Struktur und reaktivität aliphatischer diazoverbindungen—XVIII

Abstract

The rate of the acid-catalysed hydrolysis of p-nitrophenyl-diazomethane has been measured in dioxan-water/HClO4. As indicated by the solvent deuterium isotope effect kH/kD = 3·76 and the general acid catalysis in acetic acid-sodium acetate buffer solutions, the decomposition proceeds by rate-controlling protonation (A-SE2 mechanism). These results are not in accordance with the recent findings of the A-2 hydrolyses of primary and of the A-SE2 hydrolyses of secondary diazo alkanes and α-diazocarbonyl compounds. The stability toward acids is increased by substitution at the diazo C atom of IIIa. In the reaction series IIIa–g the rate of hydrolysis at 20° decreases from k2 = 82.10−1 1.mol−1.s−1 for IIIa to k2 = 11·3.10−5 1.mol−1.s−1 in the case of IIIg. A variation of the mechanism of hydrolysis was not observed. The plot of the logarithms of the rate constants k2 for the acid-catalysed hydrolysis of IIIb–g vs σ*, the Taft substituent constant, has been found to be linear with a slope ϱ* = − 1·869. With decreasing reactivity also the solvent isotope effect kH/kD decreases from a value of 3·76 in the case of IIIa to a range between 1·55 and 1·95 for IIId–g. An approximately linear relationship is indicated between the magnitude of kH/kD and the reactivity lg k2 (Fig 1). This correlation shows the theoretically expected substituent effects on transition-state symmetry for rate-determining proton-transfer reactions.