Electrophilic and Nucleophilic Addition of Alkanethiols to 2-Vinyloxyethylmethacrylate

Under acid- or base-catalyzed conditions, thiols add regio- and chemos-electively either to vinyloxy or methacrylate group of vinyloxyalkyl-methacrylates to give polyfunctional methacrylates or polyfunctional vinyl ethers, respectively.     

系统论述烯烃亲电加成反应的立体选择性

亲电加成是烯烃最具代表性的典型反应,在本科的教学中,要求学生熟练掌握和应用。烯烃的亲电加成反应不仅涉及到反应的活性、区域选择性问题,还涉及到反应的立体选择性问题。但对于立体选择性的阐述,很多教科书并不全面,甚至不够严谨,因而在教学过程中发现学生在理解这部分内容时存在一定的误解。针对这一问题进行了系统的论述,以帮助学生更好更正确地理解和掌握烯烃的亲电加成反应的立体选择性。     

Sequential One‐Pot Addition of Excess Aryl‐Grignard Reagents and Electrophiles to O‐Alkyl Thioformates

The sequential addition of aromatic Grignard reagents to O‐alkyl thioformates proceeded to completion within 30 s to give aryl benzylic sulfanes in good yields. This reaction may begin with the nucleophilic attack of the Grignard reagent onto the carbon atom of the O‐alkyl thioformates, followed by the elimination of ROMgBr to generate aromatic thioaldehydes, which then react with a second molecule of the Grignard reagent at the sulfur atom to form arylsulfanyl benzylic Grignard reagents. To confirm the generation of aromatic thioaldehydes, the reaction between O‐alkyl thioformates and phenyl Grignard reagent was carried out in the presence of cyclopentadiene. As a result, hetero‐Diels–Alder adducts of the thioaldehyde and the diene were formed. The treatment of a mixture of the thioformate and phenyl Grignard reagent with iodine gave 1,2‐bis(phenylsulfanyl)‐1,2‐diphenyl ethane as a product, which indicated the formation of arylsulfanyl benzylic Grignard reagents in the reaction mixture. When electrophiles were added to the Grignard reagents that were generated in situ, four‐component coupling products, that is, O‐alkyl thioformates, two molecules of Grignard reagents, and electrophiles, were obtained in moderate‐to‐good yields. The use of silyl chloride or allylic bromides gave the adducts within 5 min, whereas the reaction with benzylic halides required more than 30 min. The addition to carbonyl compounds was complete within 1 min and the use of lithium bromide as an additive enhanced the yields of the four‐component coupling products. Finally, oxiranes and imines also participated in the coupling reaction.     

Electrophilic Reactivities of Azodicarboxylates

The kinetics of the reactions of the azodicarboxylates 1 with the enamines 2 have been studied in CH₃CN at 20 °C. The reactions follow a second‐order rate law and can be described by the linear free energy relationship log k₂(20 °C)=s(N+E) (E=electrophilicity parameter, N=nucleophilicity parameter, and s=nucleophile‐specific slope parameter). With E parameters from ?12.2 to ?8.9, the electrophilic reactivities of 1 turned out to be comparable to those of α,β‐unsaturated iminium ions, amino‐substituted benzhydrylium ions, and ordinary Michael acceptors. While the E parameters of the azodicarboxylates 1 determined in this work also hold for their reactions with triarylphosphines, they cannot be used for estimating rate constants for their reactions with amines. Comparison of experimental and calculated rate constants for cycloadditions and ene reactions of azodicarboxylates provides information on the concertedness of these reactions.     

卤化氢与烯烃亲电加成反应的机理及立体选择性*

卤化氢与烯烃的亲电加成反应既是烯烃的重要反应之一,也是一种制备卤代烷的重要方法。现行教材中通常只介绍其分步的碳正离子中间体机理和加成的区域选择性,未涉及反应的立体选择性。介绍了文献中关于该类反应的机理及其立体选择性,希望能够从教学上更新和提高对这类反应的认识。     

含邻手性碳原子双键亲电加成反应的立体选择性模型

本文主要介绍了预测邻位具有手性中心π体系亲电加成反应的立体选择性模型,尤其是α,β-不饱和羰基化合物亲核共轭加成形成的烯醇负离子及其类似物和硝基烯烃亲核加成形成的氮酸根类中间体的烷基化和动力学控制质子化反应的立体选择性模型。立体位阻效应控制的Zimmerman前过渡态模型主要适用于底物构象受限的环状或联烯型烯醇负离子的动力学质子化。对于构象转化相对灵活的直链底物,Houk基于量子化学计算研究提出了亲电试剂的进攻角度或接触角在经过环状过渡态时是锐角还是钝角决定了亲电加成反应的立体选择性控制(Houk模型)。而Fleming模型首次将烯丙基A-1,3张力引入各类烯醇负离子烷基化和质子化的过渡态。Mohrig模型在考虑A-1,3张力的基础上,将吸电子取代基与即将形成的σ键反式共平面,主要阐述了立体电子效应对酯的烯醇负离子动力学控制质子化立体选择性的影响,后来又提出了金属离子参与的立体电子效应控制的六元环状半椅式模型。本文希望对理解亲电加成反应的立体化学控制提供一些有用的信息。     

Selectfluor: Mechanistic Insight and Applications

The replacement of hydrogen atoms with fluorine substituents in organic substrates is of great interest in synthetic chemistry because of the strong electronegativity of fluorine and relatively small steric footprint of fluorine atoms. Many sources of nucleophilic fluorine are available for the derivatization of organic molecules under acidic, basic, and neutral conditions. However, electrophilic fluorination has historically required molecular fluorine, whose notorious toxicity and explosive tendencies limit its application in research. The necessity for an electrophilic fluorination reagent that is safe, stable, highly reactive, and amenable to industrial production as an alternative to very hazardous molecular fluorine was the inspiration for the discovery of selectfluor. This reagent is not only one of the most reactive electrophilic fluorinating reagents available, but it is also safe, nontoxic, and easy to handle. In this Review we document the many applications of selectfluor and discuss possible mechanistic pathways for its reaction.     

Electrophilic Addition to Alkenes: The Relation between Reactivity and Enthalpy of Hydrogenation: Regioselectivity is Determined by the Stability of the Two Conceivable Products

Although electrophilic addition to alkenes has been well studied, some secrets still remain. Halogenations, hydrohalogenations, halohydrin formations, hydrations, epoxidations, other oxidations, carbene additions, and ozonolyses are investigated to elucidate the relation of alkene reactivities with their enthalpies of hydrogenation (ΔH_hyd). For addition of electrophiles to unconjugated hydrocarbon alkenes, ln(k) is a linear function of ΔH_hyd, where k is the rate constant. Linear correlation coefficients are about 0.98 or greater. None of the many previously proposed correlations of ln(k) with the properties of alkenes or with linear free‐energy relationships match the generality and accuracy of the simple linear relationship found herein. A notable exception is acid‐catalyzed hydration in water or in solvents stabilizing relatively stable carbocation intermediates (e.g., tertiary, benzylic, or allylic). ¹³C NMR chemical shifts of the two alkene carbons also predict regioselectivity. These effects have not been noted previously and are operative in general, including addition to heteroatom‐substituted alkenes.     

单壁碳纳米管与卤代烷的亲电加成反应

主要考察了以路易斯酸为催化剂,几种卤代烷烃为反应试剂的单壁碳纳米管侧壁的亲电加成反应,并通过傅立叶红外光谱、热失重分析和拉曼光谱验证了实验所得产物。此反应的目的是在单壁碳纳米管的侧壁连上烷基基团以提高其溶解性和分散性,并可使其更好地与聚烯烃相结合从而提高复合材料的性能,因而具有较高的研究和应用价值。     

磷化氢与烯烃反应的原位NMR谱研究

本文应用加压原位(in-situ)核磁共振技术,在反应温度70～75℃,压力1.0～2.0MPa,氘代苯为溶剂,偶氮二异丁腈(AIBN)为引发剂的条件下探索了磷化氢与丙烯腈,4-乙烯基吡啶,烯丙基胺,丙烯酰胺,氨基吡啉和N-乙烯基咔唑的自由基加成反应,结果表明除氨基吡啉之外的其它五种含氮化合物均能与磷化氢发生反应,从而找到合成含氮有机膦的良好路线。     

Catalytic performance of symmetrical and unsymmetrical sulfur-containing pincer complexes: synthesis and tandem catalytic activity of the first PCS-pincer palladium complex

The synthesis and catalytic applications of a new aryl-based unsymmetrical PCS-pincer complex are reported. Preparation of the robust air- and moisture-stable PCS-pincer palladium complex 5[X] started from the symmetrical alpha,alpha'-dibromo-meta-xylene and involved the selective substitution of one bromide by PPh(2)(BH(3)), followed by substitution of the second bromide by SPh and subsequent introduction of the palladium. The new PCS complexes (5[X]) were employed as catalysts in two important organic transformations. Firstly, complex 5[Cl] displays high catalytic activity in aldol reactions but enters the catalytic cycle as a precatalyst. Secondly, complex 5[BF(4)] displays tandem catalytic activity in the coupling of allyl chlorides with aldehydes and imines in the presence of hexamethylditin. In these tandem catalytic reactions the first process is the conversion of allyl chlorides into trimethylallyltin (and trimethyltin chloride) with Sn(2)Me(6), which is followed by catalytic allylation of aldehyde and sulfonimine substrates. In addition, we present a new catalytic process for the one-pot allylation of 4-nitrobenzaldehyde with vinyloxirane. The catalytic performance of the novel PCS-pincer palladium complex was compared to those of its symmetrical PCP- and SCS-pincer complex analogues. It was concluded that the unsymmetrical PCS complex advantageously unifies the attractive catalytic features of the corresponding symmetrical pincer complexes including both (pi-) electron-withdrawing (such as phosphorus) or (sigma-) electron-donating (such as sulfur and nitrogen) heteroatoms. Thus, in the aldol reaction the PCS-pincer palladium complex 5[X] provides a high turnover frequency, while in the tandem process both reactions are catalysed with sufficiently high activity.     

Addition of thiols to (-)-carvone

Polyfunctional thioterpenoids were formed in the presence of a Lewis-acid catalyst by reaction of (-)-carvone with thiols that involved the endocyclic double bond and the carbonyl.Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 393–395, September–October, 2004.     

Stereoselective Iodolactonization of 4‐Allenoic Acids with Efficient Chirality Transfer: Development of a New Electrophilic Iodination Reagent

A highly stereoselective iodolactonization of 4‐allenoic acids with a new sterically demanding electrophilic iodination reagent to afford optically active γ‐butyrolactones has been developed. The reaction shows high efficiency of axial chirality transfer and excellent Z/E selectivity and has been applied to the synthesis of chiral cis‐β,γ‐disubstituted γ‐butyrolactones to give very high diastereomeric and enantiomeric excess values. The reaction has been successfully utilized in the synthesis of naturally occurring compounds (+)‐cis‐whisky lactone and (+)‐cis‐3‐methyl‐4‐decanolide.     

Electrophilic Reactions of Dithio-Substituted o-Methoxycinnamyllithium

The dithio-substituted o-methoxycinnamyllithium 3L reacted at the α-site with D₂O and most of carbonyl compounds in extents of 70–100 %. while it reacted with halides at either the α or γ-site, depending on the nature of respective halide.     

Electrophilic Cyanation of Boron Enolates: Efficient Access to Various β‐Ketonitrile Derivatives

The highly efficient electrophilic cyanation of boron enolates using readily available cyanating reagents, N‐cyano‐N‐phenyl‐p‐toluenesulfonamide (NCTS) and p‐toluenesulfonyl cyanide (TsCN), is reported. Various β‐ketonitriles were prepared by this new protocol, which has a remarkably broad substrate scope compared to existing methods. The present method also allowed efficient synthesis of β‐ketonitriles containing a quaternary α‐carbon center. In addition, a preliminary result with the use of a chiral boron enolate for the enantioselective cyanation reaction is described.     

通过加成反应制备纳米材料的新方法

通过加成反应,结合再沉淀法,制备得到Ni(DMG)₂的单晶纳米结构,并且通过调节反应物浓度可实现不同形貌的可控生长,在浓度低的条件下,可得到六方纳米管材,而随着浓度的升高,逐渐变为实心的六方纳米棒,这种制备方法提供了一种潜在的新方法——即通过加成反应来制备纳米材料。     

Oxidative Addition of Haloheteroarenes to Palladium(0): Concerted versus SNAr‐Type Mechanism

The kinetics of the oxidative additions of haloheteroarenes HetX (X=I, Br, Cl) to [Pd⁰(PPh₃)₂] (generated from [Pd⁰(PPh₃)₄]) have been investigated in THF and DMF and the rate constants have been determined. In contrast to the generally accepted concerted mechanism, Hammett plots obtained for substituted 2‐halopyridines and solvent effects reveal a reaction mechanism dependent on the halide X of HetX: an unprecedented S_NAr‐type mechanism for X=Br or Cl and a classical concerted mechanism for X=I. These results are supported by DFT studies.