Mechanisms Of Benzyl Group-Transfer In The Decay Of (E)-Arylmethanediazoates And Aryldiazomethanes In Aqueous-Solutions

Rate constants are reported for the buffer-independent decay of ten (E)-arylmethanediazontes in aqueous media at 25 degrees 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(2)/k(5), for partitioning of the benzyl groups in the decay of (E)-(3,5-bis(trifluoromethyl)phenyl)methanediazonate 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 k(1), the pH independent rate constant, against sigma that has a slope rho = -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-(16)-diazoat decomposes in O-16/O-18 water to give alcohol that has an ''excess'' abundance of O-16 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, similar to 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(2)/k(5) similar to 0.2 M(-1) and k(T)/k(H) similar to 0.5-0.6 for the trifluoroethanol-water selectivity and k(ET)/k(T) similar to 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 product-determining 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.