Wang Justin Y, Villalona Eris, Knowles Robert R
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States.
J Am Chem Soc. 2025 May 7;147(18):15307-15317. doi: 10.1021/jacs.5c00847. Epub 2025 Apr 22.
We report a photoredox-enabled deracemization of cyclic α-aryl ketones that occurs with high stereoselectivity and yield and proceeds by mechanistically distinct proton transfer reactions. This reaction is jointly mediated by a visible-light photocatalyst and a chiral phosphate base cocatalyst under blue light irradiation. Notably, the extent of deracemization for this reaction exhibits an unexpected dependence on the identity of the photocatalyst and the concentration of a chiral base cocatalyst, wherein the extent of deracemization can be increased by employing photocatalysts with more positive ground-state reduction potentials, raising the concentration of the chiral base cocatalyst, or by a combination of these factors. This effect is attributed to two competing processes, back-electron transfer and deprotonation, which consume the same reaction intermediate, and we propose a kinetic model that rationalizes this behavior. We also demonstrate that the redox properties of the photocatalyst impact the stereoselectivity of the product-forming step, which is the dominant stereoselective step in this transformation. Together, these mechanistic insights facilitate a deeper understanding of the complexity of light-driven deracemization reactions involving reversible electron transfer and suggest approaches by which the stereoselectivity of these processes may be increased.
我们报道了一种光催化实现的环状α-芳基酮的消旋化反应，该反应具有高立体选择性和产率，通过机理上不同的质子转移反应进行。在蓝光照射下，该反应由可见光光催化剂和手性磷酸碱共催化剂共同介导。值得注意的是，该反应的消旋化程度对光催化剂的种类和手性碱共催化剂的浓度呈现出意想不到的依赖性，其中通过使用具有更正基态还原电位的光催化剂、提高手性碱共催化剂的浓度或这些因素的组合，可以提高消旋化程度。这种效应归因于两个相互竞争的过程，即回电子转移和去质子化，它们消耗相同的反应中间体，并且我们提出了一个动力学模型来解释这种行为。我们还证明了光催化剂的氧化还原性质会影响产物形成步骤的立体选择性，这是该转化过程中的主要立体选择性步骤。总之，这些机理见解有助于更深入地理解涉及可逆电子转移的光驱动消旋化反应的复杂性，并提出了提高这些过程立体选择性的方法。
