Substituenteneffekte auf die CC‐Bindungsstärke, 14. Kinetische und thermodynamische Stabilität von 2,3‐Bis(dialkylamino)‐1,4‐diketonen — Stabilisierungsenergie capto‐dativ substituierter a‐Dialkylamino‐a‐Carbonylalkyl‐Radikale

Abstract

Substituent Effects on the Strength of CC Bonds, 14[1]. — Kinetic and Thermodynamic Stability of 2,3‐Bis(dialkylamino)‐1,4‐diketones — Energy of Stabilization of α‐Dialkylamino α‐Carbonylalkyl Radicals with Capto‐dative SubstituentsProf. Dr. H.‐G. Viehe zum 65. Geburtstag gewidmet. The equilibrium constants and rate constants for the dissociation of the 2,3‐bis(dialkylamino)‐1,4‐diketone diastereomers meso‐ and DL‐7a and 7b were measured over a temperature range of 40°C. From the enthalpies of dissociation ΔHDiss and enthalpies of activation ΔH≠ and the strain enthalpies of 7 the bond dissociation enthalpies BDE(CC) of 7 were determined. By comparison with the dissociation enthalpies of Ct‐Ct alkanes[21] the change of these BDEs(CC) by the cap‐to‐dative substitution was determined to be 85.4 kJ mol−1 (20.4 kcal mol−1). The heats of formation ΔH (g) of a series of amino ketones 8 were determined from their heats of combustion and their heats of evaporation. From the ΔH(g) values in combination with MM2 calculations of their strain enthalpies strain‐free increments CHn[N, CO, C2 ‐ n] with n With n = 0, 1, 2 were derived and geminal interaction enthalpies in the ground states were obtained thereof. The radical stabilization enthalpy RSE of 6 was deduced from the ΔBDE(CC) values and the ground state effect to be 73.6 kJ mol−1 (17.6 kcal mol−1). From these data and the radical stabilization enthalpies RSE of α‐aminoalkyl radicals (4.2 kJ mol−1) and α‐carbonyl radicals (28.9 kJ mol−1) a synergetic radical stabilization enthalpy of 40.5 kJ mol−1 (9.7 kcal mol−1) is deduced. This number combines “extra” resonance stabilization and general inductive or anomeric geminal substituent interaction in the radicals. The crystal structure of meso‐7a has been determined by X‐ray diffraction methods.