Enantiomerization Study of Some α-Nitroketones by Dynamic High-Resolution Gas Chromatography

The kinetics of the reversible enantiomer interconversion of 3-nitrobutan-2-one (1), 3-nitropentan-2-one (2), and 2-nitropentan-3-one (3) have been studied by dynamic high-resolution gas chromatography (DHRGC) by using a β-cyclodextrin derivative chiral stationary phase; the process occurs via enolization of the keto forms. The DHRGC experiments involving the studied nitroketones and the chiral stationary phase show chromatographic profiles with a typical interconversion plateau in the temperature range between 130 and 160 °C. Computer simulation of the experimental chromatographic elution profiles was employed for the determination of rate constants and the corresponding enantiomerization barriers (k, ΔG^#(T), ΔH^#, and ΔS^#). The highly negative entropy of activation (ΔS^# values from −19 to −37 cal mol^-1 K^-1) points to a transition state (TS) with large charge separation. The obtained results for 1−3 show the dramatic effect of an α-nitro-substituent on the rate of enolization of simple ketones, when compared with those for 3-chloro-2-butanone and 3-methyl-2-pentanone. To get some information on the separate contributions of the stationary liquid phase and the mobile gas phase on the studied process, some DFT ab initio calculations have been performed for the same compounds.