Azetidines with All-Carbon Quaternary Centers: Merging Relay Catalysis with Strain Release Functionalization

Synthesis of 2-(Trifluoromethyl)Azetidines by Strain-Release Reactions of 2-(Trifluoromethyl)-1-Azabicyclo[1.1.0]Butanes

Substituted azetidines are privileged heterocyclic scaffolds in medicinal chemistry and have become synthetic targets of high interest in recent years. With the goal of developing a new access to azetidines incorporating the pharmaceutically relevant trifluoromethyl group, the reactivity of 2-(trifluoromethyl)-1-azabicyclo[1.1.0]butanes was investigated in polar strain-release reactions. By using benzyl chloroformate or trifluoroacetic anhydride as reacting partners, diversely substituted 3-chloroazetidines, 3-substituted azetidines and azetidin-3-ols bearing a trifluoromethyl group at C2 could be readily synthesized. In addition, palladium-catalyzed hydrogenolysis reactions provided an entry to cis-3-aryl-2-trifluoromethyl azetidines.

Rapid access to azetidines via allylation of azabicyclo[1.1.0]butanes by dual copper/photoredox catalysis

Recently, the strain-release-driven synthesis of N1/C3 functionalized azetidines from azabicyclo[1.1.0]butanes has generated significant interest in the fields of medicinal and synthetic chemistry. This paper presents a mild and efficient dual copper/photoredox-catalyzed multi-component allylation of azabicyclo[1.1.0]butanes using a radical-relay strategy. This strategy enables the synthesis of C3 quaternary center-containing azetidines via a radical relay mechanism with a high yield of 91%. The method's utility is highlighted by late-stage derivatization of bioactive molecules, demonstrating exceptional functional group compatibility.

Modular Synthesis of 3,3-Disubstituted Azetidines via Azetidinylation Reagents

Azetidines represent an attractive and emerging design option in medicinal chemistry owing to their small size and polar nature, as well as their potential to significantly impact the physicochemical properties of drug molecules. However, traditional methods for the synthesis of 3,3-disubstituted azetidines usually require higher step counts or exhibit poor functional group compatibility. Herein, we report a modular synthesis strategy for 3,3-disubstituted azetidines based on azetidinylation reagents. The practicality of this method is further exemplified by the use of readily available starting materials, mild reaction conditions, and a very broad substrate scope.

In Situ SERS Monitoring and Heterogeneous Catalysis Verification of Pd-Catalyzed Suzuki-Miyaura Reaction via Bifunctional "Black Raspberry-like" Plasmonic Nanoreactors

Constructing materials that possess both catalytic properties and surface plasmonic active structures presents an essential requirement in the field of in situ surface-enhanced Raman spectroscopy (SERS) monitoring. In this work, we developed a novel black raspberry-like bifunctional nanoreactor platform (HPRS@Au-Pd) with enhanced surface-enhanced Raman spectroscopy (SERS) activity (EF = 3.11 × 109) and excellent catalytic performance (e.g., |K| = 1.72 × 10-2 s-1 for 4-BTP). This platform enables sensitive and real-time monitoring of Suzuki reactions, providing an efficient and stable system for mechanistic investigations. By fitting a first-order kinetic model, we quantitatively analyzed the reaction kinetics of haloarenes on Pd NPs, revealing the direct involvement of surface-bound Pd NPs in the catalytic process. Furthermore, 4MPBA-blocking experiments using haloarenes devoid of functional groups demonstrated the absence of catalytic activity from dissolved Pd species, conclusively supporting the heterogeneous nature of the reaction. This work extends the applicability of the conclusion regarding heterogeneous catalysis to a broader range of substrates, offering valuable insights into the design and optimization of dual-functional nanoreactors. Beyond the Suzuki reactions, the versatility of this platform opens avenues for applications in other catalytic reactions, showcasing its potential in advanced catalytic studies and real-time monitoring.

Redox-Neutral Carboxylation of Benzylic Tertiary C-H Bonds with Carbon Dioxide

The direct catalytic carboxylation of benzylic tertiary C-H bonds with CO2 for the synthesis of all-carbon quaternary carboxylic acids represents a significant challenge. Here, we present a redox-neutral approach to address this difficulty by leveraging the synergistic interplay between photocatalysis and cascade hydrogen abstraction cycles. Remarkably, this strategy eliminates the need for sacrificial electron donors, electron acceptors, or stoichiometric additives, offering enhanced atom economy and environmental sustainability. It is particular that the combination of α-amino alkyl radicals with sulfur radicals generated in situ from the decomposition of DMSO was employed to realize the abstraction of benzylic tertiary C-H bonds. Our method enables the direct synthesis of a diverse array of benzylic quaternary carboxylic acids with excellent functional group tolerance as well as the derivatization of bioactive molecules and the gram-scale synthesis of pharmaceuticals under mild reaction conditions.

Harnessing Oxetane and Azetidine Sulfonyl Fluorides for Opportunities in Drug Discovery

Four-membered heterocycles such as oxetanes and azetidines represent attractive and emergent design options in medicinal chemistry due to their small and polar nature and potential to significantly impact the physiochemical properties of drug molecules. The challenging preparation of these derivatives, especially in a divergent manner, has severely limited their combination with other medicinally and biologically important groups. Consequently, there is a substantial demand for mild and effective synthetic strategies to access new oxetane and azetidine derivatives and molecular scaffolds. Here, we report the development and use of oxetane sulfonyl fluorides (OSFs) and azetidine sulfonyl fluorides (ASFs), which behave as precursors to carbocations in an unusual defluorosulfonylation reaction pathway (deFS). The small-ring sulfonyl fluorides are activated under mild thermal conditions (60 °C), and the generated reactive intermediates couple with a broad range of nucleophiles. Oxetane and azetidine heterocyclic, -sulfoximine, and -phosphonate derivatives are prepared, several of which do not have comparable carbonyl analogs, providing new chemical motifs and design elements for drug discovery. Alternatively, a SuFEx pathway under anionic conditions accesses oxetane-sulfur(VI) derivatives. We demonstrate the synthetic utility of novel OSF and ASF reagents through the synthesis of 11 drug analogs, showcasing their potential for subsequent diversification and facile inclusion into medicinal chemistry programs. Moreover, we propose the application of the OSF and ASF reagents as linker motifs and demonstrate the incorporation of pendant groups suitable for common conjugation reactions. Productive deFS reactions with E3 ligase recruiters such as pomalidomide and related derivatives provide new degrader motifs and potential PROTAC linkers.

Catalytic Asymmetric Reactions of Ketimines and Alkenes via [2 + 2] Cycloaddition: Chemical Reactivity Controlled by Switching a Heteroatom

Azetidine units are commonly found in natural products and biologically active drugs. The [2 + 2] cycloaddition of imines and alkenes has been extensively used in the synthesis of such structures, while enantioselective approaches remain elusive. Herein, an efficient B(C6F5)3/chiral phosphoric acid-catalyzed asymmetric [2 + 2] cycloaddition of ketimines and aryl vinyl selenides was presented, delivering valuable chiral azetidines with excellent stereoselectivities (>20:1 dr and up to 96:4 er). What's even more interesting was that when a "Se" atom was switched to an "S" atom, the reaction proceeded through a [2 + 2] cycloaddition/ring-opening cascade process, affording a range of chiral thioacetals with high enantioselectivities (up to 98:2 er), which were also important organic sulfur compounds. Mechanistic experiments, coupled with density functional theory (DFT) calculations, shed light on a mechanism involving stepwise [2 + 2] cycloaddition and ring-opening processes, with the initial alkenylation step identified as crucial for achieving stereoselective control.

Synthesis of 1-Azabicyclo[2.1.1]hexanes via Formal Single Electron Reduction of Azabicyclo[1.1.0]butanes under Photochemical Conditions

C(sp3)-rich heterocycles are privileged building blocks for pharmaceuticals and agrochemicals. Therefore, synthetic methods that provide access to novel saturated nitrogen-containing heterocycles are in high demand. Herein, we report a general synthesis of 1-azabicyclo[2.1.1]hexanes (1-aza-BCH) via a formal cycloaddition of azabicyclo[1.1.0]butanes (ABB) with styrenes under photochemical conditions. To overcome the challenging direct single electron reduction of ABBs, we designed a polar-radical-polar relay strategy that leverages a fast acid-mediated ring-opening of ABBs to form bromoazetidines, which undergo efficient debrominative radical formation to initiate the cycloaddition reaction. The reaction is applicable to a broad range of ABB-ketones and we demonstrate the 1-aza-BCH products can be further functionalized to access larger saturated, conformationally rigid heterocycles.

Isolation and Reactivity of Arylnickel(II) Complexes in Nickel-Catalyzed Borylation of Aryl Fluorosulfates

Aryl fluorosulfates have emerged as versatile SuFExable substrates, harnessing the reactivity of the S-F bond. In this study, we unveil their alternative synthetic utility in nickel-catalyzed borylation via C-O bond activation. This method highlights mild reaction conditions, a broad substrate scope, and moderate functional group tolerance, rendering it a practical and appealing approach for synthesizing a diverse array of aryl boronate esters. Furthermore, computational analysis sheds light on the reaction pathways, uncovering the participation of LNi(0) and LNi(II)ArX species. This insight is supported by the 31P NMR reaction monitoring along with isolation and single-crystal X-ray structural elucidation of well-defined arylnickel(II) intermediates obtained from the oxidative addition of aryl fluorosulfates. A comprehensive investigation, merging experimental and computational approaches, deepens our understanding of the alternative reactivity of SuFExable substrates.