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51.
52.
Philip Eckert Prof. Michael G. Organ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(35):e202200665
Bulky Pd−N-heterocyclic carbene (NHC) catalysts (e. g., N-(di-2,6-(3-pentyl)phenyl), IPent) have been shown to have significantly higher reactivity in a wide variety of cross-coupling applications (i. e., C−C, C−S, C−N) than less hindered variants (e. g., N-(di-2,6-(isopropyl)phenyl), IPr). Further, chlorinating the backbone of the NHC ring sees an even greater increase in reactivity. In the cross-coupling of (hetero)aryl electrophiles to secondary alkyl nucleophiles, making the N-aryl groups larger reduces the amount of β-hydride elimination leading to alkene byproducts and chlorinating the NHC core had an even greater effect, all but eliminating alkene formation. In the present study involving the cross-coupling of primary alkyl electrophiles and nucleophiles, a sharp and surprising reversal of all of the above trends was observed. Bulkier catalysts had generally slower rate of reaction and β-hydride elimination worsened leading to extensive amounts of alkene byproducts. 相似文献
53.
54.
Allylic amines are synthesized in good to excellent yields by treatment of aziridine-2-alcohols with PPh3/I2/imidazole in THF as solvent under mild conditions. 相似文献
55.
Partial substitution of cations and anions in perovskite-type materials is a powerful way to tune the desired properties. The systematic variation of the cations size, the partial exchange of O2− for N3− and their effect on the size of the optical band gap and the thermal stability was investigated here. The anionic substitution resulted in the formation of the orthorhombic perovskite-type oxynitrides Mg0.25Ca0.65Y0.1Ti(O,N)3, Ca1-xYxZr(O,N)3, and Sr1–xLaxZr(O,N)3. A two-step synthesis protocol was applied: i) (nano-crystalline) oxide precursors were synthesized by a Pechini method followed by ii) ammonolysis in flowing NH3 at T = 773 K (Ti) and T = 1273 K (Zr), respectively. High-temperature synthesis of such oxide precursors by solid–state reaction generally resulted in phase separation of the different A-site cations. Changes of the crystal structures were investigated by Rietveld refinements of the powder XRD data, thermal stability by DSC/TG measurements in oxygen atmosphere, oxygen and nitrogen contents by O/N analysis using hot gas extraction technique, and optical band gaps by photoluminescence spectroscopy. By moving from Mg0.25Ca0.65Y0.1Ti(O,N)3 via Ca1–xYxZr(O,N)3 to Sr1–xLaxZr(O,N)3, the degree of tilting of the octahedral network is reduced, as observed by an increase in the B–X–B angles caused by the simultaneously increasing effective ionic radius of the A-site cation(s). In general, increasing substitution levels on the A-site (Y3+ and La3+) are accompanied by an enhanced replacement of O2− by N3−. In all three systems, this anionic substitution resulted in a reduction of the optical band gap by approximately 1 eV (Ti) and up to 2.1 eV (Zr) compared to the respective oxides. For Mg0.25Ca0.65Y0.1Ti(O,N)3 an optical band gap of 2.2 eV was observed, appropriate for a solar water splitting photocatalyst. The Zr-based oxynitrides required a by a factor of 2 higher nitrogen contents to significantly reduce the optical band gap and the measured values of 2.9 eV–3.2 eV are larger compared to the Ti-based oxynitride. Bulk thermal stability was revealed up to T = 881 K. In general, the thermal stability decreased with increasing substitution levels due to an increasing deviation from the ideal anionic composition as demonstrated by O/N analysis. 相似文献
56.
57.
Sakae Uemura 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3-4):721-728
Retrospect of organoselenium and tellurium chemistry for these 30 years is described focusing on our novel findings in this field: (1) telluroxide elimination leading to alkenes and allylic compounds, (2) Pd-catalyzed or –mediated carbodetelluration for a new C–C bond formation, (3) synthesis of chiral diferrocenyl dichalcogenides and their use as chiral auxiliaries, (4) asymmetric selenoxide elimination for making optically active allenes and alkenes, (5) meta chloroperbenzoic acid (MCPBA) oxidation of organic selenides and tellurides leading to a substitution of a PhSe or PhTe moiety, as well as (6) preparation of chalcogen-bridged diruthenium complexes and their catalytic use for propargylic substitution reactions. 相似文献
58.
《Green Chemistry Letters and Reviews》2013,6(4):243-247
Abstract A protocol for regeneration of key intermediate of aprepitant from its undesired diastereomers is described. This work features the recycling of at least one-third of the undesired isomers (ent-6, 7 and ent-7) to desired isomer 4 as the key early intermediate for the synthesis of aprepitant 1. The key step in our strategy involves diastereomeric salt preparation. 相似文献
59.
Julia Intemann Dr. Jan Spielmann Dr. Peter Sirsch Prof. Dr. Sjoerd Harder 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(26):8478-8489
A new tetranuclear magnesium hydride cluster, [{ NN ‐(MgH)2}2], which was based on a N? N‐coupled bis‐β‐diketiminate ligand ( NN 2?), was obtained from the reaction of [{ NN ‐(MgnBu)2}2] with PhSiH3. Its crystal structure reveals an almost‐tetrahedral arrangement of Mg atoms and two different sets of hydride ions, which give rise to a coupling in the NMR spectrum (J=8.5 Hz). To shed light on the relationship between the cluster size and H2 release, the thermal decomposition of [{ NN ‐(MgH)2}2] and two closely related systems that were based on similar ligands, that is, an octanuclear magnesium hydride cluster and a dimeric magnesium hydride species, have been investigated in detail. A lowering of the H2‐desorption temperature with decreasing cluster size is observed, in line with previously reported theoretical predictions on (MgH2)n model systems. Deuterium‐labeling studies further demonstrate that the released H2 solely originates from the oxidative coupling of two hydride ligands and not from other hydrogen sources, such as the β‐diketiminate ligands. Analysis of the DFT‐computed electron density in [{ NN ‐(MgH)2}2] reveals a counterintuitive interaction between two formally closed‐shell H? ligands that are separated by 3.106 Å. This weak interaction could play an important role in H2 desorption. Although the molecular product after H2 release could not be characterized experimentally, DFT calculations on the proposed decomposition product, that is, the low‐valence tetranuclear Mg(I) cluster [( NN ‐Mg2)2], predict a structure with two almost‐parallel, localized Mg? Mg bonds. As in a previously reported β‐diketiminate MgI dimer, the Mg? Mg bond is not characterized by a bond critical point, but instead displays a local maximum of electron density midway between the atoms, that is, a non‐nuclear attractor (NNA). Interestingly, both of the NNAs in [( NN ‐Mg2)2] are connected through a bond path that suggests that there is bonding between all four MgI atoms. 相似文献
60.
Azetidinium triflates were reacted in a one-pot two-steps sequence involving, generation of an azetidinium ylide, its alkylation with an halide, and final regioselective Hofmann elimination of the produced alkylated azetidinium ion to yield substituted α,β-unsaturated nitriles bearing an aminoethyl side-chain. The scope of this sequence was examined, and was found to depend both on the steric hindrance around the reactive center in the starting azetidinium salt, and on the nature of the reacting halide. Produced acrylonitriles were further used in DBU-catalyzed conjugate addition of amines, to yield 4-amino-2-aminomethyl-butyronitriles with fair diastereoselectivity, or, alternatively, to give C2 symmetrical cyclopropanes. 相似文献