Asymmetric Transfer Hydrogenation of 1-Aryl-3,4-Dihydroisoquinolines Using a Cp*Ir(TsDPEN) Complex

We report herein a simple alternative method for the asymmetric transfer hydrogenation (ATH) of 1-aryl-3,4-dihydroisoquinolines (1-Ar-DHIQs) that are known to be challenging substrates owing to their poor reactivity. The hydrogenation protocol employs the readily available Cp*Ir(TsDPEN) {where Cp* = pentamethylcyclopentadienyl and TsDPEN = (S,S)-HNCHPhCHPhNTs2-} catalytic complex, 2-propanol and HCOOH/triethylamine mixture as the solvent and hydrogen donor, and anhydrous phosphoric acid as an inexpensive additive. The series of examined substrates shows a favorable tolerance to various functional groups. Unlike 1-alkyl-DHIQs, where the enantiomeric excess (ee) starkly changes during the course of hydrogenation, 1-Ar-DHIQs exhibit a constant ee value, which makes the method practical and useful for the production of fine chemicals containing the 1,2,3,4-tetrahydroisoquinoline motif.