Metal-mediated cleavage of unsaturated C-C bonds

Enchanting Realm of Five-Membered Rare-Earth Metallacycles

ConspectusMetallacycles, derivatives of carbocyclic compounds wherein a metal atom replaces at least one carbon center, have been a constant powerhouse in organic synthesis. While metallacycles of main-group, transition, and actinide metals have been extensively studied, those incorporating rare-earth (RE) elements (Sc, Y, and lanthanides) have remained elusive primarily due to synthetic challenges. Nevertheless, the electropositive character of these elements and the resulting polarization of RE-C bonds, along with the intrinsic synergistic effects within metallacycles, endow RE metallacycles with unique properties and a rich, yet largely untapped, reaction chemistry. In this Account, we present the development and applications of five-membered RE metallacycles.Over the past decade, we have successfully synthesized a variety of five-membered all-carbon rare-earth metallacycles using two key synthetic strategies: (i) transmetalation, which has been employed to prepare RE metallacyclopentadienes and spiro-metallacyclopentadienes, which, featuring various ligand systems, provide distinct coordination environments around the RE center, significantly influencing their reactivity, and (ii) transmetalation and reduction, enabling the synthesis of RE spiro-metallacyclopentenes and 2-butene tetraanion (BTA)-bridged dinuclear RE metallacyclopentenes. The reduction process proceeds via either self-disproportionation of metallacyclopentadienes or reduction by divalent RE centers or alkali metals. These metallacycles represent the first instances of such RE-containing metallacyclic ring structures.Our investigations into these metallacycles have uncovered unique reactivities and new reaction modes. The high intrinsic reactivity and multiple reactive sites of rare-earth metallacycles enable them not only to activate small molecules efficiently but also to exhibit distinct activation modes for some small molecules. For instance, reactions of RE metallacyclopentadienes with carbodiimides showcase diverse insertion/rearrangement chemistry, influenced by various factors such as number of equivalents of carbodiimide and the solvent choice. The RE metallacyclopentadiene-mediated [3 + 1] fragmentation of white phosphorus demonstrates an activation mode markedly different from that observed with main-group and transition metal analogs. Moreover, the discovery of cross-carbanion coupling at RE centers and RE-metal-mediated ring-opening metathesis of benzene introduces new reaction modes, demonstrating that, with rational design, RE metals can exhibit properties similar to or even surpassing those of transition metals. These reaction modes have further led to the development of applications for RE metallacycles in synthetic chemistry.Additionally, some novel properties of these rare-earth metallacycles have been uncovered, stemming from their unique geometric and electronic structures. Structural analysis and theoretical calculations have revealed the nonplanar aromaticity of BTA-bridged dinuclear RE metallacyclopentenes, extending the concept of nonplanar aromaticity into the chemistry of carbon-RE metallacycles. Furthermore, benefiting from the redox capabilities of butadiene dianion and BTA ligands, the ligand-based redox chemistry of BTA-bridged dinuclear RE metallacyclopentenes demonstrates diverse and efficient multielectron transfer processes, highlighting the potential of these metallacycles for redox chemistry.The studies of rare-earth metallacycles, encompassing their construction, characterization, properties, reactivity, and synthetic applications, have greatly enriched the field of f-block metallacycles. We hope that this Account will inspire further exploration into the synthesis of new organometallic reagents and metallacycle-mediated transformations, fueling continued progress in rare-earth chemistry.

Molecular Ring Remodeling through C-C Bond Cleavage

ConspectusStable and inert C-C bonds form the fundamental framework of organic compounds. Consequently, direct transformations involving C-C bond cleavage present an innovative approach for the rapid modification and remodeling of molecular skeletons. In recent years, the concept of molecular skeletal editing has garnered widespread attention and has been significantly developed, providing new opportunities for the late-stage modification of bioactive molecules, the high-value transformation of bulk chemicals, and a revolution in the traditional fragment coupling strategies of chemical synthesis. Notable advancements in this field have focused on C-C bond cleavage and the remodeling of cyclic molecules, including ring expansion, ring contraction, and ring-opening reactions, thereby enriching the synthetic toolbox available to chemists. However, selective C-C bond transformation remains a formidable challenge, especially in the remodeling of complex molecules, due to the high bond dissociation energy and the difficulty in achieving precise selectivity control. Over the past few years, our group has made efforts to address these challenges. We have demonstrated the potential of cyclic molecule remodeling reactions as an efficient strategy for the synthesis and modification of complex molecules.Herein, we present two major thematic advancements achieved by our group, utilizing cascade activation and entropy-driven reconstruction strategies for molecular ring remodeling via C-C bond cleavage. These strategies are characterized by mild conditions, the accessibility of catalysts and reagents, and exceptional functional group compatibility, thereby emerging as novel approaches for molecular ring remodeling through atom-incorporation reactions mainly on nitrogenation, oxygenation, and halogenation to synthesize pharmaceuticals, natural products, and material molecules. (1) Ring expansion reactions: We developed novel reactions that enable the insertion of C-, N-, and O-containing units into molecular rings. These methodologies offer practical and efficient routes for synthesizing amides, amines, lactones, and nitrogen-containing heterocycles. (2) Ring-opening reactions: C-C bond cleavage in ring-opening reactions enables the efficient construction of distally difunctionalized molecular frameworks. By utilizing a transition metal catalysis and radical-mediated process, we have successfully achieved the cleavage of both C-C single bonds and C═C double bonds within molecular rings. Furthermore, we have tackled the highly challenging arene ring-opening (ARO) reaction, enabling the construction of stereoselective conjugated systems through the unsaturation liberation of aromatic systems. Mechanistic studies and DFT calculations have provided critical insights into these processes. We have also identified key intermediates involved in C-C bond cleavage, including benzyl azide, O-acetyl hydroxylamine, β-azido peroxyl radical, copper bisnitrene, and 2-nitrene indazole. These findings have deepened our understanding of the mechanisms and the entropy-driven reconstruction strategy, which has further promoted the discovery of related C-C bond transformations of acyclic substrates.

Activation of Molecular Oxygen and Selective Oxidation with Metal Complexes

ConspectusSelective oxidation with molecular oxygen is one of the most appealing approaches to functionalization of organic molecules and, yet at the same time, one of the most challenging reactions facing organic synthesis due to poor selectivity control. Molecular oxygen is a green and inexpensive oxidant, producing water as the only byproduct in oxidation. Not surprisingly, it has been used in the manufacturing of many commodity chemicals in the industry. It is also nature's choice of oxidant and drives a variety of oxidation reactions critical to life and various other biologic processes. While the past decades have witnessed great progress in understanding, both structurally and mechanistically, how nature exploits metalloenzymes, i.e., monooxygenases and dioxygenases, to tackle some of the most challenging oxidation reactions, e.g., methane oxidation to methanol, there are only a small number of well-defined, man-made metal complexes that have been reported to enable selective oxidation with molecular oxygen of compounds more relevant to fine chemical and pharmaceutical synthesis.In the past 10 years or so, our laboratories have developed several transition metal complexes and shown that they are capable of catalyzing selective oxidation under 1 atm of O2. Thus, we have shown that an Fe(II)-bisimidazolidinyl-pyridine complex catalyzes selective oxygenation of C-H bonds in ethers with concomitant release of hydrogen gas instead of water, and when the iron center is replaced with Fe(III), selective oxidative cleavage of C═C bonds of olefins becomes feasible. To address the low activity of the iron complex in oxidizing less active olefins, we have developed a Mn(II)-bipyridine complex, which catalyzes oxidative cleavage of C═C bonds in aliphatic olefins, C-C bonds in diols, and carboxyl units in carboxylic acids under visible light irradiation. Light is necessary in the oxidation to cleave an off-cycle, inactive manganese dimer into a catalytically active Mn═O oxo species. Furthermore, we have found that a binuclear salicylate-bridged Cu(II) complex enables the C-H oxidation of tetrahydroisoquinolines as well as C═C bond cleavage, and when a catalytic vitamin B1 analogue is brought in, oxygenation of tetrahydroisoquinolines to lactams takes place via carbene catalysis. Still further, we have found that a readily accessible binuclear Rh(II)-terpyridine complex catalyzes the oxidation of alcohols, and being water-soluble, the catalyst can be easily separated and reused multiple times. In addition, we recently unearthed a simple protocol that allows waste polystyrene to be depolymerized to isolable, valuable chemicals. A cheap Brønsted acid acts as the catalyst, activating molecular oxygen to a singlet state through complexation with the polymer under light irradiation, thereby depolymerizing the polymer.

Rare-Earth Metal-Enabled Ring-Opening Metathesis of Benzene

Activation and transformation of inert C-C bonds within arenes are challenging and important topics in synthetic chemistry. While there have been some reports on the activation of C-C bonds in arene rings, the realm of metathesis reactions involving arene C-C bonds remains unexplored. Here, we report a rare-earth metal-enabled intramolecular metathesis reaction of one benzene C-C bond and another C-C single bond, assisted by the high reactivity and unique synergistic effect of rare-earth metallacycles. Mechanistic studies disclose an intriguing pathway that forms a fused tricyclic intermediate bearing a 4-membered ring through stepwise [2 + 2] cycloaddition, followed by stepwise [2 + 2] cycloreversion. The distinct reaction discovered here extends the reactivity of arenes and is expected to inspire the development of aromatic ring-opening metathesis reactions.

Carbene-Assisted Arene Ring-Opening

Despite the significant achievements in dearomatization and C-H functionalization of arenes, the arene ring-opening remains a largely unmet challenge and is underdeveloped due to the high bond dissociation energy and strong resonance stabilization energy inherent in aromatic compounds. Herein, we demonstrate a novel carbene assisted strategy for arene ring-opening. The understanding of the mechanism by our DFT calculations will stimulate wide application of bulk arene chemicals for the synthesis of value-added polyconjugated chain molecules. Various aryl azide derivatives now can be directly converted into valuable polyconjugated enynes, avoiding traditional synthesis including multistep unsaturated precursors, poor selectivity control, and subsequent transition-metal catalyzed cross-coupling reactions. The simple conditions required were demonstrated in the late-stage modification of complex molecules and fused ring compounds. This chemistry expands the horizons of carbene chemistry and provides a novel pathway for arene ring-opening.

Catalytic Upgrading of a Mixed Hydroxy Acid Feedstock Derived from Kraft Black Liquor

Lignocellulosic feedstocks are widely studied for sustainable liquid fuel and chemical production. The pulp and paper industry generates large amounts of kraft black liquor (BL) from which a high volume of hydroxy acids (HAs) can be separated for further catalytic processing. Here, we explore the catalytic upgrading of HAs, including the conversion of (1) a model HA, gluconic acid; (2) a model mixture of HAs, and (3) a real mixture of HAs derived from kraft BL on M/Nb2O5 (M = Pd, Pt, Rh, and Ru). The hydrodeoxygenation of model gluconic acid reveals that "volatile" carboxylic acids (mainly C2 and C3), levulinic acid, and cyclic esters are significant products over all the catalysts, with Pd/Nb2O5 showing superior activity and selectivity toward valuable intermediates. The model mixture of HAs shows a wide range of reactivity over the supported metal catalyst, with the product selectivity strongly correlating to reaction temperature. Utilizing a 0.25% Pd/Nb2O5 catalyst, a real mixture of HAs derived from kraft BL is successfully dehydroxylated to produce a mixture rich in C3-C8 carboxylic acids that may be amenable for further upgrading, e.g., catalytically to ketones with high carbon chain lengths. Despite the feedstock complexity, we selectively cleaved the C-OH bonds of HAs, while successfully preserving most of the -COOH groups and minimizing C-C and C=O bond scission reactions under the operating conditions tested. The BL-derived HA stream is thus proposed to be a suitable platform for producing mixed carboxylic acid products from an overoxygenated byproduct feed.

Reversible Boron-Insertion into Aromatic C-C Bonds

Formation of borabicyclo[3.2.0]heptadiene derivatives was achieved via boron-insertion into aromatic C-C bonds in the photo-promoted skeletal rearrangement reaction of triarylboranes bearing an ortho-phosphino substituent (ambiphilic phosphine-boranes). The borabicyclo[3.2.0]heptadiene derivatives were fully characterized by NMR and X-ray analyses. The dearomatized products were demonstrated to undergo the reverse reaction in the dark at room temperature, realizing photochemical and thermal interconversion between triarylboranes and boron-doped bicyclic systems. Experimental and theoretical studies revealed that sequential two electrocyclic reactions involving E/Z-isomerization of an alkene moiety proceed via a highly strained trans-borepin intermediate.

Swift C-C bond insertion by a 12-electron palladium(0) surrogate

The selective activation of C-C bonds holds vast promise for catalysis. So far, research has been primarily directed at rhodium and nickel under harsh reaction conditions. Herein, we report C-C insertion reactions of a 12-electron palladium(0) surrogate stabilized by a cyclic(alkyl)(amino) carbene (CAAC) ligand. Benzonitrile (1), biphenylene (2), benzocyclobutenone (3), and naphtho[b]cyclopropene (4) were studied. These substrates allow elucidation of the effect of ring strain as well as hybridization encompassing sp3, sp2 and sp hybridized carbon atoms. All reactions proceed quantitatively at or below room temperature. This work therefore outlines perspectives for mild C-C bond functionalization catalysis.

Aerobic oxidative C-C bond cleavage and functionalization for the synthesis of value-added chemicals

The aerobic oxidative cleavage of C-C bonds is an attractive and sustainable route for constructing valuable molecules such as esters, nitriles, and amides. Traditionally homogeneous catalytic systems for C-C bond cleavage required harsh conditions, stoichiometric oxidants, and noble metal catalysts to overcome the thermodynamic and kinetic barriers of C-C bonds, imposing environmental concerns of the transformation. Therefore, developing efficient, low-cost, and environmentally benign methods for C-C bond cleavage is of great importance and a cutting-edge area in modern chemistry. This feature article summarizes the sustainable aerobic oxidative C-C bond cleavage method developed by our group in the past 5 years. Fundamental principles in catalyst design, substrate scope, and mechanism for C-C bond cleavage are also discussed.

Breaking bonds and breaking rules: inert-bond activation by [(iPr3P)Ni]5H4 and catalytic stereospecific norbornene dimerization

The facile carbon atom abstraction reaction by [(ⁱPr₃P)Ni]₅H₆ (1) with various terminal alkenes to give [(ⁱPr₃P)Ni]₅H₄(μ₅-C) (2) occurs via a common highly reactive intermediate [(ⁱPr₃P)Ni]₅H₄ (3), which was isolated by the reaction of 1 with norbornene. Temperature dependent ¹H and ³¹P{¹H} NMR chemical shifts of 3 are consistent with a thermally populated triplet excited state only 2 kcal mol^-1 higher energy than the diamagnetic ground state. Complex 3 catalyzes the dimerization of norbornene to stereoselectively provide exclusively (Z) anti-(bis-2,2'-norbornylidene).

Selective Cleavage of the Strong or Weak C-C Bonds in Biphenylene Enabled by Rare-Earth Metals

Selective cleavage of C-C bonds within arene rings is of great interest but remains elusive, especially for the molecules possessing the active and inert C-C bonds. Here, we report that the active and inert C-C bonds of biphenylene could be controllably cleaved by the reaction of biphenylene, potassium graphite, and rare-earth complexes with different metal centers. For scandium, the bond activation occurs at the C_aryl-C_aryl single bond, yielding 9-scandafluorene. For Lu, the reaction goes through ring contraction of the aromatic ring in biphenylene to provide benzopentalene dianionic lutetium. The origin of the selectivity and the reaction mechanism were illustrated by the isolation of intermediates and DFT calculations.

Room Temperature Intermolecular Dearomatization of Arenes by an Acyclic Iminosilylene

A novel nontransient acyclic iminosilylene (1), bearing a bulky super silyl group (-Si^tBu₃) and N-heterocyclic imine ligand with a methylated backbone, was prepared and isolated. The methylated backbone is the feature of 1 that distinguishes it from the previously reported nonisolable iminosilylenes, as it prevents the intramolecular silylene center insertion into an aromatic C-C bond of an aryl substituent. Instead, 1 exhibits an intermolecular Büchner-ring-expansion-type reactivity; the silylene is capable of dearomatization of benzene and its derivatives, giving the corresponding silicon analogs of cycloheptatrienes, i.e. silepins, featuring seven-membered SiC₆ rings with nearly planar geometry. The ring expansion reactions of 1 with benzene and 1,4-bis(trifluoromethyl)benzene are reversible. Similar reactions of 1 with N-heteroarenes (pyridine and DMAP) proceed more rapidly and irreversibly forming the corresponding azasilepins, also with nearly planar seven-membered SiNC₅ rings. DFT calculations reveal an ambiphilic nature of 1 that allows the intermolecular aromatic C-C bond insertion to occur. Additional computational studies, which elucidate the inherent reactivity of 1, the role of the substituent effect, and reaction mechanisms behind the ring expansion transformations, are presented.

PhIO-Mediated Oxidative C═C Bond Cleavage and Reassembly toward Highly Functionalized Oxazolones

An efficient PhIO-mediated oxidative C═C bond cleavage and reassembly of enaminone toward oxazolone with high regioselectivity has been reported. DFT calculations revealed that the reaction proceeded through an oxygen atom transfer, C═C bond cleavage, alkylthio migration, and reassembly cascade. This strategy is highlighted by high atom and step economy with formation of five bonds in one pot and generation of a high-valued oxazolone skeleton under mild conditions.

Copper-catalyzed switchable cyclization of alkyne-tethered α-bromocarbonyls: selective access to quinolin-2-ones and quinoline-2,4-diones

Copper-catalyzed cyclization of alkynes has played a significant role in modern catalytic chemistry.

Supported CuII Single-Ion Catalyst for Total Carbon Utilization of C2 and C3 Biomass-Based Platform Molecules in the N-Formylation of Amines

The shift from fossil carbon sources to renewable ones is vital for developing sustainable chemical processes to produce valuable chemicals. In this work, value‐added formamides were synthesized in good yields by the reaction of amines with C₂ and C₃ biomass‐based platform molecules such as glycolic acid, 1,3‐dihydroxyacetone and glyceraldehyde. These feedstocks were selectively converted by catalysts based on Cu‐containing zeolite 5A through the in situ formation of carbonyl‐containing intermediates. To the best of our knowledge, this is the first example in which all the carbon atoms in biomass‐based feedstocks could be amidated to produce formamide. Combined catalyst characterization results revealed preferably single Cu^II sites on the surface of Cu/5A, some of which form small clusters, but without direct linking via oxygen bridges. By combining the results of electron paramagnetic resonance (EPR) spin‐trapping, operando attenuated total reflection (ATR) IR spectroscopy and control experiments, it was found that the formation of formamides might involve a HCOOH‐like intermediate and ^.NHPh radicals, in which the selective formation of ^.OOH radicals might play a key role.

Palladium-Catalyzed Cascade Carbonylation to α,β-Unsaturated Piperidones via Selective Cleavage of Carbon-Carbon Triple Bonds

A direct and selective synthesis of α,β-unsaturated piperidones by a new palladium-catalyzed cascade carbonylation is described. In the presented protocol, easily available propargylic alcohols react with aliphatic amines to provide a broad variety of interesting heterocycles. Key to the success of this transformation is a remarkable catalytic cleavage of the present carbon-carbon triple bond by using a specific catalyst with 2-diphenylphosphinopyridine as ligand and appropriate reaction conditions. Mechanistic studies and control experiments revealed branched unsaturated acid 11 as crucial intermediate.

Direct Metal-Free Transformation of Alkynes to Nitriles: Computational Evidence for the Precise Reaction Mechanism

Density functional theory calculations elucidated the precise reaction mechanism for the conversion of diphenylacetylenes into benzonitriles involving the cleavage of the triple C≡C bond, with N-iodosuccinimide (NIS) as an oxidant and trimethylsilyl azide (TMSN3) as a nitrogen donor. The reaction requires six steps with the activation barrier ΔG‡ = 33.5 kcal mol-1 and a highly exergonic reaction free-energy ΔGR = -191.9 kcal mol-1 in MeCN. Reaction profiles agree with several experimental observations, offering evidence for the formation of molecular I2, interpreting the necessity to increase the temperature to finalize the reaction, and revealing thermodynamic aspects allowing higher yields for alkynes with para-electron-donating groups. In addition, the proposed mechanism indicates usefulness of this concept for both internal and terminal alkynes, eliminates the option to replace NIS by its Cl- or Br-analogues, and strongly promotes NaN3 as an alternative to TMSN3. Lastly, our results advise increasing the solvent polarity as another route to advance this metal-free strategy towards more efficient processes.

Selective cleavage of unactivated arene ring C-C bonds by iridium: key roles of benzylic C-H activation and metal-metal cooperativity

The cleavage of aromatic C–C bonds is central for conversion of fossil fuels into industrial chemicals and designing novel arene functionalisations through ring opening, expansion and contraction. However, the current progress is hampered by both the lack of experimental examples of selective oxidative addition of aromatic C–C bonds and limited understanding of the factors that favour insertion into the C–C rather than the C–H bonds. Here, we describe the comprehensive mechanism of the only reported chemo- and regioselective insertion of a transition metal into a range of substituted arene rings in simple iridium(i) complexes. The experimental and computational data reveal that this ring cleavage requires both reversible scission of a benzylic C–H bond and cooperativity of two Ir centres sandwiching the arene in the product-determining intermediate. The mechanism explains the chemoselectivity and scope of this unique C–C activation in industrially important methylarenes and provides a general insight into the role of metal–metal cooperativity in the cleavage of unsaturated C–C bonds.

The detailed mechanism of iridium-mediated C–C cleavage in unactivated arenes reveals the key factors enabling the process and helps predict the scope of the cleavage reaction.

Cleavage of the Inert C(sp2)-Ar σ-Bond of Alkenes by a Spatial Constrained Interaction with Phosphinidene

[1 + 2] cycloaddition is a classical reaction between the electrophilic phosphinidene and an alkene. However, a spatial constraint blocks this well-known reaction and enables an unprecedented chemoselective C(sp²)-Ar σ-bond insertion of the alkene. The theoretical calculations demonstrate that this C-C bond cleavage is energetically feasible and thermodynamically favored through an electrophilic rearrangement and concomitant 1,9-aryl migration without involving any strained intermediate.

An organocatalytic C–C bond cleavage approach: a metal-free and peroxide-free facile method for the synthesis of amide derivatives

An organocatalyzed C–C bond cleavage approach has been envisioned towards the synthesis of amide derivatives from their corresponding amines and 1,3-dicarbonyls.