4 > 1 > 2 > 3. However, in this study no appropriate attempts to identify products were exerted. E.g., for 2 only benzene and toluene are mentioned as products. This study is primarily product oriented and no mechanistic investigation. Reaction schemes and reaction pathways are to be understood as purely illustrative. Results and Discusion The compounds were pyrolysed in an argon stream at atmospheric pressure at 900 o C. In addition, quinoline (5) was pyrolysed at 1100 o C. The pyrolysate was analysed by GC-MS. The results are summarized in Tables 1-5. Non-volatile products were analysed by direct inlet (heated probe) MS. Pyrrole (1) The thermal fragmentation of pyrrole (1) has been investigated meticulously. Patterson et al. 9 have pyrolysed 1 under continuous flow (CFP) conditions at 850 o C and identified about 40 products. Axworthy et al. 10 confirmed these results. Cullis and Norris 11 analyzed only some of the gaseous products. Lifshitz et al. 12 have studied the thermal fragmentation reactions of 1 by shock-wave experiments over the temperature range 1050-1450 K, and Mackie et al. 13 a little later by the same technique at temperatures from 1200 bis 1700 K. In both studies, ciscrotonitrile (10), allylcyanide (11), hydrogen cyanide, and propyne (9) were found as main products. Further products are acetylene, ketene imine (13), and allene (8). Fragmentation of 1 is initiated by 1,2-hydrogen shift affording 2H-pyrrole (7) 13,14 which is followed by cleavage of the N1-C2 bond (Scheme 1). This reactive intermediate opens four reaction channels: a) 1,4-hydrogen shift affording cis-crotonitrile (10); b) 1,2-hydrogen shift affording allylcyanide (11); c) 1,3-hydrogen shift affording an intermediate that cleaves into General Papers ARKIVOC 2000 (iv) 576-602 acetylene and ketene imine (13); d) cleavage into hydrogen cyanide and a fragment C 3 H 4 that isomerizes mainly to propyne (9) and to a lesser extent to allene (8). At higher pyrolysis temperatures also trans-crotonitrile was found. General Papers ARKIVOC 2000 (iv) 576-602 PHAs like acridine (23) and benzo[b]acridine (43) as well as their isomers are generated from 5 or 6, respectively, and one or two butadiyne fragments (16). Compounds like indole (2), carbazole (3), benzo[b]carbazole (40), and their isomers require formation of a five-membered heterocycle that can be formed in a reaction of propyne (9) with HCN. Since pyrrole (1) was not detected in the pyrolysate, it has to be assumed that instead of 1 a reactive intermediate is generated that subsequently reacts with 16. Alternatively, it seems possible that indole (2) arises from benzene (15) and ketene imine (13). Compared with earlier investigations, 10,26 several additional products were found. These are 9-cyanoanthracene (20) and its isomers, cyanoquinoline or cyanoisoquinoline, phenylquinoline, phenylnaphthalene, benzo[b]carbazole (40), acridine (23), benzo[b]acridine (43) and isomers. Results and Discussion The compounds investigated in this study are structurally too different for a uniform pyrolysis pattern. However, there are obviously common features in their CFP that allow some general conclusions. First of all, this relates to the annulation behaviour of 1-6. While for 1, 2, 4-6 between 15 and 24 products were identified by GC-MS, 3 is rather stable under the reaction conditions, and naphthalene (18) was found as the only significant product. Except for 3, about 10 compounds are always present in the pyrolysate, benzonitrile (14) (6-24%) and naphthalene (18) (2-21%) being the most prominent products. Compounds 4-6 have even 16 common pyrolysis products. Among the major products are N heterocycles like quinoline (5) (4-13%), isoquinoline (6) (0-8%), and indole (2) (0.5-3%), cyano-substituted aromatic hydrocarbons like 14, 1-cyanonaphthalene (17) (0-16%), and 9-cycanoanthracene (19) (0-6%), as well as the corresponding unsubstituted parent compounds like naphthalene (18), phenanthrene (31) (2-10%), pyrene (38) (1-5%), and fluoranthene (22) (2-8%). From these results it may be concluded that similar reaction pathways with similar intermediates are followed in the CFP of 1-6. The distinct tendency to form PAHs and their cyano derivatives under CFP conditions at 900 o C can apparently be explained by a mechanism in which primarily the skeleton of a certain PAH is generated that can then react with HCN to afford its cyano derivative. Formation of nitrogen-containing five-and six-membered rings is of minor importance, because generally only General Papers ARKIVOC 2000 (iv) 576-602
doi:10.3998/ark.5550190.0001.410 fatcat:o5q3marn6ncn7gnhugredbtf2q