Fe-Catalyzed C-C Bond Construction from Olefins via Radicals

A Radical Approach to Anionic Chemistry: Synthesis of Ketones, Alcohols, and Amines

Historically accessed through two-electron, anionic chemistry, ketones, alcohols, and amines are of foundational importance to the practice of organic synthesis. After placing this work in proper historical context, this Article reports the development, full scope, and a mechanistic picture for a strikingly different way of forging such functional groups. Thus, carboxylic acids, once converted to redox-active esters (RAEs), can be utilized as formally nucleophilic coupling partners with other carboxylic derivatives (to produce ketones), imines (to produce benzylic amines), or aldehydes (to produce alcohols). The reactions are uniformly mild, operationally simple, and, in the case of ketone synthesis, broad in scope (including several applications to the simplification of synthetic problems and to parallel synthesis). Finally, an extensive mechanistic study of the ketone synthesis is performed to trace the elementary steps of the catalytic cycle and provide the end-user with a clear and understandable rationale for the selectivity, role of additives, and underlying driving forces involved.

Ligand-Promoted Iron(III)-Catalyzed Hydrofluorination of Alkenes

An iron-catalyzed hydrofluorination of unactivated alkenes has been developed. The use of a multidentate ligand and the fluorination reagent N-fluorobenzenesulfonimide (NFSI) proved to be critical for this reaction, which afforded various fluorinated compounds in up to 94 % yield.

A free radical alkylation of quinones with olefins

We demonstrated herein an Fe(iii)-mediated radical alkylation of quinones. A wide range of bioactive molecules with quinone motifs can be rapidly synthesized by using readily available and inexpensive NaBH4/NaBD4 with alkenes at room temperature under open flask conditions.

Versatile Synthetic Route to Cycloheximide and Analogues That Potently Inhibit Translation Elongation

Cycloheximide (CHX) is an inhibitor of eukaryotic translation elongation that has played an essential role in the study of protein synthesis. Despite its ubiquity, few studies have been directed towards accessing synthetic CHX derivatives, even though such efforts may lead to protein synthesis inhibitors with improved or alternate properties. Described here is the total synthesis of CHX and analogues, and the establishment of structure-activity relationships (SAR) responsible for translation inhibition. The SAR studies aided the design of more potent compounds, one of which irreversibly blocks ribosomal elongation, preserves polysome profiles, and may be a broadly useful tool for investigating protein synthesis.

Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

A hydrate salt-promoted reductive coupling reaction of nitrodienes with unactivated alkenes

Transition metal-catalyzed reductive coupling has emerged as a powerful method for the construction of C-C bonds. Herein, a crystalline hydrate, Na2HPO4·7H2O, has been disclosed as an effective promoter for the reductive coupling of nitrodienes with unactivated alkenes to afford diverse dienes with various functionalities in an open-flask manner. The mechanism study has revealed that Na2HPO4·7H2O accelerates the in situ generation of active silane PhSi(OEt)H2 and prevents the deactivation of catalyst. The approach can increase the efficiency of previous reductive coupling reactions as well.

Metal-free visible light photoredox enables generation of carbyne equivalents via phosphonium ylide C-H activation

Carbyne, an interesting synthetic intermediate, has recently been generated from hypervalent iodine precursors via photoredox catalysis. Given the underexplored chemistry of carbyne, due to the paucity of carbyne sources, we are intrigued to discover a new source for this reactive species from classical reagents - phosphonium ylides. Our novel strategy employing phosphonium ylides in an olefin hydrocarbonation reaction features a facile approach for constructing carbon-carbon bonds through metal-free and benign reaction conditions. Moreover, the hydrocarbonation products were delivered in a highly regioselective manner.

Forging C(sp3)-C(sp3) Bonds with Carbon-Centered Radicals in the Synthesis of Complex Molecules

Radical fragment coupling reactions that unite intricate subunits have become an important class of transformations within the arena of complex molecule synthesis. This Perspective highlights some of the early contributions in this area, as well as more modern applications of radical fragment couplings in the preparation of natural products. Additionally, emphasis is placed on contemporary advances that allow for radical generation under mild conditions as a driving force for the implementation of radical fragment couplings in total synthesis.

Reductive coupling of alkenes with unsaturated imines via a radical pathway

A reductive coupling of alkenes with unsaturated imines via a radical pathway is described.

Multi-component heteroarene couplings via polarity-reversed radical cascades

A multi-component radical addition strategy enables difunctionalization of alkenes with heteroarenes and a variety of radical precursors, including N3, P(O)R2, and CF3. This unified approach for coupling diverse classes of electrophilic radicals and heteroarenes to vinyl ethers allows for direct, vicinal C-C as well as C-N, C-P, and C-Rf bond formation.

A metal-free desulfurizing radical reductive C–C coupling of thiols and alkenes

An intermolecular reductive C–C coupling of electrophilic alkyl radicals and alkenes has been developed.

Green oxidant H2O2 as a hydrogen atom transfer reagent for visible light-mediated Minisci reaction

A metal-free protocol for oxidative coupling of heteroarenes and aliphatic C–H components has been achieved via a radical pathway.

Visible-light-initiated manganese-catalyzed Giese addition of unactivated alkyl iodides to electron-poor olefins

Visible-light-initiated Giese addition of unactivated alkyl iodides to electron-poor olefins with catalysis by decacarbonyl dimanganese, Mn₂(CO)₁₀ was reported.

Iron-catalyzed reductive cyclization reaction of 1,6-enynes for the synthesis of 3-acylbenzofurans and thiophenes

A facile synthesis of 3-acylbenzofurans and thiophenes via iron(ii)-catalyzed reductive cyclization of 1,6-enynes has been developed.

A general synthesis of unnatural α-amino acids by iron-catalysed olefin–olefin coupling via generated radicals

A general protocol for the asymmetric synthesis of unnatural α-amino acids with γ-tertiary and quaternary carbon centers via generated radicals is reported.

Three-component difluoroalkylamination of alkenes mediated by photoredox and iron cooperative catalysis

A new three-component difluoroalkylamination of alkenes mediated by photoredox and iron cooperative catalysis has been developed with a broad substrate scope.

Metal-, photocatalyst-, and light-free late-stage C–H alkylation of N-heteroarenes with organotrimethylsilanes using persulfate as a stoichiometric oxidant

Minisci C–H alkylation of N-heteroarenes with readily available benzylsilanes and heteroatom substituted silanes was developed.

Oxidative allene amination for the synthesis of nitrogen-containing heterocycles

The prevalence of stereochemically complex amines in natural products, pharmaceuticals and other bioactive compounds, coupled with the challenges inherent in their preparation, has inspired our work to develop new and versatile methodologies for the synthesis of amine-containing stereotriads ('triads'). The key step is a highly chemo-, regio-, and stereoselective transition-metal catalyzed nitrene transfer reaction that transforms one of the cumulated double bonds of an allene precursor into a bicyclic methyleneaziridine intermediate. This account summarizes our strategies to rapidly elaborate such intermediates into stereochemically rich, densely functionalized amine triads, nitrogen heterocycles, aminated carbocycles and other useful synthetic building blocks.

Studies toward the Total Synthesis of Caribbean Ciguatoxin C-CTX-1: Synthesis of the LMN-Ring Fragment through Reductive Olefin Cross-Coupling

Synthesis of the LMN-ring fragment of Caribbean ciguatoxin C-CTX-1, the principal causative toxin for ciguatera fish poisoning around the Caribbean Sea areas, is described. The key feature of the synthesis is the stereoselective introduction of an angular methyl group on the sterically encumbered seven-membered M-ring by the application of a hydrogen atom transfer-based reductive olefin coupling.

Two Types of Cross-Coupling Reactions between Electron-Rich and Electron-Deficient Alkenes Assisted by Nucleophilic Addition Using an Organic Photoredox Catalyst

Two types of photoreactions between electronically differentiated donor and acceptor alkenes assisted by nucleophilic addition using an organic photoredox catalyst efficiently afforded 1:1 or 2:1 cross-coupling adducts. A variety of alkenes and alcohols were employed in the photoreaction. Control of the reaction pathway (i.e., the formation of the 1:1 or 2:1 adduct) was achieved by varying the concentration of the alcohol used. Detailed mechanistic studies suggested that the organic photoredox catalyst acts as an effective electron mediator to promote the formation of the cross-coupling adducts.

Hydrogen Atom Transfer-Mediated Cyclisations of Nitriles

Hydrogen atom transfer-mediated intramolecular C-C coupling reactions between alkenes and nitriles, using PhSiH₃ and catalytic Fe(acac)₃ , are described. This introduces a new strategic bond disconnection for ring-closing reactions, forming ketones via imine intermediates. Of note is the scope of the reaction, including formation of sterically hindered ketones, spirocycles and fused cyclic systems.

Ti-Catalyzed Radical Alkylation of Secondary and Tertiary Alkyl Chlorides Using Michael Acceptors

Alkyl chlorides are common functional groups in synthetic organic chemistry. However, the engagement of unactivated alkyl chlorides, especially tertiary alkyl chlorides, in transition-metal-catalyzed C-C bond formation remains challenging. Herein, we describe the development of a TiIII-catalyzed radical addition of 2° and 3° alkyl chlorides to electron-deficient alkenes. Mechanistic data are consistent with inner-sphere activation of the C-Cl bond featuring TiIII-mediated Cl atom abstraction. Evidence suggests that the active TiIII catalyst is generated from the TiIV precursor in a Lewis-acid-assisted electron transfer process.

Twelve-Step Asymmetric Synthesis of (-)-Nodulisporic Acid C

A short, enantioselective synthesis of (-)-nodulisporic acid C is described. The route features two highly diastereoselective polycyclizations en route to the terpenoid core and the indenopyran fragment and a highly convergent assembly of a challenging indole moiety. Application of this chemistry allows for a 12-step synthesis of the target indoloterpenoid from commercially available material.

A Protocol To Transform Sulfones into Nitrones and Aldehydes

A simple method to transform sulfones into nitrones and therefore into the corresponding carbonyl derivatives has been developed. Some examples demonstrate that it is a new reliable and versatile reaction in the toolbox of sulfones that has great synthetic potential. NMR and computational studies were used to elucidate the mechanism.

Iron-Nickel Dual-Catalysis: A New Engine for Olefin Functionalization and the Formation of Quaternary Centers

Alkene hydroarylation forms carbon-carbon bonds between two foundational building blocks of organic chemistry: olefins and aromatic rings. In the absence of electronic bias or directing groups, only the Friedel-Crafts reaction allows arenes to engage alkenes with Markovnikov selectivity to generate quaternary carbons. However, the intermediacy of carbocations precludes the use of electron-deficient arenes, including Lewis basic heterocycles. Here we report a highly Markovnikov-selective, dual-catalytic olefin hydroarylation that tolerates arenes and heteroarenes of any electronic character. Hydrogen atom transfer controls the formation of branched products and arene halogenation specifies attachment points on the aromatic ring. Mono-, di-, tri-, and tetra-substituted alkenes yield Markovnikov products including quaternary carbons within nonstrained rings.

Catalytic Alkene Difunctionalization via Imidate Radicals

The first catalytic strategy to harness imidate radicals has been developed. This approach enables alkene difunctionalization of allyl alcohols by photocatalytic reduction of their oxime imidates. The ensuing imidate radicals undergo consecutive intra- and intermolecular reactions to afford (i) hydroamination, (ii) aminoalkylation, or (iii) aminoarylation, via three distinct radical mechanisms. The broad scope and utility of this catalytic method for imidate radical reactivity is presented, along with comparisons to other N-centered radicals and complementary, closed-shell imidate pathways.

Cobalt bis(acetylacetonate)-tert-butyl hydroperoxide-triethylsilane: a general reagent combination for the Markovnikov-selective hydrofunctionalization of alkenes by hydrogen atom transfer

We show that cobalt bis(acetylacetonate) [Co(acac)₂], tert-butyl hydroperoxide (TBHP), and triethylsilane (Et₃SiH) constitute an inexpensive, general, and practical reagent combination to initiate a broad range of Markovnikov-selective alkene hydrofunctionalization reactions. These transformations are believed to proceed by cobalt-mediated hydrogen atom transfer (HAT) to the alkene substrate, followed by interception of the resulting alkyl radical intermediate with a SOMOphile. In addition, we report the first reductive couplings of unactivated alkenes and aryldiazonium salts by an HAT pathway. The simplicity and generality of the Co(acac)₂–TBHP–Et₃SiH reagent combination suggests it as a useful starting point to develop HAT reactions in complex settings.

Total Synthesis of Aplysiasecosterol A

Aplysiasecosterol A (1) is a structurally unusual 9,11-secosteroid isolated from the sea hare Aplysia kurodai. We have accomplished the first and asymmetric total synthesis of 1 in a convergent fashion. The left-hand segment bearing three adjacent stereocenters was constructed through desymmetrizing reduction, ketalization, and radical cyclization. A strategy of asymmetric 2-bromoallylation followed by spontaneous desymmetrizing lactolization enabled a more expeditious access to this segment. The right-hand segment was prepared through two different approaches: one featuring Myers alkylation and Suzuki-Miyaura coupling and the other relying upon Aggarwal lithiation-borylation and Zweifel-Evans olefination. The two fragments were coupled by a Reformatsky type reaction. The three consecutive stereocenters embedded in the central domain of 1 were generated by an iron-mediated, hydrogen atom transfer based radical cyclization reaction.

Radical chain repair: The hydroalkylation of polysubstituted unactivated alkenes

The concept of repair is widely used by nature to heal molecules such as proteins, lipids, sugars, and DNA that are damaged by hydrogen atom abstraction resulting from oxidative stress. We show that this strategy, rather undocumented in the field of synthetic organic chemistry, can be used in a radical chain reaction to enable notoriously intractable transformations. By overcoming the radical chain inhibitor properties of substituted alkenes, the radical-mediated hydroalkylation of mono-, di-, tri-, and even tetrasubstituted unactivated olefins could be performed under mild conditions. With a remarkable functional group tolerance, this reaction provides a general coupling method for the derivatization of olefin-containing natural products.

Total Synthesis of Tetarimycin A, (±)-Naphthacemycin A9, and (±)-Fasamycin A: Structure-Activity Relationship Studies against Drug-Resistant Bacteria

Making use of a reductive olefin coupling reaction and Michael-Dieckmann condensation as two key operations, we have completed a concise total synthesis of tetarimycin A, (±)-naphthacemycin A₉, and (±)-fasamycin A in a highly convergent and practical protocol. Synthetic procedures thus developed have also been applied to provide related analogues for structure-activity relationship studies, thereby coming to the conclusion that the free hydroxyl group at C-10 is essential for exerting inhibitory activities against a panel of Gram-positive bacteria, including drug-resistant strains VRE and MRSA.

Photolytic Reactivity of Organometallic Chromium Bipyridine Complexes

Known stable [Cr(bpy)₂(Ph)₂](BPh₄) complexes undergo reductive elimination of biphenyl with visible-light photolysis using household incandescent or compact fluorescent light bulbs. A series of [Cr(R-bpy)₂(Ar)₂](X) complexes (R = H or CMe₃; Ar = Ph, C₆H₄-CMe₃, or C₆H₄-OMe; X = I, BPh₄, or PF₆) were prepared, and the effect of varying the bipyridine and aryl ligands on the UV-visible spectra and electrochemistry of the chromium(III) complexes was investigated. Photolysis of a mixture of two different bis(aryl) complexes gave only the homocoupled biaryl products by ¹H NMR and gas chromatography/mass spectrometry analysis. The initial product of photoinduced reductive elimination of [Cr(bpy)₂(Ar)₂](PF₆) was trapped with bipyridine to generate [Cr(bpy)₃](PF₆) and with benzoyl peroxide to form [Cr(bpy)₂(O₂CPh)₂](PF₆). The latter chromium(III) bis(benzoate) complex was also synthesized by the addition of bipyridine and PhCO₂H to Cp₂Cr, followed by air oxidation. The neutral Cr(bpy)(S₂CNMe₂)Ph₂ complex also generated biphenyl upon visible-light photolysis. While the treatment of Cr(^tBu-bpy)(dpm)Cl₂ [dpm = (OC^tBu)₂CH] with AgO₂CPh gave trans-Cr(^tBu-bpy)(dpm)(O₂CPh)₂, reaction of the dichloro precursor with PhMgCl produced anionic [Cr(^tBu-bpy)Ph₃]^- with [Mg(dpm)(THF)₄]⁺ as the countercation, with both complexes characterized by single-crystal X-ray diffraction. Protonolysis of Cr(bpy)Ph₃(THF) with 8-hydroxyquinoline produced Cr(bpy)(quin)Ph₂, which generated biphenyl under visible-light photolysis, and the initial product of reductive elimination was trapped by bipyridine or benzoyl peroxide. A related Cr(bpy)(quin)₂ complex was synthesized by protonolysis of Cr(bpy)[N(SiMe₃)₂]₂ and characterized by single-crystal X-ray diffraction.

Modular radical cross-coupling with sulfones enables access to sp3-rich (fluoro)alkylated scaffolds

Cross-coupling chemistry is widely applied to carbon-carbon bond formation in the synthesis of medicines, agrochemicals, and other functional materials. Recently, single-electron-induced variants of this reaction class have proven particularly useful in the formation of C(sp²)-C(sp³) linkages, although certain compound classes have remained a challenge. Here, we report the use of sulfones to activate the alkyl coupling partner in nickel-catalyzed radical cross-coupling with aryl zinc reagents. This method's tolerance of fluoroalkyl substituents proved particularly advantageous for the streamlined preparation of pharmaceutically oriented fluorinated scaffolds that previously required multiple steps, toxic reagents, and nonmodular retrosynthetic blueprints. Five specific sulfone reagents facilitate the rapid assembly of a vast set of compounds, many of which contain challenging fluorination patterns.