Mechanisms of benzyl group transfer in the decay of (E)-arylmethanediazoates and aryldiazomethanes in aqueous solutions

Article abstract of DOI:10.1021/ja00120a004

Rate constants are reported for the buffer-independent decay of ten (E)-arylmethanediazoates in aqueous media at 25 °C, ionic strength 1 M (NaClO₄), 4% 2-propanol, in the region of pH 4-12. The rate constants are proportional to hydrogen ion concentration at high pH and become pH independent in the low-pH region. Varying concentrations of oxyanion, amine, and hydrazine buffers over the range 0.05-0.2 M increased the pseudo-first-order rate constant for decay of the diazoates by less than 10%. The azide - water selectivities, k_a/k_s, for partitioning of the benzyl groups in the decay of (E)-(3,5-bis(trifluoromethyl)phenyl)methanediazoate and the (3,5-bis(trifluoromethyl)phenyl)diazomethane are determined to be 0.20 and 0.21 M^-1, respectively, in phosphate buffered water and 0.27 and 0.26 M^-1, respectively, in 20/80 DMSO-water. It is concluded that these two reactants decompose, in these media, via a common free diazonium ion intermediate that is formed in the case of the diazoate upon unassisted N-O bond cleavage of the diazoic acid. A common rate-limiting step is indicated for all the diazoates by the correlation line for the plot of log k₁, the pH independent rate constant, against σ that has a slope q = -1.23. Product ratios for trapping of benzyl groups derived from other pairs of arylmethanediazoates and aryldiazomethanes with less electron withdrawing groups are different outside experimental error, indicating the importance of different nitrogen-separated ion pairs in these reactions. The (E)-(p-methoxy)phenyl)methane-¹⁶O-diazoate decomposes in ¹⁶O/¹⁸O water to give alcohol that has an "excess" abundance of ¹⁶O compared to solvent. Decomposition of the same compound in 50/50 trifluoroethanol-water with varying concentrations of azide indicates that azide ion appears to trap a limiting amount, ～80%, of the p-methoxybenzyl group. Quantitative analysis of the data indicates that 16% of the p-methoxybenzyl cation is trapped by solvent at the nitrogen-separated ion pair stage, in the absence of azide ion. There is a 9-fold enhancement of selectivity for trifluoroethanol at the ion pair stage that is ascribed to a proton switch initiated by the leaving hydroxide ion in the ion pair. The values of K_a/k_s ～ 0.2 M^-1 and k_T/k_H ～ 0.5-0.6 for the trifluoroethanol-water selectivity and k_ET/k_T ～ 1 for the ethanol-trifluoroethanol selectivity are independent of substituent in the decay of arylmethanediazoates (X = H and EWG) in water, water-trifluoroethanol (50/50), and water-trifluoroethanol-ethanol (50/40/10), respectively. It is concluded from this that the productdetermining steps do not involve chemical bonding but rather rotational/translational reorientation of the nucleophiles in the first solvation sphere of the carbocation intermediates. It is concluded that the values of k_H/k_T = 0.5-0.6 indicate preferential solvation of the cation precursor by trifluoroethanol. It is shown that a preferential interaction for trifluoroethanol of <1 kcal/mol is required to generate the observed selectivities.