Isothiourea-catalyzed enantioselective addition of 4-nitrophenyl esters to iminium ions

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 upon the nature of the iminium counteri-on. 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 dr and er_{major (2R,1′S)} 99.5:0.5.