Bioinspired desaturation of alcohols enabled by photoredox proton-coupled electron transfer and cobalt dual catalysis

In the biosynthesis sterols an enzyme-catalyzed demethylation is achieved via a stepwise oxidative transformation of alcohols to olefins. The overall demethylation proceeds through two sequential monooxygenation reactions and a subsequent dehydroformylative saturation. To mimic the desaturation processes observed in nature, we have successfully integrated photoredox proton-coupled electron transfer (PCET) and cobaloxime chemistry for the acceptorless dehydrogenation of alcohols. The state-of-the-art remote and precise desaturation of ketones proceeds efficiently through the activation of cyclic alcohols using bond-dissociation free energy (BDFE) as thermodynamic driving force. The resulting transient alkoxyl radical allows C-C bond scission to generate the carbon-centered radical remote to the carbonyl moiety. This key intermediate is subsequently combined with cobaloxime photochemistry to furnish the alkene. Moreover, the mild protocol can be extended to desaturation of linear alcohols as well as aromatic hydrocarbons. Application to bioactive molecules and natural product derivatives is also presented.

Dehydrogenative reactions can provide alkenes, which are among the most useful handles for synthetic organic chemists. Here the authors integrated photoredox proton-coupled electron transfer and cobaloxime chemistry for the acceptorless dehydrogenation of alkyl alcohols.

Photoredox/nickel dual catalyzed stereospecific synthesis of distal alkenyl ketones

The selective C-C bond deconstruction/refunctionalization via a photoredox/nickel dual-catalyzed hydroalkylation of alkynes is developed under mild reaction conditions. In this protocol, a broad range of alkyl- and aryl-alkynes could react smoothly with cycloalkanols, affording the corresponding distal and site-specific vinyl-substituted ketones with high yields and excellent regioselectivities. Moreover, DFT calculations verified that the electron-rich behavior of aromatics and weak Brønsted bases have a common effect on the photocatalytic oxidant ring-opening of cyclobutanols.

Photoinduced Ligand-to-Metal Charge Transfer (LMCT) of Fe Alkoxide Enabled C-C Bond Cleavage and Amination of Unstrained Cyclic Alcohols

We report an alkoxy radical process for the C-C bond cleavage and functionalization of unstrained tertiary and secondary cyclic alcohols. In the absence of a chlorine atom, the readily available iron catalysts [Fe(OBu-t)₃ or Fe(acac)₃/t-BuONa] facilitate alkoxy radical formation via the direct ligand-to-metal charge transfer of Fe alkoxide and further enable the ring opening and amination of cyclic alcohols. The remote amino carbonyl compounds could be obtained with a broad scope in up to excellent yields under the mildly redox-neutral system. Light-driven electron transfer, alkoxy radical formation, and subsequent C-C bond cleavage via β-scission were the keys to the transformation.

Cerium-Oxo clusters for photocatalytic aerobic oxygenation of sulfides to sulfoxides

Two cerium-oxo clusters (COCs) 1 and 2 are constructed by self-assembly of cerium ions and carboxylate ligands. Both clusters feature spherical structures resembling the key moiety of fluorite phase CeO2,...

Recent Applications of Rare Earth Complexes in Photoredox Catalysis for Organic Synthesis

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In recent years, photoredox catalysis has appeared as a new paradigm for forging a wide range of chemical bonds under mild conditions using abundant reagents. This approach allows many organic transformations through the generation of various radical species, enabling the valorization of non-traditional partners. A continuing interest has been devoted to the discovery of novel radical-generating procedures. Over the last ten years, strategies using rare-earth complexes as either redox-active centers or as redox-neutral Lewis acids have emerged. This review provides an overview of the recent accomplishments made in this field. It especially aims to demonstrate the utility of rare-earth complexes for ensuring photocatalytic transformations and to inspire future developments.

Photoredox/Persistent Radical Cation Dual Catalysis for Alkoxy Radical Generation from Alcohols

In this report, we present a mild and general strategy for the direct generation of alkoxy radical from simple aliphatic alcohols enabled by visible-light-induced photoredox/persistent radical cation dual catalysis. The...

Visible-light-promoted defluorinative ring-opening gem-difluoroallylation of cycloalkanols using 1-trifluoromethyl alkenes

A photocatalytic redox-neutral defluorinative ring-opening coupling of cycloalkanols with α-trifluoromethyl alkenes for the synthesis of gem-difluoroalkenes bearing a distal carbonyl group was developed.

Lewis-Acid-Catalyzed Tandem Cyclization by Ring Expansion of Tertiary Cycloalkanols with Propargyl Alcohols

A new method for the efficient synthesis of hexahydro-1H-fluorene and octahydrobenzo[a]azulene derivatives through a ring-expansion strategy is reported. With an appropriate combination of thulium(III) trifluoromethanesulfonate and 13X molecular sieves, a range of unsaturated polycyclic compounds were obtained in good yields. Mechanism studies reveal that the reaction is more likely to undergo Meyer-Schuster rearrangement, ring expansion, and Friedel-Crafts-type pathways, which provide a conceptually different strategy for the ring opening of tertiary cycloalkanols.

Iron-Catalyzed Photoinduced Remote C(sp3)-H Amination of Free Alcohols

We report a general photocatalytic protocol for the remote C(sp³)-H bond amination of free aliphatic alcohols. The electron transfer between the abundant and inexpensive catalyst FeCl₃ and simple alkanols under blue LED irradiation enables the alkoxy radical formation under mild redox-neutral conditions, with no need for additional oxidant and prefunctionalization. The subsequent selective 1,5-hydrogen atom transfer (HAT) and amination provide a simple and efficient way to access molecular complexity from readily available and bulk alcohols.

Iron-Catalyzed C-C Single-Bond Cleavage of Alcohols

An iron-catalyzed deconstruction/hydrogenation reaction of alcohols through C-C bond cleavage is developed through photocatalysis, to produce ketones or aldehydes as the products. Tertiary, secondary, and primary alcohols bearing a wide range of substituents are suitable substrates. Complex natural alcohols can also perform the transformation selectively. A investigation of the mechanism reveals a procedure that involves chlorine radical improved O-H homolysis, with the assistance of 2,4,6-collidine.

Alkoxy Radicals See the Light: New Paradigms of Photochemical Synthesis

Alkoxy radicals are highly reactive species that have long been recognized as versatile intermediates in organic synthesis. However, their development has long been impeded due to a lack of convenient methods for their generation. Thanks to advances in photoredox catalysis, enabling facile access to alkoxy radicals from bench-stable precursors and free alcohols under mild conditions, research interest in this field has been renewed. This review comprehensively summarizes the recent progress in alkoxy radical-mediated transformations under visible light irradiation. Elementary steps for alkoxy radical generation from either radical precursors or free alcohols are central to reaction development; thus, each section is categorized and discussed accordingly. Throughout this review, we have focused on the different mechanisms of alkoxy radical generation as well as their impact on synthetic utilizations. Notably, the catalytic generation of alkoxy radicals from abundant alcohols is still in the early stage, providing intriguing opportunities to exploit alkoxy radicals for diverse synthetic paradigms.

Recent Advances in Photoredox-Mediated Radical Conjugate Addition Reactions: An Expanding Toolkit for the Giese Reaction

Photomediated Giese reactions are at the forefront of radical chemistry, much like the classical tin-mediated Giese reactions were nearly forty years ago. With the global recognition of organometallic photocatalysts for the mild and tunable generation of carbon-centered radicals, chemists have developed a torrent of strategies to form previously inaccessible radical intermediates that are capable of engaging in intermolecular conjugate addition reactions. This Review summarizes advances in photoredox-mediated Giese reactions since 2013, with a focus on the breadth of methods that provide access to crucial carbon-centered radical intermediates that can engage in radical conjugate addition processes.

Visible-Light-Induced Homolysis of Earth-Abundant Metal-Substrate Complexes: A Complementary Activation Strategy in Photoredox Catalysis

The mainstream applications of visible-light photoredox catalysis predominately involve outer-sphere single-electron transfer (SET) or energy transfer (EnT) processes of precious metal RuII or IrIII complexes or of organic dyes with low photostability. Earth-abundant metal-based Mn Ln -type (M=metal, Ln =polydentate ligands) complexes are rapidly evolving as alternative photocatalysts as they offer not only economic and ecological advantages but also access to the complementary inner-sphere mechanistic modes, thereby transcending their inherent limitations of ultrashort excited-state lifetimes for use as effective photocatalysts. The generic process, termed visible-light-induced homolysis (VLIH), entails the formation of suitable light-absorbing ligated metal-substrate complexes (Mn Ln -Z; Z=substrate) that can undergo homolytic cleavage to generate Mn-1 Ln and Z. for further transformations.

Reduction of Amides to Amines with Pinacolborane Catalyzed by Heterogeneous Lanthanum Catalyst La(CH2C6H4NMe2-o)3@SBA-15

Hydroboration of amides is a useful synthetic strategy to access the corresponding amines. In this contribution, it was found that the supported lanthanum benzyl material La(CH₂C₆H₄NMe₂-o)₃@SBA-15 was highly active for the hydroboration of primary, secondary, and tertiary amides to amines with pinacolborane. These reactions selectively produced target amines and showed good tolerance for functional groups such as -NO₂, -halogen, and -CN, as well as heteroatoms such as S and O. This reduction procedure exhibited the recyclable and reusable property of heterogeneous catalysts and was applicable to gram-scale synthesis. The reaction mechanisms were proposed based on some control experiments and the previous literature. This is the first example of hydroborative reduction of amides to amines mediated by heterogeneous catalysts.

Solvent-controlled photocatalytic divergent cyclization of alkynyl aldehydes: access to cyclopentenones and dihydropyranols

Divergent synthesis is a powerful strategy for the fast assembly of different molecular scaffolds from identical starting materials. We describe here a solvent-controlled photocatalytic divergent cyclization of alkynyl aldehydes with sulfonyl chlorides for the direct construction of highly functionalized cyclopentenones and dihydropyranols that widely exist in bioactive molecules and natural products. Density functional theory calculations suggest that a unique N,N-dimethylacetamide-assisted 1,2-hydrogen transfer of alkoxy radicals is responsible for the cyclopentenone formation, whereas a C–C cleavage accounts for the selective production of dihydropyranols in acetonitrile and water at 50 °C. Given the simple and mild reaction conditions, excellent functional group compatibility, forming up to four chemical bonds, and tunable selectivity, it may find wide applications in synthetic chemistry.

A solvent-controlled photocatalytic divergent cyclization of alkynyl aldehydes is developed, providing a facile access to sulfonylated cyclopentenones and dihydropyranols under mild conditions.

Electrophotochemical Ring-Opening Bromination of tert-Cycloalkanols

An electrophotochemical ring-opening bromination of unstrained tert-cycloalkanols has been developed. This electrophotochemical method enables the oxidative transformation of cycloalkanols with 5- to 7-membered rings into synthetically useful ω-bromoketones without the use of chemical oxidants or transition-metal catalysts. Alkoxy radical species would be key intermediates in the present transformation, which generate through homolysis of the O-Br bond in hypobromite intermediates under visible light irradiation.

Photo-induced oxidative cleavage of C-C double bonds for the synthesis of biaryl methanone via CeCl3 catalysis

A Ce-catalyzed strategy is developed to produce biaryl methanones via photooxidative cleavage of C-C double bonds at room temperature. This reaction is performed under air and demonstrates high activity as well as functional group tolerance. A synergistic Ce/ROH catalytic mechanism is also proposed based on the experimental observations. This protocol should be the first successful Ce-catalyzed photooxidation reaction of olefins with air as the oxidant, which would provide inspiration for the development of novel Ce-catalyzed photochemical synthesis processes.

Copper-catalyzed radical ring-opening halogenation with HX

An efficient copper-catalyzed radical ring-opening halogenation with HX (aq) is described. This protocol features redox-neutral conditions, green halogen sources, and a broad substrate scope, providing practical access to distally chlorinated, brominated and iodinated alkyl ketones and alkyl nitriles with moderate to good yields.

Visible-Light-Driven Selective Air-Oxygenation of C-H Bond via CeCl3 Catalysis in Water

Visible-light-induced C-H aerobic oxidation is an important chemical transformation that can be applied for the synthesis of aromatic ketones. High-cost catalysts and toxic solvents were generally needed in the present methodologies. Here, an efficient aqueous C-H aerobic oxidation protocol was reported. Through CeCl₃ -mediated photocatalysis, a series of aromatic ketones were produced in moderate to excellent yields. With air as the oxidant, this reaction could be performed under mild conditions in water and demonstrated high activity and functional group tolerance. This method is economical, highly efficient, and environmentally friendly, and it will provide inspiration for the development of aqueous photochemical synthesis reactions.

Renaissance of Homogeneous Cerium Catalysts with Unique Ce(IV/III) Couple: Redox-Mediated Organic Transformations Involving Homolysis of Ce(IV)-Ligand Covalent Bonds

Recent advances in the catalytic application of cerium complexes were achieved through controlling the Ce(IV/III) redox couple. Although Ce(IV) complexes have been extensively investigated as stoichiometric oxidants in organic synthesis on the basis of their highly positive redox potentials, these complexes can be used as catalysts, not only by introducing supporting ligands around the coordination sphere of cerium, but also by taking advantage of the photoresponsive properties of Ce(IV) and Ce(III) species. Cerium is highly abundant, comparable to that of some first-row transition metals such as copper, nickel, and zinc. Cerium complexes are new and promising homogeneous catalyst candidates for a variety of organic transformations under mild reaction conditions. They are typically used to activate dioxygen to oxidize organic compounds and applied for organic radical generation using the photoresponsive character of Ce(IV) carboxylates and alkoxides as well as electronic transition of Ce(III), in which homolysis of Ce(IV)-ligand covalent bonds is an important step for the overall catalytic cycle. In this Perspective, we first review the early discovery of Ce(OAc)₄-mediated oxidative transformations to emphasize the importance of Ce(IV)-OAc bond homolysis in various C-C bond-forming reactions and its relation to recent developments. We then focus on the fundamental importance of Ce(IV) reactivity involving thermal and photoassisted homolysis of the Ce(IV)-ligand covalent bond and the developments regarding Ce(IV/III) redox changes in catalytic reactions together with our recent findings on cerium-based catalysis.

Photocatalytic C-H activation and the subtle role of chlorine radical complexation in reactivity

The functionalization of methane, ethane, and other alkanes derived from fossil fuels is a central goal in the chemical enterprise. Recently, a photocatalytic system comprising [Ce^IVCl₅(OR)]^2- [Ce^IV, cerium(IV); OR, -OCH₃ or -OCCl₂CH₃] was disclosed. The system was reportedly capable of alkane activation by alkoxy radicals (RO•) formed by Ce^IV-OR bond photolysis. In this work, we present evidence that the reported carbon-hydrogen (C-H) activation of alkanes is instead mediated by the photocatalyst [NEt₄]₂[CeCl₆] (NEt₄ ⁺, tetraethylammonium), and RO• are not intermediates. Spectroscopic analyses and kinetics were investigated for C-H activation to identify chlorine radical (Cl•) generation as the rate-limiting step. Density functional theory calculations support the formation of [Cl•][alcohol] adducts when alcohols are present, which can manifest a masked RO• character. This result serves as an important cautionary note for interpretation of radical trapping experiments.

Oxidative α-C-C Bond Cleavage of 2° and 3° Alcohols to Aromatic Acids with O2 at Room Temperature via Iron Photocatalysis

The selective α-C-C bond cleavage of unfunctionalized secondary (2°) and tertiary alcohols (3°) is essential for valorization of macromolecules and biopolymers. We developed a blue-light-driven iron catalysis for aerobic oxidation of 2° and 3° alcohols to acids via α-C-C bond cleavages at room temperature. The first example of oxygenation of the simple tertiary alcohols was reported. The iron catalyst and blue light play critical roles to enable the formation of highly reactive O radicals from alcohols and the consequent two α-C-C bond cleavages.

Radical Decarboxylative Carbometalation of Benzoic Acids: A Solution to Aromatic Decarboxylative Fluorination

Abundant aromatic carboxylic acids exist in great structural diversity from nature and synthesis. To date, the synthetically valuable decarboxylative functionalization of benzoic acids is realized mainly by transition-metal-catalyzed decarboxylative cross couplings. However, the high activation barrier for thermal decarboxylative carbometalation that often requires 140 °C reaction temperature limits both the substrate scope as well as the scope of suitable reactions that can sustain such conditions. Numerous reactions, for example, decarboxylative fluorination that is well developed for aliphatic carboxylic acids, are out of reach for the aromatic counterparts with current reaction chemistry. Here, we report a conceptually different approach through a low-barrier photoinduced ligand to metal charge transfer (LMCT)-enabled radical decarboxylative carbometalation strategy, which generates a putative high-valent arylcopper(III) complex, from which versatile facile reductive eliminations can occur. We demonstrate the suitability of our new approach to address previously unrealized general decarboxylative fluorination of benzoic acids.

Halogen-Substituted Allenyl Ketones through Ring Opening of Nonstrained Cycloalkanols

An efficient synthesis of halogen-substituted allenyl ketones via Ag-catalyzed oxidative ring opening of allenyl cyclic alcohols under mild reaction conditions has been achieved. The reaction features a wide substrate scope and excellent regioselectivity. The synthetic potential of the products has been demonstrated by their conversion to stereodefined alkenes and heterocyclic compounds.

Photocatalytic Dehydroxymethylative Arylation by Synergistic Cerium and Nickel Catalysis

Under mild reaction conditions with inexpensive cerium and nickel catalysts, easily accessible free alcohols can now be utilized as operationally simple and robust carbon pronucleophiles in selective C(sp³)-C(sp²) cross-couplings. Facilitated by automated high-throughput experimentation, sterically encumbered benzoate ligands have been identified for robust cerium complexes, enabling the synergistic cooperation of cerium catalysis in the emerging metallaphotoredox catalysis. A broad range of free alcohols and aromatic halides can be facilely employed in this transformation, representing a new paradigm for the C(sp³)-C(sp²) bond construction between free alcohols and aryl halides with the extrusion of formaldehyde. Moreover, mechanistic investigations have been conducted, leading to the identification of a tribenzoate cerium(III) complex as a viable intermediate.

Rapid and scalable synthesis of fluoroketones via cerium-mediated C-C bond cleavage

Ketones with remote fluorination are an important motif in the synthesis of bioactive molecules. Here we demonstrate that ceric ammonium nitrate (CAN) is able to produce this functionality under incredibly mild conditions and short reaction times (30 min) while eliminating the need for precious metals in previous methods. Importantly, this method allows the efficient synthesis of a wide variety of γ-fluoroketones and is highly scalable. Preliminary mechanistic studies suggest this reaction proceeds through a radical pathway.

Oxidizing Cerium(IV) Alkoxide Complexes Supported by the Kläui Ligand [Co(η5-C5H5){P(O)(OEt)2}3]-: Synthesis, Structure, and Redox Reactivity

Tetravalent cerium alkoxide complexes supported by the Kläui tripodal ligand [Co(η⁵-C₅H₅){P(O)(OEt)₂}₃]^- (L_OEt^-) have been synthesized, and their nucleophilic and redox reactivity have been studied. Treatment of the Ce(IV) oxo complex [Ce^IV(L_OEt)₂(O)(H₂O)]·MeCONH₂ (1) with ⁱPrOH or reaction of [Ce^IV(L_OEt)₂Cl₂] (2) with Ag₂O in ⁱPrOH afforded the Ce(IV) dialkoxide complex [Ce^IV(L_OEt)₂(OⁱPr)₂] (3-iPr). The methoxide and ethoxide analogues [Ce^IV(L_OEt)₂(OR)₂] (R = Me (3-Me), Et (3-Et)) have been prepared similarly from 2 and Ag₂O in ROH. Reaction of 3-iPr with an equimolar amount of 2 yielded a new Ce(IV) complex that was formulated as the chloro-alkoxide complex [Ce^IV(L_OEt)₂(OⁱPr)Cl] (4). Treatment of 3-iPr with HX and methyl triflate (MeOTf) afforded [Ce(L_OEt)₂X₂] (X^- = Cl^-, NO₃^-, PhO^-) and [Ce^IV(L_OEt)₂(OTf)₂], respectively, whereas treatment with excess CO₂ in hexane led to isolation of the Ce(IV) carbonate [Ce^IV(L_OEt)₂(CO₃)]. 3-iPr reacted with water in hexane to give a Ce(III) complex and a Ce(IV) species, presumably the reported tetranuclear oxo cluster [Ce^IV₄(L_OEt)₄(O)₅(OH)₂]. The Ce(IV) alkoxide complexes are capable of oxidizing substituted phenols, possibly via a proton-coupled electron transfer pathway. Treatment of 3-iPr with ArOH afforded the Ce(III) aryloxide complexes [Ce^III(L_OEt)₂(OAr)] (Ar = 2,4,6-tri-tert-butylphenyl (5), 2,6-diphenylphenyl (6)). On the other hand, a Ce(III) complex containing a monodeprotonated 2,2'-biphenol ligand, [Ce^III(L_OEt)₂(^tBu₄C₁₂H₄O₂H)] (7) (^tBu₄C₁₂H₄O₂H₂ = 4,4',6,6'-tetra-tert-butyl-2,2'-biphenol), was isolated from the reaction of 3-iPr with 2,4-di-tert-butylphenol. The crystal structures of complexes 3-iPr, 3-Me, 3-Et, and 5-7 have been determined.

Ag-Catalyzed ring-opening of tertiary cycloalkanols for C-H functionalization of cyclic aldimines

We firstly describe a silver-catalyzed direct C-H functionalization of cyclic aldimines with cyclopropanols and cyclobutanols via a radical-mediated C-C bond cleavage strategy. The desired products were generated in decent yields with wide substrate scope under mild reaction conditions. In addition, a gram-scale reaction and synthetic transformation of the product were performed.

Photocatalytic Aerobic Oxidative Ring Expansion of Cyclic Ketones to Macrolactones by Cerium and Cyanoanthracene Catalysis

We describe a cerium-catalyzed aerobic oxidative ring expansion for the expedient construction of synthetically challenging macrolactones under visible-light conditions. Cyanoanthracene has been employed as co-catalyst to accelerate the turnover of the cerium cycle leading to a fast conversion within 20 min of irradiation. Taking advantage of the high efficiency and operationally simple conditions, a collection of over 100 macrolactones equipped with ring systems ranging from 9- to 19-membered macrocycles have been prepared from simple building blocks. Moreover, the enabling potential of this strategy to simplify the generation of molecular complexity has been demonstrated through the concise synthesis of sonnerlactone.

Visible light photocatalysis in the late-stage functionalization of pharmaceutically relevant compounds

The late stage functionalization (LSF) of complex biorelevant compounds is a powerful tool to speed up the identification of structure-activity relationships (SARs) and to optimize ADME profiles. To this end, visible-light photocatalysis offers unique opportunities to achieve smooth and clean functionalization of drugs by unlocking site-specific reactivities under generally mild reaction conditions. This review offers a critical assessment of current literature, pointing out the recent developments in the field while emphasizing the expected future progress and potential applications. Along with paragraphs discussing the visible-light photocatalytic synthetic protocols so far available for LSF of drugs and drug candidates, useful and readily accessible synoptic tables of such transformations, divided by functional groups, will be provided, thus enabling a useful, fast, and easy reference to them.

Catalytic generation of alkoxy radicals from unfunctionalized alcohols

Alkoxy radicals have long been recognized as powerful synthetic intermediates with well-established reactivity patterns. Due to the high bond dissociation free energy of aliphatic alcohol O-H bonds, these radicals are difficult to access through direct homolysis, and conventional methods have instead relied on activation of O-functionalized precursors. Over the past decade, however, numerous catalytic methods for the direct generation of alkoxy radicals from simple alcohol starting materials have emerged and created opportunities for the development of new transformations. This minireview discusses recent advances in catalytic alkoxy radical generation, with particular emphasis on progress toward the direct activation of unfunctionalized alcohols enabled by transition metal and photoredox catalysis.

Transition Metal-Mediated C-C Single Bond Cleavage: Making the Cut in Total Synthesis

Transition-metal-mediated cleavage of C-C single bonds can enable entirely new retrosynthetic disconnections in the total synthesis of natural products. Given that C-C bond cleavage inherently alters the carbon framework of a compound, and that, under transition-metal catalysis, the generated organometallic or radical intermediate is primed for further complexity-building reactivity, C-C bond-cleavage events have the potential to drastically and rapidly remodel skeletal frameworks. The recent acceleration of the use of transition-metal-mediated cleavage of C-C single bonds in total synthesis can be ascribed to a communal recognition of this fact. In this Review, we highlight ten selected total syntheses from 2014 to 2019 that illustrate how transition-metal-mediated cleavage of C-C single bonds at either the core or the periphery of synthetic intermediates can streamline synthetic efforts.

Cleavage of Carbon-Carbon σ-Bonds of Four-Membered Rings

This article reviews synthetic transformations involving cleavage of a carbon-carbon bond of a four-membered ring, with a particular focus on the examples reported during the period from 2011 to the end of 2019. Most significant is the progress of catalytic reactions involving oxidative addition of carbon-carbon bonds onto transition metals or β-carbon elimination of transition metal alkoxides. When they are looked at from synthetic perspectives, they offer unique and efficient methods to build complex natural products and structures that are difficult to construct by conventional methods. On the other hand, β-scission of radical intermediates has also attracted increasing attention as an alternative elementary step to cleave carbon-carbon bonds. Its site-selectivity is often complementary to that of transition metal-catalyzed reactions. In addition, Lewis acid-mediated and thermally induced ring-opening of cyclobutanone derivatives has garnered renewed attention. On the whole, these examples demonstrate unique synthetic potentials of structurally strained four-membered ring compounds for the construction of organic skeletons.

C-C Bond Cleavage Approach to Complex Terpenoids: Development of a Unified Total Synthesis of the Phomactins

The rearrangement of carbon-carbon (C-C) single bonds in readily available carbocyclic scaffolds can yield uniquely substituted carbocycles that would be challenging to construct otherwise. This is a powerful and often non-intuitive approach for complex molecule synthesis. The transition-metal-mediated cleavage of C-C bonds has the potential to broaden the scope of this type of skeletal remodeling by providing orthogonal selectivities compared to more traditional pericyclic and carbocation-based rearrangements. To highlight this emerging technology, a unified, asymmetric, total synthesis of the phomactin terpenoids was developed, enabled by the selective C-C bond cleavage of hydroxylated pinene derivatives obtained from carvone. In this full account, the challenges, solutions, and intricacies of Rh(I)-catalyzed cyclobutanol C-C cleavage in a complex molecule setting are described. In addition, details of the evolution of strategies that ultimately led to the total synthesis of phomactins A, K, P, R, and T, as well as the synthesis and structural reassignment of Sch 49027, are given.

Photo-mediated selective deconstructive geminal dihalogenation of trisubstituted alkenes

Selective deconstructive functionalization of alkenes, other than the well-established olefin metathesis and ozonolysis, to produce densely functionalized molecular scaffolds is highly attractive but challenging. Here we report an efficient photo-mediated deconstructive germinal dihalogenation of carbon-carbon double bonds. A wide range of geminal diiodoalkanes and bromo(iodo)alkanes (>40 examples) are directly prepared from various trisubstituted alkenes, including both cyclic and acyclic olefins. This C=C cleavage is highly chemoselective and produces geminal dihalide ketones in good yields. Mechanistic investigations suggest a formation of alkyl hypoiodites from benzyl alcohols and N-iodoimides, which undergo light-induced homolytic cleavage to generate active oxygen radical species.

Efficient synthetic pathways to geminal dihalides are quite limited, despite their versatility as chemical building blocks. Here, the authors report a photo-mediated deconstructive fragmentation of cyclic and acyclic trisubstituted alkenes to access a variety of geminal dihalides.

DMSO-Enabled Selective Radical O-H Activation of 1,3(4)-Diols

Control of selectivity is one of the central topics in organic chemistry. Although unprecedented alkoxyl-radical-induced transformations have drawn a lot of attention, compared to selective C-H activation, selective radical O-H activation remains less explored. Herein, we report a novel selective radical O-H activation strategy of diols by combining spatial effects with proton-coupled electron transfer (PCET). It was found that DMSO is an essential reagent that enables the regioselective transformation of diols. Mechanistic studies indicated the existence of the alkoxyl radical and the selective interaction between DMSO and hydroxyl groups. Moreover, the distal C-C cleavage was realized by this selective alkoxyl-radical-initiation protocol.