Recent advances in alkylidene carbene chemistry

Carbenes from cyclopropanated aromatics

Although a ripe old discipline by now, carbene chemistry continues to flourish as both theorists and experimentalists have shown sustained interest in this area of research. While there are numerous ways of generating carbenes, the thermal and/or photochemical decomposition of diazo compounds and diazirines remains, by far, the most commonly used method of producing these intermediates. There is no disputing the fact that these nitrogenous precursors have served carbene researchers well, but their use is not without problems. They are often sensitive and hazardous to handle and, sometimes, the desired nitrogenous precursor simply may not be available, e.g., for synthetic reasons, to study the particular carbene of interest. Furthermore, there is a legitimate concern that the photochemical generation of carbenes in solution from diazo compounds and diazirines may be contaminated by reactions in the excited states (RIES) of the precursors themselves. As an alternative, several laboratories, including ours, have used cyclopropanated aromatic systems to generate a wide range of carbenes. In each case, the cheleotropic extrusion of carbenes is accompanied by the formation of stable aromatic by-products such as phenanthrene, indane, naphthalene, and 1,4-dihydronaphthalene. The emergence of these "non-traditional" carbene sources, their versatility, and promise are reviewed in this work.

Adamantylidenecarbene: Photochemical Generation, Trapping, and Theoretical Studies

Photolysis of 1-(2-adamantylidene)-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene in benzene (or benzene-d6) at ambient temperature produces adamantylidenecarbene. The carbene undergoes dimerization to a cumulene and may also be trapped in a stereospecific fashion by cis- and trans-4-methyl-2-pentene. No products attributable to 4-homoadamantyne, resulting from ring expansion of the carbene, could be detected. Coupled cluster/density functional theory calculations place the singlet carbene ∼49 kcal/mol below the triplet and show that the former must overcome a barrier of ∼13.5 kcal/mol to rearrange into 4-homoadamantyne.

Wittig/B─H insertion reaction: A unique access to trisubstituted Z-alkenes

The Wittig reaction, which is one of the most effective methods for synthesizing alkenes from carbonyl compounds, generally gives thermodynamically stable E-alkenes, and synthesis of trisubstituted Z-alkenes from ketones presents notable challenges. Here, we report what we refer to as Wittig/B─H insertion reactions, which innovatively combine a Wittig reaction with carbene insertion into a B─H bond and constitute a promising method for the synthesis of thermodynamically unstable trisubstituted Z-boryl alkenes. Combined with the easy transformations of boryl group, this methodology provides efficient access to a variety of previously unavailable trisubstituted Z-alkenes and thus provides a platform for discovery of pharmaceuticals. The unique Z-selectivity of the reaction is determined by the maximum overlap of the orbitals between the B─H bond of the borane adduct and the alkylidene carbene intermediate in the transition state.

Diazoalkenes: From an Elusive Intermediate to a Stable Substance Class in Organic Chemistry

Over decades diazoalkenes (R2 C=C=N2 ) were postulated as reactive intermediates in organic chemistry even though their direct spectroscopic detection proved very challenging. In the 1970/80ies several groups probed their existence mainly indirectly by trapping experiments or directly by matrix-isolation studies. In 2021, our group and the Severin group reported independently the synthesis and characterization of the first room-temperature stable diazoalkenes, which initiated a rapidly expanding research field. Up to now four different classes of N-heterocyclic substituted room-temperature stable diazoalkenes have been reported. Their properties and unique reactivity, such as N2 /CO exchange or utilization as vinylidene precursors in organic and transition metal chemistry are presented. This review summarizes the early discoveries of diazoalkenes from their initial postulation as transient, elusive species up to the recent findings of the room-temperature stable derivatives.

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

Zeolite chemistry and catalysis are expected to play a decisive role in the next decade(s) to build a more decentralized renewable feedstock-dependent sustainable society owing to the increased scrutiny over carbon emissions. Therefore, the lack of fundamental and mechanistic understanding of these processes is a critical "technical bottleneck" that must be eliminated to maximize economic value and minimize waste. We have identified, considering this objective, that the chemistry related to the first-generation reaction intermediates (i.e., carbocations, radicals, carbenes, ketenes, and carbanions) in zeolite chemistry and catalysis is highly underdeveloped or undervalued compared to other catalysis streams (e.g., homogeneous catalysis). This limitation can often be attributed to the technological restrictions to detect such "short-lived and highly reactive" intermediates at the interface (gas-solid/solid-liquid); however, the recent rise of sophisticated spectroscopic/analytical techniques (including under in situ/operando conditions) and modern data analysis methods collectively compete to unravel the impact of these organic intermediates. This comprehensive review summarizes the state-of-the-art first-generation organic reaction intermediates in zeolite chemistry and catalysis and evaluates their existing challenges and future prospects, to contribute significantly to the "circular carbon economy" initiatives.

Alkylidene Carbene from Silyl Vinyl Iodide Provides Mechanistic Insights on Trimethylenemethane Diyl-Mediated Tandem Cyclizations

We describe a method to generate alkylidene carbenes via tetramethylammonium-fluoride-induced desilylation of silyl vinyl iodides. The reversible carbene generation from an iodovinyl anion enabled us to unearth mechanistic aspects of the trimethylenemethane (TMM) diyl cyclization reaction that could not be explored via previous methods. We observed that a slow diyl-diylophile cycloaddition can induce the reversible formation of an alkylidene carbene from the TMM diyl intermediate via a retro-cyclopropanation at ambient temperature.

High Magnetic Anisotropy of a Square-Planar Iron-Carbene Complex

Among Earth-abundant catalyst systems, iron-carbene intermediates that perform C-C bond forming reactions such as cyclopropanation of olefins and C-H functionalization via carbene insertion are rare. Detailed descriptions of the possible electronic structures for iron-carbene bonds are imperative to obtain better mechanistic insights and enable rational catalyst design. Here, we report the first square-planar iron-carbene complex (^MesPDP^Ph)Fe(CPh₂), where [^MesPDP^Ph]^2- is the doubly deprotonated form of [2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine]. The compound was prepared via reaction of the disubstituted diazoalkane N₂CPh₂ with (^MesPDP^Ph)Fe(thf) and represents a rare example of a structurally characterized, paramagnetic iron-carbene complex. Temperature-dependent magnetic susceptibility measurements and applied-field Mössbauer spectroscopic studies revealed an orbitally near-degenerate S = 1 ground state with large unquenched orbital angular momentum resulting in high magnetic anisotropy. Spin-Hamiltonian analysis indicated that this S = 1 spin system has uniaxial magnetic properties arising from a ground M_S = ±1 non-Kramers doublet that is well-separated from the M_S = 0 sublevel due to very large axial zero-field splitting (D = -195 cm^-1, E/D = 0.02 estimated from magnetic susceptibility data). This remarkable electronic structure gives rise to a very large, positive magnetic hyperfine field of more than +60 T for the ⁵⁷Fe nucleus along the easy magnetization axis observed by Mössbauer spectroscopy. Computational analysis with complete active space self-consistent field (CASSCF) calculations provides a detailed electronic structure analysis and confirms that (^MesPDP^Ph)Fe(CPh₂) exhibits a multiconfigurational ground state. The majority contribution originates from a configuration best described as a singlet carbene coordinated to an intermediate-spin Fe^II center with a (d_xy)²{(d_xz),(d_z²)}³(d_yz)¹(d_x²-y²)⁰ configuration featuring near-degenerate d_xz and d_z² orbitals.

Detection of Ylide Formation between an Alkylidenecarbene and Acetonitrile by Femtosecond Transient Absorption Spectroscopy

Femtosecond laser flash photolysis of 3-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-ylidene)tetrahydrofuran produces singlet 3-oxacyclopentylidenecarbene which reacts with acetonitrile solvent to form an ylide. This is the first direct detection of ylide formation by an alkylidenecarbene. This new type of ylide was observed to have a broad absorption band in the visible region with λ_max ∼450 nm and a lifetime of ∼13.5 ps. As with other "conventional" carbenes (the divalent carbon atom is separately bound to two substituents), this ylide formation method could be also useful for detecting alkylidenecarbenes, especially those that do not absorb at wavelengths suitable for direct observation. Furthermore, the mechanisms by which 3-oxacyclopentylidenecarbene forms the ylide and the overall favorability of ylide formation, vis-à-vis ring expansion of the carbene to strained 3-oxacyclohexyne, were supported by results from density functional theory calculations.

α-Bromoacrylic Acids as C1 Insertion Units for Palladium-Catalyzed Decarboxylative Synthesis of Diverse Dibenzofulvenes

Herein α-bromoacrylic acids have been employed as C1 insertion units to achieve the palladium-catalyzed [4 + 1] annulation of 2-iodobiphenyls, which provides an efficient platform for the construction of diverse dibenzofulvenes. This protocol enables the formation of double C(aryl)-C(vinyl) bonds via a C(vinyl)-Br bond cleavage and decarboxylation. It is particularly noteworthy that the method features a broad substrate scope, and various interesting frameworks, such as bridged ring, fused (hetero)aromatic ring, and divinylbenzene, can be successfully incorporated into the products.

Systematic Evaluation of 1,2-Migratory Aptitude in Alkylidene Carbenes

Alkylidene carbenes undergo rapid inter- and intramolecular reactions and rearrangements, including 1,2-migrations of β-substituents to generate alkynes. Their propensity for substituent migration exerts profound influence over the broader utility of alkylidene carbene intermediates, yet prior efforts to categorize 1,2-migratory aptitude in these elusive species have been hampered by disparate modes of carbene generation, ultrashort carbene lifetimes, mechanistic ambiguities, and the need to individually prepare a series of ¹³C-labeled precursors. Herein we report on the rearrangement of ¹³C-alkylidene carbenes generated in situ by the homologation of carbonyl compounds with [¹³C]-Li-TMS-diazomethane, an approach that obviates the need for isotopically labeled substrates and has expedited a systematic investigation (¹³C{¹H} NMR, DLPNO-CCSD(T)) of migratory aptitudes in an unprecedented range of more than 30 alkylidene carbenes. Hammett analyses of the reactions of 26 differentially substituted benzophenones reveal several counterintuitive features of 1,2-migration in alkylidene carbenes that may prove of utility in the study and synthetic application of unsaturated carbenes more generally.

A palladium-catalyzed Heck/[4 + 1] decarboxylative cyclization cascade to access diverse heteropolycycles by using α-bromoacrylic acids as C1 insertion units

A novel palladium-catalyzed Heck/[4 + 1] decarboxylative cyclization cascade of alkene-tethered aryl iodides by employing α-bromoacrylic acids as C1 insertion units is reported.

Insertion of Alkylidene Carbenes into B-H Bonds

We have developed a protocol for insertion of alkylidene carbenes into the B-H bonds of amine-borane adducts, enabling, for the first time, the construction of C(sp²)-B bonds by means of carbene-insertion reactions. Various acyclic and cyclic alkenyl borane-amine adducts were prepared from readily accessible starting materials in good to high yields and were subsequently subjected to a diverse array of functional group transformations. The unprecedented spiro B-N heterocycles prepared in this study have potential utility as building blocks for the synthesis of pharmaceuticals. Preliminary mechanistic studies suggest that insertion of the alkylidene carbenes into the B-H bonds of the amine-borane adducts proceeds via a concerted process involving a three-membered-ring transition state.

A Chiral Naphthyridine Diimine Ligand Enables Nickel-Catalyzed Asymmetric Alkylidenecyclopropanations

A novel class of chiral naphthyridine diimine ligands (NDI*) readily accessible from C₂ -symmetric 2,6-di-(1-arylethyl)anilines is described. The utility of these ligands, particularly one with fluorinated aryl side arms, is demonstrated by a reductive Ni-catalyzed enantioselective alkylidene transfer reaction from 1,1-dichloroalkenes to olefins. This transformation provides direct access to a broad range of synthetically valuable alkylidenecyclopropanes in high yields and enantioselectivities.

Nickel-Catalyzed Anionic Cross-Coupling Reaction of Lithium Sulfonimidoyl Alkylidene Carbenoids With Organolithiums

The mechanistic platform for a novel nickel0 -catalyzed anionic cross-coupling reaction (ACCR) of lithium sulfonimidoyl alkylidene carbenoids (metalloalkenyl sulfoximines) with organometallic reagents is reported herein, affording substituted alkenylmetals and lithium sulfinamides. The Ni0 -catalyzed ACCR of three different types of metalloalkenyl sulfoximines, including acyclic, axially chiral and exocyclic derivatives, with sp2 organolithiums and sp2 and sp3 Grignard reagents has been studied. The ACCR of metalloalkenyl sulfoximines with PhLi in the presence of the Ni0 -catalyst and precatalyst Ni(PPh3 )2 Cl2 afforded alkenyllithiums, under inversion of configuration at the C atom and complete retention at the S atom. In a combination of experimental and DFT studies, we propose a catalytic cycle of the Ni0 -catalyzed ACCR of lithioalkenyl sulfoximines. Computational studies reveal two distinctive pathways of the ACCR, depending on whether a phosphine or 1,5-cyclooctadiene (COD) is the ligand of the Ni atom. They rectify the underlying importance of forming the key Ni0 -vinylidene intermediate through an indispensable electron-rich Ni0 -center coordinated by phosphine ligands. Fundamentally, we present a mechanistic study in controlling the diastereoselectivity of the alkenyllithium formation via the key lithium sulfinamide coordinated Ni0 -vinylidene complex, which consequently avoids an unselective formation of an alkylidene carbene Ni-complex and ultimately racemic alkenyllithium.

C-H Insertion by Alkylidene Carbenes To Form 1,2,3-Triazines and Anionic [3 + 2] Dipolar Cycloadditions To Form Tetrazoles: Crucial Roles of Stereoelectronic and Steric Effects

The synthesis of 1,2,3-triazines and bicyclic tetrazoles from α-azido ketones is described. The common intermediate generated from lithiated trimethylsilyldiazomethane and α-azido ketones diverges depending on the steric bulk of the substituents. The formation of 1,2,3-triazines via a C-H insertion of alkylidene carbene to form 3-azidocyclopropene, followed by its rearrangement, is supported by density functional theory calculations. Tetrazole formation proceeds via a facile anionic [3 + 2] dipolar cycloaddition between a lithiated diazo moiety and an azido group facilitated by the chelation of a lithium ion.

Development of New Synthetic Reactions Using Hetero-Heavy Atoms and Their Application to Synthesis of Biofunctional Molecules

Novel reactions using hetero-heavy atoms (P, S, Si, Se, and Sn) were developed and applied to the synthesis of biofunctional molecules and some medicine-candidates, in which the following items are covered. 1) Development of introduction of C₁-unit using cyanophosphates (CPs). 2) Carbene-generation under neutral condition from CPs and its application to organic synthesis. 3) [3,3]Sigmatropic rearrangement-ring expansion reactions of medium-sized cyclic thionocarbonates containing a sulfur atom and their application to natural product synthesis. 4) Stereoselective synthesis of novel β-imidazole C-nucleosides via diazafulvene intermediates and their application to investigating ribozyme reaction mechanism. 5) Developments of novel histamine H₃- and H₄-receptor ligands using new synthetic methods involving Se or Sn atoms.

Oxyacylation of Iodoalkynes: Gold(I)-Catalyzed Expeditious Access to Benzofurans

(Acetonitrile)[1,3-bis(2,6-diisopropylphenyl)-imidazole-2-ylidene] gold(I) catalyzes the cycloisomerization of 2-(iodoethynyl)aryl esters to give 3-iodo-2-acyl benzofurans. This catalytic transformation is the result of an unprecedented selective synthetic event, which comprises a [1,2]-iodine shift, a C-O ring-closure step, and a C-C bond-formation that installs the ketone functionality into the new ring. Experimental evidence supports the involvement of a β-iodo-substituted gold vinylidene as the intermediate species. The reaction tolerates different substitution patterns at the phenol moiety and a wide diversity of groups at the carboxylic fragment, including not only alkyl but also alkenyl, aryl, and heteroaryl groups.

Transition Metal Vinylidene- and Allenylidene-Mediated Catalysis in Organic Synthesis

With their mechanistic novelty and various modalities of reactivity, transition metal unsaturated carbene (alkenylidene) complexes have emerged as versatile intermediates for new reaction discovery. In particular, the past decade has witnessed remarkable advances in the chemistry of metal vinylidenes and allenylidenes, leading to the evolution of a diverse array of new catalytic transformations that are mechanistically distinct from those developed in the previous two decades. This review aims to provide a survey of the recent achievements in the development of organic reactions that make use of transition metal alkenylidenes as catalytic intermediates and their applications to organic synthesis.

Ring Expansion of Alkylidenecarbenes Derived from Lactams, Lactones, and Thiolactones into Strained Heterocyclic Alkynes: A Theoretical Study

Strained cycloalkynes are of considerable interest to theoreticians and experimentalists, and possess much synthetic value as well. Herein, a series of cyclic alkylidenecarbenes—formally obtained by replacing the carbonyl oxygen of four-, five-, and six-membered lactams, lactones, and thiolactones with a divalent carbon—were modeled at the CCSD(T)/cc-pVTZ//B3LYP/6-311+G** and CCSD(T)/cc-pVTZ//CCSD/6-311+G** levels of theory. The singlet carbenes were found to be more stable than the triplets. The strained heterocyclic alkynes formed by ring expansion of these singlet carbenes were also modeled. Interestingly, the C≡C bonds in the five-membered heterocycles, obtained from the rearrangement of β-lactam- and β-lactone-derived alkylidenecarbenes, displayed lengths intermediate between formal double and triple bonds. Furthermore, 2-(1-azacyclobutylidene)carbene was found to be nearly isoenergetic with its ring-expanded isomer, and 1-oxacyclopent-2-yne was notably higher in energy than its precursor carbene. In all other cases, the cycloalkynes were lower in energy than the corresponding carbenes. The transition states for ring-expansion were always lower for the 1,2-carbon shifts than for 1,2-nitrogen or oxygen shifts, but higher than for the 1,2-sulfur shifts. These predictions should be verifiable using carbenes bearing appropriate isotopic labels. Computed vibrational spectra for the carbenes, and their ring-expanded isomers, are presented and could be of value to matrix isolation experiments.

Access to Fully Substituted Thiazoles and 2,3-Dihydrothiazoles via Copper-Catalyzed [4 + 1] Heterocyclization of α-(N-Hydroxy/aryl)imino-β-oxodithioesters with α-Diazocarbonyls

An efficient chemoselective practical route to fully substituted thiazoles and 2,3-dihydrothiazoles has been devised by [4 + 1] heterocyclization of α-(N-hydroxy/aryl)imino-β-oxodithioesters with in situ generated Cu-carbenoids of diazocarbonyls. The α-(N-hydroxy/aryl)imino-β-oxodithioesters are readily accessible by the reaction of β-oxodithioesters with nitrous acid/nitrosoarenes. The overall transformation involves sequential N-O/C-N bonds cleavage followed by cascade C-N/C-S bonds formation in one-pot. This new strategy allows full control over the introduction of various sensitive functional groups at different positions of the thiazole ring, broadening the arsenal of synthetic methods to obtain such scaffolds.

Transformation of Carbonyl Compounds into Homologous Alkynes under Neutral Conditions: Fragmentation of Tetrazoles Derived from Cyanophosphates

Cyanophosphates (CPs) can be easily prepared from either ketones or aldehydes, and their reaction with NaN₃-Et₃N·HCl results in the formation of azidotetrazoles. Under microwave irradiation, successive fragmentation of the azidotetrazoles generates alkylidene carbenes that undergo [1,2]-rearrangement and are transformed into homologous alkynes. Treatment of ketone-derived CPs with TMSN₃ and Bu₂SnO as catalyst in toluene at reflux directly yields the corresponding internal alkynes, whereas the reaction of aldehyde-derived CPs with NaN₃-Et₃N·HCl in THF at reflux or TMSN₃-Bu₂SnO (cat.) in toluene at reflux provides homologous terminal alkynes in good yields. These reactions take place under neutral conditions and can be successfully extended to obtain alkynes that are not usually accessible from the corresponding carbonyl compounds by the Ohira-Bestmann or Shioiri procedures, which require basic conditions.

Photochemical generation and trapping of 3-oxacyclohexyne

The strained heterocyclic alkyne, 3-oxacyclohexyne, has been generated for the first time by a photochemical route.

Synthesis of homopropargylamines from 2-cyanoazetidines

Mild generation of vinylidene carbene from α-amino nitrile is the basis of a new synthesis of homopropargylamines.