Comminges Clement, Barhdadi Rachid, Doherty Andrew P, O'Toole Sarah, Troupel Michel
Institut de Chimie et des Materiaux Paris Est (ICMPE - equipe ESO), UMR 7182, CNRS - Universite Paris 12, Val de Marne, 2-8 rue H. Dunant, 94320 Thiais, France.
J Phys Chem A. 2008 Aug 28;112(34):7848-55. doi: 10.1021/jp801253n. Epub 2008 Aug 2.
The mechanism of 2,2'6,6'-tetramethylpiperidin- N-oxyl (TEMPO)-mediated oxidation of alcohols to aldehydes and ketones in ionic liquids has been investigated using cyclic voltammetry and rotating disk electrode voltammetry. It is shown that the presence of bases (B) and their conjugate acids (BH (+)), as well as their p K as, strongly influences the rate of reaction. Data indicated that the first step in the oxidation is the formation of the alcoholate species via acid-base equlibrium with B. The alcoholate subsequently reacts with the active form of TEMPO (T (+), i.e., the one-electron oxidized form) forming an intermediate that further reacts with T (+) and B returning TEMPO catalytically, BH (+), and the carbonyl product. A kinetic model incorporating this pre-equilibrium step has been derived, which accounts for the experimentally observed reaction kinetics. Overall, the rate of reaction is controlled by the equilibrium constant for the pre-equilibrium step; as such, strong bases are required for more kinetically efficient transformations using this redox catalyst.
利用循环伏安法和旋转圆盘电极伏安法，研究了2,2′,6,6′-四甲基哌啶氮氧化物（TEMPO）在离子液体中将醇氧化为醛和酮的机理。结果表明，碱（B）及其共轭酸（BH(+)）的存在以及它们的pKa对反应速率有很大影响。数据表明，氧化的第一步是通过与B的酸碱平衡形成醇盐物种。醇盐随后与TEMPO的活性形式（T(+)，即单电子氧化形式）反应形成中间体，该中间体进一步与T(+)和B反应，催化地使TEMPO、BH(+)和羰基产物再生。推导了一个包含该预平衡步骤的动力学模型，该模型解释了实验观察到的反应动力学。总体而言，反应速率由预平衡步骤的平衡常数控制；因此，使用这种氧化还原催化剂进行动力学上更有效的转化需要强碱。
