Aqueous Amine-Tolerant [2+2] Photocycloadditions of Unactivated Olefins

Synthesis of bicyclo[3.2.0]heptane lactones via a ligand-enabled Pd-catalyzed C(sp3)-H activation cascade

Bicyclo[3.2.0]heptane lactones represent an important scaffold in bioactive molecules. Herein, we report a diastereoselective synthetic disconnection to access bicyclo[3.2.0]heptane lactones from bicyclo[1.1.1]pentane carboxylic acids, which proceeds through palladium-catalyzed C-H activation and C-C cleavage processes. By using two different classes of ligands, MPAA and pyridone-amine, either all-syn arylated bicyclo[3.2.0]heptane lactones or non-arylated ones can be synthesized. The bicyclo[3.2.0]heptane lactone products were converted into multiple substituted cyclobutane, γ-lactone, and oxobicyclo[3.2.0]heptane derivatives to showcase the synthetic versatility of this method.

Photoexcited Copper-Catalyzed Intramolecular [2+2] Cycloaddition To Construct Bicyclo[3.2.0]heptanes

The bicyclo[3.2.0]heptane scaffold is frequently found in a wide range of bioactive molecules and plays a pivotal role in constructing key modules of these compounds. In this study, we present a strategy for the synthesis of bicyclo[3.2.0]heptanes using a copper/BINAP complex to facilitate an intramolecular [2+2] cycloaddition under visible light irradiation. This methodology offers several advantages, including the use of a cost-effective copper catalyst and the ability to achieve high yields (up to 98%) and diastereoselectivity (>20:1 dr). Our approach provides an efficient strategy for constructing the bicyclo[3.2.0]heptane framework.

Modular synthesis of bis-α-chiral amines using Ellman sulfinamide for consecutive S-to-C chirality induction/transfer

Amines are ubiquitous components in pharmaceuticals. Increasing saturated substitutions (sp3-hybridized carbon) at the amino center and the number of chiral centers can enrich the molecular diversity and chemical space, ultimately enhancing the success of drug development. However, the synthesis of such advanced amines is challenging due to a higher level of structural complexity and stereo-control. Here, we report a modular protocol for short de novo synthesis of bis-α-chiral amines. This protocol uses commercially available Ellman sulfinamide, tert-butanesulfinamide (tBS), as the exclusive chiral source to selectively produce all possible stereoisomers. Sequential formation of contiguous α-amino chiral carbons is achieved by chirality induction and transfer mechanisms that are both enabled by tBS, the stereoselective imine functionalization and alkyne-participated rearrangement reaction. The second step we developed is crucial for high diastereoselectivity, which is problematic in previous methods. The other coupling partners used in this protocol are abundant feedstocks, providing desirable chemical diversity in the products.

Catalysis in the Excited State: Bringing Innate Transition Metal Photochemistry into Play

Transition metal catalysis is an indispensable tool for organic synthesis that has been harnessed, modulated, and perfected for many decades by careful selection of metal centers and ligands, giving rise to synthetic methods with unparalleled efficiency and chemoselectivity. Recent developments have demonstrated how light irradiation can also be recruited as a powerful tool to dramatically alter the outcome of catalytic reactions, providing access to innovative pathways with remarkable synthetic potential. In this context, the adoption of photochemical conditions as a mainstream strategy to drive organic reactions has unveiled exciting opportunities to exploit the rich excited-state framework of transition metals for catalytic applications. This Perspective examines advances in the application of transition metal complexes as standalone photocatalysts, exploiting the innate reactivity of their excited states beyond their common use as photoredox catalysts. An account of relevant examples is dissected to provide a discussion on the electronic reorganization, the orbitals involved, and the associated reactivity of different types of excited states. This analysis aims to provide practitioners with fundamental principles and guiding strategies to understand, design, and apply light-activation strategies to homogeneous transition metal catalysis for organic synthesis.

Palladium-Catalyzed Alkoxycarbonylation of Alcohols for the Synthesis of Cyclobutanecarboxylates with α-Quaternary Carbon Centers

A palladium-catalyzed alkoxycarbonylation with two different alcohols for the synthesis of cyclobutanecarboxylates bearing an α-quaternary carbon center is presented. The reaction utilizes readily accessible starting materials, tolerates a broad scope of functional groups, and provides a straightforward and efficient approach for the synthesis of a diverse array of cyclobutanecarboxylates bearing an α-quaternary carbon. Meanwhile, this strategy effectively prevents the transition-metal-catalyzed ring-opening of cyclobutanols, preserves the cyclobutane framework, and affords 1,1-disubstituted cyclobutanecarboxylates in high yields with excellent regioisomeric ratios.

Metal-free [2+2]-photocycloaddition of unactivated alkenes enabled by continuous flow processing

We report a continuous flow approach generating bicyclic cyclobutanes from unactivated alkenes in a metal-free manner that is inspired by the Kochi-Salomon reaction. A filtered Hg-lamp in combination with a simple flow set-up and acetone as UV light-absorbing co-solvent are crucial for this method thus showcasing attractive features for further exploitations.

Ligand-controlled regiodivergent aminocarbonylation of cyclobutanols toward 1,1- and 1,2-substituted cyclobutanecarboxamides

Four-membered carbocycles are among the most sought-after backbones which are commonly found in biologically active molecules. However, difficulties on their producing are existing due to its highly strained ring system. On the other hand, cyclobutanols can be straightforwardly prepared and can serves as precursors for synthesizing cyclobutane derivatives. Here we report an example of regioselective aminocarbonylation of cyclobutanols in which the cyclobutane core remained intact. The method exhibits good functional group compatibility, as well as high regio- and stereoselectivity, offering new pathways for synthesizing several pharmaceuticals. Furthermore, this strategy enables the rapid installation of cyclobutane as a conformational restricted skeleton, greatly facilitating direct access to valuable drug molecules that require conformational restriction.

B/Pd Synergistic Catalysis for the Decarboxylative Allylation of 2-(2-Azaaryl)acetic Acids

We describe an allylation reaction between 2-(2-azaaryl)acetic acids and allylic electrophiles catalyzed synergistically by a dual system consisting of borinic acid and a Pd complex under mild conditions. The decarboxylative allylation proceeds via a boron-bound enamine intermediate, which then interacts with a π-allylpalladium intermediate from the allylic electrophile. High yields of diallylation products highlight the method's efficiency. Intriguingly, when using 2-(2-pyridyl)acetic acid with a C3 substituent on the pyridyl ring, the reaction exclusively yields monoallylation products.

Evidence for Dearomatizing Spirocyclization and Dynamic Effects in the Quasi-stereospecific Nitrogen Deletion of Tetrahydroisoquinolines

Selectivity in organic chemistry is generally presumed to arise from energy differences between competing selectivity-determining transition states. However, in cases where static density functional theory (DFT) fails to reproduce experimental product distributions, dynamic effects can be examined to understand the behavior of more complex reaction systems. Previously, we reported a method for nitrogen deletion of secondary amines which relies on the formation of isodiazene intermediates that subsequently extrude dinitrogen with concomitant C-C bond formation via a caged diradical. Herein, a detailed mechanistic analysis of the nitrogen deletion of 1-aryl-tetrahydroisoquinolines is presented, suggesting that in this system the previously determined diradical mechanism undergoes dynamically controlled partitioning to both the normal 1,5-coupling product and an unexpected spirocyclic dearomatized intermediate, which converges to the expected indane by an unusually facile 1,3-sigmatropic rearrangement. This mechanism is not reproduced by static DFT but is supported by quasi-classical molecular dynamics calculations and unifies several unusual observations in this system, including partial chirality transfer, nonstatistical isotopic scrambling at the ethylene bridge, the isolation of spirocyclic dearomatized species in a related heterocyclic series, and the observation that introduction of an 8-substituent dramatically improves enantiospecificity.

Synthesis of dienes from pyrrolidines using skeletal modification

Saturated N-heterocyclic pyrrolidines are common in natural products, medicinal compounds and agrochemicals. However, reconstruction of their skeletal structures creating new chemical space is a challenging task, and limited methods exist for this purpose. In this study, we report a skeletal modification strategy for conversion of polar cyclic pyrrolidines into nonpolar linear dienes through a N-atom removal and deconstruction process. This involves N-sulfonylazidonation followed by rearrangement of the resulting sulfamoyl azide intermediates. This can be an energetically unfavorable process, which involves the formation of active C-C π bonds, the consumption of inert C-N and C-C σ bonds and the destruction of stable five-membered rings, but we have used it here to produce versatile conjugated and nonconjugated dienes with links of varying lengths. We also studied the application of this method in late-stage skeletal modification of bioactive compounds, formal traceless C(sp2)-H functionalization and formal N-atom deletion.

Direct Alkylative Amination Using 1-Allylsilatrane

Homoallylic amines prepared via addition of allylsilanes often require preformed imine substrates, metal catalysts, fluoride activators, or use of protected amines. In this metal-free, air- and water-tolerant procedure, aromatic aldehyde and aniline substrates undergo direct alkylative amination using easily accessible 1-allylsilatrane.

A Metal-Free Cyclobutadiene Reagent for Intermolecular [4 + 2] Cycloadditions

Cyclobutadiene is a highly reactive antiaromatic hydrocarbon that has fascinated chemists for over 60 years. However, its preparation and uses in chemical synthesis are sparing, in part due to its lengthy synthesis that generates hazardous byproducts including excess heavy metals. Herein, we report a scalable, metal-free cyclobutadiene reagent, diethyldiazabicyclohexene dicarboxylate, and explore its intermolecular [4 + 2] cycloaddition with various electron-deficient alkenes. We also demonstrate its utility in a three-step synthesis of dipiperamide G and a diverse array of product derivatizations including bromocyclobutadiene.