Alkene Hydrobenzylation by a Single Catalyst That Mediates Iterative Outer-Sphere Steps

Heterocoupling Two Similar Benzyl Radicals by Dual Photoredox/Cobalt Catalysis

Transition-metal-regulated radical cross coupling enables the selective bonding of two distinct transient radicals, whereas the catalytic method for sorting two almost identical transient radicals, especially similar benzyl radicals, is still rare. Herein, we show that leveraging dual photoredox/cobalt catalysis can selectively couple two similar benzyl radicals. Using easily accessible methylarenes and phenylacetates (benzyl N-hydroxyphthalimide (NHPI) esters) as benzyl radical sources, a range of unsymmetrical 1,2-diarylethane classes via the 1°-1°, 1°-2°, 1°-3°, 2°-2°, 2°-3° and 3°-3° couplings were obtained with broad functional group tolerance. Besides the photochemical continuous flow synthesis, the one-pot procedure that directly uses phenylacetic acids and NHPI as the starting materials to avoid the pre-preparation of benzyl NHPI esters for the gram-scale synthesis is also feasible and affords good yields, showcasing the synthetic utility of our protocol.

Fe(III)-Catalyzed Ring Expansion of Cyclopropenone from Olefins via Radicals to Access Pyrone and Indanone Derivatives

A novel approach for the synthesis of pyrone and indanone derivatives utilizing Fe(III)-catalyzed reductive radical ring expansion of olefins and cyclopropenone has been proposed. The preliminary mechanism study shows that the alkyl radical is formed by hydrogen atom transfer, which can open the tension ring and then generate the intermediate. There are two paths for the intermediate: when there is a hydroxyl group at the β-position of the olefin, the reaction produces pyrones, and otherwise 1-indanone is generated. This method has mild conditions and wide substrate adaptability and allows the indanone fragment to be conveniently accessed.

Aminoalkylation of Alkenes Enabled by Triple Radical Sorting

The direct synthesis of C(sp3)-rich architectures is a driving force for innovation in synthetic organic chemistry. Such scaffolds impart beneficial properties onto drug molecules that correlate with greater clinical success. Consequently, there is a strong impetus to develop new methods by which to access sp3-rich molecules from commercial feedstocks, such as alkenes. Herein, we report a three-component aminoalkylation reaction that utilizes the principles of triple radical sorting to regioselectively add N-centered and C-centered radicals across alkenes. This process relies upon photoredox catalysis to transform alkyl bromides and reductively activated N-centered radical precursors into high-energy radical species in a redox-neutral fashion. A broad scope of coupling partners is demonstrated, with multiple synthetic applications, including facile syntheses of pharmacophoric substituted N-heterocycles.

Nickel-Catalyzed α-selective Hydroalkylation of Vinylarenes

C(sp3) centers adjacent to (hetero) aryl groups are widely present in physiologically active molecules. Metal-hydride-catalyzed hydroalkylation of alkenes represents an efficient means of forging C(sp3)-C(sp3) bonds, boasting advantages as a wide source of substrates, mild reaction conditions, and facile selectivity manipulation. Nevertheless, the hydroalkylation of vinylarenes encounters constraints in terms of substrate scope, necessitating the employment of activated alkyl halides or alkenes containing chelating groups, remains a challenge. In this context, we report a general nickel-hydride-catalyzed hydroalkylation protocol for vinylarenes. Remarkably, this system enables α-selective hydroalkylation of both aryl and heteroaryl alkenes under an extra ligand-free condition, demonstrating excellent coupling efficiency and selectivity. Furthermore, through the incorporation of chiral bisoxazoline ligands, we have achieved regio- and enantioselective hydroalkylation of vinylpyrroles, thereby facilitating the synthesis of α-branched alkylated pyrrole derivatives.

Metal-Hydride C-C Cross-Coupling of Alkenes Through a Double Outer-Sphere Mechanism

This Synopsis covers recent reports of metal-catalyzed alkene functionalizations that likely involve iterative outer-sphere reactions in which the substrate reacts directly with a metal ligand instead of with the metal center itself. Traditional metal hydride-catalyzed alkene functionalizations involve this latter pathway whereby the alkene forms part of the metal ligand sphere (i.e. an inner-sphere reaction). In contrast, alkenes do not ligate the metal in so-called outer-sphere reactions and instead react with a metal ligand. These transformations have proved crucial for the synthesis of high fraction sp3 (Fsp3) targets, especially in hindered fragment couplings of relevance to natural product space.

Cost-Effective Carbon Quaternization with Redox-Active Esters and Olefins

Quaternary carbons are embedded in various natural products, pharmaceuticals, and organic materials. However, constructing this valuable motif is far from trivial. Conventional approaches mainly rely on classical polar disconnections and encounter bottlenecks concerning harsh conditions, functional group tolerance, regioselectivity, and step economy. In this context, Kawamata, Baran, Shenvi, and co-workers recently demonstrated that two feedstock chemicals, alkyl carboxylic acids and olefins, could be utilized to construct tetrasubstituted carbons in the presence of an inexpensive iron porphyrin catalyst and a suitable reductant combination through quaternization of the radical intermediates. The method enables access to various sterically encumbered quaternary carbons under mild and robust conditions. Taking a complete detour from conventional approaches, the present heteroselective radical-radical coupling simplifies the synthesis of quaternary carbon-containing molecules through an innovative and distinctive disconnection approach.

Iron-catalyzed radical Markovnikov hydrohalogenation and hydroazidation of alkenes

We herein report radical hydroazidation and hydrohalogenation of mono-, di- and trisubstituted alkenes through iron catalysis. The alkene moiety that often occurs as a functionality in natural products is readily transformed into useful building blocks through this approach. Commercially available tosylates and α-halogenated esters are used as radical trapping reagents in combination with silanes as reductants. The reported radical Markovnikov hydroazidation, hydrobromination, hydrochlorination, and hydroiodination occur under mild conditions. These hydrofunctionalizations are valuable and practical alternatives to ionic hydrohalogenations with the corresponding mineral acids that have to be run under harsher acidic conditions, which diminishes the functional group tolerance. Good to excellent diastereoselectivities can be obtained for the hydrofunctionalization of cyclic alkenes.

1,3-Difunctionalization of [1.1.1]propellane through iron-hydride catalyzed hydropyridylation

Current methodologies for the functionalization of [1.1.1]propellane primarily focus on achieving 1, 3-difunctionalized bicyclo[1.1.1]pentane or ring-opened cyclobutane moiety. Herein, we report an innovative approach for the 1, 3-difunctionalization of [1.1.1]propellane, enabling access to a diverse range of highly functionalized cyclobutanes via nucleophilic attack followed by ring opening and iron-hydride hydrogen atom transfer. To enable this method, we developed an efficient iron-catalyzed hydropyridylation of various alkenes for C - H alkylation of pyridines at the C4 position, eliminating the need for stoichiometric quantities of oxidants or reductants. Mechanistic investigations reveal that the resulting N-centered radical serves as an effective oxidizing agent, facilitating single-electron transfer oxidation of the reduced iron catalyst. This process efficiently sustains the catalytic cycle, offering significant advantages for substrates with oxidatively sensitive functionalities that are generally incompatible with alternative approaches. The strategy presented herein is not only mechanistically compelling but also demonstrates broad versatility, highlighting its potential for late-stage functionalization.

Cobalt/Photoredox Dual-Catalyzed Cross-Radical Coupling of Alkenes via Hydrogen Atom Transfer and Homolytic Substitution

Cobalt-catalyzed metal hydride hydrogen atom transfer (MHAT) in combination with photoredox catalysis has emerged as a powerful synthetic method, owing to its redox nature and applicability to various radical precursors. Herein, we describe a cross-radical coupling reaction under cobalt/photoredox dual catalysis. MHAT and homolytic substitution (SH2) processes enabled Markovnikov-selective hydrobenzylation of di/trisubstituted alkenes, affording products with a quaternary carbon center in a redox-neutral manner.

Engaging Alkenes in Metallaphotoredox: A Triple Catalytic, Radical Sorting Approach to Olefin-Alcohol Cross-Coupling

Metallaphotoredox cross-coupling is a well-established strategy for generating clinically privileged aliphatic scaffolds via single-electron reactivity. Correspondingly, expanding metallaphotoredox to encompass new C(sp3)-coupling partners could provide entry to a novel, medicinally relevant chemical space. In particular, alkenes are abundant, bench-stable, and capable of versatile C(sp3)-radical reactivity via metal-hydride hydrogen atom transfer (MHAT), although metallaphotoredox methodologies invoking this strategy remain underdeveloped. Importantly, merging MHAT activation with metallaphotoredox could enable the cross-coupling of olefins with feedstock partners such as alcohols, which undergo facile open-shell activation via photocatalysis. Herein, we report the first C(sp3)-C(sp3) coupling of MHAT-activated alkenes with alcohols by performing deoxygenative hydroalkylation via triple cocatalysis. Through synergistic Ir photoredox, Mn MHAT, and Ni radical sorting pathways, this branch-selective protocol pairs diverse olefins and methanol or primary alcohols with remarkable functional group tolerance to enable the rapid construction of complex aliphatic frameworks.

Enantioselective Construction of Quaternary Stereocenters via Cooperative Photoredox/Fe/Chiral Primary Amine Triple Catalysis

The catalytic and enantioselective construction of quaternary (all-carbon substituents) stereocenters poses a formidable challenge in organic synthesis due to the hindrance caused by steric factors. One conceptually viable and potentially versatile approach is the coupling of a C-C bond through an outer-sphere mechanism, accompanied by the realization of enantiocontrol through cooperative catalysis; however, examples of such processes are yet to be identified. Herein, we present such a method for creating different compounds with quaternary stereocenters by photoredox/Fe/chiral primary amine triple catalysis. This approach facilitates the connection of an unactivated alkyl source with a tertiary alkyl moiety, which is also rare. The scalable process exhibits mild conditions, does not necessitate the use of a base, and possesses a good functional-group tolerance. Preliminary investigations into the underlying mechanisms have provided valuable insights into the reaction pathway.

Alkene dialkylation by triple radical sorting

The development of bimolecular homolytic substitution (SH2) catalysis has expanded cross-coupling chemistries by enabling the selective combination of any primary radical with any secondary or tertiary radical through a radical sorting mechanism1-8. Biomimetic9,10 SH2 catalysis can be used to merge common feedstock chemicals-such as alcohols, acids and halides-in various permutations for the construction of a single C(sp3)-C(sp3) bond. The ability to sort these two distinct radicals across commercially available alkenes in a three-component manner would enable the simultaneous construction of two C(sp3)-C(sp3) bonds, greatly accelerating access to complex molecules and drug-like chemical space11. However, the simultaneous in situ formation of electrophilic and primary nucleophilic radicals in the presence of unactivated alkenes is problematic, typically leading to statistical radical recombination, hydrogen atom transfer, disproportionation and other deleterious pathways12,13. Here we report the use of bimolecular homolytic substitution catalysis to sort an electrophilic radical and a nucleophilic radical across an unactivated alkene. This reaction involves the in situ formation of three distinct radical species, which are then differentiated by size and electronics, allowing for regioselective formation of the desired dialkylated products. This work accelerates access to pharmaceutically relevant C(sp3)-rich molecules and defines a distinct mechanistic approach for alkene dialkylation.