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41.
风机系统工作时的一个突出问题是其进风口和出风口产生的噪声。由于风机流场非常复杂,以及实验成本、实验条件限制,基于计算流体力学(Computational Fluid Dynamics, CFD)的理论逐渐成为风机噪声估计的重要方法。本文拟通过对由风机及其冷却系统构成的封闭系统进行数值建模和仿真,判断出风机主要气动噪声源的位置和种类,为处于封闭系统内的风机的噪声大小预测,提供一个可供参考的信息。结果表明:风机出口腔体内部非定常压力波动强度最大。 相似文献
42.
用快速摄影拍摄了升液管中熔融铝合金液面的运动状态,发现在升液过程中,存在着沿液面切向的流动。这种流动具有撇除表面氧化膜、净化合金液的作用。为实现平稳充型和减少热损耗,设计了正弦速度函数。用实验方法确定了速度振幅。 相似文献
43.
本文提出了一种整数规划中的指数一对数对偶.证明了此指数-对数对偶方法具有的渐近强对偶性质,并提出了不需要进行对偶搜索来解原整数规划问题的方法.特别地,当选取合适的参数和对偶变量时,原整数规划问题的解可以通过解一个非线性松弛问题来得到.对具有整系数目标函数及约束函数的多项式整规划问题,给出了参数及对偶变量的取法. 相似文献
44.
阐述了巧妙地利用基本RS触发器输入信号由违反约束条件变化成符合约束条件的瞬间,触发器的两个输出立即会变化成稳定的互斥状态的特点,可开发出多个输入信号竞争时只有一个有效状态输出的逻辑电路;并以双CPU系统共享随机存储器原理为例,介绍利用基本RS触发器解决双方访问硬件时的冲突问题,对其中的核心硬件电路工作原理进行了剖析。 相似文献
45.
关联规则向量化挖掘算法及其在车险精算中的应用 总被引:1,自引:0,他引:1
本文首先回顾了关联规则的基本概念和传统的Apriori算法,然后利用关联规则的数据库是布尔型数据库的特点,在计算关联规则的支持度和置信度的时候引进向量数乘和向量内积的概念,得到关联规则向量化挖掘算法ARVDA,避免全数据库逐条记录模式匹配和属性分层,提高算法的速度.最后,本文利用提升度量关联规则的重要性,采用单独追踪和对比分析方法,衡量车险精算中风险因子的有效性.结果表明把车辆使用性质作为车险定价的分级因素是比较合理的,对于非运营车辆需要合理的费用附加. 相似文献
46.
47.
低压铸造充型动力学的高速摄影观察 总被引:3,自引:1,他引:2
利用高速摄影和影片运动分析技术,在透明模型上对低压铸造充型过程进行了实时观察和运动分析。结果表明:低压铸造充型的动力学特征可用二阶非线性微分方程描述。 相似文献
48.
针对带不等式约束的极大极小问题,借鉴一般约束优化问题的模松弛强次可行SQP算法思想,提出了求解不等式约束极大极小问题的一个新型模松弛强次可行SQCQP算法.首先,通过在QCQP子问题中选取合适的罚函数,保证了算法的可行性以及目标函数F(x)的下降性,同时简化QCQP子问题二次约束项参数α_k的选取,可保证算法的可行性和收敛性.其次,算法步长的选取合理简单.最后,在适当的假设条件下证明了算法具有全局收敛性及强收敛性.初步的数值试验结果表明算法是可行有效的. 相似文献
49.
We describe the synergistic utilization of titanocene/photoredox dual catalysis driven by visible light for the radical opening/spirocyclization of easily accessible epoxyalkynes. This environmentally benign process uses the organic donor–acceptor fluorophore 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) as a photocatalyst and Hantzsch ester (HE) as an electron donor instead of stoichiometric metallic reductants. The photocatalytic conditions showed exceptionally high reactivity for the synthesis of privileged and synthetically challenging spirocycles featuring a spiro all-carbon quaternary stereocenter. Cyclic voltammetry (CV) studies suggest that Cp2TiIIICl is the catalytically active species.We describe the synergistic utilization of titanocene/photoredox dual catalysis driven by visible light for radical opening/spirocyclization of easily accessible epoxyalkynes.Over the last few decades, radical-based transformations have been increasingly used in organic synthesis due to their salient features, such as ease of generation, mild reaction conditions, and broad functional group compatibility.1,2 As a mild single-electron-transfer (SET) reagent, titanocene monochloride (Cp2TiIIICl) is considered a formidable tool in contemporary radical chemistry due to its ability to promote various fundamental radical-based transformations.3–7 Cp2TiIIICl was first introduced by Nugent and RajanBabu as a very mild stoichiometric reagent for the reductive opening of epoxides.8–11 Later, the catalytic conditions developed by Gansäuer et al. (Scheme 1a)12 employing stoichiometric amounts of active metals in combination with 2,4,6-collidine·HCl further expanded its applications and led to the discovery of a number of novel transformations.13–16 The key to success was the formation of a stable complex A in reactions while decreasing the concentration of active Cp2TiIIICl.17,18 We were interested in the radical opening/cyclization reaction of epoxides which has attracted considerable attention from the synthetic community and has been used numerous times in the synthesis of natural products.19,20 Nevertheless, this reaction required stoichiometric metallic reductants and proceeded slowly particularly with sterically hindered substrates even with high catalyst loading.21 Therefore, the development of an eco-friendly and efficient catalytic system with an expanded substrate scope is highly desirable.Open in a separate windowScheme 1Cp2TiIIICl mediated radical opening/spirocyclization of epoxides; (a) generation of TiIIIvia a metal reduction approach; (b) dual titanocene/photoredox catalysis; (c) examples of drugs and natural products containing heterospirocycles.In recent years metallaphotoredox catalysis has been a new and rapidly growing research subject.22–29 Photoredox processes can directly modulate the oxidation state of metals by electron transfer (ET).30–33 Given that the generation of TiIII is a SET process, we envisioned that the reduction could be facilitated by a photoredox-controlled process while overcoming the aforementioned limitations. On the other hand, spirocycles bearing a chiral spiro all-carbon quaternary carbon are particularly attractive synthetic targets in pharmaceutical development (Scheme 1c).34–36 Such privileged rigid 3D structures offer the concomitant ability to project functionalities in all three-dimensional orientations and led to enhanced pharmacological activities of molecules. Thus significant attention has been paid to their synthesis.37,38 Against this backdrop, here we describe our efforts on the synthesis of various heterospirocycles with the aid of photoredox catalysis.We chose epoxyalkyne 2a as a model substrate for optimization of reaction conditions. After a systematic variation of different reaction parameters, we were pleased to identify the optimal reaction conditions in which a mixture of Cp2TiCl2 (5.0 mol%), [Ir(dtbbpy)(ppy)2]PF6 (1a, 1.0 mol%, EIII/II1/2 = −1.51 V vs. SCE in MeCN), HE (1.2 equiv.) and 2a (1.0 equiv.) in THF at room temperature under the irradiation of a 10 W 450 nm light emitting diode (LED) lamp for 12 hours afforded the desired product 3a in an excellent yield of 96% (13 : 1 d.r.) upon isolation (entry 1). Using a commercial 23 W compact fluorescent lamp (CFL) instead of the 10 W 450 nm LED did not compromise the overall yield of the reaction (entry 2). Notably, when the loading of Cp2TiCl2 was decreased to as low as only 2.0 mol%, the reaction still led to full conversion and produced 3a in 95% yield (entry 3). Further screening of other photosensitizers revealed that the cheap and readily obtained organic dye 4CzIPN 1b is a competent alternative, which led to full conversion with 94% isolated yield (entry 4). Importantly, the reaction did not proceed in the absence of Cp2TiCl2, HE, the photocatalyst, or visible light (entries 5–8). Various solvents, including DMF, MeOH, DMSO, and MeCN, were screened, and they all resulted in poor conversion. The use of other organic electron donors, such as triethylamine, triethanolamine, and ascorbic acid, afforded the product in poor yield.With satisfactory reaction conditions established, we then explored the scope of the cyclization reaction using 4CzIPN as the photosensitizer. Positively, the cyclization reaction worked well and afforded the desired variably heterospirocyclic products in good to excellent yield (Tables 2 and and3).3). The reaction allows the rapid construction of various 5/5, 5/6, 5/7 and 5/8 spiro-ring fused systems (3a–3k) bearing tetrahydrofuran or pyrrolidine motifs via the 5-exo cyclization pathway. Interesting, the diastereoselectivity of the cyclization reaction is highly correlated with the ring size in the substrates. Heterospirocycles containing a 5/5 spiro-ring fused system (3a–3f) were obtained with surprisingly high diastereoselectivity. In some cases (3b, 3c, and 3e) only a single isomer was obtained. The product 3d with a sterically hindered t-butyloxy carbonyl (Boc) protecting group on the N atom was obtained with reduced diastereoselectivity (5 : 1 d.r.). The diastereoselectivities dropped in 5/6, 5/7 and 5/8 spiro-ring fused systems. Given that enantioenriched epoxides could be easily obtained (e.g. via sharpless asymmetric epoxidation), this strategy provides access to optically active spirocycles featuring an all-carbon quaternary stereocenter with the transfer of stereochemical information from epoxides (3c, 3e and 3f). Bis-heterospirocyclic scaffolds were frequently employed in pharmaceutical chemistry. For example, bis-heterospirocyclic 3d is the core structure of DLK inhibitors39 and XEN402 (ref. 40) (scheme 1c), which are used for treating neurodegeneration and congenital erythromelalgia respectively. Furthermore, 6-exo cyclization was also investigated under the standard conditions and smoothly produced a serious of drug-like 6-(trifluoromethyl)-3-pyridinesulfonyl piperidine derivatives including 6/5, 6/6 and 6/7 spiro-ring fused systems (5a–5k) in generally excellent yields. Moreover, cyclization reactions with epoxy-alkynes afforded products containing exocyclic-alkenes and free alcohols which were suitable for further functionalization. This approach provides access to a broad range of novel spirocyclic piperidine and pyrrolidine spirocycles which could be of interest to synthetic and medicinal chemists.Scope of 5-exo and 6-exo cyclizationa,b,c,d
Open in a separate windowaReaction conditions: 2 and 4 (100 mg, 0.1 M in THF).bIsolated yield.c 3c, 3e and 3f were synthesized from enantiomer pure epoxides.dYields within parentheses are based on catalytic conditions using metal as a reductant: CpTi2Cl2 (5 mol%), Zn (2.0 eq.), coll·HCl (2.5 eq.), THF, 20 hours.Additive effect on Ti-catalyzed cyclizationa,b
Open in a separate windowaIn all cases, 2l was used as the substrate and the yield of 3l was determined with 1H NMR.bValues within parentheses are recovery yields of the additives determined with 1H NMR.To examine the scalability of the reaction, gram-scale synthesis of 3a and 3c was performed under the standard conditions with 23 W CFL irradiation. Pleasingly, 92% (3a) and 89% (3c) isolated yields were obtained respectively without any deterioration. Furthermore, an additive-based investigation41 was performed and the results are summarized (Table 3). From this screening, we found that 11 out of 12 additives have no adverse impact on the yield of the reaction. The additives were recovered after the reaction, including benzoxazole 6a, quinazolinone 6b, collidine 6c, tetrahydroquinoline 6d, benzothiazole 6e, indole 6f, and benzofuran 6h. However, quinoxaline 6l strongly inhibited Ti catalysis and 3l was produced in only 23% yield. Notably, the reaction is compatible with various functional groups including phenols 6g, free alcohols and alkene 6j, iodobenzene 6i, ester 3m, ether 3n, dioxolane 5i, lactone 5k, and alkyne 3e. The generality of the dual Ti/4CzIPN catalysis system was further demonstrated by a highly related hydrogen transfer reaction of epoxides which could exclusively provide anti-Markovnikov alcohols (see the ESI S8†). The low-cost of 4CzIPN, broad compatibility with sensitive functional groups, and simple operation conditions are appealing for laboratory and industrial applications.Importantly, the reactions were re-subjected to metal reduction catalytic conditions for comparison with the photocatalytic conditions and the yields are shown within parentheses (Table 2d). It clearly showed that the yields were generally lower. Particularly, we found that the yields dropped dramatically as the steric encumbrance of the substrates increased. For example, substrates containing dioxolane (2g, 4i) or a 7/8-membered-ring (2j, 2k, 4j) afforded the products (3g, 5i, 3j, 3k, 5j) in less than 10% yield with the recovery of the starting materials. 相似文献
50.
A new expression for contact deformation is given, and the normal contact stiffness between single asperities is derived according to Hooke’s law. A contact model between two ellipsoidal asperities is simulated by the FE method, the result compared with the theoretical solution. It is found that the curves of the normal contact stiffness versus the included angle in the principal curvature direction show similar trends and evolve as a cosine feature. The effects of the parameters on normal contact stiffness are found to show that normal contact stiffness increases and reaches the upper limit gradually with an increase in these parameters. 相似文献