Micellar effects on dediazoniation and on azo coupling reactions

The effects of a sodium dodecyl sulfate, SDS, micellar solution on the coupling rates of two arenediazonium ions, ArN(2)(+), with the hydrophobic 1-naphthylamine, 1NA and N-(1-naphthyl) ethylenediamine, NED, coupling agents and with the hydrophilic Na salt of 2-naphthol-6-sulfonic acid, 2N6S, have been studied. First, we explored the micellar effects on the thermal decomposition of the arenediazonium ions. The observed rate constants are slightly depressed or increased, depending on the nature of ArN(2)(+), compared to those in pure water upon increasing [SDS]. Estimations of the corresponding association constant to the micelle indicate that a significant fraction of the arenediazonium ions are incorporated into the micelles even at low surfactant concentrations. The sulfonate group in 2N6S prevents its incorporation into the micellar aggregate due to the electrostatic barrier imposed by the micelles and, in consequence, the coupling reaction is inhibited. In contrast, when employing the naphthylamine derivatives, the observed rate constant increase rapidly up to a maximum at [SDS]相似文献   

Ligand effects in palladium‐catalyzed Suzuki and Heck coupling reactions

A range of sterically hindered diimine ligands and their palladium (II) complexes were synthesized. These compounds were fully characterized by elemental analysis, ¹H and ¹³C‐NMR spectroscopy. The use of the palladium complexes as catalysts for Suzuki and Heck coupling has been studied in an attempt to demonstrate the effect of side groups on catalytic activity. It was clearly seen that the location of side ? CH₃ groups which bound to benzene ring had little effect on catalytic activity. Interestingly when we changed these ? CH₃ groups with ? Cl groups the activity of the complexes increased. On the other hand, side groups which bound to imine nitrogen also had a large effect on catalytic activity. Copyright © 2008 John Wiley & Sons, Ltd.     

Hydrogen abstraction from organosilicon compounds. The reactions of fluoromethyl radicals with tetramethylsilane. Polar effects in gas phase reactions

The photolysis of 1,1,3,3-tetrafluoroacetone has been reinvestigated as a source of CHF₂ radicals at temperatures up to 578°K, and the following rate constant ratio was determined for the reactions

1 θ= 2.303 RT in kcal/mole.

1,1-Difluoro- and 1,1,3,3-tetrafluoroacetone were photolyzed in the presence of tetramethylsilane, and Arrhenius parameters were measured for the hydrogen abstraction reactions: R + Me₄Si → RH + Me₃SiCH₂

A perchlorate-promoted electrochemically induced nitride coupling reaction

The osmium nitride complex [Os^VI(NH₃)₄N]³⁺ undergoes a one-electron reduction in acetonitrile to give [Os^V(N)(NH₃)₄]²⁺, which further reacts by nitride coupling to give the μ-dinitrogen osmium complex [(CH₃CN)(NH₃)₄Os^II(N₂)Os^II(NH₃)₄(CH₃CN)]⁴⁺. The formation of the μ-dinitrogen osmium complex is promoted by the presence of perchlorate anion, which causes the deposition of [(CH₃CN)(NH₃)₄Os^II(N₂)Os^II(NH₃)₄(CH₃CN)](ClO₄)₄ on the electrode surface upon repetitive voltammetric scans.     

Substituent effects on carbohydrate coupling reactions promoted by insoluble silver salts

Coupling reactions were performed in the α-D-glucopyranosyl bromide series (i.e. compounds $\text{1}$ a–c, $\text{2}$ a, b, $\text{3}$ and $\text{4}$) and the α-D-mannopyranosyl bromide series (i.e. compounds $\text{7}$ a–c, $\text{8}$ a, b, $\text{9}$) with aglycons $\text{5}$ or $\text{10}$ in the presence of insoluble silver salt promotors. (i.e. silver silicate and silver zeolite). The insoluble silver salt promotes the formation of the β-glycosidic linkage, while a non-participating group is present at C-2 of the glycon. It was found that in both series 4-O-acyl functions increase the β/α ratio of the glycosidic bond formation relative to 4-O-alkyl functions, whereas 3-O and 6-O-acyl functions decrease this ratio. We assume that inductive effects are responsible for the influence exerted by 3-O-substituents, but that through-bond interactions are essential for the effects exerted by 4-O and 6-O substituents. Another unexpected finding was that coupling of α-D-mannopyranosyl bromide derivatives gave much higher β/α ratios than the corresponding reactions of α -D-glucopyranosyl bromides.     

Theoretical study on solvation effects in chemical reactions: A vibrational coupling model

A vibrational coupling model to treat the solvation effects in chemical reaction rate calculations is proposed and applied to the intramolecular hydrogen transfer reaction CH₃O· → ·CH₂OH in the condensed phase. The effect of solvation is taken into account in two ways: (1) the solvent effect on the activation energy of the reaction is simulated by including 39 surrounding water molecules, represented by fractional charges at the assumed atomic positions, in the potential energy surface calculation; and (2) the vibrational couplings between the 10 nearest solvent molecules and the molecules constituting the reaction system are explicitly included in a vibrational frequency calculation. RRKM theory with Miller's tunneling correction included is employed to calculate the rate constants. The effect of solvation causes a significant change in the chemical reaction rate, mainly through a lowering of the activation energy. The vibrational coupling causes a slight increase of the rate constant in the tunneling region by perturbing the vibrational frequencies of the reactant and transition states, which appear in the rate-constant expression, but has little effect at higher temperatures.     

Abnormal salt effects on reactions between ions: The coupling of salt and solvent effects

Salt effects on the reactions [Fe(CN)₆]^4? + S₂O⁼₈ and [Ru(NH₃)₅py]²⁺ + S₂O have been studied in media of different dielectric constants constituted by mixtures of water with organic cosolvents. It is known that salt and solvent effects are coupled and, consequently, cannot be treated separately. This implies that salt (and solvent) effects need to be carefully analyzed before using them as a tool for mechanistic discrimination. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 582–588, 2009     

Preparation and reactions of trifluoromethyl-sulfur oxide nitride

In the reaction of CF₃SOCl with NaN₃ or (CH₃)₃SiN₃, azide, CF₃SON₃ (I) is formed:

Upon gentle warming to room temperature, (I) decomposes under evolution of N₂ to give CF₃SON (II),

which however oligomerizes instantaneously, The monomeric nitride(II) can be captured in the presence of reactive halogene compounds to yield N-substituted trifluoromethan-imino sulfonyl chlorides. With CH₃CN, a six-membered ring compound is obtained:

     

Components and coupling in enzyme-catalyzed reactions

Many enzyme-catalyzed reactions involve coupling of two or more reactions that could otherwise be catalyzed separately. When biochemical reactions are coupled, the equilibrium composition is very different from that when the reactions are not coupled. The number of components in a chemical reaction is equal to the number of independent conservation equations for atoms of elements, but the number of components in an enzyme-catalyzed reaction that is coupled is larger than the number of independent conservation equations for atoms of elements. The investigation of these additional conservation equations by use of linear algebra is complicated by the fact that in dilute aqueous solutions, the activity of water is taken to be unity. This causes an incompatibility of conservation matrices and stoichiometric number matrices that can be avoided by use of the further transformed Gibbs energy G' ' that provides the criterion for spontaneous change and equilibrium when the standard transformed Gibbs energy of water is constant. In the most striking example discussed, the enzyme mechanism of a ligase reaction introduces three constraints in addition to conservation of atoms of elements. This is completely unheard of in chemical reaction thermodynamics.     

Polar effects in free-radical reactions. New selective alkylations of heteroaromatic bases by benzoylperoxide and olefins.

The decomposition of benzoylperoxide in the presence of cyclohexene and 1-methylcyclohexene and protonated lepidine provides a new type of selective homolytic aromatic substitution.     

Polar and solvation effects in reactions of oxygen atoms and hydroxyl and alkoxyl radicals with oxygen-containing compounds

Experimental data on the activation energies of reactions of H-abstraction from oxygencontaining compounds by oxygen atoms and hydroxyl and alkoxyl radicals in the gas and liquid phases have been analyzed by means of the parabolic model of the transition state. The contribution of polar interaction to the activation energies of the reactions has been calculated. The contribution of solvation to the activation energy has been calculated by comparison of the reaction parameters of the respective reaction in the liquid and gas phases.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 38–42, January, 1994.     

Polar effects in free-radical reactions. New synthetic developments in the functionalization of heteroaromatic bases by nucleophilic radicals

The synthetic interest of the direct substitution of protonated heteroaromatic bases by nucleophilic carbon-centered radicals is furtheron developed by the following new achievements: i) utilization of the redoc system

in several solvents; ii) utilization of benzoyl peroxide in alcohols; iii) carbamoylation by HCONH₂ and H₂O₂ in the presence of catalytic amounts of Fe(II). These systems allow to obtain either substitution till now tried without success or reactions of industrial interest. Polar effects play a dominant role in determining reactivity, selectivity and synthetic applications; in particular the role of the strongly nucleophilic intermediate radicals of pyridinyl type in the rearomatization step is emphasized.     

Directing effects of tethered alkenes in nickel-catalyzed coupling reactions of 1,6-enynes and aldehydes

Nickel-catalyzed reductive coupling reactions of aldehydes and 1,6-enynes proceed in excellent regioselectivity in the absence of a phosphine, and the use of a monodentate phosphine additive leads to the formation of the opposite regioisomer with equally high selectivity. Both products are the result of the same fundamental mechanism, with the inversion of regioselectivity being the result of stereospecific ligand substitution at the metal center.     

Response reactions: equilibrium coupling

It is pointed out and illustrated in the present paper that if a homogeneous multiple equilibrium system containing k components and q species is composed of the reactants actually taken and their reactions contain only k + 1 species, then we have a unique representation with (q - k) stoichiometrically independent reactions (SIRs). We define these as coupling reactions. All the other possible combinations with k + 1 species are the coupled reactions that are in equilibrium when the (q - k) SIRs are in equilibrium. The response of the equilibrium state for perturbation is determined by the coupling and coupled equilibria. Depending on the circumstances and the actual thermodynamic data, the effect of coupled equilibria may overtake the effect of the coupling ones, leading to phenomena that are in apparent contradiction with Le Chatelier's principle.     

可见光参与的氧化偶联反应研究进展

实现资源和能源利用高效化、操作简单化、条件温和化、环境友好化以及产物高效选择性是有机合成的重要研究方向,而探索绿色温和条件下构建化学键的有效方法是有机合成领域的基本挑战之一。伴随着金属有机化学的发展,过渡金属催化的偶联反应已经成为构建碳-碳和碳-杂键的有效手段,而传统的交叉偶联一般是基于亲核试剂与亲电试剂之间的反应,需要进行预官能团化和再官能团化的步骤。近年来,在此基础上发展起来的氧化偶联反应利用合适的氧化剂实现两个亲核试剂直接构建化学键也得到了国内外有机化学家的广泛关注。氧化偶联反应的发展极大地提高了构建碳-碳键及碳-杂键的效率,尤其是利用交叉脱氢偶联实现直接的碳-氢键或杂-氢键的活化直接构建化学键,避免了传统偶联过程中的预官能团化步骤,为直接利用简单的原料实现高效、复杂的有机合成开辟了一条新的道路。越来越多的第一过渡金属催化的氧化偶联反应涉及到单电子转移的过程,这种自由基氧化偶联模式在绿色化学的发展中具有光明的前景。可见光是一种可再生的资源,可见光催化符合绿色合成、环境友好和可持续发展的理念,在有机合成领域中引起了广泛的关注。建立在光诱导的单电子转移过程的基础上,可见光催化为实现温和条件下构建化学键提供了一种新的思路。虽然很多的有机分子不能有效吸收可见光,但是利用可见光催化剂(光敏剂)在电子和能量转移过程中的独特优势,能够有效实现在可见光作用下的一系列光化学反应。光催化是有机合成构建新物质的一种有效的手段,随着光催化在有机合成领域中的发展,一系列新型温和而有效的可见光催化的氧化偶联反应也逐渐被报道。虽然过渡金属催化的氧化偶联反应已经取得了重大的进展,但是利用可见光催化的策略以实现两个亲核试剂之间化学键的构建作为一个新的领域,近年来才刚刚发展起来。尤其是结合可见光催化条件温和、环境友好的特点和氧化偶联反应的原子经济性特点,将可见光诱导的单电子转移过程运用到交叉脱氢偶联反应,引起了广泛的关注。光催化氧化偶联反应不仅解决了传统偶联中的步骤经济性和原子经济性的问题,同时也具备了可见光反应温和环保的特点。近几年来已经发展了多种类型的光催化氧化偶联反应,包括光催化氧化含氮化合物的偶联反应、光催化氧化脱羧偶联反应、以及光催化交叉偶联放氢反应等。但是在光催化氧化偶联领域的相关报道大多局限于含氮化合物和易氧化的底物的活化氧化,因此任然存在着很多的挑战。本文总结了最近几年来的不同反应类型的可见光催化的氧化偶联的研究和进展。而对这些反应的理解和认识,可以为人们发展更多的高效率和高选择性的可见光催化氧化偶联反应提供帮助。