New Directions in Ketene Chemistry: The Land of Opportunity

The lithium effect in ketenyl anion chemistry

Ketenyl lithium compounds of type [RC(Li)CO] (with R = Ph2P(E), E = O, S, Se) were found to exhibit lower thermal stabilities than their potassium analogues due to the stronger coordination of the oxygen of the ketene moiety to the harder metal cation, resulting in a more pronounced ynolate character. Using additional ligands allows manipulation of the O-Li interaction, thereby influencing the stability and reactivity of the ketenyl anions.

Mechanistic Rationale for Ketene Formation during Dabbing and Vaping

Ketene is one of the most toxic vaping emissions identified to date. However, its high reactivity renders it relatively challenging to identify. In addition, certain theoretical studies have shown that realistic vaping temperature settings may betoo low to produce ketene. Each of these issues is addressed herein. First, an isotopically labeled acetate precursor is used for the identification of ketene with enhanced rigor in vaped aerosols. Second, discrepancies between theoretical and experimental findings are explained by accounting for the effects of aerobic (experimental) versus anaerobic (simulated and theoretical) pyrolysis conditions. This finding is also relevant to explaining the relatively low-temperature production of aerosol toxicants beyond ketene. Moreover, the study presented herein shows that ketene formation during vaping is not limited to molecules possessing a phenyl acetate substructure. This means that ketene emission during vaping, including from popular flavorants such as ethyl acetate, may be more prevalent than is currently known.

Double Strain-Release [2π+2σ]-Photocycloaddition

In pursuit of potent pharmaceutical candidates and to further improve their chemical traits, small ring systems can serve as a potential starting point. Small ring units have the additional merit of loaded strain at their core, making them suitable reactants as they can capitalize on this intrinsic driving force. With the introduction of cyclobutenone as a strained precursor to ketene, the photocycloaddition with another strained unit, bicyclo[1.1.0]butane (BCB), enables the reactivity of both π-units in the transient ketene. This double strain-release driven [2π+2σ]-photocycloaddition promotes the synthesis of diverse heterobicyclo[2.1.1]hexane units, a pharmaceutically relevant bioisostere. The effective reactivity under catalyst-free conditions with a high functional group tolerance defines its synthetic utility. Experimental mechanistic studies and density functional theory (DFT) calculations suggest that the [2π+2σ]-photocycloaddition takes place via a triplet mechanism.

Comparative Analysis of Nickel-Phosphine Complexes with Cumulated Double Bond Ligands: Structural Insights and Electronic Interactions via ETS-NOCV and QTAIM Approaches

This study presents a comprehensive analysis of nickel-phosphine complexes, specifically Ni(PH3)2(OCCH2), Ni(PH3)2(H2CCO), Ni(PH3)2(H2CCCH2), Ni(PH3)2(NNCH2), and Ni(PH3)2(η1-H2CNN). Utilizing ETS-NOCV analysis, we explored orbital energy decomposition and the Hirshfeld charges of the ligands, providing insights into the electronic structures and donor-acceptor interactions within these complexes. The interactions in the ketene and allene complexes exhibit similar deformation densities and NOCV orbital shapes to those calculated for Ni(PH3)2(NNCH2), indicating consistent interaction characteristics across these complexes. The total interaction energy for all η2 complexes is observed to be over 60 kcal/mol, slightly exceeding that of the analogous carbon dioxide complex reported earlier. Furthermore, the study highlights the stronger back-donation as compared to donor interactions across all η2 complexes. This is further corroborated by Hirshfeld analysis, revealing the charge distribution dynamics within the ligand fragments. The research offers new perspectives on the electron distribution and interaction energies in nickel-phosphine complexes, contributing to a deeper understanding of their catalytic and reactive behaviors.

Selectivity Control of the Ligand Exchange at Carbon in α-Metallated Ylides as a Route to Ketenyl Anions

α-Metallated ylides have recently been reported to undergo phosphine by CO exchange at the ylidic carbon atom to form isolable ketenyl anions. Systematic studies on the tosyl-substituted yldiides, R3 P=C(M)Ts (M=Li, Na, K), now reveal that carbonylation may lead to a competing metal salt (MTs) elimination. This side-reaction can be controlled by the choice of phosphine, metal cation, solvent and co-ligands, thus enabling the selective isolation of the ketenyl anion [Ts-CCO]M (2-M). Complexation of 2-Na by crown ether or cryptand allowed structure elucidation of the first free ketenyl anion [Ts-CCO]- , which showed an almost linear Ts-C-C linkage indicative for a pronounced ynolate character. However, DFT studies support a high charge at the ketenyl carbon atom, which is reflected in the selective carbon-centered reactivity. Overall, the present study provides important information on the selectivity control of ketenyl anion formation which will be crucial for future applications.

Stereo- and regiocontrol in intermolecular [2+2] cycloadditions between diarylketenes and allenamides to access substituted α-methylenecyclobutanones

The development of intermolecular [2+2] cycloadditions between allenamides and diarylketenes is described. α-Aryldiazo arylketones are employed as ketene precursors that react smoothly with allenamides in the presence of a P(C6F5)3 promoter. High diastereoselectivity (dr > 20 : 1) with endo-regioselectivity can be achieved when two aryl groups of ketenes have opposite electronic properties. The role of P(C6F5)3 is to increase the reaction yields, but also enhancing stereoselectivity.

Synthesis of tetracyclic dibenzo[b,f][1,4]oxazepine-fused β-lactams via visible-light-induced Staudinger annulation

An efficient visible-light-induced Staudinger [2 + 2] annulation reaction between α-diazo ketones and dibenzo[b,f][1,4]oxazepine/thiazepine-imines under catalyst-free conditions has been developed. This protocol provides a facile method to synthesize tetracyclic dibenzo[b,f][1,4]oxazepine/thiazepine-fused β-lactams bearing a quaternary carbon center with a broad substrate scope and high efficiency (37 examples, up to >99% yield).

Lewis Acid Catalyzed Formal (3+2)-Cycloaddition of Bicyclo[1.1.0]butanes with Ketenes

Design, synthesis and application of benzene bioisosteres have attracted a lot of attention in the past 20 years. Recently, bicyclo[2.1.1]hexanes have emerged as highly attractive bioisosteres for ortho- and meta-substituted benzenes. Herein we report a mild, scalable and transition-metal-free protocol for the construction of highly substituted bicyclo[2.1.1]hexan-2-ones through Lewis acid catalyzed (3+2)-cycloaddition of bicyclo[1.1.0]-butane ketones with disubstituted ketenes. The reaction shows high functional group tolerance as documented by the successful preparation of various 3-alkyl-3-aryl as well as 3,3-bisalkyl bicyclo[2.1.1]hexan-2-ones (26 examples, up to 89 % yield). Postfunctionalization of the exocyclic ketone moiety is also demonstrated.

Taming Keteniminium Reactivity by Steering Reaction Pathways: Computational Predictions and Experimental Validations

Keteniminium ions, the nitrogen analogues of ketenes, exhibit high reactivity toward olefins and π-systems. Previous results from the Maulide group demonstrated an unexpected propensity for an alternative intramolecular Belluš-Claisen-type rearrangement rather than an expected intramolecular (2 + 2) cycloaddition. We have conducted a cooperative density functional theory/experimental investigation of this process, seeking insights into the competition between the observed Claisen-type reaction and the historically expected (2 + 2) cyclization. Our calculations revealed a surprisingly small difference in the free energy barrier between these two intramolecular reactions. Further theoretical and experimental investigations probe the electronics of the substrate, rationalize a competing deallylation side reaction, and demonstrate the proof-of-concept for an enantioselective (2 + 2) variant.

Asymmetric Cascade Carbonylation/Annulation of Benzyl Bromides, CO, and Vinyl Benzoxazinanones Enabled by Pd/Chiral Lewis-Base Relay Catalysis

A highly enantioselective cascade carbonylation/annulation of benzyl bromides, CO, and vinyl benzoxazinanones under mild conditions has been established by Pd/chiral Lewis base relay catalysis, providing an efficient method to assemble chiral quinolinones from readily available starting materials in good yields with excellent diastereo- and enantioselectivities. The palladium catalyst plays two roles in this reaction, enabling both the carbonylation process and the generation of the zwitterionic π-allyl palladium intermediate.

Thermal and Microwave-Assisted Synthesis of New Highly Functionalized Bis-β-lactams from Available Compounds via Bisketene as an Intermediate

The synthesis of highly functionalized bis-β-lactams containing aromatic rings was achieved by thermal and microwave-assisted methods starting from easily available 2-(4-hydroxyphenyl)acetic acid and 2,2'-(propane-2,2-diyl)diphenol precursors. The approach to these valuable heterocyclic scaffolds involved formal [2π + 2π] cycloadditions between Schiff bases and novel bisketenes, which were generated in situ, followed by an electrocyclic reaction of zwitterionic intermediates. Reactions carried out under microwave irradiation were clean and gave high yields with significantly reduced reaction times. Interestingly, in the thermal method, the reaction proceeded in a stereospecific manner, and only the trans-cis or cis-cis isomers were formed. However, under the microwave conditions, the reaction proceeded stereoselectively, and other possible isomers such trans-trans and cis-trans isomers were formed in addition to the product formed under thermal conditions. More interestingly, when the two compounds that did not produce any products under thermal conditions were reacted under microwave conditions, one formed the trans-cis isomer and the other formed the cis-trans and trans-trans isomers as two products .

Reactions of thioalkynes with diarylketenes via [3+2]-annulation versus benzannulation using Au and P(C6F5)3 catalysts

Two catalytic annulations of non-symmetric diarylketenes with thioalkynes are described using gold and phosphine catalysts respectively. We employed α-aryldiazo ketones to generate gold-π-ketenes, ultimately yielding azulen-1-one derivatives. With the same reactants, we utilized P(C₆F₅)₃ to increase the yields of 1-naphthols, notably with a complete regioselectivity.

1,4-Aryl migration in ketene-derived enolates by a polar-radical-crossover cascade

The arylation of carboxylic acid derivatives via Smiles rearrangement has gained great interest in recent years. Both radical and ionic approaches, as well as radical-polar crossover concepts, have been developed. In contrast, a reversed polar-radical crossover approach remains underexplored. Here we report a simple, efficient and scalable method for the preparation of sterically hindered and valuable α-quaternary amides via a polar-radical crossover-enolate oxidation-aryl migration pathway. A variety of easily accessible N-alkyl and N-arylsulfonamides are reacted with disubstituted ketenes to give the corresponding amide enolates, which undergo upon single electron transfer oxidation, a 1,4-aryl migration, desulfonylation, hydrogen atom transfer cascade to provide α-quaternary amides in good to excellent yields. Various mono- and di-substituted heteroatom-containing and polycyclic arenes engage in the aryl migration reaction. Functional group tolerance is excellent and substrates as well as reagents are readily available rendering the method broadly applicable.

The α-arylation of amides via aryl migration has attracted considerable interest in recent years. Here, the authors report a method for the preparation of bulky α-quaternary amides via a polar-radical crossover enolate oxidation-aryl migration cascade.

Two-colour light activated covalent bond formation

We introduce a photochemical bond forming system, where two colours of light are required to trigger covalent bond formation. Specifically, we exploit a visible light cis/trans isomerization of chlorinated azobenzene, which can only undergo reaction with a photochemically generated ketene in its cis state. Detailed photophysical mapping of the reaction efficiencies at a wide range of monochromatic wavelengths revealed the optimum irradiation conditions. Subsequent small molecule and polymer ligation experiments illustrated that only the application of both colours of light affords the reaction product. We further extend the functionality to a photo reversible ketene moiety and translate the concept into material science. The presented reaction system holds promise to be employed as a two-colour resist.

Dual-wavelength photochemical systems open up new avenues for novel lithographic techniques but currently only few wavelength-orthogonal photoreactive compounds undergoing reversible photoreaction are known. Here, the authors exploit cis/trans photoisomerization of azobenzenes and demonstrate photoligation of the cis state with a photochemically generated ketene.

Oxindole synthesis via polar-radical crossover of ketene-derived amide enolates in a formal [3 + 2] cycloaddition

Herein we introduce a simple, efficient and transition-metal free method for the preparation of valuable and sterically hindered 3,3-disubstituted oxindoles via polar-radical crossover of ketene derived amide enolates. Various easily accessible N-alkyl and N-arylanilines are added to disubstituted ketenes and the resulting amide enolates undergo upon single electron transfer oxidation a homolytic aromatic substitution (HAS) to provide 3,3-disubstituted oxindoles in good to excellent yields. A variety of substituted anilines and a 3-amino pyridine engage in this oxidative formal [3 + 2] cycloaddition and cyclic ketenes provide spirooxindoles. Both substrates and reagents are readily available and tolerance to functional groups is broad.

Deconstructive Insertion of Oximes into Coumarins: Modular Synthesis of Dihydrobenzofuran-Fused Pyridones

In the presence of a copper catalyst, a series of oximes undergo deconstructive insertion into coumarins to afford structurally interesting dihydrobenzofuran-fused pyridones in moderate to good yields with good functional group compatibility. The reaction likely involves a radical relay annulation, leading to the ring opening of the lactone moiety of the coumarins, and simultaneous formation of three new bonds. The investigation of photoluminescent properties reveals that several obtained compounds may have potential as fluorescent materials.

Stereoselective synthesis of chiral sultam-fused dihydropyridinones via photopromoted NHC catalyzed [4 + 2] annulation

A photopromoted NHC catalyzed asymmetric [4+2] annulation of saccharine-derived azadienes and α-diazoketones was developed, affording the corresponding sultam-fused dihydropyridinones efficiently (up to 80% yield, 99% ee and >20 : 1 d.r.).

Stereoelectronic Features of a Complex Ketene Dimerization Reaction

The amidation reaction of a tetrahydroisoquinolin-1-one-4-carboxylic acid is a key step in the multi-kilogram-scale preparation of the antimalarial drug SJ733, now in phase 2 clinical trials. In the course of investigating THIQ carboxamidations, we found that propanephosphonic acid anhydride (T3P) is an effective reagent, although the yield and byproducts vary with the nature and quantity of the base. As a control, the T3P reaction of a 3-(2-thienyl) THIQ was performed in the absence of the amine, and the products were characterized: among them are three dimeric allenes and two dimeric lactones. A nucleophile-promoted ketene dimerization process subject to subtle steric and stereoelectronic effects accounts for their formation. Two novel monomeric products, a decarboxylated isoquinolone and a purple, fused aryl ketone, were also isolated, and mechanisms for their formation from the ketene intermediate are proposed.

Light-Triggered Click Chemistry

The merging of click chemistry with discrete photochemical processes has led to the creation of a new class of click reactions, collectively known as photoclick chemistry. These light-triggered click reactions allow the synthesis of diverse organic structures in a rapid and precise manner under mild conditions. Because light offers unparalleled spatiotemporal control over the generation of the reactive intermediates, photoclick chemistry has become an indispensable tool for a wide range of spatially addressable applications including surface functionalization, polymer conjugation and cross-linking, and biomolecular labeling in the native cellular environment. Over the past decade, a growing number of photoclick reactions have been developed, especially those based on the 1,3-dipolar cycloadditions and Diels-Alder reactions owing to their excellent reaction kinetics, selectivity, and biocompatibility. This review summarizes the recent advances in the development of photoclick reactions and their applications in chemical biology and materials science. A particular emphasis is placed on the historical contexts and mechanistic insights into each of the selected reactions. The in-depth discussion presented here should stimulate further development of the field, including the design of new photoactivation modalities, the continuous expansion of λ-orthogonal tandem photoclick chemistry, and the innovative use of these unique tools in bioconjugation and nanomaterial synthesis.

DFT Study on the Mechanism of Iron-Catalyzed Diazocarbonylation

The mechanism of the carbonylation of diazomethane in the presence of iron–carbonyl–phosphine catalysts has been investigated by means of DFT calculations at the M06/def-TZVP//B97D3/def2-TZVP level of theory, in combination with the SMD solvation method. The reaction rate is determined by the formation of the coordinatively unsaturated doublet-state Fe(CO)₃(P) precursor followed by the diazoalkane coordination and the N₂ extrusion. The free energy of activation is predicted to be 18.5 and 28.2 kcal/mol for the PF₃ and PPh₃ containing systems, respectively. Thus, in the presence of less basic P-donor ligands with stronger π-acceptor properties, a significant increase in the reaction rate can be expected. According to energy decomposition analysis combined with natural orbitals of chemical valence (EDA–NOCV) calculations, diazomethane in the Fe(CO)₃(phosphine)(η¹-CH₂N₂) adduct reveals a π-donor–π-acceptor type of coordination.

Oxidation of Alkynes via Catalytic Metal-Vinylidenes

Metal-vinylidenes, generated by treatment of terminal alkynes with transition metals, are very useful intermediates in modern synthetic chemistry as shown by the high number of transformations in which they are involved. When a metal-vinylidene is generated in the presence of an oxidant, its immediate oxidation to a ketene occurs. In this short review, recent synthetic applications of the oxidation of alkynes via ketene intermediates from initially formed metal-vinylidenes­ are highlighted.

1 Introduction

2 Oxidation of Metal-Vinylidenes with Internal Oxidants

3 Oxidation of Metal-Vinylidenes with External Oxidants

4 Conclusions

Infrared spectroscopic detection of ketene formation from carbene and CO sources: an amide synthesis

An effective methodology to detect the highly reactive ketene intermediate, which is formed in situ during the course of organic transformation.

Ruthenium-Catalyzed Oxidative Amidation of Alkynes to Amides

Complex CpRuCl(PPh₃)₂ catalyzes reactions of terminal alkynes with 4-picoline N-oxide and primary and secondary amines to afford the corresponding amides. The reactions occur in chlorinated solvent and aqueous medium, showing applications in peptide chemistry. Stoichiometric studies reveal that the true catalysts of the processes are the vinylidene cations [CpRu(═C═CHR)(PPh₃)₂]⁺ which are oxidized to the Ru(η²-CO)-ketenes by the N-oxide. Finally, nucleophilic additions of primary and secondary amines to the free ketenes yield the corresponding amides.

Exploring the generation and use of acylketenes with continuous flow processes

The generation and use of acyl ketenes under continuous flow reaction conditions is reported. Several reaction classes of these reactive intermediates have been studied. Under zero headspace conditions, a ketone exchange process is possible between volatile ketones. The process can be readily scaled to deliver gram quantities of product.

Evidence for an umpolung type of [2+2] cycloaddition of 2-carbamoyl ketenes

2-Carbamoyl ketenes react as nucleophiles with iminium salts and the process leads to the formation of alkylidene malonamides.

Pyrolysis of azetidinones. Part 2. Kinetics and mechanism of thermolysis of β-lactams and β-thiolactams

The kinetics of the gas-phase thermolysis reaction of seven β-lactams and their thione analogues were investigated over the temperature range 533–603 K for the β-lactams and 463–542 K for the β-thiolactams. The average values of the energy of activation (Ea) (kJ mol−1) and Arrhenius log A (s–1) were, respectively, 170.8 ± 18.6 and 12.4 ± 1.6 for the lactams and 131.7 ± 18.2 and 11.0 ± 2.0 for the thione analogues. The entropy of activation (ΔS#) was negative for of the substrates and slightly positive for three. The rate constants (k) (s−1) were calculated for 510 K and compared for the two series of azetidinones. The effects of substituents on rates and the novel role played by the C=O and C=S moieties on the relative reactivities of the cyclic amides are rationalized on the basis of a formal retro[2+2]cycloaddition mechanism used earlier to explain the products of the gas-phase thermolysis reaction of the present azetidinones.

Stereochemistry and Mechanistic Insight in the [2(k)+2(i)+2(i)] Annulations of Ketenes and Imines

The stereochemistry and mechanistic insight in the annulations of one ketene molecule with two imine molecules ([2(k)+2(i)+2(i)] annulation) are studied by using six-membered 3,4-dihydroisoquinoline as an imine probe. A concerted hetero-Diels-Alder cycloaddition mechanism is proposed to explain the stereochemical outcomes. In most cases, the zwitterionic 2-aza-1,3-butadiene-type intermediates, generated from ketenes and imines, undergo endo hetero-Diels-Alder cycloaddition with the second imine molecule. For ketenes with electron-donating substituents, (2,4)-cis-(4,5)-cis-[2(k)+2(i)+2(i)] annuladducts formed stereospecifically, while, for ketenes with electron-accepting substituents, (2,4)-cis-(4,5)-trans-[2(k)+2(i)+2(i)] annuladducts are generated stereospecifically. The [2(k)+2(i)+2(i)] annulations of aryloxyketenes and 3,4-dihydroisoquinoline give stereodivergent products due to the occurrence of the stepwise nucleophilic annulation. However, in the [2(k)+2(i)+2(i)] annulations of seven-membered cyclic imine dibenzo[b,f][1,4]oxazepine, the zwitterionic aza-butadiene-type intermediates exclusively undergo exo hetero-Diels-Alder cycloadditions with another molecule of imine to yield (2,4)-trans-(4,5)-trans-[2(k)+2(i)+2(i)] annuladducts stereospecifically, regardless of the ketene substituents. The mechanistic model not only discloses the nature of the [2(k)+2(i)+2(i)] annulations, but also can be used to explain and predict the stereochemistry of the [2(k)+2(i)+2(i)] annuladducts from different ketenes and imines.

A telescopic one-pot synthesis of β-lactam rings using amines as a convenient source of imines

A facile synthetic approach to substituted β-lactams was designed, using secondary benzylic amines and acid chlorides as starting materials. The reactions proceeded smoothly and all the products were obtained in good yields.