| 1. |
- M, Yu, et al.
(författare)
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Enol Ethers as Substrates for Efficient Z- and Enantioselective Ring-Opening/Cross-Metathesis Reactions Promoted by Stereogenic-at-Mo Complexes; Utility in Chemical Synthesis and Mechanistic Attributes
- 2012
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 134:5, s. 2788-2799
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Tidskriftsartikel (refereegranskat)abstract
- The first examples of catalytic enantioselective ring-opening/cross- metathesis (EROCM) reactions that involve enol ethers are reported. Specifically, we demonstrate that catalytic EROCM of several oxa-and azabicycles, cyclobutenes and a cyclopropene with an alkyl-or aryl-substituted enol ether proceed readily in the presence of a stereogenic-at-Mo monopyrrolide-monoaryloxide. In some instances, as little as 0.15 mol % of the catalytically active alkylidene is sufficient to promote complete conversion within 10 min. The desired products are formed in up to 90% yield and >99:1 enantiomeric ratio (er) with the disubstituted enol ether generated in >90% Z selectivity. The enol ether of the enantiomerically enriched products can be easily differentiated from the terminal alkene through a number of functionalization procedures that lead to the formation of useful intermediates for chemical synthesis (e.g., efficient acid hydrolysis to afford the enantiomerically enriched carboxaldehyde). In certain cases, enantioselectivity is strongly dependent on enol ether concentration: larger equivalents of the cross partner leads to the formation of products of high enantiomeric purity (versus near racemic products with one equivalent). The length of reaction time can be critical to product enantiomeric purity; high enantioselectivity in reactions that proceed to >98% conversion in as brief a reaction time as 30 s can be nearly entirely eroded within 30 min. Mechanistic rationale that accounts for the above characteristics of the catalytic process is provided. © 2012 American Chemical Society.
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| 2. |
- Ryan, K.M., et al.
(författare)
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Three Dimensional Architectures of Ultra-high Density Semiconducting Nanowires Deposited on Chip
- 2003
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Ingår i: Journal of the American Chemical Society. - 0002-7863 .- 1520-5126. ; 125:20, s. 6284-6288
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Tidskriftsartikel (refereegranskat)abstract
- We report a "clean" and fast process, utilizing supercritical carbon dioxide, for producing ultrahigh densities, up to 10(12) nanowires per square centimeter, of ordered germanium nanowires on silicon and quartz substrates. Uniform mesoporous thin films were employed as templates for the nucleation and growth of unidirectional nanowire arrays orientated almost perpendicular to a substrate surface. Additionally, these nanocomposite materials display room-temperature photoluminescence (PL), the energy of which is dependent on the diameter of the encased nanowires. The ability to synthesis ultrahigh-density arrays of semiconducting nanowires on-chip is a key step in future "bottom-up" fabrication of multilayered device architectures for nanoelectronic and optoelectronic devices.
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| 3. |
- Zhu, Can, et al.
(författare)
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Efficient Stereoselective Carbocyclization to cis-1,4-Disubstituted Heterocycles Enabled by Dual Pd/Electron Transfer Mediator (ETM) Catalysis
- 2020
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 142:12, s. 5751-5759
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Tidskriftsartikel (refereegranskat)abstract
- An efficient Pd/ETM (ETM = electron transfer mediator)-cocatalyzed stereoselective oxidative carbocyclization of dienallenes under aerobic oxidation conditions has been developed to afford six-membered heterocycles. The use of a bifunctional cobalt complex [Co(salophen)-HQ] as hybrid ETM gave a faster aerobic oxidation than the use of separated ETMs, indicating that intramolecular electron transfer between the hydroquinone unit and the oxidized metal macrocycle occurs. In this way, a class of important cis-1,4-disubstituted six-membered heterocycles, including dihydropyran and tetrahydropyridine derivatives were obtained in high diastereoselectivity with good functional group compatibility. The experimental and computational (DFT) studies reveal that the pendent olefin does not only act as an indispensable element for the initial allene attack involving allenic C(sp(3))-H bond cleavage, but it also induces a face-selective reaction of the olefin of the allylic group, leading to a highly diastereoselective formation of the product. Finally, the deuterium kinetic isotope effects measured suggest that the initial allenic C(sp(3))-H bond cleavage is the rate-limiting step, which was supported by DFT calculations.
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