Transition metal-free decarboxylative olefination of carboxylic acid salts

The cost-effective and efficient synthesis of alkenes is highly significant due to their extensive applications in both synthetic and polymer industries. A transition metal-free approach has been devised for the chemoselective olefination of carboxylic acid salts. This modular approach provides direct access to valuable electron-deficient styrenes in moderate to good yields. Detailed mechanistic studies suggest anionic decarboxylation is followed by halogen ion transfer. This halogen transfer leads to an umpolung of reactant electronics, allowing for a rate-limiting rebound elimination.

Ion Pair Catalyst - Pentanidinium

In this account, we further describe our already developed N-sp² hybrid guanidinium as an efficient phase-transfer catalyst and ion pair catalysis based on N-sp² hybrid pentanidinium and its application in some new reactions. The sp³ hybrid quaternary ammonium salt has a tetrahedral structure, which means that three sides of it can be effectively steric, allowing the remaining side to be close to the substrate. However, the sp² hybrid ammonium salt allows the substrate to form ion pairs from both directions respectively, so it is a greater challenge to control the stereoselectivity of the reaction. Van der Waals forces, such as hydrogen bonds and π - π ${\pi -\pi }$ interactions, have been used to make electrophiles approach from a certain direction, leading to a higher enantioselectivity. Based on the above idea, we designed an N-sp² hybrid phase-transfer catalyst, pentanidinium. Pentanidinium has five conjugated nitrogen atoms, one of which has a formal positive charge, which is necessary for it to become an ion pair catalyst. We have confirmed that pentanidinium can catalyze α-hydroxylation of 3-substituted-2-oxindoles, Michael addition of 3-alkyloxindoles with vinyl sulfone, and alkylation reactions of sulfenate anions and dihydrocoumarins, desymmetrization of pro-chiral sulfinate to afford enantioenriched sulfinate esters. Pentanidinium with side chain structure changes can also be catalyzed efficiently with enantioconvergent halogenophilic nucleophilic substitution, including azidation and thioesterification. In the reaction catalyzed by pentanidinium, it always attracts us with the advantages of low catalytic load and good enantioselectivity.

Molecular Understanding and Practical In Silico Catalyst Design in Computational Organocatalysis and Phase Transfer Catalysis-Challenges and Opportunities

Through the lens of organocatalysis and phase transfer catalysis, we will examine the key components to calculate or predict catalysis-performance metrics, such as turnover frequency and measurement of stereoselectivity, via computational chemistry. The state-of-the-art tools available to calculate potential energy and, consequently, free energy, together with their caveats, will be discussed via examples from the literature. Through various examples from organocatalysis and phase transfer catalysis, we will highlight the challenges related to the mechanism, transition state theory, and solvation involved in translating calculated barriers to the turnover frequency or a metric of stereoselectivity. Examples in the literature that validated their theoretical models will be showcased. Lastly, the relevance and opportunity afforded by machine learning will be discussed.

Site- and Enantioselective Manganese-Catalyzed Benzylic C-H Azidation of Indolines

A manganese-catalyzed highly site- and enantioselective benzylic C-H azidation of indolines has been described. The practical method is applicable for azidation of a tertiary benzylic C-H bond with good functional group tolerance, allowing facile access to structurally diverse tertiary azide-containing indolines in high efficiency with excellent site-, chemo-, and enantioselectivity. The generality of the method was further demonstrated by site- and enantioselective azidation of the secondary benzylic C-H bond for a range of secondary azide-containing indolines. The benzylic C-H azidation method allows to straightforwardly and enantioselectively install a variety of nitrogen-based functional groups and diverse bioactive molecules at the C³ position of indoline frameworks through post-azidation manipulations. Gram-scale synthesis was also demonstrated, further highlighting the synthetic potential of the method. Mechanistic studies by combined experiments and computations elucidated the reaction mechanism and origins of stereoselectivity.

Stereoinvertive Nucleophilic Substitution at Quaternary Carbon Stereocenters of Cyclopropyl Ketones and Ethers

A highly regio- and diastereoselective nucleophilic substitution at the quaternary carbon stereocenter of cyclopropyl ketones and cyclopropyl carbinol derivatives using TMSBr, DMPSCl and TMSN3 as nucleophiles has been developed. A variety of acyclic tertiary alkyl bromides, chlorides and azides were therefore prepared with excellent diastereopurity. The substitution occurs at the most substituted quaternary carbon center in a stereoinvertive manner, which may be attributed to the existence of a bicyclobutonium species.

Kinetic resolution of cyclic benzylic azides enabled by site- and enantioselective C(sp3)-H oxidation

Catalytic nonenzymatic kinetic resolution (KR) of racemates remains one of the most powerful tools to prepare enantiopure compounds, which dominantly relies on the manipulation of reactive functional groups. Moreover, catalytic KR of organic azides represents a formidable challenge due to the small size and instability of the azido group. Here, an effective KR of cyclic benzylic azides through site- and enantioselective C(sp³)-H oxidation is described. The manganese catalyzed oxidative KR reaction exhibits good functional group tolerance, and is applicable to a range of tetrahydroquinoline- and indoline-based organic azides with excellent site- and enantio-discrimination. Computational studies elucidate that the effective chiral recognition is derived from hydrogen bonding interaction between substrate and catalyst.

Direct SN2 or SN2X Manifold─Mechanistic Study of Ion-Pair-Catalyzed Carbon(sp3)-Carbon(sp3) Bond Formation

Density functional theory (DFT) is used in this work to predict the mechanism for constructing congested quaternary-quaternary carbon(sp³)-carbon(sp³) bonds in a pentanidium-catalyzed substitution reaction. Computational mechanistic studies were carried out to investigate the proposed S_N2X manifold, which consists of two primary elementary steps: halogen atom transfer (XAT) and subsequent S_N2. For the first calculated model on original experimental substrates, XAT reaction barriers were more kinetically competitive than an S_N2 pathway and connect to thermodynamically stable intermediates. Extensive computational screening modeling was then done on various substrate combinations designed to study the steric influence and to understand the mechanistic rationale, and calculations reveal that sterically congested substrates prefer the S_N2X manifold over S_N2. Different halides as leaving groups were also screened, and it was found that the reactivity increases in the order of I > Br > Cl > F, in agreement with the strength of C-X bonds. However, DFT modeling suggests that chlorides can be a viable substrate for the S_N2X process, which should be further explored experimentally. ONIOM calculations on the full catalyst model predicted the correct stereochemical outcome, and further catalyst screening with cationic Me₄N⁺ and K⁺ predicted that pentanidium is still the choice for S_N2X C-C bond formation.

Organocatalytic asymmetric azidation of sulfoxonium ylides: mild synthesis of enantioenriched α-azido ketones bearing a labile tertiary stereocenter

Disclosed here is a catalytic asymmetric azidation reaction for the efficient synthesis of α-azido ketones bearing a labile tertiary stereocenter. With a superb chiral squaramide catalyst, a mild asymmetric formal H–N₃ insertion of α-carbonyl sulfoxonium ylides proceeded with excellent efficiency and enantioselectivity. This organocatalytic process not only complements the previous α-azidation approaches for the formation of quaternary stereocenters and mostly for 1,3-dicarbonyl compounds, but also has advantages over the well-known metal-catalyzed asymmetric carbene insertion chemistry using α-diazocarbonyl compounds. Detailed mechanistic studies via control reactions and NMR studies provided important insights into the reaction pathway, which features reversible protonation and dynamic kinetic resolution. The curiosity in mechanism also led to the development of a simplified alternative protocol with a cheaper HN₃ source.

An organocatalytic asymmetric H–N₃ insertion of α-carbonyl sulfoxonium ylides has been developed, providing efficient access to α-azido ketones bearing labile tertiary stereocenters and complementing the metal carbene insertion chemistry.

Exploiting Configurational Lability in Aza-Sulfur Compounds for the Organocatalytic Enantioselective Synthesis of Sulfonimidamides

Methods for establishing the absolute configuration of sulfur‐stereogenic aza‐sulfur derivatives are scarce, often relying on cumbersome protocols and a limited pool of enantioenriched starting materials. We have addressed this by exploiting, for the first time, a feature of sulfonimidamides in which it is possible for tautomeric structures to also be enantiomeric. Such sulfonimidamides can readily generate prochiral ions, which we have exploited in an enantioselective alkylation process. Selectivity is achieved using a readily prepared bis‐quaternized phase‐transfer catalyst. The overall process establishes the capability of configurationally labile aza‐sulfur species to be used in asymmetric catalysis.

The azidosulfonylation of terminal alkynes leading to β-azidovinyl sulfones

We report herein the first example of the N₃ radical-mediated azidosulfonylation of alkynes, affording the β-azidovinyl sulfone products with a broad substrate scope, excellent functional group compatibility, and high yield. DFT calculations suggest that the mechanism of the reaction proceeds through an unprecedented sequential N₃ radical addition and sulfonyl radical coupling pathway.

Ni-Catalyzed asymmetric reduction of α-keto-β-lactams via DKR enabled by proton shuttling

Chiral α-hydroxy-β-lactams are key fragments of many bioactive compounds and antibiotics, and the development of efficient synthetic methods for these compounds is of great value. The highly enantioselective dynamic kinetic resolution (DKR) of α-keto-β-lactams was realized via a novel proton shuttling strategy. A wide range of α-keto-β-lactams were reduced efficiently and enantioselectively by Ni-catalyzed asymmetric hydrogenation, providing the corresponding α-hydroxy-β-lactam derivatives with high yields and enantioselectivities (up to 92% yield, up to 94% ee). Deuterium-labelling experiments indicate that phenylphosphinic acid plays a pivotal role in the DKR of α-keto-β-lactams by promoting the enolization process. The synthetic potential of this protocol was demonstrated by its application in the synthesis of a key intermediate of Taxol and (+)-epi-Cytoxazone.