Ruthenium‐Catalyzed CC Bond Cleavage in Lignin Model Substrates

Ruthenium–triphos complexes exhibited unprecedented catalytic activity and selectivity in the redox‐neutral C? C bond cleavage of the β‐O‐4 lignin linkage of 1,3‐dilignol model compounds. A mechanistic pathway involving a dehydrogenation‐initiated retro‐aldol reaction for the C? C bond cleavage was proposed in line with experimental data and DFT calculations.     

Protection Group Effects During α,γ‐Diol Lignin Stabilization Promote High‐Selectivity Monomer Production

Protection groups were introduced during biomass pretreatment to stabilize lignin's α,γ‐diol group during its extraction and prevent its condensation. Acetaldehyde and propionaldehyde stabilized the α,γ‐diol without any aromatic ring alkylation, which significantly increased final product selectivity. The subsequent hydrogenolysis catalyzed by Pd/C generated lignin monomers at near‐theoretical yields based on Klason lignin (48 % from birch, 20 % from spruce, 70 % from high‐syringyl transgenic poplar), and with high selectivity to a single 4‐n‐propanolsyringol product (80 %) in the case of the poplar. Unlike direct hydrogenation of native wood, hydrogenolysis of protected lignin with Ni/C also led to high selectivity to this single product (78 %), paving the way to high‐selectivity lignin upgrading with base metal catalysts. The use of extracted lignin facilitated valorization of polysaccharides, leading to high yields of all three major biomass polymers to a single major product.     

Nickel‐Catalyzed Enantioselective CC Bond Formation through CO Cleavage in Aryl Esters

We report the first enantioselective C? C bond formation through C? O bond cleavage using aryl ester counterparts. This method is characterized by its wide substrate scope and results in the formation of quaternary stereogenic centers with high yields and asymmetric induction.     

Lewis Acid Mediated Tandem Reaction of Propargylic Alcohols to Tetrazoles Involving CO‐ and CC‐Bond Cleavage Reactions and a CN‐Bond Formation

A novel and direct synthesis of 1‐aryl‐5‐arylvinyl‐tetrazoles from easily prepared propargylic alcohols and TMSN₃ is developed in the presence of TMSCl under mild conditions (TMS=trimethylsilyl). The process involves an allenylazide intermediate, followed by a C?C‐bond cleavage and C?N‐bond formation to afford the desired products. Moreover, this method offers a good functional‐group applicability and can be scaled‐up to grams (yield up to 85 %).     

Peracetic Acid Depolymerization of Biorefinery Lignin for Production of Selective Monomeric Phenolic Compounds

Lignin is the largest source of renewable material with an aromatic skeleton. However, due to the recalcitrant and heterogeneous nature of the lignin polymer, it has been a challenge to effectively depolymerize lignin and produce high‐value chemicals with high selectivity. In this study, a highly efficient lignin‐to‐monomeric phenolic compounds (MPC) conversion method based on peracetic acid (PAA) treatment was reported. PAA treatment of two biorefinery lignin samples, diluted acid pretreated corn stover lignin (DACSL) and steam exploded spruce lignin (SESPL), led to complete solubilization and production of selective hydroxylated monomeric phenolic compounds (MPC‐H) and monomeric phenolic acid compounds (MPC‐A) including 4‐hydroxy‐2‐methoxyphenol, p‐hydroxybenzoic acid, vanillic acid, syringic acid, and 3,4‐dihydroxybenzoic acid. The maximized MPC yields obtained were 18 and 22 % based on the initial weight of the lignin in SESPL and DACSL, respectively. However, we found that the addition of niobium pentoxide catalyst to PAA treatment of lignin can significantly improve the MPC yields up to 47 %. The key reaction steps and main mechanisms involved in this new lignin‐to‐MPC valorization pathway were investigated and elucidated.     

Cuprous Oxide Catalyzed Oxidative CC Bond Cleavage for CN Bond Formation: Synthesis of Cyclic Imides from Ketones and Amines

Selective oxidative cleavage of a C? C bond offers a straightforward method to functionalize organic skeletons. Reported herein is the oxidative C? C bond cleavage of ketone for C? N bond formation over a cuprous oxide catalyst with molecular oxygen as the oxidant. A wide range of ketones and amines are converted into cyclic imides with moderate to excellent yields. In‐depth studies show that both α‐C? H and β‐C? H bonds adjacent to the carbonyl groups are indispensable for the C? C bond cleavage. DFT calculations indicate the reaction is initiated with the oxidation of the α‐C? H bond. Amines lower the activation energy of the C? C bond cleavage, and thus promote the reaction. New insight into the C? C bond cleavage mechanism is presented.     

Copper‐Catalyzed Aerobic Oxidative Inert CC and CN Bond Cleavage: A New Strategy for the Synthesis of Tertiary Amides

A copper‐catalyzed aerobic oxidative amidation reaction of inert C?C bonds with tertiary amines has been developed for the synthesis of tertiary amides, which are significant units in many natural products, pharmaceuticals, and fine chemicals. This method combines C?C bond activation, C?N bond cleavage, and C?H bond oxygenation in a one‐pot protocol, using molecular oxygen as the sole oxidant without any additional ligands.     

Extended Reaction Scope of Thiamine Diphosphate Dependent Cyclohexane‐1,2‐dione Hydrolase: From CC Bond Cleavage to CC Bond Ligation

ThDP‐dependent cyclohexane‐1,2‐dione hydrolase (CDH) catalyzes the C? C bond cleavage of cyclohexane‐1,2‐dione to 6‐oxohexanoate, and the asymmetric benzoin condensation between benzaldehyde and pyruvate. One of the two reactivities of CDH was selectively knocked down by mutation experiments. CDH‐H28A is much less able to catalyze the C? C bond formation, while the ability for C? C bond cleavage is still intact. The double variant CDH‐H28A/N484A shows the opposite behavior and catalyzes the addition of pyruvate to cyclohexane‐1,2‐dione, resulting in the formation of a tertiary alcohol. Several acyloins of tertiary alcohols are formed with 54–94 % enantiomeric excess. In addition to pyruvate, methyl pyruvate and butane‐2,3‐dione are alternative donor substrates for C? C bond formation. Thus, the very rare aldehyde–ketone cross‐benzoin reaction has been solved by design of an enzyme variant.     

Rhodium(I)‐Catalyzed Decarbonylative Spirocyclization through CC Bond Cleavage of Benzocyclobutenones: An Efficient Approach to Functionalized Spirocycles

The rhodium‐catalyzed formation of all‐carbon spirocenters involves a decarbonylative coupling of trisubstituted cyclic olefins and benzocyclobutenones through C? C activation. The metal–ligand combination [{Rh(CO)₂Cl}₂]/P(C₆F₅)₃ catalyzed this transformation most efficiently. A range of diverse spirocycles were synthesized in good to excellent yields and many sensitive functional groups were tolerated. A mechanistic study supports a hydrogen‐transfer process that occurs through a β‐H elimination/decarbonylation pathway.     

Selective Functionalization of P4 by Metal‐Mediated CP Bond Formation

A new and selective one‐step synthesis was developed for the first activation stage of white phosphorus by organic radicals. The reactions of NaCp^R with P₄ in the presence of CuX or FeBr₃ leads to the clean formation of organic substituted P₄ butterfly compounds Cp^R₂P₄ (Cp^R: Cp^BIG=C₅(4‐nBuC₆H₄)₅ ( 1 a ), Cp′′′=C₅H₂tBu₃ ( 1 b ), Cp*=C₅Me₅ ( 1 c ) und Cp^4iPr=C₅HiPr₄ ( 1 d )). The reaction proceeds via the activation of P₄ by Cp^R radicals mediated by transition metals. The newly formed organic derivatives of P₄ have been comprehensively characterized by NMR spectroscopy and X‐ray crystallography.     

Rhodium(III)‐Catalyzed CC and CO Coupling of Quinoline N‐Oxides with Alkynes: Combination of CH Activation with O‐Atom Transfer

[Cp*Rh^III]‐catalyzed C? H activation of arenes assisted by an oxidizing N? O or N? N directing group has allowed the construction of a number of hetercycles. In contrast, a polar N? O bond is well‐known to undergo O‐atom transfer (OAT) to alkynes. Despite the liability of N? O bonds in both C? H activation and OAT, these two important areas evolved separately. In this report, [Cp*Rh^III] catalysts integrate both areas in an efficient redox‐neutral coupling of quinoline N‐oxides with alkynes to afford α‐(8‐quinolyl)acetophenones. In this process the N? O bond acts as both a directing group for C? H activation and as an O‐atom donor.     

Heterogeneously Porous γ‐MnO2‐Catalyzed Direct Oxidative Amination of Benzoxazole through CH Activation in the Presence of O2

Oxidative amination of azoles through catalytic C? H bond activation is a very important reaction due to the presence of 2‐aminoazoles in several biologically active compounds. However, most of the reported methods are performed under homogeneous reaction conditions using excess reagents and additives. Herein, we report the heterogeneous, porous γ‐MnO₂‐catalyzed direct amination of benzoxazole with wide range of primary and secondary amines. The amination was carried under mild reaction conditions and using molecular oxygen as a green oxidant, without any additives. The catalyst can easily be separated by filtration and reused several times without a significant loss of its catalytic performance. Of note, the reaction tolerates a functional group such as alcohol, thus indicating the broad applicability of this reaction.     

Synthesis of Silaphenalenes by Ruthenium‐Catalyzed Annulation between 1‐Naphthylsilanes and Internal Alkynes through CH Bond Cleavage

Ruthenium‐catalyzed annulation of 1‐naphthylsilanes with internal alkynes afforded silaphenalenes through cleavage of the C?H bond at the 8‐position of the naphthalene. [RuH₂(CO){P(p‐FC₆H₄)₃}₃] efficiently catalyzed the reaction. The use of 1‐naphthyldiphenylsilane as a substrate resulted in a better yield of the annulation product compared to the use of silanes with alkyl groups on the silicon atom. Internal alkynes with both aryl and alkyl groups were tolerated in this reaction.     

Rhodium‐Catalyzed ortho‐Selective CF Bond Borylation of Polyfluoroarenes with BpinBpin

An ortho‐selective C? F bond borylation between N‐heterocycle‐substituted polyfluoroarenes and Bpin‐Bpin with simple and commercially available [Rh(cod)₂]BF₄ as a catalyst is now reported. The reaction proceeds under mild reaction conditions with high efficiency and broad substrate scope, even toward monofluoroarene, thus providing a facile access to a wide range of borylated fluoroarenes that are useful for photoelectronic materials. Preliminary mechanistic studies reveal that a Rh^III/V catalytic cycle via a key intermediate rhodium(III) hydride complex [(H)Rh^IIIL_n(Bpin)] may be involved in the reaction.     

Zinc‐Catalyzed Alkyne Oxidation/CH Functionalization: Highly Site‐Selective Synthesis of Versatile Isoquinolones and β‐Carbolines

An efficient zinc(II)‐catalyzed alkyne oxidation/C? H functionalization sequence was developed, thus leading to highly site‐selective synthesis of a variety of isoquinolones and β‐carbolines. Importantly, in contrast to the well‐established gold‐catalyzed intermolecular alkyne oxidation, over‐oxidation can be completely suppressed in this system and the reaction most likely proceeds by a Friedel–Crafts‐type pathway. Mechanistic studies and theoretical calculations are described.     

Palladium‐Catalyzed Arylation of Olefins by Triarylphosphines via CP Bond Cleavage

C? H Arylation of olefins by triarylphosphines via C? P bond cleavage has been achieved with either Pd⁰ or Pd^II catalysts. A variety of olefins and triarylphosphines are tolerated, and we inferred that both Pd⁰ and Pd^II could function directly without pre‐oxidation or pre‐reduction.     

Nickel‐Catalyzed Chemo‐, Regio‐ and Diastereoselective Bond Formation through Proximal CC Cleavage of Benzocyclobutenones

The first catalytic intermolecular proximal C1? C2 cleavage of benzocyclobutenones (BCB) without prior carbonyl activation or employing noble metals has been developed. This protocol operates at room temperature and is characterized by an exquisite chemo‐, regio‐ and diastereoselectivity profile, constituting a unique platform for preparing an array of elusive carbocyclic skeletons.     

The Mechanism of NO Bond Cleavage in Rhodium‐Catalyzed CH Bond Functionalization of Quinoline N‐oxides with Alkynes: A Computational Study

Metal‐catalyzed C?H activation not only offers important strategies to construct new bonds, it also allows the merge of important research areas. When quinoline N‐oxide is used as an arene source in C?H activation studies, the N?O bond can act as a directing group as well as an O‐atom donor. The newly reported density functional theory method, M11L, has been used to elucidate the mechanistic details of the coupling between quinoline N?O bond and alkynes, which results in C?H activation and O‐atom transfer. The computational results indicated that the most favorable pathway involves an electrophilic deprotonation, an insertion of an acetylene group into a Rh?C bond, a reductive elimination to form an oxazinoquinolinium‐coordinated Rh^I intermediate, an oxidative addition to break the N?O bond, and a protonation reaction to regenerate the active catalyst. The regioselectivity of the reaction has also been studied by using prop‐1‐yn‐1‐ylbenzene as a model unsymmetrical substrate. Theoretical calculations suggested that 1‐phenyl‐2‐quinolinylpropanone would be the major product because of better conjugation between the phenyl group and enolate moiety in the corresponding transition state of the regioselectivity‐determining step. These calculated data are consistent with the experimental observations.     

Double CF Bond Activation through β‐Fluorine Elimination: Nickel‐Mediated [3+2] Cycloaddition of 2‐Trifluoromethyl‐1‐alkenes with Alkynes

The nickel‐mediated [3+2] cycloaddition of 2‐trifluoromethyl‐1‐alkenes with alkynes afforded fluorine‐containing multi‐substituted cyclopentadienes in a regioselective manner. This reaction involves the consecutive two C? F bond cleavage of a trifluoromethyl or a pentafluoroethyl group through β‐fluorine elimination.