Light-Mediated Cross-Coupling of Anomeric Trifluoroborates

Abstract

Stereoselective reactions at the anomeric carbon constitute the cornerstone of preparative carbohydrate chemistry. Here, we report the synthesis of axial C1 trifluoroborates and stereoselective C-arylation and etherification reactions under photoredox conditions. These reactions are characterized by high anomeric selectivities for 2-deoxysugars and broad substrate scope (24 examples), including disaccharides and trifluoroborates with free hydroxyl groups. Computational studies show that high axial selectivities for these reactions originate from a combination of kinetic anomeric effect of the intermediate C1 radical and stereoelectronic stabilization of Ni(III) through the metallo-anomeric effect. Taken together, this new class of carbohydrate reagents adds the palette of anomeric nucleophile reagents suitable for efficient installation C-C and Cheteroatom bonds.

Metallo-Anomeric Effect

“…2.2 配体对金属异头碳效应的影响 通过配体可以有效调节金属络合物的电性和空间位阻，可以调节金属异头碳效应。与其他配体 相比，具有强给电子能力的卡宾配体 [14] 另一个值得注意的地方是在链状体系中，邻位交叉效应受氮杂环卡宾给电子能力的削弱更为明 显，氮杂环卡宾配位的金属取代基则倾向于对位构象(ΔE < 0)。 2.4 金属异头碳效应的应用 金属异头碳效应在指导发展高效、高选择性的糖类分子修饰方面有着十分广阔的应用前景。例 如，Walczak课题组巧妙地设计了一类金属Ni/Ir协同催化体系，实现了糖分子异头碳的立体选择性芳 基化修饰(图6) [15] 。首先，底物a和Ir催化剂在光照条件下产生异头碳自由基中间体b，b与Ni(II)中间 体结合形成具有直立键构象的金属有机化合物中间体c， 再经过立体专一性的还原消除反应得到最终 产物d。反应的高立体选择性一方面来源于高氧化态的Ni和异头碳自由基中间体b结合是以立体构型 保持的方式进行的。另一方面，后续生成的金属有机化合物中间体c也表现出较强的金属异头碳效 应，这些机理过程均得到了相应的DFT计算的验证 [16]…”