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.

本研究采用β-环糊精衍生物手性固定相（β-cyclodextrin derivative chiral stationary phase），借助动态高分辨气相色谱（dynamic high-resolution gas chromatography, DHRGC），对3-硝基丁-2-酮（1）、3-硝基戊-2-酮（2）与2-硝基戊-3-酮（3）的可逆对映体互变动力学开展了系统研究；该互变过程经由酮式结构的烯醇化（enolization）路径实现。针对上述硝基酮与手性固定相开展的DHRGC实验结果显示，在130 ℃至160 ℃的温度区间内，色谱洗脱图谱呈现出典型的互变平台特征。研究人员通过计算机模拟实验色谱洗脱曲线，测定了该过程的速率常数及对应的对映异构化能垒（k、ΔG#(T)、ΔH#与ΔS#）。高度负值的活化熵（ΔS#值介于-19至-37 cal·mol⁻¹·K⁻¹之间）表明，其过渡态（transition state, TS）存在显著的电荷分离现象。相较于3-氯-2-丁酮与3-甲基-2-戊酮的相关研究结果，针对化合物1~3得到的实验数据揭示了α-硝基取代基对简单酮类烯醇化速率的显著调控作用。为探明固定液相与流动气相在本研究体系中的独立贡献，研究人员针对上述相同化合物开展了密度泛函理论从头算（DFT ab initio）计算。