37 research outputs found
MICROWAVE SPECTRUM, BARRIER TO INTERNAL ROTATION AND DIPOLE MOMENT OF CIS 3-PENTEN-1-YNE
No full textAuthor Institution: University of Southern California, Los AngelesThe microwave spectrum of cis 3-penten-1-yne has been observed in the region 8-30 Hz. The barrier to internal rotation of the methyl group was determined from splitting of the rotational transitions. The barrier is 1120 cal.
MODE-SPECIFICITY AND NON-RRKM KINETICS IN THE CONFORMATIONAL ISOMERIZATION OF 4-CHLOROBUT-1-YNE
No full textAuthor Institution: Department of Chemistry, University of VirginiaWe have measured the rate of conformational isomerization of 4-chlorobut-1-yne at an energy of above the zero-point level through the rotational spectroscopy of single molecular eigenstates. From the spectrum of the J=3-J=4 rotational transition of several eigenstates in this energy region we measure a unimolecular cunformational isomerization rate of . This rate is more than two orders-of-magnitude slower than the rate calculated using RRKM theory: . Furthermore, from the eigenstate-resolved rovibrational spectrum of the acetylenic C-H stretch we find that there is strong mode-specificity in the isomerization rate. Coherent preparation of the acetylenic C-H stretch of 4-chlorobut-1-yne leads to even slower isomerization with an upper limit of the rate set by the measured rate of intramolecular vibrational energy redistribution:
Cu(I)-Catalyzed Cross-Coupling of Conjugated Ene-yne-ketones and Terminal Alkynes: Synthesis of Furan-Substituted Allenes
No full textA new method for the synthesis of furan-substituted allenes via Cu(I)-catalyzed coupling of conjugated ene-yne-ketones with terminal alkynes has been developed. A wide range of functional groups are tolerated, and the products are obtained in good to excellent yields under mild conditions. A copper carbene migratory insertion is proposed as the key step in this transformation with conjugated ene-yne-ketones as carbene precursors.Chemistry, OrganicSCI(E)[email protected]; [email protected]
Palladium-Catalyzed Carbene Migratory Insertion Using Conjugated Ene-Yne-Ketones as Carbene Precursors
No full textPalladium-catalyzed cross-coupling reactions between benzyl, aryl, or allyl bromides and conjugated ene-yne-ketones lead to the formation of 2-alkenyl-substituted furans. This novel coupling reaction involves oxidative addition, alkyne activation-cyclization, palladium carbene migratory insertion, beta-hydride elimination, and catalyst regeneration. Palladium (2-furyl)carbene is proposed as the key intermediate, which is supported by DFT calculations. The palladium carbene character of the key intermediate is validated by three aspects, including bond lengths, Wiberg bond order indices, and molecular orbitals, by comparison to those reported for stable palladium carbene species. Computational-studies also revealed that the rate-limiting step is ene-yne-ketone cyclization, which leads to the formation of the palladium (2-furyl)carbene, while the subsequent carbene migratory insertion is a facile process with a low energy barrier (<5 kcal/mol).Chemistry, MultidisciplinarySCI(E)PubMed23ARTICLE3613502-1351113
Density Functional Theory Study of the Mechanisms and Stereochemistry of the Rh(I)-Catalyzed Intramolecular [3+2] Cycloadditions of 1-Ene- and 1-Yne-Vinylcyclopropanes
No full textThe mechanisms, structures of all stationary points involved, and kinetic and thermodynamic parameters of the Rh(I)-catalyzed intramolecular [3+2] cycloaddition reactions of 1-ene- and 1-yne-vinylcyclopropanes (1-ene-VCPs and 1-yne-VCP5) have been investigated using density functional theory (DFT) calculations. The computational results showed that the [3+2] reactions of 1-ene/yne-VCPs studied here occur through a catalytic cycle of substrate catalyst complex formation, cyclopropane cleavage, alkene/alkyne insertion, and reductive elimination. Alkene/alkyne insertion is the rate-and stereoselectivity-determining step of these multistep [3+2] cycloadditions. The experimentally observed high reactivity of 1-yne-VCPs compared to 1-ene-VCPs is well rationalized by the differences of steric effects in the alkyne/alkene insertion transition states. DFT calculations unveiled that the relative orientation of the tethers in the 1-ene/yne-VCPs plays a key role in controlling the stereochemistry of the [3+2] cycloadducts. In addition, DFT calculation results are used to explain why, in some cases, the formation of the beta-hydride elimination byproduct can compete with the [3+2] pathway
DETERMINATION OF THE CONFORMATIONAL ISOMERIZATION RATE OF 4-FLUOROBUT-1-YNE USING DYNAMICAL ROTATIONAL SPECTROSCOPY
No full textAuthor Institution: University of Virginia; Department of Chemistry, University of VirginiaThe conformational isomerization rate of 4-fluorobut-1-yne has been measured at an energy of above the zero point energy through the rotational spectroscopy of single vibrationally-excited eigenstates. 4-Fluorobut-1-yne exist as two conformers (trans and gauche) with a barrier to isomerization from of . Using infrared-microwave double-resonance and infrared-microwave-microwave triple-resonance spectroscopy the rotational spectra of several different eigenstates were obtained for and . Each of the conformers has characteristic rotational frequencies, known from ground-state microwave spectroscopy. When the molecule is excited above the barrier to isomerization both conformers contribute to the rotational spectra and the isomerization rate is determined from the overall line shape of the dynamic rotational spectra. Peculiarities of the line shape due to the large difference (factor of 10) in the dipole moment, , of the two conformers will be discussed. From these rotational spectra we measure a conformational isomerization rate of . This rate is significantly slower than the rate calculated using RRKM theory, and orders of magnitude faster than the upper limit given by the measured intramolecular vibrational energy redistribution rate:
CONFORMATIONAL ISOMERIZATION KINETICS OF 4-BROMOBUT1-YNE FROM HIGH-RESOLUTION INFRARED SPECTROSCOPY AND DYNAMICAL ROTATIONAL SPECTROSCOPY
No full textAuthor Institution: University of Virginia; Department of Chemistry, University of Virginia; Department of Chemistry, National Institute of StandardsThe rate of conformational isomerization between the more stable trans and less stable gauche conformers of 4-bromobut-1-yne is measured by dynamical rotational spectroscopy. The high-resolution, molecular-beam infrared spectrum of the acetylenic C-H stretch of the trans conformer of 4-bromobut-1-yne is rotationally assigned using microwave-infrared double-resonance spectroscopy in an electric resonance optothermal spectrometer. The vibrational spectrum of the gauche conformer is not observed. The existence of intramolecular vibrational energy redistribution (IVR) is indicated by the extensive local perturbations to the spectrum that fragment the vibrational spectrum. The time scale for IVR following coherent preparation of the acetylenic C-H stretch of the trans conformer is about 2 ns (for both isotopes). The isomerization rate at is measured through the single-eigenstate rotational spectrum. This spectrum is obtained using a saturation spectroscopy infrared-microwave double-resonance technique. From a line shape analysis of the spectrum, the time scale for isomerization is determined to be 120 ps. This measurement shows that the isomerization process is highly mode-specific with a strong reduction in the rate for coherent excitation of the acetylenic C-H stretch of the trans conformer. This result is compared to isomerization rate measurements on other halobutynes
Pd-catalyzed cross-coupling of terminal alkynes with ene-yne-ketones: access to conjugated enynes via metal carbene migratory insertion
No full textA novel strategy for alkyne-alkyne cross-coupling has been developed under the palladium catalysis. In this reaction, ene-yne-ketones are employed as carbene precursors, which couple with terminal alkynes through the metal carbene migratory insertion process. Furan-substituted enynes are obtained in good yields and in a stereoselective manner.973 Program [2012CB821600]; NSFC [21472004, 21332002]SCI(E)[email protected]
Formal Synthesis of Gracilamine Using Rh(I)-Catalyzed [3 + 2 + 1] Cycloaddition of 1‑Yne–Vinylcyclopropanes and CO
No full textReported
here is a formal synthesis of gracilamine using Rh(I)-catalyzed
[3 + 2 + 1] reaction of yne-VCP (±)-4 and CO. The
key reaction gave the cycloadduct (±)-trans-3 with the A–B–C core structure of gracilamine.
This advanced intermediate was further transformed to Gao’s
intermediate (±)-2 via regular transformations to
realize the formal synthesis of gracilamine. The present strategy
was used to accomplish the asymmetric formal synthesis of gracilamine
using chiral substrate (+)-4
Stapling of unprotected helical peptides via photoinduced intramolecular thiol-yne hydrothiolation
No full textPeptide stapling emerged as a versatile strategy to recapitulate the bioactive helical conformation of unstructured short peptides in water to improve their therapeutic properties in targeting intracellular "undruggable" targets. Here, we describe the development of photo-induced intramolecular thiol-yne macrocyclization for rapid access to short stapled peptides with enhanced biophysical properties. This new peptide stapling technique provides rapid access to conformationally constrained helices with satisfying functional group tolerance. Notably, the vinyl sulfide linkage shows distinct lipophilicity with reduced membrane toxicity compared to the corresponding all-hydrocarbon analogue. As a proof of principle, we constructed stabilized helices modulating intracellular estrogen receptor (ER)-coactivator interactions with a nanomolar binding affinity, enhanced serum stability, a diffuse cellular distribution and selective cytotoxicity towards ER-positive MCF-7 cells.National Natural Science Foundation of China [21102007, 21372023]; MOST [2015DFA31590]; Shenzhen Science and Technology Innovation Committee [SW201110018, SGLH20120928095602764, ZDSY20130331145112855, JSGG20140519105550503]; Shenzhen Peacock Program [KQTD201103]SCI(E)[email protected]; [email protected]
