| 1. |
- Alvi, Muhammad Rouf, et al.
(författare)
-
Highly Efficient and Convenient Acid Catalyzed Hypersilyl Protection of Alcohols and Thiols by Tris(trimethylsilyl)silyl-N,N-dimethylmethaneamide
- 2012
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Tris(trimethylsilyl)silyl-N,N-dimethylmethaneamide, herein named hypersilylamide, is a convenient and efficient source of the hypersilyl group in the first widely applicable acid catalyzed protocol for silyl group protection of primary, secondary, tertiary alkyl as well as aryl alcohols and thiols in high yields. The sole by-product is N,N-dimethylformamide (DMF) and a range of solvents can be used, including DMF. A high selectivity in the protection of diols can be achieved, also for diols with very small differences in the steric demands at the two hydroxyl groups. Moreover, in the protection of equivalent alcohol and thiol sites the protection of the alcohol is faster, allowing for selective protection in high yields. Quantum chemical calculations at the M062X hybrid meta density functional theory level give insights on the mechanism for the catalytic process. Finally, the hypersilyl group is easily removed from all protected alcohols and thiols examined herein by irradiation at 254 nm.
|
|
| 2. |
- Emanuelsson, Rikard, et al.
(författare)
-
Configuration- and Conformation-Dependent Electronic-Structure Variations in 1,4-Disubstituted Cyclohexanes Enabled by a Carbon-to-Silicon Exchange
- 2014
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 20:30, s. 9304-9311
-
Tidskriftsartikel (refereegranskat)abstract
- Cyclohexane, with its well-defined conformers, could be an ideal force-controlled molecular switch if it were to display substantial differences in electronic and optical properties between its conformers. We utilize sigma conjugation in heavier analogues of cyclohexanes (i.e. cyclohexasilanes) and show that 1,4-disubstituted cyclohexasilanes display configuration-and conformation-dependent variations in these properties. Cis- and trans-1,4-bis(trimethylsilylethynyl)-cyclohexasilanes display a 0.11 V difference in their oxidation potentials (computed 0.11 V) and a 0.34 eV difference in their lowest UV absorption (computed difference between first excitations 0.07 eV). This is in stark contrast to differences in the corresponding properties of analogous all-carbon cyclohexanes (computed 0.02 V and 0.03 eV, respectively). Moreover, the two chair conformers of the cyclohexasilane trans isomer display large differences in electronic-structure-related properties. This enables computational design of a mechanically force-controlled conductance switch with a calculated single-molecule ON/OFF ratio of 213 at zero-bias voltage.
|
|
| 3. |
- Emanuelsson, Rikard, et al.
(författare)
-
Optimization of the Cyclic Cross-Hyperconjugation in 1,4-Ditetrelcyclohexa-2,5-dienes
- 2014
-
Ingår i: Organometallics. - : American Chemical Society (ACS). - 0276-7333 .- 1520-6041. ; 33:12, s. 2997-3004
-
Tidskriftsartikel (refereegranskat)abstract
- Cyclic cross-hyperconjugation can exist to variable extents in 1,4-ditetrelcyclohexa-2,5-dienes, i.e., all-carbon cyclohexa-1,4-dienes and 1,4-disila/digerma/distanna/diplumbacyclohexa-2,5-dienes. In this study we first use density functional theory (DFT) computations to optimize the conjugation strength by seeking the optimal atom E and substituent group E'Me-3 in the two saturated E(E'Me-3)(2) moieties (E and E' as the same or different tetrel (group 14) elements). We reveal that the all-carbon cyclohexadienes with gradually heavier E'Me-3 substituents at the two saturated carbon atoms display significant cross-hyperconjugation. The first electronic excitations in these compounds, which formally have two isolated C=C bonds, are calculated to reach wavelengths as long as 400 nm (excitation energies of 3.1 eV). These transitions are mostly forbidden, and the lowest allowed transitions are found at 387 nm (3.2 eV). The silicon analogues are also cross-hyperconjugated, while a decline is observed in the 1,4-digerma/distanna/diplumbacyclohexa-2,5-diene. Experiments on two substituted 1,4-disilacyclohexa-2,5-dienes confirm the effect of the E'Me3 substituents, with regard to both electronic excitations and geometries as determined by UV absorption spectroscopy and X-ray crystallography, respectively. At the end, we reveal through computations how electron-donating and electron-withdrawing substituents at the C=C double bonds influence the electronic properties of the all-carbon ring. We find that the first calculated excitation, which is forbidden, can be shifted to 440 nm (2.83 eV). This shows to what extent cyclic cross-hyperconjugation can affect the electronic and optical properties of a compound with two formally isolated C=C double bonds.
|
|
| 4. |
- Szarek, Michael, et al.
(författare)
-
Total Cardiovascular and Limb Events and the Impact of Polyvascular Disease in Chronic Symptomatic Peripheral Artery Disease
- 2022
-
Ingår i: Journal of the American Heart Association. - : John Wiley & Sons. - 2047-9980. ; 11:11
-
Tidskriftsartikel (refereegranskat)abstract
- Background: Peripheral artery disease (PAD) is associated with heightened risk for major adverse cardiovascular and limb events, but data on the burden of risk for total (first and potentially subsequent) events, and the association with polyvascular disease, are limited. This post hoc analysis of the EUCLID (Examining Use of Ticagrelor in Peripheral Artery Disease) trial evaluated total cardiovascular and limb events among patients with symptomatic PAD, overall and by number of symptomatic vascular territories.Methods and Results: In the EUCLID trial, patients with symptomatic PAD (lower extremity revascularization >30 days before randomization or ankle-brachial index ≤0.80) were randomized to treatment with ticagrelor or clopidogrel. Relative effects on total events (cardiovascular death; nonfatal myocardial infarction and ischemic stroke; acute limb ischemia, unstable angina, and transient ischemic attack requiring hospitalization; coronary, carotid, and peripheral revascularization procedures; and amputation for symptomatic PAD) were summarized by hazard ratios (HRs), whereas absolute risks were estimated by incidence rates and mean cumulative functions. Among 13 885 randomized patients, 7600 total cardiovascular and limb events occurred during a median 2.7 years of follow-up, translating to 60.0 and 62.5 events per 100 patients through 3 years for the ticagrelor and clopidogrel groups, respectively (HR, 0.96; 95% CI, 0.89-1.03; P=0.27). Among 1393 patients with disease in 3 vascular territories, event accrual rates through 3 years for the ticagrelor and clopidogrel groups were 87.3 and 97.7 events per 100 patients, respectively. Absolute risk reductions for ticagrelor relative to clopidogrel at 3 years were -0.2, 6.7, and 10.3 events per 100 patients for 1, 2, and 3 affected vascular territories, respectively (Pinteraction=0.09).Conclusions: Patients with symptomatic PAD have nearly double the number of total events than first events, with rates reflecting the number of affected vascular territories. These findings highlight the clinical relevance of quantifying disease burden in terms of total events and the need for long-term preventive treatments in high-risk patient populations. Registration URL: https://clinicaltrials.gov/; Unique identifier: NCT01732822.
|
|
| 5. |
- Tibbelin, Julius, et al.
(författare)
-
1,4-Disilacyclohexa-2,5-diene : a molecular building block that allows for remarkably strong neutral cyclic cross-hyperconjugation
- 2014
-
Ingår i: Chemical Science. - : Royal Society of Chemistry (RSC). - 2041-6520. ; 5:1, s. 360-371
-
Tidskriftsartikel (refereegranskat)abstract
- 2,3,5,6-Tetraethyl-1,4-disilacyclohexa-2,5-dienes with either four chloro (1a), methyl (1b), or trimethylsilyl (TMS) (1c) substituents at the two silicon atoms were examined in an effort to design rigid compounds with strong neutral cross-hyperconjugation between pi- and sigma-bonded molecular segments arranged into a cycle. Remarkable variations in the lowest electronic excitation energies, lowest ionization energies, and the first oxidation potentials were observed upon change of substituents, as determined by gas phase ultraviolet (UV) absorption spectroscopy, ultraviolet photoelectron spectroscopy (UPS), and cyclic voltammetry. A particularly strong neutral cyclic cross-hyperconjugation was observed in 1c. Its lowest electron binding energy (7.1 eV) is distinctly different from that of 1b (8.5 eV). Molecular orbital analysis reveals a stronger interaction between filled pi(C=C) and pi(SiR2) group orbitals in 1c than in 1a and 1b. The energy shift in the highest occupied molecular orbital is also reflected in the first oxidation potentials as observed in the cyclic voltammograms of the respective compounds (1.47, 0.88, and 0.46 V for 1a, 1b and 1c, respectively). Furthermore, 1,4-disilacyclohexadiene 1c absorbs strongly at 273 nm (4.55 eV), whereas 1a and 1b have no symmetry allowed excitations above 215 nm (below 5.77 eV). Thus, suitably substituted 1,4-disilacyclohexa-2,5-dienes could represent novel building blocks for the design of larger cross-hyperconjugated molecules as alternatives to traditional purely cross-p-conjugated analogues, and could allow for design of molecules with properties that are not accessible to those that are exclusively pi-conjugated.
|
|
| 6. |
|
|
| 7. |
- Wallner, Andreas, et al.
(författare)
-
Coupling of Disilane and Trisilane Segments Through Zero, One, Two, and Three Disilanyl Bridges in Cyclic and Bicyclic Saturated Carbosilanes
- 2013
-
Ingår i: Organometallics. - : American Chemical Society (ACS). - 0276-7333 .- 1520-6041. ; 32:2, s. 396-405
-
Tidskriftsartikel (refereegranskat)abstract
- Several six-membered cyclic and [2.2.2]bicyclic organo-silanes with varying proportions of silicon atoms in the bridges have been prepared following a stepwise approach that exploits dianionic polysilanes. Focus in our analysis was placed on the bicyclic compounds which all have silicon atoms at the bridgehead positions. Quantum chemical calculations of these compounds revealed the possibility to enhance the coupling through a single cisoid tetrasilane cage segment by replacing one or two of the other -SiMe2SiMe2- bridges with -CH2CH2- bridges. UV absorption spectroscopy revealed a red shift in the lowest visible transitions when going from a bicyclo[2.2.2]octane with three -SiMe2SiMe2- bridges to those with two or one such bridge. However, these red shifts are deceptive, as the lowest vertically excited singlet states, which are dark according to TD-DFT calculations, do not display the same trend. Still, since these compounds have (i) excellent structural rigidity, (ii) provide potentials for functionalization through their exocyclic trimethylsilyl groups, and (iii) display electronic structure variations with the number of -SiMe2SiMe2- bridges, they could be interesting for further studies: e.g., in single-molecule electronics.
|
|
| 8. |
- Wallner, Andreas, et al.
(författare)
-
Formation and NMR Spectroscopy of ω-Thiol Protected α,ω-Alkanedithiol Coated Gold Nanoparticles and Their Usage in Molecular Charge Transport Junctions
- 2011
-
Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 27:14, s. 9057-9067
-
Tidskriftsartikel (refereegranskat)abstract
- Gold nanoparticles (AuNPs) coated with stabilizing molecular monolayers are utilized in areas ranging from life sciences to nanoelectronics. Here we present a novel and facile one-pot single phase procedure for the preparation of stable AuNPs with good dispersity, which are coated with α,ω-alkanedithiols whose outer ω-thiol is protected by a triphenylmethyl group. Using dielectrophoresis we were able to trap these AuNPs, coated with ω-thiol protecting groups, in a 20 nm gold electrode nanogap. The ω-thiol group was then deprotected under acidic conditions in situ once the AuNPs were correctly positioned in the device. The subsequent deprotection resulted in an increase of conductance by three orders of magnitude, indicating that the isolated dithiol coated AuNPs were fused into a covalently bonded network with AuNP-molecule-AuNP as well as electrode-molecule-AuNP linkages. Furthermore, complete characterization of the AuNP surface-bonded alkanedithiols was achieved using a series of one- and two-dimensional NMR spectroscopy techniques. Our spectra of the molecule-coated AuNPs show well resolved signals, only slightly broader than for free molecules in solution, in contrast to many earlier reported NMR spectral data of molecules attached to AuNPs. Complementary diffusion NMR spectroscopic experiments were performed to prove that the observed alkanedithiols are definitely surface bonded species and do not exist in free and unattached form.
|
|
