Carbon Suboxide in Preparative Organic Chemistry. New synthetic methods (1)

Carbon Suboxide, C2O3: A Hidden σ0π2 Carbene

Electronic structure of carbon suboxide, C3O2, has been recently described as OC→C←CO having two lone pairs at the central carbon atom, thereby called as "Carbones". Although it has a linear geometry, the presence of two lone pairs comes to fore when its reactivity is analyzed with two protons. However, no attention had been paid on its alternating reactivity with hydride ions. Herein, detailed quantum chemical calculations predict that carbon suboxide can also have significant hydride ion affinity. This reactivity is in tune with σ0π2 carbene character of carbon suboxide. This study also shows that such a σ0π2 carbene character is also prevalent in carbodiphosphorane, C(PH3)2. This bonding situation has been hitherto unexplored in "Carbone" chemistry.

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.

Formation and infrared spectroscopic characterization of carbon suboxide complexes TM-η1 -C3 O2 and the inserted ketenylidene complexes OCTMCCO (TM=Cu, Ag, Au) in solid neon

The reactions of coinage metal atoms Cu, Ag and Au with carbon suboxide (C₃ O₂ ) are studied by matrix isolation infrared spectroscopy. The weakly bound complexes TM-η¹ -C₃ O₂ (TM=Cu, Ag, Au), in which the carbon suboxide ligand binds to the metal center in the monohapto fashion are formed as initial reaction products. The complexes subsequently isomerize to the inserted products OCTMCCO upon visible light (λ = 400-500 nm) excitation. The analysis of the electronic structure using modern quantum chemistry methods suggests that the linear OCTMCCO complexes are best described by the bonding interactions between the TM⁺ cation in the electronic singlet ground state and the [OC…CCO]^- ligands in the doublet state forming two TM⁺  ← ligands σ donation and two TM⁺  → ligands π backdonation bonding components. In addition, the CuCCO, AgCCO and AuCCO complexes are also formed, which are predicted to be bent.

Reactivity of an N-heterocyclic silylene with a 1,1'-ferrocenediyl backbone towards carbonyl compounds, including carbon suboxide

Reactions of a silylene with a ketene and with carbon suboxide are reported, respectively leading to the first silaallene oxide and to a silylketene, whose reaction with water affords the first structurally characterised stable methyleneketene and constitutes a unique type of single-crystal-to-single-crystal transformation of a molecular solid by a stoichiometric gas-solid chemical reaction.

N2/CO Exchange at a Vinylidene Carbon Center: Stable Alkylidene Ketenes and Alkylidene Thioketenes from 1,2,3-Triazole Derived Diazoalkenes

We present a new class of room-temperature stable diazoalkenes featuring a 1,2,3-triazole backbone. Dinitrogen of the diazoalkene moiety can be thermally displaced by an isocyanide and carbon monoxide. The latter alkylidene ketenes are typically considered as highly reactive compounds, traditionally only accessible by flash vacuum pyrolysis. We present a new and mild synthetic approach to the first structurally characterized alkylidene ketenes by a substitution reaction. Density functional theory calculations suggest the substitution with isocyanides to take place via a stepwise addition/elimination mechanism. In the case of carbon monoxide, the reaction proceeds through an unusual concerted exchange at a vinylidene carbon center. The vinylidene ketenes react with carbon disulfide via a four-membered thiete intermediate to give vinylidene thioketenes under release of COS. We present spectroscopic as well as structural data for the complete isoelectronic series (R₂C═C═X; X = N₂, CO, CNR, CS) including ¹J(¹³C-¹³C) data. As N₂, CO, and isocyanides belong to the archetypical ligands in transition-metal chemistry, this process can be interpreted in analogy to coordination chemistry as a ligand exchange reaction at a vinylidene carbon center.

Differences between carbon suboxide and its heavier congeners as ligands in transition metal complexes: a theoretical study

The lowest energy structures for the [M](C₃E₂) complexes ([M] = Ir, Ni, Pt surrounded by ligands such as phosphines and halides; E = O, S, Se) are compared.

Spherical Oligo-Silicic Acid SOSA Disclosed as Possible Endogenous Digitalis-Like Factor

The Na(+)/K(+)-ATPase is a membrane ion-transporter protein, specifically inhibited by digitalis glycosides used in cardiac therapy. The existence in mammals of some endogenous digitalis-like factors (EDLFs) as presumed ATPase ligands is generally accepted. But the chemical structure of these factors remained elusive because no weighable amounts of pure EDLFs have been isolated. Recent high-resolution crystal structure data of Na(+)/K(+)-ATPase have located the hydrophobic binding pocket of the steroid glycoside ouabain. It remained uncertain if the EDLF are targeting this steroid-receptor or another specific binding site(s). Our recently disclosed spherical oligo-silicic acids (SOSA) fulfill the main criteria to be identified with the presumed EDL factors. SOSA was found as a very potent inhibitor of the Na(+)/K(+)-ATPase, Ca(2+)-ATPase, H(+)/K(+)-ATPase, and of K-dp-ATPase, with IC50 values between 0.2 and 0.5 μg/mL. These findings are even more astonishing while so far, neither monosilicic acid nor its poly-condensed forms have been remarked biologically active. With the diameter ϕ between 1 and 3 nm, SOSA still belong to molecular species definitely smaller than silica nano-particles with ϕ > 5 nm. In SOSA molecules, almost all Si-OH bonds are displayed on the external shell, which facilitates the binding to hydrophilic ATPase domains. SOSA is stable for long term in solution but is sensitive to freeze-drying, which could explain the failure of countless attempts to isolate pure EDLF. There is a strong resemblance between SOSA and vanadates, the previously known general inhibitors of P-type ATPases. SOSA may be generated endogenously by spherical oligomerization of the ubiquitously present monosilicic acid in animal fluids. The structure of SOSA is sensitive to the concentration of Na(+), K(+), Ca(2+), Mg(2+), and other ions suggesting a presumably archaic mechanism for the regulation of the ATPase pumps.

Divalent carbon(0) compounds

Quantum chemical studies show that there is a class of carbon compounds with the general formular CL2 where the carbon atom retains its four valence electrons as two lone pairs. The C-L bonds come from L → C donor-acceptor interactions where L is a strong σ-donor. Divalent C(0) compounds (carbones) are conceptually different from divalent C(II) compounds (carbenes) and tetravalent carbon compounds, but the bonding situation in a real molecule may be intermediate between the three archetypes. There are molecules like tetraaminoallenes which may be described in terms of two double bonds (R2N)2C=C=C(NR2)2 where the extraordinary donor strength of the dicoordinated carbon atom comes only to the fore through the interactions with protons and Lewis acids. They may be considered as "hidden divalent C(0) compounds". The donor strength of divalent C(0) molecules has been investigated by calculations of the binding energies with protons and with main-group Lewis acids and the bond dissociation energies (BDEs) of transition-metal complexes.

Collisionally induced fragmentation of [M-H](-) species of resveratrol and piceatannol investigated by deuterium labelling and accurate mass measurements

Resveratrol (3,5,4'-trihydroxystilbene) and piceatannol (3,4,3',5'-tetrahydroxy-trans-stilbene) are phytoalexins present in red grapes and wines. In vitro studies have revealed that piceatannol blocks LMP2A, a viral protein-tyrosine kinase implicated in leukemia, non-Hodgkin's lymphoma and other diseases associated with the Epstein-Barr virus, and has an antimelanoma effect on human melanoma cells. Resveratrol has several beneficial effects on human health, such as anticancer, cardioprotection, antioxidant, inhibition of platelet aggregation and anti-inflammatory activity. In this investigation, the collisional behaviour of deprotonated resveratrol and piceatannol obtained under electrospray conditions is described. The mechanisms involved in the fragmentation pattern of [M-H](-) species of the two compounds were investigated by performing MS(n) experiments, deuterium labelling and accurate mass measurements.

First and second proton affinities of carbon bases

Quantum chemical calculations at the MP2/TZVPP//BP86/SVP level are reported for the first and second proton affinities (PAs) of divalent carbon-donor molecules. The molecules investigated are imidazol-2-ylidenes ("normal" NHCs) and the tautomeric imidazol-4/5-ylidenes ("abnormal" NHCs). PAs are also calculated for acyclic and cyclic carbodiphosphoranes, carbophosphoranesulfide, unsaturated and saturated carbodicarbenes, tetraaminoallenes and carbon suboxide. The results are discussed in terms of divalent carbon(II) compounds (carbenes) CR(2), which have one lone electron pair at carbon, and carbon(0) compounds CL(2), which have two lone pairs at carbon and two C<--L donor-acceptor bonds. Divalent C(0) compounds (carbones) not only have very high first PAs, but the second PA is also large and strong enough to isolate doubly protonated C(0) species as salts in a condensed phase. The first PA of divalent carbon(II) compounds (carbenes) are also large. However, they have much smaller second PAs than the divalent carbon(0) compounds. The divalent C(0) character of a compound is not always obvious when the bonding situation in the equilibrium geometry is considered. This is the case, for example, for tetraaminoallenes (TAAs). Protonation of TAAs changes the bonding situation of the central moiety from doubly bonded (R(2)N)(2)C=C=C(NR(2))(2) to a donor-acceptor description (R(2)N)(2)C-->C(H(+))(n)<--C(NR(2)) [n = 1, 2]. The atomic partial charge at the carbon donor atom does not correlate with the PA and the trend of the second PA may be quite different from the trend of the first. The trends of the first and second PA correlates quite well with the eigenvalues of the highest-lying carbon lone-pair orbitals.

Carbon suboxide, a highly reactive intermediate from the abiotic degradation of aromatic compounds in soil

The formation of volatile compounds during abiotic degradation processes of aromatic compounds in soil has been the subject of many experimental studies but should be examined further. In this context, the present work investigates the natural formation of carbon suboxide using the model compounds catechol and 3,5-dichlorocatechol and also a soil sample from a peat bog. The measurements were performed with a purge and trap GC/ MS system following various optimization steps. Under certain conditions, we obtained 16.7 ng of carbon suboxide from a 250 mg soil sample. We also found that the formation of carbon suboxide requires a definite activation energy and that it is rather short-lived in the natural environment. A subsequent reaction to malonic acid is expected in the presence of water. It is shown that iron-(III), hydrogen peroxide, and chloride are prerequisites for its formation. Experimental parameters for the highest yield of carbon suboxide depend on the precise molecular structure of the model compound or on the individual soil sample, respectively. The presented results point to a new degradation process for aromatic compounds in soil.

An Approach to the Isoschizozygane Alkaloid Core Using a 1,4-Dipolar Cycloaddition of a Cross-Conjugated Heteroaromatic Betaine

[reaction: see text] A new strategy for the synthesis of the isoschizozygane alkaloid core has been developed that is based on a 1,4-dipolar cycloaddition reaction of a cross-conjugated heteroaromatic betaine. The resulting cycloadduct undergoes loss of COS, and further reduction delivers a 5a-aza-acenaphthylene intermediate that was transformed into the isoschizozygane skeleton upon treatment with acid.

Characterization of the macrocyclic carbon suboxide factors as potent Na,K-ATPase and SR Ca-ATPase inhibitors

Recently discovered macrocyclic carbon suboxide (MCS) factors with the general formula (C(3)O(2))(n) were found to strongly inhibit rabbit and rat Na,K-ATPase as well as SR Ca-ATPase. Highly active MCS factors were obtained by a base/acid treatment of their lipophilic precursor isolated from plants. In the ESI-MS spectra, the dominant molar mass ion of 431 Da corresponds to a 1:1 complex of the carbon suboxide hexamer (n=6; M(r)=408 Da) with a Na(+) ion. Additional mass ions identified in positive and negative ion mode were assigned as complexes of the MCS hexamer (n=6) and octamer (n=8) with Na(+) or with TFA(-) in various ratios. The dominant mass ion values of these active MCS factors from plants are also found in mass spectra of previously described endogenous digitalis-like factors (EDLF) from animals. This would suggest that ubiquitously distributed MCS factors may function as putative endogenous regulatory substances of Na,K-ATPase and possibly of other ATPases. With the symmetric display of several equivalent carbonyl or hydroxy groups, the structure of MCS factors is particularly suited for interactions with proteins and other bio-molecules. This could explain the high biological activity and the unusual properties of the MCS factors.

Chemistry of stable iminopropadienones, RN=C=C=C=O

The synthesis, spectroscopic properties, and chemical reactions of the stable (neopentylimino)-, (mesitylimino)-, and (o-tert-butylphenylimino)propadienones (6) are reported. Nucleophilic addition of amines affords the malonic amidoamidines 7 and 8. 3,5-Dimethylpyrazole reacts analogously to form 9b. Addition of 1,2-dimethylhydrazine produces pyrazolinones 10-12. Addition of N,N'-dimethyldiaminoethane, -propane, and -butane gives diazepine, diazocine, and diazonine derivatives 13-15, respectively (X-ray structures of 13c, 14a, and 15a are available). The mesoionic pyridopyrimidinium olates 18 are obtained by addition of 2-(methylamino)pyridine (X-ray structure of 18b available). Primary 2-aminopyridines afford the pyridopyrimidininones 20-29 and 31 (X-ray structure of 21a available), and 2-aminopyrimidines and 2-aminopyrazine afford pyrimidopyrimidinones and pyrazinopyrimidinones 33-35. Pyrimidoisoquinolinone 36 results from 1-aminoisoquinoline and pyridoquinolinone 40 from 8-aminoquinoline. 2-Aminothiazoline and 2-aminothiazole afford thiazolopyrimidinone derivatives 41-43 (X-ray structure of 43a available).

Cycloaddition Reaction of Mesoionic Betaines as an Approach toward Trialkylindoline Alkaloids

Intramolecular 1,4-dipolar cycloaddition of an anhydro-4-hydroxy-2-oxo-1,3-thiazium hydroxide across a tethered indole pi-bond has been used for the construction of the pentacyclic skeleton of epi-16,17-dihydroeburnamenine. The reaction of 3-ethyl-3-(alkenyl)piperidinones with diketene and trimethylsilyl triflate in benzene at ambient temperature produced annulated pyridones in good yield. The initial reaction involved formation of a N-acetoacetylated amide which was further converted to the pyridone with TMSOTf. The overall process was found to proceed with complete stereospecificity. Treating a sample of 3-ethyl-3-[(E)-4-phenyl-3-butenyl]-2-piperidone with diketene and TMSOTf produced a cycloadduct in 63% yield whose stereochemistry was elucidated by a X-ray crystallographic study. The epimeric Z-isomer produced a different stereoisomer of the annulated dihydropyridone. The mechanism of the annulation involves a TMSOTf induced cyclization followed by proton removal and generation of a cross-conjugated heteroaromatic betaine. This 1,4-dipole undergoes a subsequent intramolecular dipolar cycloaddition across the neighboring pi-bond, and the resulting cycloadduct is subsequently converted to the annulated lactam. A related annulation sequence leading to a key intermediate previously utilized in the synthesis of the (+/-)-vallesamidine has been developed which is based on the intramolecular dipolar cycloaddition of a mesoionic betaine intermediate.