Mechanism of decomposition of (E)-methanediazoate in aqueous solutions

Article abstract of DOI:10.1021/ja00052a032

Rate constants for the decomposition of (E)-methanediazoate (1) at 25°C, ionic strength 1 M (NaClO₄), are independent of the concentration of nucleophile in up to 0.25 M propylamine base, 0.5 M methoxyamine base, 0.20 M thiosulfate dianion, and 0.50 M azide ion. When the CH₃ group of 1 is transferred to benzoate ions, benzyl alcohol, and water upon decomposition in near neutral D₂O solutions, the isotopic integrity of the methyl group is maintained to the extent of 70-90%. The rate constant for the decomposition of protonated 1 in ethanol is 680 times slower than that in aqueous 4% ethanol under identical buffering and ionic strength conditions. Trapping of the methyl group derived from 1 by two pairs of nucleophiles gives the same ratios of methylated products as those determined from the decomposition of diazomethane. The solvent deuterium isotope effect for the decomposition of the protonated form of 1 is k_H2o/k_H2O = 1.49 ± 0.09, and the activation parameters for its decomposition are ΔH? = 69.6 ± 1.5 kJ/mol and ΔS? = -4.5 ± 9.5 J/deg mol. It is concluded that 1 decomposes via equilibrium protonation on oxygen with subsequent rate-limiting cleavage of the O-N bond of the diazoic acid to yield the methanediazonium ion. This conclusion and measurements of the rate constants for decomposition of diazomethane under comparable conditions (25°C, aqueous 5% acetonitrile, ionic strength 0.20 M) permit the first semiquantitative description of the decomposition of a simple alkanediazoate and associated intermediates in wholly or predominately aqueous media.