Isothioureas catalyze the enantioselective addition of 4-nitrophenyl esters to tetrahydroisoquinoline-derived iminium ions. 4-Nitrophenoxide, generated in situ from initial N-acylation of the isothiourea by the 4-nitrophenyl ester, is used to facilitate catalyst turnover in this reaction process. Optimization showed that 4-nitrophenyl esters give the best reactivity in this protocol over a range of alternative aryl esters, with the observed enantioselectivity markedly dependent on the nature of the iminium counterion. Highest yields and enantioselectivity were obtained using iminium bromide ions generated in situ via photoredox catalysis using BrCCl₃ and Ru(bpy)₃Cl₂ (0.5 mol %) and commercially available tetramisole (5 mol %) as the Lewis base catalyst. The scope and limitations of this procedure was developed, giving the desired β-amino amide products in up to 96% yield, 79:21 diastereomeric ratio (dr), and er_{major (2R,1′S)} 99.5:0.5.

中文翻译：

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异硫脲催化的4-硝基苯基酯与亚胺离子的对映选择性加成反应

异硫脲催化将4-硝基苯基酯对映选择性加成到四氢异喹啉衍生的亚胺离子上。由4-硝基苯基酯由异硫脲的初始N-酰化反应原位产生的4-硝基苯氧化物可用于促进该反应过程中的催化剂转化。优化表明，在一系列替代芳基酯中，4-硝基苯基酯在该方案中具有最佳反应性，观察到的对映选择性明显取决于亚胺抗衡离子的性质。使用BrCCl ₃和Ru（bpy）₃ Cl ₂通过光氧化还原催化原位产生的溴化亚胺离子获得最高的收率和对映选择性（0.5mol％）和市售的四咪唑（5mol％）作为路易斯碱催化剂。研究了该方法的范围和局限性，从而以高达96％的收率，79：21的非对映异构体比率（dr）和_{主要（2 R，1'S）} 99.5：0.5给出了所需的β-氨基酰胺产物。