Revisiting Markovnikov addition to alkenes via molecular electrostatic potential

Molecular electrostatic potentials (MESP) surrounding the pi-region of several substituted ethylenes (CH(2)CHR) have been characterized by locating the most negative-valued point (V(min)) in that region. The substituents have been classified as electron donating and withdrawing on the basis of the increase or decrease in the negative character of V(min) in these systems as compared to ethylene. The values of V(min) show a good linear correlation with the Hammett sigma(p) constants, suggesting that the substituent electronic effects in substituted ethylenes and substituted benzenes are basically similar. With electron-donating substituents, the position of MESP minimum is closer to the unsubstituted carbon facilitating the pi-complex formation of it with HCl at this site. Such a regiospecific pi-complex formation is found to favor the formation of Markovnikov-type transition state for the addition of HCl to CH(2)CHR. For the electron-withdrawing substituents, the V(min) location is almost equidistant and farther from the ethylenic carbon atoms. This and the less negative V(min) values account for the less regiospecific CH(2)CHR...HCl pi-complexes as well as the transition states for the HCl addition to CH(2)CHR when R is an electron-withdrawing group. The interaction energy (E(int)) between CH(2)CHR and HCl for the formation of the CH(2)CHR...HCl pi-complex shows a good linear correlation with the corresponding V(min) value.     

Measurement of the diffusion coefficient of atomic chlorine in molecular chlorine

A scheme for measuring the diffusion coefficients of reactive atomic species through their radiative recombination reactions is proposed. The method was employed to measure the rate of diffusion of chlorine atoms in chlorine molecules and led to an observed value for the diffusion coefficient of 0.149 ± 0.025 cm² s⁻¹ at 298 K and 1 atm.     

Anti-Markovnikov addition to alkenes with a neighbouring thioacetal function

The acid-induced cyclisation of unsaturated thioacetals 6 gives anti-Markovnikov products 9, apparently involving sulfur elimination and readdition.     

Regioselectivity in addition of atomic fluorine to alkenes

Irradiation of N₂F₄(NF₂) with propene and with 1,1-difluoroethene in the gas phase atroom temperature leads to the addition of F and NF₂ across the double bond. The ratioof terminal to internal addition of F to propene was found to be 1.4:1 after correctingfor the decomposition of vibrationally excited intermediates. In the case of 1,1-difluoroethene,the corrected ratio of F addition is 5:1 with addition at the methylene sitepredominating. The difference in selectivity of F addition to propene and to 1,1-difluoroethenecorrelates with spin density difference in the π-π* triplet state of the alkeneand with the difference in the HOMO coefficients of the substrate alkene, i.e. the patternof selectivity is that predicted by the State Correlation Diagram model of Shaik andCanadell.     

Synthesis of a fluorous ligand and its application for asymmetric addition of dimethylzinc to aldehydes

A new fluorous ligand was synthesized from the acetonide of dimethyl tartarate, which showed excellent asymmetric induction on the addition of dimethylzinc to aldehydes. This ligand will be useful for synthesis of bioactive compounds with a methyl carbinol moiety. It could be recycled without using a fluorous solvent or a fluorous column.     

Substituent effects in addition of iodine thiocyanate to alkenes

The plots of logarithms of relative rates of ISCN addition to alkenes versus alkene IPs and versus alkene HOMO energies reveal that the alkene relative reactivity depends upon both electronic and steric effects of the substituents. Steric effects are related not only to the degree of substitution on the CC bond but also to the relative position, size, and branching of alkyl substituents.     

Stereochemistry of addition of disulfonium dications to alkenes

Stereochemistry of the addition of a cyclic disulfonium dication generated from 1,4-dithiane to alkenes was studied. The reaction is nonstereospecific, which can be attributed to a stepwise mechanism.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1660–1662, August, 2004.     

Regioselective cobalt-catalyzed addition of sulfides to unactivated alkenes

A novel method to synthesize tertiary alkyl/aryl sulfides in a mild and regioselective manner from unactivated alkenes using cobalt catalysis is described. The methodology is compatible with sensitive functionalities and is successful with several different types of alkenes and sulfides.     

Ab initio quantum chemical calculation of the mechanism for molecular addition of chlorine to acetylene in the gas phase

We have investigated the mechanism of gas-phase chlorination of acetylene using anab initio quantum chemical method in a 3–21G° basis. We show that formation ofcis andtrans addition products may be connected with the appearance of carbene in the reaction medium, formed upon addition of a chlorine molecule to one of the acetylene carbon atoms. We have found that intermediate carbene may also be a source for the appearance of radicals in the reaction medium.Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5 ul. Murmanskaya, Kiev 253094, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 1, pp. 17–19, January–February, 1996. Original article submitted April 6, 1995.     

Mechanism of the addition of tributyltin iodoacetate to alkenes

The mechanism of the formation of lactones by free radical additions of tri-n-butyltin iodoacetate to alkenes occurs through a two-step process: a homolytic addition of the ester (addition of ^·CH₂CO₂SnBu₃, iodine transfer), followed by a fast ionic cyclization with elimination of tri-n-butyltin iodide.     

Factors controlling the addition of carbon-centered radicals to alkenes

The addition rate constants of radicals to alkenes are strongly substituent dependent because of enthalpic, polar and steric effects. Recent absolute experimental and high level ab initio data for many prototype additions of small radicals are analyzed with the aid of the state correlation diagram. This leads to a unifying rationalization of the various effects and allows the prediction of rate constants to one order of magnitude or better. Propagation rate coefficients of homo- and copolymerizations and penultimate effects are also discussed.     

Polar addition of perfluoro-tert-hexyl iodide to alkenes and alkynes

The effective charges on the atoms in molecules of perfluoroalkyl halides of general formula (CF₃)_nCF_3–nX (X=Cl, Br, I) have been calculated by the AM1 semiempirical method. In polar solvents perfluoro-tert-hexyl iodide is reduced under the action of alkenes and aromatic hydrocarbons to form 2-hydroperfluoro-2-methyl-pentane and perfluoro-2-methyl-2-pentene. In ethyl acetate the regio- and stereo-specific addition of perfluoro-tert-hexyl iodide to alkenes, butadiene, and alkynes takes place, which is associated with the realization of a polar ion-radical mechanism for the reaction.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2057–2061, September, 1989.     

A synthesis of trisubstituted alkenes by a Ru-catalyzed addition

Catalyzed by ruthenium trisacetonitrile hexafluorophosphate 4, the Alder-ene type reaction of alkenes and internal alkynes provides an effective way to synthesize trisubstituted alkenes. Unlike most typical olefination protocols, this reaction is atom economical, and affords trisubstituted alkenes with defined olefin geometry. The regioselectivity can be explained invoking a steric argument based on the proposed mechanism. The first C-C bond formation generally involves sterically less hindered carbons of the alkenes and alkynes. Modest to very high regioselectivity can be achieved depending on the steric difference of the two substituents of alkynes.     

Direct addition of Zr-C bonds of alkylzirconocenes to activated alkenes

Direct addition of alkylzirconocenes to activated alkenes was found, for the first time. Octyl- and decylzirconocene chloride reacted with benzylidenemalononitrile to give the corresponding addition products after hydrolysis in 86% and 79% yield, respectively. Zirconacyclopentanes showed a similar reactivity toward activated alkenes with a two-electron-withdrawing group. On the other hand, treatment of the reaction mixture of zirconacyclopentanes and ylidenemalononitriles with iodine afforded six-membered cyclic compounds in high yields. The diastereoselectivity of the cyclized compound was remarkably high and the high selectivity originated from the Zr-promoted cyclization. The structures of cyclic compounds 10b and the major diastereoisomer of 10d were determined by X-ray analysis. Zirconacyclopentenes reacted with ylidenemalononitrile with high chemoselectivity in which the sp(3)-carbon attached to zirconium reacted with ylidenemalononitrile.     

Absolute rate constants for the addition of hydroxymethyl radicals to alkenes in methanol solution

Absolute rate constants and their temperature dependencies were determined for the addition of hydroxymethyl radicals (CH₂OH) to 20 mono- or 1,1-disubstituted alkenes (CH₂ = CXY) in methanol by time-resolved electron spin resonance spectroscopy. With the alkene substituents the rate constants at 298 K (k₂₉₈) vary from 180 M^?1s^?1 (ethyl vinylether) to 2.1 middot; 10⁶ M^?1s^?1 (acrolein). The frequency factors obey log A/M^?1s^?1 = 8.1 ± 0.1, whereas the activation energies (E_a) range from 11.6 kJ/mol (methacrylonitrile) to 35.7 kJ/mol (ethyl vinylether). As shown by good correlations with the alkene electron affinities (EA), log k₂₉₈/M^?1s^?1 = 5.57 + 1.53 · EA/eV (R² = 0.820) and E_a = 15.86 ? 7.38 · EA/eV (R² = 0.773), hydroxymethyl is a nucleophilic radical, and its addition rates are strongly influenced by polar effects. No apparent correlation was found between E_a or log k₂₉₈ with the overall reaction enthalpy. © 1995 John Wiley & Sons, Inc.     

The stereochemistry of metal catalysed hydrogen cyanide addition to alkenes

Reactions of both terminal and cyclic alkenes have been shown to occur in a stereospecifically $\text{cis}$ - manner with deutrerium cyanide when a catalyst system based on Pd(DIOP)₂ was used. Reactions of cyclohexene give only the products of equatorial cyanide incorporation.     

A convenient AIBN-initiated radical addition of ethyl iododifluoroacetate to alkenes

The AIBN-initiated addition of ethyl 4-iodo-2,2-difluoroacetate to a variety of alkene substrates is described. The addition generally led to the corresponding addition products in good to excellent yields and various functional groups could be tolerated under the reaction conditions.