171,132 research outputs found
Crystal chemistry rationale and DFT investigations of novel hard tetragonal C6 built from tetrahedral C(sp3) lattice embedding allene-like sp2 linear tricarbon.
No full textFrom crystal chemistry rationale novel tetragonal carbon allotrope: C6 featuring allene-like C(sp2) tricarbon unit >C=C=C< embedded in diamond-like structure C(sp3) lattice is proposed. Quantum mechanics DFT calculations onto ground state let identify C6 as cohesive with localized charge around tetrahedral C-sp3 and smeared charge density along linear allene-like tricarbon unit characteristic of C-sp2 hybridization. Novel C6 is found mechanically stable from elastic properties with large Vickers hardness: HV= 58 GPa. Dynamically stable with positive phonon acoustic and optic frequencies, C6 is characterized with particularly high optical phonon frequency =60 THz assigned to antisymmetric stretching mode (C----C-C), found to characterize I.R. active mode in C3H4 allene isolated molecule. The electronic band structure reveals semi-conducting electronic properties
Dinuclear Coordination of a Novel Four-electron Reduced μ-η1,η3-allene Ligand with β-agostic C–H⋯Ta Interaction: Direct Addition of allene to (C5Me5)2Ta2(μ-X)4 (X = Cl, Br), and Molecular Structure of (C5Me5)2Ta2(μ-η1,η3-C3H4)(μ-Cl)Cl3
No full text[[abstract]]The organoditantalum complexes (η-C5Me5)2Ta2(μ-X)4 (X = Cl, Br) react with 1–5 equiv. of allene under mild conditions to afford the fluxional allene adducts (C5Me5)2Ta2(μ-η1,η3-C3H4)(μ-X)X3 as shown by spectroscopic and spectrometric data. Low-temperature-limit proton NMR data at −95 °C for (C5Me5)2Ta2(μ-η1,η3-C3H4)(μ-Cl)Cl3 show a large chemical shift range for the inequivalent allene hydrogens, with a high-field value for one hydrogen at δ −0.36, and inequivalent Cp* groups. The low-temperature-limit 13C NMR resonances for the allene ligand are found at δ 204.4 (central C), δ 70.3 (CH2, 1JCH = 154 Hz), and the substantially upfield value of δ 42.9. The latter resonance has remarkably different coupling constants (1JCH = 173, 154 Hz). The allene coordinates in a novel alkylidene-diyl fashion in the solid-state, as shown by X-ray diffractometry on (C5Me5)2Ta2(μ-η1,η3-C3H4)(μ-Cl)Cl3 at 200 K. One tantalum is doubly-bonded to the central allenic carbon (Ta1C22 distance, 2.011(7) Å) and interacts with an allene hydrogen in a probable β-agostic manner (Ta1–H21A distance of 2.25(10) Å). The second tantalum is coordinated to the allene in a hybrid of η3-allylic and sigma bonding, with Ta2–C distances to the allene methylenes of 2.206(8) and 2.276(9) Å and a Ta2–C22 bond distance to the central allenic carbon of 2.253(7) Å. The allene hydrogens are oriented above and below the C–C–C plane, consistent with appreciable σ-bonding between the Ta(2) and the allene methylenes. The bent allene ligand has a C–C–C angle of 106.0(6)° and can be viewed as reduced by four electrons, with concomitant oxidation of the two d2 tantalum centers of the organoditantalum(III) reactant. The long Ta⋯Ta nonbonded distance of 3.3052(8) Å is consistent with two d0 tantalum centers in an organoditantalum(V) product
Allene Synthesis via C−C Fragmentation: Method and Mechanistic Insight
No full textAllene Synthesis via C−C Fragmentation: Method and Mechanistic Insigh
1,4-Iron Migration for Expedient Allene Annulations through Iron-Catalyzed C−H/N−H/C−O/C−H Functionalizations
No full textAbstract C−H activation bears great potential for enabling sustainable molecular syntheses in a step‐ and atom‐economical manner, with major advances having been realized with precious 4d and 5d transition metals. In contrast, we employed earth abundant, nontoxic iron catalysts for versatile allene annulations through a unique C−H/N−H/C−O/C−H functionalization sequence. The powerful iron catalysis occurred under external‐oxidant‐free conditions even at room temperature, while detailed mechanistic studies revealed an unprecedented 1,4‐iron migration regime for facile C−H activations.Deutsche Forschungsgemeinschaft https://doi.org/10.13039/501100001659China Scholarship Council https://doi.org/10.13039/50110000454
Planar Tetracoordinate Carbons in Allene-Type Structures
No full textThe exhaustive exploration of the potential energy surfaces of CE2M2 (E = Si-Pb, M = Li, and Na) revealed seven global minima containing a planar tetracoordinate carbon (ptC). The design was based on a π-localization strategy, resulting in a ptC embedded in a linear or a bent allene-type E=C=E motif. The magnetic response showed a σ-aromaticity in the bent E=C=E fragments. The bonding analysis indicated that the ptCs form C-E covalent bonds and C-M electrostatic interactions.</p
Electrooxidative Allene Annulations by Mild Cobalt-Catalyzed C–H Activation
No full textVersatile cobalt catalysis enabled
electrochemical C–H activation with allenes. Thus, allene annulations
were accomplished in terms of C–H/N–H functionalizations
with excellent levels of chemoselectivity, site selectivity, and regioselectivity
under exceedingly mild conditions. Detailed mechanistic studies were
conducted, including reactions with isotopically labeled compounds,
kinetic investigations, and in-operando infrared spectroscopic studies.
Further, computational studies were supportive of a non-rate-determining
C–H cleavage and gave key insights into the regioselectivity
of the allene annulation. The practical utility of the user-friendly
approach was furthermore highlighted by gram-scale electrocatalysis
Electrooxidative Allene Annulations by Mild Cobalt-Catalyzed C–H Activation
No full textVersatile cobalt catalysis enabled
electrochemical C–H activation with allenes. Thus, allene annulations
were accomplished in terms of C–H/N–H functionalizations
with excellent levels of chemoselectivity, site selectivity, and regioselectivity
under exceedingly mild conditions. Detailed mechanistic studies were
conducted, including reactions with isotopically labeled compounds,
kinetic investigations, and in-operando infrared spectroscopic studies.
Further, computational studies were supportive of a non-rate-determining
C–H cleavage and gave key insights into the regioselectivity
of the allene annulation. The practical utility of the user-friendly
approach was furthermore highlighted by gram-scale electrocatalysis
The rotation-vibration spectra of allene and allene-d<SUB>4</SUB>
No full textThe infrared absorption spectra of allene and allene-d4 vapor have been measured over the spectral range 250-5000 cm−1 with a prism spectrometer. The infrared data, together with previously obtained Raman spectra, enable a complete vibrational analysis that satisfies the product rule well for all vibrational species. The torsional vibration is located at 865 cm−1 in allene and 615 cm−1 in allene-d4. Fine
structure was resolved for all perpendicular fundamental bands except that of the lowest frequency in both compounds. With help of the Ζ-sum rule, a modified equation of H. H. Nielsen and the statistical weights of the rotational levels, the fine structure is analyzed to yield a complete set of Ζ-values for both molecules and a moderately accurate value for the moment of inertia about the figure axis. The latter gives for the HCH angle the figure 113°; ± 1°; if one assumes a C∃ H distance of 1.071A
Allene-based approach to the synthesis of De Novo erythromycinoids
No full textWe prepared an advanced synthetic module (bis[allene] macrolactone at center) equipped with two allenes embedded in a macrolactone scaffold. The plan was to effect heterogeneous derivatization of the allenes, in tandem or separately. In addition to diversity, this approach is maximally concise and economic, especially in terms of steps. Moreover, immediate derivatives of the macrocyclic bis[allene] can be taken into further steps, thus providing canonical build-up of erythromycin analogs. To date, our bis[allene] macrolactone has been converted to over 30 novel macrolides. These de novo analogs serve to validate the strategy and lay the ground work for further work. Taken together, the allene-based reactions/transformations employed in this study, such as DMDO oxidation/nucleophile addition, allene osmylation/electrophile addition, bromination, allene oxide rearrangement, spirodiepoxide rearrangement, benzylic migration/elimination, mono- and bis-oxidation of bis[allene], chelation-controlled reduction and oxime formation, demonstrate that 8 of the 11 modifiable carbons in this antibiotic can be modified. It is especially noteworthy that each congener was made in less than three steps from the bis[allene] macrolactone.Ph. D.Includes bibliographical referencesIncludes vitaby Hiyun Ki
Medium and Large N-Heterocycle Formation via Allene Hydroamination with a Bimetallic Rh(I) Catalyst
No full textWe report the synthesis of a bimetallic Rh(I) complex containing a bridging CO ligand that facilitates Rh–Rh bond formation. This bimetallic complex enables intramolecular allene hydroamination to form seven to ten-member rings in high yield. Monometallic Rh complexes, in contrast, fail to achieve any product formation. We demonstrate a broad substrate scope for formation of a variety of N-heterocycles in good to excellent yields. Macrocyclization reactions that form eleven to fifteen-membered heterocycles are also demonstrated. Mechanistic studies show that the reaction likely proceeds via catalyst protonation by trifluoroacetic acid, followed by reversible allene insertion and C–N bond-forming reductive elimination. The difference in product selectivity observed with our bimetallic catalyst vs monometallic Rh complexes may result from cooperativity between the two metals
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