Kinetic studies of retinol addition radicals

Retinol neutral radicals (RS-retinol˙), generated from the reaction of retinol with 4-pyridylthiyl and 2-pyridylthiyl radicals in argon-saturated methanol, undergo β-elimination, which can be monitored via the slow secondary absorption rise at 380 nm attributed to the rearrangement of the unstable retinol neutral addition radicals to the more stable addition radicals. Rate constants for the β-elimination reactions (k(β)) of 4-PyrS-retinol˙ were measured at different temperatures and the Arrhenius equation for the reaction is described by log (k(β)/s(-1)) = (12.7 ± 0.2) - (54.3 ± 1.3)/θ, where θ = 2.3RT kJ mol(-1). The reactivities of retinol addition radicals (RS-retinol˙), generated from the reaction of retinol with various thiyl radicals, towards oxygen have also been investigated in methanol. In the presence of oxygen, the decay of RS-retinol˙ fits to biexponential kinetics and both observed rate constants for the RS-retinol˙ decay are oxygen-concentration dependent. This suggests that at least two thiyl addition radicals, formed from the reaction of RS˙ with retinol, undergo oxygen addition reactions. In light of the estimated rate constants for oxygen addition to RS-retinol˙ and RS-CAR˙ (CAR: carotenoid), the antioxidant-prooxidant properties of retinol are discussed.     

Silylzincation of carbon-carbon multiple bonds revisited

The first investigation of the copper-catalyzed silylzincation of alkynes as well as a diene and styrene using bis(triorganosilyl) zinc reagents led to the development of an efficient procedure and the disclosure of an unexpected bissilylation and unforeseen regioselectivity.     

Transition-metal complex catalyzed polyfluoroalkylation. I. Facile addition of polyfluoroalkyl halides to carbon-carbon multiple bonds

Addition of polyfluoroalkyl halides to alkynes and alkenes bearing a variety of substituents is effected by the catalysis of iron, cobalt and ruthenium carbonyl complexes to give the corresponding adducts in good to excellent yields under mild conditions.     

Addition of ArSSAr to carbon-carbon multiple bonds using electrochemistry

ArS(ArSSAr)⁺ (arylbis(arylthio)sulfonium ions), which were generated and accumulated by the electrochemical oxidation of diaryl disulfides (ArSSAr) in CH₂Cl₂ at −78 °C, reacted with alkenes to give the corresponding diarylthio-substituted compounds in a stereospecific manner in good yields, when the reaction was quenched with a soft nucleophile, such as allylsilanes, ketene silyl acetals, and triethylamine. A mechanism involving the initial formation of an episulfonium ion followed by ring-opening by the attack of ArSSAr has been suggested. The reactions of ArS(ArSSAr)⁺ with alkynes also took place to give 1,2-diorganothio-substitued alkenes stereoselectively under similar conditions.     

Asymmetric hydroamination of non-activated carbon-carbon multiple bonds

The catalysed enantioselective formation of carbon-nitrogen bonds by the hydroamination reaction is reviewed. All examples deal with substrates containing non-activated carbon-carbon multiple bonds which are transformed either via intramolecular or intermolecular reactions. Structurally different complexes already provided nitrogen containing compounds/heterocycles with high enantioselectivities.     

碳碳不饱和键氟化加成反应的研究进展

选择性地向有机物中引入氟原子越来越受到重视，近年来通过碳碳不饱和键的 氟化加成来引入氟原子发展迅速。综述了碳碳不饱和键氟化加成反应的研究进展。     

Nickel-catalyzed addition of benzenethiol to alkynes: Formation of carbon-sulfur and carbon-carbon bonds

Nickel-catalyzed addition of benzenethiol to alkynes leads to alkenyl and dienyl sulfides; the direction of the process can be controlled by varying the PhSH/alkyne ratio. An advanced procedure, which ensures higher yields of 2-phenylsulfanylalkenes, includes gradual addition of alkyne to the other reactants. The structures of conjugated dienyl sulfides formed in the reaction were determined by 2D NMR spectroscopy. Dedicated to Academician O. M. Nefedov on the occasion of his 75th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2030–2034, November, 2006.     

A novel approach for the formation of carbon-nitrogen bonds: azidation of alkyl radicals with sulfonyl azides.

Two preparatively attractive methods for the azidation of alkyl radicals are described. Secondary and tertiary alkyl iodides and dithiocarbonates are easily converted into the corresponding azides, either by reaction with ethanesulfonyl azide in the presence of dilauroyl peroxide, or by treatment with benzenesulfonyl azide and hexabutylditin in the presence of a radical initiator. Interestingly, intramolecular tandem radical cyclization-azidation processes can be performed in high yields.     

Kinetic study for addition of the phenylthio radicals to vinylpyridines

The absolute rate constants for reversible addition of the substituted phenylthio radicals to vinylpyridines in solution at 23°C have been determined by flash photolysis method. For each phenylthio radical, the reactivities of vinylpyridines is 3–5 times lower than that of styrene. The relative equilibrium constants estimated with flash photolysis indicate that the stabilities of the adduct carbon-centered radicals generated from vinylpyridines are similar to that from styrene. The Hammett plots suggest that the difference in the reactivities originates in the polar transition state; the nitrogen atom in 4-vinylpyridine withdraws the electrons of vinylic double bond.     

Formation of carbon-carbon bonds with radicals derived from the esters of thiohydroxamic acids

Radicals formed from the esters of thiohydroxamic acids readily add to electron deficient olefins to give adducts of potential synthetic value in variable yield. In certain cases the added sulphur function is easily eliminated with reformation of olefin.     

Tandem catalysis: generating multiple contiguous carbon-carbon bonds through a ruthenium-catalyzed ring-closing metathesis/Kharasch addition

Tandem catalysis can offer unique and powerful strategies for converting simple starting materials into more complex products in a single reaction vessel while generating less waste and minimizing handling. In this regard, Grubbs' ruthenium alkylidene (Cy3P)2Cl2Ru=CHPh is shown to catalyze two mechanistically distinct transformations to offer a unique protocol that effects multiple bond changes in a single operation. A tandem ruthenium-catalyzed olefin ring-closing metathesis (RCM)/Kharasch addition allows for the facile preparation of bicyclic [3.3.0], [4.3.0], and [5.3.0] ring systems in one step from the appropriately functionalized acyclic precursors. For substrates where the intramolecular Kharasch addition fails, an intermolecular Kharasch addition is possible. By combining the intra- and intermolecular Kharasch additions with RCM, three new contiguous carbon-carbon bonds with multiple stereocenters can be generated by the ruthenium catalyst in a controlled fashion in one operation through two mechanistically distinct pathways.     

Theoretical analysis of the alternative additions of radicals to multiple bonds

The alternative additions of the hydrogen atom and methyl, aminyl, and methoxyl radicals to the double bond of CH₂=Y (Y = CHR, CR₂, CHCH=CH₂, CHPh, NH, O) compounds are theoretically analyzed using the intersecting parabolas method and DFT. The enthalpies, activation energies, and geometric parameters of the transition state in the reactions R^· + CH₂=Y → RCH₂Y^· and R^· + CH₂=Y → RYC^·H₂ are calculated. The results obtained by the two methods are compared with experimental data. The competing alternative radical additions to the multiple bonds are governed by the enthalpies of the reactions.     

Kinetic studies on the addition of dipropylamine to starch acrylate

Summary The kinetics of nucleophilic addition of dipropylamine to vinyl groups of starch acrylate has been investigated. The reactions were carried out under pseudo-first order conditions in tetrahydrofuran with excess amine within the temperature range from 25 to 45^oC. Pseudo-first-order rate constants for addition and the activation energy have been determined.     

Kinetic studies on the addition of phenol to S-phenyl-S-vinyl-N-p-tosylsulfilimine derivatives

The rate constants of nucleophilic addition of phenyl to vinylsulfilimine(VSI) and its derivatives were determined by an ultraviolet spectrophotometer at 25°C, and the rate equations which can be applied over a wide pH range were obtained. On the basis of the pH-rate profile, product analysis, general base catalysis, and the substituent effect, a plausible mechanism of this addition reaction was proposed: Below pH 3.0, the reaction proceeded via the addition of a neutral molecule to the carbon-carbon double bond after protonation at the nitrogen atom of the sulfilimine. Above pH 10.0, the addition of a phenoxide ion to the carbon-carbon double bond was rate controlling. However, in the pH range of 3.0 to 10.0, these two reactions occurred competitively.     

Catalytic addition of C-H bonds to multiple bonds with the aid of ruthenium complexes

Ruthenium complexes, e.g., RuH2(CO)(PPh3)3, have been found to catalyze the direct addition of ortho carbon-hydrogen bonds of aromatic ketones to olefins and acetylenes with high efficiency and selectivity. The C-H/olefin coupling reaction is applicable to not only C-H bonds in aromatic ketones but also to those in a,b-enones and aro-matic esters. Catalytic addition of ortho carbon-hydrogen bonds of aromatic imines to olefins is found to be catalyzed by Ru3(CO)12.     

Kinetic studies on the addition of dipropylamine to glucose acrylate

The kinetics of the nucleophilic addition reaction of dipropylamine to the vinyl double bonds of glucose acrylate (GA) has been investigated. The reactions were carried out under pseudo first-order conditions in tetrahydrofuran with excess amine within the temperature range from 25 to 45 °C. The activation energy of dipropylamine addition to the vinyl groups of GA is 21.1 kJ/mol.     

Catalysis of hydrosilylation of carbon-carbon multiple bonds: Recent progress

Recent progress in the catalytic hydrosilylation of organic and organosilicon compounds containing carbon-carbon multiple bonds is reviewed. Related to this, dehydrogenative silylation is also discussed. During the last decade new hydrosilylation catalysts, predominantly homogenous and heterogenous transition methal complexes, have been developed. These catalysts offer not only increased efficiency and turnover rate but also improved regioselectivity and stereoselectivity; moreover, there has been development in the mechanistic rationale behind these improvements. Application and extension of these basic chemical advances are found in many areas including polyorganosiloxane curing, hydrosilylation polymerization, polysiloxane functionalization, and silicon-containing dendrimer development.     

Surveying approaches to the formation of carbon-carbon bonds between a pyran and an adjacent ring

We have examined several methods for the stereoselective formation of carbon-carbon bonds between contiguous rings where a stereogenic center is already present. The approaches investigated were a [1,3] oxygen to carbon rearrangement of cyclic vinyl acetals, an intermolecular enolsilane addition into an in situ generated oxocarbenium ion, an intramolecular conjugate addition of tethered alkoxy enones, and epimerization of several α-pyranyl cycloalkanones. These routes have been found to be complementary in several cases and have enabled formation of both the trans:anti and cis:anti stereoisomers in good to excellent yields and varying diastereoselectivities. We have proven C2-C2′ relative stereochemistry of 1-2 via a chemical correlation.     

Nucleophilic addition of hydroxymethyl-1,2,3-triazoles to multiple bonds

The addition of C- and N-hydroxymethyl-1,2,3-triazoles to the activated multiple bonds of acrylonitrile and benzoylacetylene was studied. The structures of the compounds obtained were proved by means of the PMR and IR spectra.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1417–1420, October, 1982.     

A new method for the synthesis of acyltitanium complexes and their application to copper-mediated acylmetallation of carbon-carbon multiple bonds in aqueous media

Treatment of alpha, beta-unsaturated carbonyl compounds or methyl propargyl ether with acylchlorobis(cyclopentadienyl)titanium in the presence of triethylamine and a copper salt in aqueous THF resulted in acylation of the carbon-carbon multiple bond, yielding the corresponding 1,4-diketones in good yields.