A reinvestigation of the reactions between diazomethane and 1-phenylurazole

Exhaustive methylation of 1‐phenylurazole (1), using ethereal diazomethane as the methylating agent, results in the formation of three products: 1‐phenyl‐3,5‐dimethoxy‐1,2,4‐triazole (2), 1‐phenyl‐3‐methoxy‐4‐methyl‐Δ2‐1,2,4‐triazolinev5‐one (3) and 1‐phenyl‐2,4‐dimethylurazole (4). Based on the amounts of 2–4 isolated, the overall yields in these reactions are typically >90%. When 1 and diazomethane were allowed to react in a fashion that resulted in the formation of monomethylated analogues of 1, 1‐phenyl‐3‐methoxy‐Δ2‐1,2,4‐triazoline‐5‐one (5) and 1‐phenylv2‐methylurazole (6) were formed. In separate experiments, the monomethylated species 5 and 6 were allowed to react with diazomethane in efforts to develop a sequence of reactions that reasonably accounts for the formation of 2–4. Whereas N‐ and O‐methylated products were obtained when 5 was treated with ethereal diazomethane, the reaction of 6 and diazomethane produced only the N‐alkylated product 1‐phenyl–2,4‐dimethylurazole (in quantitative yield). The outcomes of these experiments are consistent with a sequence of reactions in which the treatment of 1 with diazomethane results, initially, in the formation of the monomethylated species 5 and 6. In the presence of sufficient diazomethane, 5 and 6 then undergo further mothylation, forming the dimethylated species 2–4.