N-amino heterocycles : applications in flash vacuum pyrolysis

Abstract

Routes to N-amino heterocycles were reviewed and findings applied to generate flash vacuum pyrolysis (FVP) precursors of two types - ketene generators and azol-1-yl radical generators. N-Amino heterocycles can be used as nitrogen radical generators, the N-N bond being homolytically cleaved at furnace temperatures of approximately 850 °C. A number of 2-substituted benzimidazoles were synthesised and subsequently Naminated. The 2-arylbenzimidazole precursors 1-amino-2-(2-methylphenyl)-1Hbenzimidazole and 1-amino-2-(2-ethylphenyl)-1H-benzimidazole were synthesised and subjected to FVP. The hydrogen transfer processes of the resulting azol-1-yl radicals were investigated. Pyrolysis of 1-amino-2-(2-methylphenyl)-1Hbenzimidazole resulted in three products; 2-(2-methylphenyl)-1H-benzimdazole, 11H-benzo[4,5]imidazo[1,2-a]isoindole and 1-(2-methylphenyl)-1Hbenzo[ d]imidazol-2-amine. Pyrolysis of 1-amino-2-(2-ethylphenyl)-1Hbenzimidazole resulted in five products, four of which have been successfully isolated and identified as 2-(2-ethylphenyl)-1H-benzimidazole, 5,6- dihydrobenzo[4,5]imidazo[2,1-a]isoquinoline, 1-(2-ethylphenyl)-1Hbenzo[ d]imidazol-2-amine and 11-methyl-11H-benzo[4,5]imidazo[2,1-a]isoindole. The mechanism of formation of most products is initiated by hydrogen atom transfer to the azol-1-yl radical position. N-Aminopyrazole was reacted with 5-methoxymethylene-2,2-dimethyl-1,3-dioxane- 4,6-dione to form the corresponding 5-(N-aminopyrazolyl)methylene derivative, which, when subjected to FVP, eliminates acetone and carbon dioxide to form a methyleneketene. This subsequently undergoes a [1,3]-hydrogen shift giving an imidoylketene which can collapse onto the neighbouring nitrogen atom forming pyrazolo[1,2-a][1,2,3]triazin-5-ium-4-olate (a novel heterocyclic mesomeric betaine system) or cyclise onto the adjacent carbon atom to yield a pyrazolopyridazinone. On variation of the furnace temperature it was apparent the former forms at relatively moderate temperatures (~500 °C) whereas the latter begins to predominate as the furnace temperature increases (~700 °C). The relationship between these kinetic and thermodynamic products was modelled using DFT calculations. By using substituted pyrazole precursors, substituents could be incorporated into all three available positions around the pyrazole ring. Using substituted acrylic esters as alternative imidoylketene generators, substituents could also be incorporated into both available positions in the pyridazinone ring. All corresponding betaine and pyrazolopyridazinone products were isolated and characterised.