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21.
The new dinuclear half-sandwich complexes of titanium with xylene bridge, [Ti(η5-cyclopentadienyl)Cl2L]2[CH2-C6H4-CH2] (L = Cl (3), L = O-2,6-iPr2C6H3 (4), L = N(SiMe3)(2,6-Me2C6H3) (5)), have been synthesized. The complexes 4 and 5 have been prepared by the reaction of the complex 3 with the corresponding lithium salts of aryloxy and anilide. Structure of these complexes has been characterized by 1H and 13C NMR. The change of substituent from chloride, 3, to anilide, 5, at titanium resulted in chemical shift change of cyclopentadienyl protons from 6.92 and 6.79 to 6.13 and 5.95 ppm probably due to the positive electron density delivery from the anilide group. It was found that all three half-titanocenes were effective catalyst for the generation of SPS (syndiotactic polystyrene). Xylene bridged dinuclear catalyst (4) with aryloxy substituent exhibited very high activity (458 kg of SPS/(mol of [Ti])h), at 40 °C, whereas the analogous hexamethylene bridged dinuclear half-titanocene catalyst (7) showed a lower activity (80.7 kg of SPS/(mol of [Ti])h) under the same conditions. While the catalyst 3 was the most active catalyst among three complexes less than 40 °C the catalyst 5 exhibited the highest activity at 70 °C. Xylene linkage was suggested to be too stiff to permit any kind of intramolecular interaction between two active centers. Lack of steric disturbance due to the rigidity of the xylene bridge might give rise to the similar properties of dinuclear metallocene to the corresponding mononuclear metallocene to result in not only the facile coordination of monomer at the active center to lead high activity but also the easier β-H elimination comparing to the dinuclear catalysts with the flexible bridge to result in the formation of lower molecular weight polymer. 相似文献
22.
We propose a method for evaluating the size of the laser-induced breakdown region in water based on the detection and analysis of optodynamic waves. The breakdown region is an optodynamic source of pressure waves that propagate into the surrounding liquid as an ultrasonic pulse. In the experiment the optical breakdown was generated by a standard ophthalmic Nd:YAG laser with a pulse duration of 10 ns and a maximum energy per pulse of 10 mJ. The pulses were detected inside the liquid with a laser-beam deflection probe. The waveforms were captured in the far-field and analyzed. The analysis provides information about the apparent size of the optodynamic source, which is directly related to the size of the breakdown region. The proposed method can be adapted for online monitoring. 相似文献
23.
Model cross and ring-closing metathesis strategies toward the C1–C8-linear carbon skeleton are presented. The introduction of a four-atom tether enables the formation of 12-membered rings in good-to-excellent yields and stereoselectivity. Furthermore, the study revealed that the cross-metathesis approach and the formation of medium ring sizes via ring-closing metathesis are much less favorable. 相似文献
24.
The efficient total synthesis of biologically interesting (+/-)-daurichromenic acid is accomplished starting from 2,4-dihydroxy-6-methylbenzoic acid or 2,4-dihydroxy-6-methylbenzaldehyde in one or two steps. 相似文献
25.
26.
Young Jin Cho Seung Hwan Lee Jong Woo Bae Sung Hoon Kim Sam Rok Keum Cheol Min Yoon 《合成通讯》2013,43(12):2205-2211
The 7-acetoxyspiropyrans (4a and 4b) have been prepared by the reaction of Fischer's bases (3) with 2,4-diacetoxybenzaldehyde in refluxing ethanol in moderate yields. The acetyl group of 7-acetoxy-spiropyran 4a was easily removed to give a 7-hydroxyspiropyran 6 in high yield under the basic condition. 相似文献
27.
Heung Soo Baek Byung Woo Yoo Sam Rok Keum Cheol Min Yoon Sung Hoon Kim Joong Hyup Kim 《合成通讯》2013,43(1):31-38
α-Haloketones reacted with acyl cyanides to form 1,3-diketones in the presence of samarium diiodide. The reaction was assumed to proceed via a mechanism involving samarium enolates formed in situ from α-haloketones. 相似文献
28.
Drnovšek Rok Milavec Kapun Marija Rajkovič Uroš 《Central European Journal of Operations Research》2021,29(3):1021-1036
Central European Journal of Operations Research - Ventilator-associated pneumonia is a hospital-acquired infection of the lungs occurring in mechanically ventilated patients. An active risk... 相似文献
29.
Alenka Vesel Rok Zaplotnik Gregor Primc Xiangyu Liu Kaitian Xu Kevin C. Chen Chiju Wei Miran Mozetic 《Plasma Chemistry and Plasma Processing》2016,36(3):835-848
Samples of porous foam from polyurethane/urea copolymers based on polyethylene glycol (PURPEG) were prepared in the form of 1-mm-thick discs of diameter 10 cm and exposed to ammonia plasma created by inductively coupled radiofrequency discharge in either low density (E mode) or high density (H mode). The evolution of surface composition and structure upon plasma treatment was characterized by X-ray photoelectron spectroscopy. Treatment in the H mode caused depletion of oxygen even after 2 s of treatment, whereas treatment in the E mode caused gentle functionalization with amide groups. The concentration of functional groups depended on the discharge power, and the best results were obtained at moderately high power just before the transition from E to H modes. 相似文献
30.
The first total synthesis of naturally occurring mappain has been achieved by a convergent sequence. The key strategy involved in the synthesis of mappain was a (E)‐stilbene formation by Horner–Wadsworth–Emmons reaction of the corresponding prenylated benzaldehyde with a geranylated benzyl phosphonate. 相似文献