Aminocatalytic Enantioselective 1,6 Additions of Alkyl Thiols to Cyclic Dienones: Vinylogous Iminium Ion Activation

Palladium(0) and Brønsted Acid Co-Catalyzed Enantioselective Hydro-Cyclization of 2,4-Dienyl Hydrazones and Oximes

The transition metal-catalyzed asymmetric hydro-functionalization of 1,3-dienes has been well explored, but most reactions focus on electron-neutral substrates in an intermolecular manner. Here we first demonstrate that readily available 2,4-dienyl hydrazones and oximes can be efficiently utilized in the hydro-cyclization reaction under co-catalysis of a Brønsted acid and a chiral palladium complex, furnishing multifunctional dihydropyrazones and dihydroisoxazoles, respectively. Diverse substitution patterns for both types of electron-deficient diene compounds are tolerated, and corresponding heterocycles were generally constructed with moderate to excellent enantioselectivity, which can be elaborated to access products with higher molecular complexity and diversity. Control experiments and density functional theory calculations support that α-regioselective protonation of dienyl substrates by acid and concurrent π-Lewis base activation of Pd0 complex is energetically favoured in the formation of active π-allylpalladium intermediates, and an outer-sphere allylic amination or etherification mode is adopted to deliver the observed cyclized products enantioselectively.

Unlocking the Molecular Behavior of Natural Amine-Targeted Carbon Quantum Dots for the Synthesis of Diverse Pharmacophore Scaffolds via an Unusual Nanoaminocatalytic Route

Despite the fact that carbon quantum dots (CQDs) have significant catalytic potential, only emblematic applications that rely on simple acid-base or hydrogen-bonding activation pathways have been reported. In this study, natural amine-targeted CQDs (NAT-CQDs) have been successfully fabricated using a sustainable technique that harnesses a renewable green source. Based on a holistic sustainable assessment, the present approach for the synthesis of NAT-CQDs surpasses previously reported methods in terms of estimated circular and good-manufacturing-practice metrics. A set of spectroscopic and analytical techniques, including FTIR, XPS, conductometric assay, pH titration, 19FNMR, and 13CNMR confirms the presence of the assessable amino-rich groups (0.0083N) at the surface of NAT-CQDs. The occurrence of surface amine groups unlocked the molecular behavior of as-prepared NAT-CQDs and makes them an unprecedented nanoaminocatalytic platform for the synthesis of diverse pharmacophore scaffolds (>40 examples) via a one-pot Knoevenagel/(aza) Michael addition reaction in water at room temperature. The assessable amine group can covalently activate carbonyl groups through nucleophilic iminium activation modes in water and facilitate the ability to build valuable and therapeutic scaffolds on a gram scale. By transferring significant molecular primacy at the frontier of nanoscale materials, NAT-CQDs can thus bridge the gap between the nanoscale and molecular domains. This protocol can also be applied for the preparation of therapeutic anticoagulant drugs, warfarin, and coumachlor. All the reactions exhibited a high atom economy, low E-factor, low process mass intensity (PMI), high reaction mass efficiency (RME), high carbon efficiency (CE), and high catalyst reusability with overall high sustainable values. NAT-CQDs show high recyclability, and the spectral data of reused catalysts indicate that the NAT-CQDs maintained their surface chemistry and electronic properties, suggesting their stability under the tested conditions. This study presents a remarkable instance of NAT-CQDs showcasing covalent catalysis. Expanding on the aforementioned design concept, the utilization of NAT-CQDs' "potential" as distinct colloidal organocatalysts in aqueous environments at the molecular level introduces valuable prospects for aminocatalytic pathways.

Umpolung Asymmetric 1,5-Conjugate Addition via Palladium Hydride Catalysis

Electronically matched nucleophilic 1,6-conjugate addition has been well studied and widely applied in synthetic areas. In contrast, nucleophilic 1,5-conjugate addition represents an electronically forbidden process and is considered unfeasible. Here, we describe modular protocols for 1,5-conjugate addition reactions via palladium hydride catalysis. Both palladium and synergistic Pd/organocatalyst systems are developed to catalyze 1,5-conjugate reaction, followed by inter- or intramolecular [3+2] cyclization. A migratory 1,5-addition protocol is established to corroborate the feasibility of this umpolung concept. The 1,5-addition products are conveniently transformed into a series of privileged enantioenriched motifs, including polysubstituted tetrahydrofuran, dihydrofuran, cyclopropane, cyclobutane, azetidine, oxetane, thietane, spirocycle and bridged rings. Preliminary mechanistic studies corroborate the involvement of palladium hydride catalysis.

Direct Catalytic Asymmetric Doubly Vinylogous Michael Addition of α,β-Unsaturated γ-Butyrolactams to Dienones

An asymmetric doubly vinylogous Michael addition (DVMA) of α,β-unsaturated γ-butyrolactams to sterically congested β-substituted cyclic dienones with high site-, diastereo-, and enantioselectivity has been achieved. An unprecedented DVMA/vinylogous Michael addition/isomerization cascade reaction affords chiral fused tricyclic γ-lactams with four newly formed stereocenters.