Enantioselective Imine Reduction of Dihydro-β-carbolines by Fe-Thiosquaramide Catalyst.

Enantioselective imine reduction of dihydro-β-carbolines (DHBCs) is a reliable and powerful tool to construct bioactive chiral tetrahydro-β-carbolines (THBCs). Here, we report an efficient enantioselective imine reduction employing <i>in situ</i> generated Fe-thiosquaramides (Fe-TSQs) <b>3a</b> and <b>3b</b> as asymmetric organometallic catalysts to produce chiral THBCs (<b>2a</b>-<b>h</b>). The catalyst <b>3a</b> at 15 mol % was found to be suitable for the substrates with alkyl and aryl groups which afford corresponding chiral THBCs with excellent enantioselectivities (up to ee 99%).