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

Isolation and Structure Elucidation of the Heterocumulene Anions [NCC-L]- (L=CO, CS, N2)

Cumulenes are molecules characterized by a series of consecutive double bonds. They serve as important reagents and intermediates in the synthesis of polymers and a wide variety of functionalized compounds, including various heterocycles. Understanding the properties of cumulenes and developing synthetic routes to these often highly reactive species is essential for unlocking new applications. Here, we report the synthesis and isolation of the cyanodiazomethanide [NCCNN]- and cyanothioketenyl anion [NCCCS]-. These 5-atomic anions exhibit unexpected stabilities but distinct structural differences. Despite the explosive nature of diazoacetonitrile, the [NCCNN]- anion was sufficiently stable at 0 °C to allow for first reactivity studies and its structure elucidation revealing a bent structure. The thioketenyl anion is stable at room-temperature and can be accessed from the cyanoketenyl anion [NCCCO]- via a [2+2] cycloaddition and cycloreversion sequence with COS elimination. Comparative structural, spectroscopic and computational studies including those on the cyanoketenyl anion [NCCCO]-, demonstrate that the degree of bending of these heterocumulene anions [NCC-L]- can be explained by a transition in the bonding situation from a cumulene structure to an anionic carbone, with the strongly π accepting CS ligand leading to a linear structure of the thioketenyl anion.

In Crystallo Wolff Rearrangement of a Metalated Diazoester: Structural Confirmation of the Singlet Carbene Wolff-Intermediate

The Wolff rearrangement (WR) is widely used for the synthesis of ketenes from diazoketones and -esters. Stepwise WR reactions are proposed to proceed through transient carbonylcarbene (R-C-C(O)-R') intermediates, which so far have evaded structural characterization. Here, a Wolff metallocarbene (PtII-C-C(O)-OEt) is reported as a fleeting intermediate in the photoinitiated fragmentation of a diazoester ligand. Frozen solution and crystal matrix isolation experiments enabled the spectroscopic, magnetic, crystallographic, and computational characterization of this highly reactive species. All methods confirmed a singlet ground state for the WR metallocarbene, which is stabilized by π interactions with the carboxyl substituent, thus complementing computational and transient spectroscopy studies for classic organic WR reactions.

Exploring Singlet Carbyne Anions and Related Low-Valent Carbon Species Utilizing a Cyclic Phosphino Substituent

ConspectusThe advancement of synthetic methodologies is fundamentally driven by a deeper understanding of the structure-reactivity relationships of reactive key intermediates. Carbyne anions are compounds featuring a monovalent anionic carbon possessing four nonbonding valence electrons, which were historically confined to theoretical constructs or observed solely within the environment of gas-phase studies. These species possess potential for applications across diverse domains of synthetic chemistry and ancillary fields. This Account details our focused efforts to isolate singlet carbyne anions and explores our isolation of a range of related low-valent carbon species. Our achievements include the synthesis and characterization, under normal laboratory conditions, of gold-substituted free carbenes, copper carbyne anion complexes, ketenyl anions, keteniminyl anions, and a free stannyne. These have been accomplished using a bulky cyclic phosphino substituent, which effectively stabilizes these reactive species.Our journey commenced with the isolation of gold-substituted phosphinocarbenes, characterized by a robust carbon-gold covalent single bond, and progressed to the isolation of copper carbyne anion complexes featuring a carbon-copper ionic bond. This was realized through the synergistic combination of a bulky cyclic phosphino group and a closed-shell electron-rich late transition metal. The robustness of the carbon-gold bond contrasts markedly with the susceptibility of the carbon-copper bond, which imparts to the copper complexes the behavior characteristic of a phosphinocarbyne anion within the coordination sphere of copper, thereby enabling unique carbyne anion transfer reactions. The tri-active ambiphilic nature of the anionic carbon in these copper carbyne complexes enables the formation of three chemical bonds at the carbon center through one-pot reactions. Subsequent investigations unveiled ligand exchange reactions at the carbyne anion site, leading to the generation of stable crystalline ketenyl and keteniminyl anions. These species emerge as potent synthons capable of producing a diverse array of derivatives. In addition, we isolated a free phosphinostannyne, a rare species featuring a carbon-tin multiple bond and two adjacent ambiphilic centers. Collectively, these compounds have demonstrated a remarkable propensity for engaging in a spectrum of unique reactions, underscoring their versatility and confirming their utility in synthesizing uncharted, unique main group species.The methodologies and insights derived from our research contribute to the broader understanding of low-valent carbon species and may provide a platform for future innovations in synthetic chemistry, catalytic processes, and novel materials. As we continue to explore and develop this area of study, we hope that these low-valent carbon species might follow in the footsteps of stable singlet carbenes, potentially finding applications across various fields in the future.

Cleavage of Carbodicarbenes with N2O for Accessing Stable Diazoalkenes: Two-Fold Ligand Exchange at a C(0)-Atom

The cleavage of carbophosphinocarbenes and carbodicarbenes with nitrous oxide (N2O) leads to the formation of room-temperature stable diazoalkenes. The utility of Ph3P/N2 and NHC/N2 ligand exchange reactions were demonstrated by accessing novel benzimidazole- and benzothiazole derived diazoalkenes, which are not accessible by the current state-of-the-art methods. The stable diazoalkenes subsequently allow further ligand exchange reactions at C(0) with carbon monoxide, isocyanide, or a diamidocarbene (DAC). Overall, the combination of hitherto unknown NHC/N2 and N2/L (L = DAC, CO, R-NC) ligand exchange reactions at a C(0) center allow the selective functionalization of the carbodicarbene ligand structure which represents a new methodology to rapidly assemble novel carbodicarbenes or cumulenic compounds.

Cerium(III) yldiide complexes with divergent CO reactivity

Synthesis of cerium yldiide complexes and their reactivity with CO is demonstrated. In the case of the sulphur-tethered yldiide, the ketenyl complex is formed with release of PPh3, while Ph3PCCO is formed along with a sulfinato ligand in the case of the tosyl-substituted yldiide. Computational analysis shows that this diverging reactivity is due to the stability of the two isomers in the first step of each mechanism.

An Azide-Free Synthesis of Metallodiazomethanes Using Nitrous Oxide

Diazo compounds are valuable reagents in synthesis but usually require the use of potentially explosive or toxic starting materials. Here, we report the synthesis and isolation of alkali metal diazomethanides by the reaction of metalated ylides with nitrous oxide, resulting in a formal exchange of the phosphine ligand by dinitrogen. The reaction proceeds through a Wittig-like mechanism via a [3 + 2] cycloaddition of N2O across the ylide bond with release of phosphine oxide. The metalated diazomethanes exhibit an increased thermal stability due to the stronger binding of N2 compared to neutral diazomethanes. This is reflected in short C-N distances and red-shifted N-N vibrations and enables versatile applications such as for the preparation of transition metal diazomethanide complexes and the synthesis of 1,2,3-triazoles from nitriles, diazoacetates from carbon dioxide, or alkynes from aldehydes.

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.

Ph3PCN2: A stable reagent for carbon-atom transfer

Precise modification of a chemical site in a molecule at the single-atom level is one of the most elegant yet difficult transformations in chemistry. A reagent specifically designed for chemoselective introduction of monoatomic carbon is a particularly formidable challenge. Here, we report a straightforward, azide-free synthesis of a crystalline and isolable diazophosphorus ylide, Ph3PCN2, a stable compound with a carbon atom bonded to two chemically labile groups, triphenylphosphine (PPh3) and dinitrogen (N2). Without any additives, the diazophosphorus ylide serves as a highly selective transfer reagent for fragments, including Ph3PC, to deliver phosphorus ylide-terminated heterocumulenes and CN2 to produce multisubstituted pyrazoles. Ultimately, even exclusive carbon-atom transfer is possible. In reactions with aldehydes and acyclic and cyclic ketones (R2C=O), the carbon-atom substitution forms a vinylidene (R2C=C:) en route to alkynes or butatrienes.

Ligand Exchange at Carbon: Synthetic Entry to Elusive Species and Versatile Reagents

How different is carbon compared to other elements in the periodic table? Can carbon compounds be regarded as coordination complexes with carbon as the central element undergoing a facile exchange of its ligands? Although carbon clearly plays a special role among the elements of the periodic table, recent studies have drawn parallels between the bonding situation and the reactivity of carbon compounds to transition metal complexes. This Perspective summarizes recent reports about ylidic and zwitterionic compounds that were shown to exhibit ambiguous bonding situations that can be interpreted as donor-acceptor interactions similar to the bond between a metal and a neutral ligand. Based on this conception, ligand exchange reactions prototypical of transition metal complexes were realized at carbon atoms, enabling new synthetic strategies for the synthesis of reactive species and building blocks. In particular, the exchange of N2, CO, and phosphine ligands led to the development of a mild method for accessing new compounds and reagents with unusual properties, such as vinylidene ketenes or stable ketenyl anions, that open up a diverse but still poorly explored follow-up chemistry.

Synthesis and Reactivity of the [NCCCO]- Cyanoketenate Anion

Cyanoketene is a fundamental molecule that is actively being searched for in the interstellar medium. Its deprotonated form (cyanoketenate) is a heterocumulene that is isoelectronic to carbon suboxide whose structure has been the subject of debate. However, the investigation of cyanoketene and its derivatives is hampered by the lack of practical synthetic routes to these compounds. We report the first synthesis of the cyanoketenate anion in [K(18-crown-6)][NCCCO] (1) as a stable molecule on a multigram scale in excellent yields (>90 %). The structure of this molecule is probed crystallographically and computationally. We also explore the protonation of 1, and its reaction with triphenylsilylchloride and carbon dioxide. In all cases, anionic dimers are formed. The cyanoketene could be synthesized and crystallographically characterized when stabilized by a N-heterocyclic carbene. The cyanoketenate is a very useful unsaturated building block containing N, C and O atoms that can now be explored with relative ease and will undoubtedly unlock more interesting reactivity.

The Cyanoketenyl Anion [NC3O]. Angew Chem Int Ed Engl 2024;63:e202403766. [PMID: 38470943 DOI: 10.1002/anie.202403766] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Cumulenes and heterocumulenes with three or more cumulative multiple bonds are usually reactive species that serve as valuable building blocks for more complex molecules but tend to isomerize or cyclize and therefore are difficult to isolate. Using a mild ligand exchange reaction at the carbon in α-metalated ylides, we have now succeeded in the synthesis and gram-scale isolation of the elusive cyanoketenyl anion [NC3O]-. Despite its assumed cumulene-like structure and the delocalization of the negative charge across the whole 5-atom molecule, it features a bent geometry with a nucleophilic central carbon atom. Computational studies reveal an ambiguous bonding situation in the anion, which can be illustrated only by a combination of different resonance structures. Nonetheless, the anion features remarkable stability, thus allowing the storage of its potassium-crown ether salt and its application as a highly functional synthetic building block. The cyanoketenyl anion readily reacts with a series of small molecules to form more complex organic compounds, including industrially valuable compounds such as cyanoacetate. This work demonstrated that reactive species can be generated by novel synthesis methods and open up atom-economic pathways to complex compounds from small abundant molecules.

Boosting the π-Acceptor Property of Mesoionic Carbenes by Carbonylation with Carbon Monoxide

We report the room temperature dimerization of carbon monoxide mediated by C4/C5-vicinal anionic dicarbenes Li(ADC) (ADC = ArC{(Dipp)NC}2 ; Dipp = 2,6-iPr2 C6 H3 ; Ar = Ph, DMP (4-Me2 NC6 H4 ), Bp (4-PhC6 H4 )) to yield (E)-ethene-1,2-bis(olate) (i.e. - O-C=C-O- = COen ) bridged mesoionic carbene (iMIC) lithium compounds COen -[(iMIC)Li]2 (COen -[iMIC]2 = [ArC{(Dipp)NC}2 (CO)]2 ) in quantitative yields. COen -[(iMIC)Li]2 are highly colored stable solids, exhibit a strikingly small HOMO-LUMO energy gap, and readily undergo 2e-oxidations with selenium, CuCl (or CuCl2 ), and AgCl to afford the dinuclear compounds COon -[(iMIC)E]2 (E = Se, CuCl, AgCl) featuring a 1,2-dione bridged neutral bis-iMIC (i.e. COon -[iMIC]2 = [ArC{(Dipp)NC}2 (C=O)]2 ). COen -[(iMIC)Li]2 undergo redox-neutral salt metathesis reactions with LiAlH4 and (Et2 O)2 BeBr2 and afford COen -[(iMIC)AlH2 ]2 and COen -[(iMIC)BeBr]2 , in which the dianionic COen -moiety remains intact. All compounds have been characterized by NMR spectroscopy, mass spectrometry, and X-ray diffraction. Stereoelectronic properties of COon -[iMIC]2 are quantified by experimental and theoretical methods.

Phosphinoyl-Substituted Ketenyl Anions: Synthesis and Substituent Effects on the Structural Properties

Ketenyl anions are versatile intermediates in synthetic chemistry and have recently become accessible as isolable reagents from metalated ylides by exchange of the phosphine with CO. Herein, we report on a systematic study of substituent effects on the structure and bonding situation in ketenyl anions. A series of phosphinoyl-substituted ketenyl anions {[R2P(X)CCO]- with X = O, NTol, S, Se} were prepared by carbonylation of the corresponding yldiides and isolated as their corresponding potassium salts. NMR and IR spectroscopic analyses together with computational studies demonstrate that the more electron-withdrawing oxo- and iminophosphinoyl substituents increase the s-character in the bond to the ketene moiety and hence the ynolate character of the anion. This trend is particularly seen in solution, whereas the solid-state properties are influenced by packing effects affecting the bonding situation.