Crystallization-Enabled Henry Reactions: Stereoconvergent Construction of Fully Substituted [N]-Asymmetric Centers

Synthetic Chameleon Turns into Oximes, Nitrones, and Hydroxylamines when Exposed to Blue Light

A metal-free, user-friendly photochemical transformation of nitroalkanes to oximes, nitrones, and hydroxylamines has been developed. The visible-light-induced reactions are catalyzed by the readily available photoredox organocatalyst 4CzIPN and use inexpensive amines as reductants. Broad in scope and tolerant of multiple functional groups and heterocycles, the transformation proceeds under mild conditions. Its synthetic potential was demonstrated in the formal total synthesis of amathaspiramide F. A basic insight into the reaction mechanism was gained with the help of an NMR study.

A Catalytic Method to Activate Nitromethane by the Cooperation of Homo- and Heterogeneous Catalysis

The achievement of directly activating and utilizing bulk small molecules has remained a longstanding objective in the field of chemical synthesis. The present work reports a catalytic activation method for bulk chemical nitromethane (MeNO2 ). This method combines homogeneous Lewis acid with recyclable heterogeneous Brønsted acid catalysis, featuring practicality, sustainability, and low cost, thus solving the inherent drawbacks of previous Nef processes where stoichiometric reductants or activators were required. By combining the advantages of both homo- and heterogeneous catalysts, this chemistry may not only offer new opportunities for the further development of MeNO2 as a nitrogen source for organic synthesis, but also promote the catalysis design in synthetic chemistry.

Crystallization-Enabled Stereoconvergent Michael Additions of β-Keto Esters to Nitroolefins

Asymmetric Michael additions are powerful tools to meet the growing need for stereochemically complex products. While 1,3-dicarbonyls are common nucleophiles, the successful use of configurationally unstable β-keto esters in diastereoselective variants remains understudied. In this Letter, crystalline β-keto esters were leveraged in a two-phase, one-pot merger of an asymmetric Michael addition with a crystallization-induced diastereomer transformation. Tuning the crystallinity of β-keto ester adducts enabled stereoconvergence of the products, which were isolated by filtration.

Multistep Cascade Catalytic Reactions Employing Bifunctional Framework Compounds

Multistep cascade reactions are important to achieve atom as well as step economy over conventional synthesis. This approach, however, is limited due to the incompatibility of the available reactive centers in a catalyst. In the present study, new MOF compounds, [Zn₂(SDBA)(3-ATZ)₂]·solvent, I and II, with tetrahedral Zn centers as good Lewis acidic sites and the free amino group of the 3-amino triazole ligand as a strong Lewis base center were shown to perform 4-step cascade/tandem reaction in a facile manner. Effective conversion of benzaldehyde dimethyl acetal in the presence of excess nitromethane at 100 °C in water to 1-(1,3-dinitropropan-2-yl) benzene was achieved in 10 h with yields of ∼95% (I) and ∼94% (II). This 4-step cascade reaction proceeds via deacetalization (Lewis acid), Henry (Lewis base), and Michael (Lewis base) reactions. The present work highlights the importance of spatially separated functional groups in multistep tandem catalysis─the examples of which are not common.

Fluoroamide-Directed Regiodivergent C-Alkylation of Nitroalkanes

Herein, by exploiting different activation modes of fluoroamides, we achieved α- and δ-C(sp³)-H alkylation of nitroalkanes with switchable regioselectivity. Cu catalysis enabled the interception of a distal C-centered radical by a N-centered radical to couple nitroalkanes and unactivated δ-C-H bonds. In addition, imines generated in situ by fluoroamides were trapped by nitroalkanes to realize the α-C-H alkylation of amides. Both of those scalable protocols have broad substrate scopes and good functional group tolerance.

Enantio- and Diastereoselective Mannich Reactions of ß-Dicarbonyls by Second Stage Diastereoconvergent Crystallization

The synthesis of chiral α-monosubstituted-ß-dicarbonyls is a challenging task in asymmetric catalysis due to the rapid, typically uncontrolled, product racemization or epimerization under most reaction conditions. For this reason, diastereoselective additions of unsubstituted ß-dicarbonyls to π-electrophiles are unusual. Herein, we disclose a simple catalytic crystallization-driven enantio- and diastereoselective Mannich reaction for the synthesis of stereodefined α-monosubstituted-ß-keto esters, dissymmetric ß-diesters, dissymmetric ß-diketones, and ß-keto amides that productively leverages product epimerization in solution. Mechanistic studies suggest a scenario where the initial enantioselective, diastereodivergent skeletal assembly is catalyzed by a chiral tertiary amine organocatalyst, which then facilitates second stage crystallization-induced diastereoconvergence to provide the challenging α-stereocenter in excellent stereoselectivity.

Conglomerate Crystallization in the Cambridge Structural Database (2020-2021)

Conglomerate crystals are materials capable of undergoing spontaneous resolution and were responsible for the discovery of molecular chirality. Their relevance to modern chemical and crystallographic sciences has been hindered by the difficulty in identifying and searching materials with this characteristic ability to spontaneously bias their own enantioenrichment. With the release of the November 2021 distribution of the Cambridge Structural Database (CSD) (version 5.43), a fresh quantity of chiral conglomerate crystals is expected to have been published in the CSD without identification. Indeed, no crystals in the CSD have been identified as a spontaneously resolving conglomerate crystal in their crystallographic information file since the 2019 release, despite the deposition of over 108,000 new crystal structures into the database over the same time period. A manual inspection of crystals deposited between 2020 and 2021 was conducted to identify 343 new chiral materials which exhibit conglomerate crystallization behavior. It is hoped that the continued manual curation of this list will aid those in the crystallographic and synthetic communities to study and exploit this spontaneous enantioenrichment behavior.