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

Abstract

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 in situ generated Fe-thiosquaramides (Fe-TSQs) 3a and 3b as asymmetric organometallic catalysts to produce chiral THBCs (2a–h). The catalyst 3a 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%).

Air-Stable Cobalt(III) and Chromium(III) Complexes as Single-Component Catalysts for the Activation of Carbon Dioxide and Epoxides

“…To study the mechanism of catalyst cat2 , ESI-MS was utilized, which is a soft ionization method that prevents weakly bound ligand cleavage in online metal-catalyzed screening studies through mass spectrometry. , We have used ESI-MS in combination with tandem mass spectrometry (ESI-MS/MS) to study mechanistic aspects of metal-mediated homogeneous catalysis. − In this study, online reaction monitoring was carried out using a microreactor (Figure S20), which afforded main species participating in Co­(III)-mediated 1a /CO 2 addition through real-time screening during the reaction. Online reaction monitoring by ESI-MS gave a very clean spectrum displaying easily identified cobalt-containing and CoCln-containing cationic species, which were identified by their isotopic patterns (Figure ).…”

Nickel(II)-Catalyzed Formal [3+2] Cycloadditions between Indoles and Donor–Acceptor Cyclopropanes

“…To gain further insight into the proposed mechanism, we performed online reaction monitoring by ESI-MS, a technique of great relevance in the advancement of organic catalysis [77][78][79], since molecules and transient intermediates [80] of high polarity and moiety complexity can be easily studied by mass spectrometry. ESI-MS has proven to be a remarkable "ion-fishing" technique, as it gently transfers preformed ions in solution directly to the gas phase.…”

“…The goal of this study was to intercept the cationic species resulting from the Ni-catalyzed reaction using ESI-MS in the positive-ion mode (Scheme 3) [81][82][83]. The main advantage of using ESI is its capacity to transfer ions to the gas phase without inducing unwanted side reactions, and the composition of ESI-transferred ions often reflects that in solution [77][78][79]. The ESI-MS spectra collected for such a reaction are particularly clean and mechanistically enlightening.…”

Recent Advances in Non-Precious Metal Catalysis

“…Enantioselective imine reduction of dihydro-β-carbolines using a Fe-thiosquaramide catalyst (Fe-TSQ) was reported by Santos and colleagues (Scheme ). The iron catalyst was prepared in situ from FeCl 2 ·4H 2 O and a chiral thiosquaramide in triethyl orthoformate and then used directly in asymmetric reduction using NaCNBH 3 as the hydride source. Chiral tetrahydro-β-carbolines were obtained with enantioselectivities up to 98% ee while tolerating alkyl and aryl substituents.…”