¹H- and ¹³C-NMR investigations on σ-adduct formation of 1,2,4-triazine 4-oxides and 3-chloro-6-phenyl-1,2,4-triazine with liquid ammonia and alkylamines

¹H- and ¹³C-NMR spectra of the σ-adducts formed between 6-phenyl-1,2,4-triazine 4-oxide (1a), 3-ethyl-6-phenyl-1,2,4-triazine 4-oxide (1b) and liquid ammonia, methylamine or dimethylamine are described, together with ¹H NMR spectra of 3-chloro-6-phenyl-1,2,4-triazine (7) in liquid ammonia. The results of the NMR study have shown that the carbon C-5 in 1a-b and 7 is the preferred site for nucleophilic attack by liquid ammonia and alkylamines at low temperatures (from -75° to -20°C). The σ-adduct (5e) formed between 1a and dimethylamine at -75°C on heating to -20°C irreversibly converts to open-chain product (3a), via intermediary C-3 σ-adduct (6). The amination of 7 into 3-amino-6-phenyl-1,2,4-triazine (10) occurs via S_N(AE) mechanism involving the isomerisation of the C-5 σ-adduct (8) into the C-3 σ-adduct (9) as confirmed by a ¹⁵N study with labeled liquid ammonia.