Initial Addition Step Research Articles

Gas Phase Reactions of Radical Cations of Halogenated Arenes and Alkenes: The Radical Cation Mediated Nucleophilic Substitution at C(sp2) Atom

Radial cations of unsaturated organic hydrocarbons are reactive intermediates of many important chemical reactions. In particular, the rate constants of the reactions of these deficient species with rich reactants are increased by several orders of magnitude compared to the reaction between neutral partners. This effects has been termed electron hole catalysis. To get insight into the mechanism(s) of these reactions and the nature of the electron hole catalysis the reactions have been investigated in the gas phase using Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry, and by molecular orbital calculations. The studies included the reactions of mono- and dihalogenated benzenes and naphthalenes as well as mono- and dihalogenated alkenes. Halogen substituents were chosen for practical reasons since the loss of a halogen substituent during the process can be conveniently used as a monitor reaction. The first part of this account discusses the methods used to determine the kinetics of ion/molecule reactions, in particular FT-ICR-mass spectrometry, and the special features of ion/molecule-reactions in the diluted gas phase. The second part deals with the reactions of halogenated arene radical cations with ammonia and amines as nucleophiles. The reactions correspond to a substitution of one halogen substituent by a radical cation mediated nucleophilic aromatic substitution and are only moderately efficient. Contrary to chemical intuition, the reaction efficiency decreases in the series Cl, Br, and I as substituent. Further, a peculiar dependance on the position of a second substituent is observed. Both effects can be explained by a mechanism involving at least the two steps of an addition of the nucleophile and of an elimination of the substituent and involving a reaction intermediate corresponding to a distonic onium ion. According to the Shaik/Pross model the first step is rate determining and depends strongly on the difference of the ionization energies of the reactants. A second effect comes front the dipole moment of the (neutral) dihalogenated arene, showing a decrease in the activation energy of the addition with an increase in dipole moment. The third part presents results of reactions of the radical cations of rnono- and dihalogenated alkenes using ammonia, amines, and aliphatic alcohols as nucleophiles. These reactions are often efficient and may be collision controlled. Besides substitution of a halogen a variety of other reaction products is observed. The efficient reaction of alkene radical cations is in agreement with electron hole catalysis. An analysis by the Shaik/Pross model reveals that the initial addition step is now a very fast inner sphere transfer which occurs probably without any activation barrier. This is substantiated by molecular orbital calculations for selected reaction systems. Interestingly, experimental results show that addition of the nucleophile to the ionized C-C double bond competes even with intermolecular exothermic transfer and exothermic proton transfer. As a consequence, the beta-distonic onium ion, which is formed during the initial addition step, arises as a highly excited intermediate. The further course of the total reaction depends on the properties of this intermediate beta-distonic ion. The consequences of this reaction model for the individual reaction systems are discussed.

Reactions of 2-bromopropene radical cation with amines—a study by FT-ICR spectrometry and DFT calculation

The reactions of the 2-bromopropene radical cation 1 + with four amines (CH 3NH 2, CH 3CH 2NH 2, (CH 3) 2NH, and (CH 3) 3N) and CH 3SH were studied by Fourier transform-ion cyclotron resonance (FT-ICR) spectrometry and molecular orbital calculations. The ionization energy (IE) of (CH 3) 2NH, and (CH 3) 3N), respectively, are considerably below the IE( 1). Consequently, fast electron transfer (ET) is the only reaction detected with 1 +. The amine radical cation formed undergoes a (known) fast secondary reaction with the neutral amine and generates the corresponding ammonium ion. ET is also exothermic (0.14–0.78 eV) for the other reactants, but in these reaction systems substitution of the Br atom of 1 + by the nucleophile and oxidation of the nucleophile by hydride transfer to 1 + competes efficiently with ET. The minimum energy reaction path (MERP) for the reaction system 1 +/CH 3NH 2 was analyzed by theoretical calculation of stationary points using DFT at the level B3LYP/6-31+G(3df,2p)//B3LYP/6-31G(d). The calculated MERP agrees to the general reaction scheme developed for the reactions of ionized alkenes with nucleophiles. In particular, the initial reaction step for substitution and oxidation, which competes with ET, is addition of the nucleophile to the ionized double bond. No energy minimum along the MERP was found with B3LYP/6-31G(d) corresponding to the encounter complex of 1 + and CH 3NH 2, while both UHF/6-31(Gd) and MP2/6-31G(d) show this complex as an energy minimum. The charge and spin density of the complex is almost completely localized at the 2-bromopropene component, confuting any “hidden charge transfer” in the complex prior to further reactions. The initial addition step, which is strongly exothermic, can be viewed as an “ inner sphere” ET which combines the single ET from the nucleophile to 1 + with simultaneous bond formation between reactants. This “ inner sphere” ET competes with “ outer sphere” ET resulting only in charge exchange between the reactants. In the case of simple amines as nucleophiles, the “ outer sphere” ET requires considerable reorganization of the amine structure. This is indicated by significantly higher vertical IE v of the amines. Therefore, a fast ET prevails over fast addition of the nucleophile to the ionized double bond of 1 + only if the IE v(amine) is distinctly below IE( 1).

A density functional study on the Pt(0)-catalysed hydrosilylation of ethylene

A theoretical investigation has been performed on the reaction pathways involved in the Pt(PH 3) 2-catalysed ethylene hydrosilylation by SiH 4. We have analysed both mechanisms proposed to explain the formation of the vinyl-silane product, i.e. (i) the Chalk–Harrod mechanism, which consists of Si–H oxidative addition of SiH 4 to Pt(PH 3) 2, ethylene insertion into the Pt–H bond followed by Si–C reductive elimination; and (ii) the modified Chalk–Harrod mechanism, which consists of the same oxidative addition of SiH 4 to Pt(PH 3) 2 followed by ethylene insertion into the Pt–Si bond and by C–H reductive elimination. We characterised all the elementary steps involved in both mechanisms by gradient-corrected DFT methods, and performed ab initio molecular dynamics simulations on the initial oxidative addition step. We compute a maximum energy barrier of 25.1 and 38.2 kcal/mol for the Chalk–Harrod and modified Chalk–Harrod mechanisms, respectively, both in correspondence of the ethylene insertion-isomerisation step. Our DFT approach is found to provide comparable results to those obtained at the MP4SDQ level on the Hartree–Fock optimised geometries, except for the ethylene insertion and isomerisation steps in the modified Chalk–Harrod mechanism, for which we compute lower energy barriers at the DFT level. Our results confirm that the Chalk–Harrod mechanism is kinetically favored over its modified version.

Synthetic and kinetic studies of the reaction of amino acid esters with tricarbonyl(dienyl)iron cations

Amino acid esters add rapidly and reversibly to the dienyl iron cations [Fe(CO) 3(2-XC 6H 6)] + ( Ia, X  MeO; Ib, X  H) to give first the cation adducts II and then the neutral adducts III, as follows: [Fe(CO) 3(XC 6H 6)] + ( I) + H 2NCH(R)CO 2,Et K1 K −1[Fe(CO) 3XC 6H 6·NH 2CH( R)CO 2Et)] + ( II) K1 K− [Fe(CO) 3XC 6H 6· NHCH(R)(CO 2Et)] ( III). Results of 1H NMR spectral studies confirm that the addition of the amino acid esters to the dienyl rings of ( I) is exo. With the chiral substrate Ia, moderate chiral discrimination occurs in the attack of ( R)-amino acid esters, providing a new route to optically active Ia. Kinetic studies of the reactions reveal the general rate law, Rate = k[Fe][RNH 2]; this observation may be rationalized in terms of a mechanism involving the steady-state formation of the intermediate II. Since k 2[RNH 2] can be expected to be >> k −1 (and k −2 negligible), the second-order rate constants k equate with k 1for the initial ring addition step. The large negative entropies of activation support such a bimolecular process. The data permit amino acid esters to be placed in a quantitative order of nucleophility with some 40 other nucleophiles towards cation Ib.

Theoretical Study of the Rates and Mechanism of the Gas-Phase Reaction H + HN3 → NH2 + N2

Abstract Reaction of the hydrogen atom with hydrazoic acid HN3 in the gas phase has been investigated theoreticallly. Tracing of the reaction path by the SCF procedure based on the 6-31G (d,p) basis functions has shown that the reaction proceeds through an initial rate-determining hydrogen atom addition to HN3 giving a doublet radical H2N3, which is subsequently self-decomposed into NH2 + N2. The barrier height for the initial addition step calculated by the MRD-CI method employing the [4s2p1d] basis sets is 21 kJ mol−1 (with the vibrational zero-point energy correction). The bimolecular rate constants for the overall reaction calculated by the conventional transition state theory combined with appropriate tunneling corrections are found to agree well with the experimental data reported over the temperature range 300 to 500 K.

ChemInform Abstract: THE LEWIS‐ACIDITY OF SQUARE‐PLANAR NICKEL(II) COMPLEXES. I. A KINETIC AND THERMODYNAMIC STUDY OF THE REACTION OF BIS(O,O‐DIETHYL PHOSPHORODITHIOATO‐S,S′‐)NICKEL(II) WITH BIDENTATE NITROGEN DONORS

The kinetics of the addition reaction between bis(O,O-diethyl phosphorodithioato-S,S'-)nicke1(11) and a series of bidentate nitrogen ligands has been examined by stopped-flow techniques. The reactions are first order in ligand and nickel complex with kf, the second-order rate constant, in the range from 103 to 106 1. mol-1 s-1 at 25oC in benzene. Solvent effects on the reaction with 2,2'-bipyridyl are minor. The variation in kf with ligand is discussed in terms of the steric requirements of the ligands, the bulkiest ligand, 2,9-dimethyl-1,l0-phenanthroline, reacting most slowly. For the ligands 1,l0-phenanthroline and 2,2'-bipyridyl in the more polar solvents, successive displacement of the thiophosphate ligands follows the initial addition step.

The Lewis-acidity of square-planar nickel(II) complexes. I. A kinetic and thermodynamic study of the reaction of Bis(O,O-diethyl phosphorodithioato-S,S'-)nickel(II) with bidentate nitrogen donors

The kinetics of the addition reaction between bis(O,O-diethyl phosphorodithioato-S,S'-)nicke1(11) and a series of bidentate nitrogen ligands has been examined by stopped-flow techniques. The reactions are first order in ligand and nickel complex with kf, the second-order rate constant, in the range from 103 to 106 1. mol-1 s-1 at 25�C in benzene. Solvent effects on the reaction with 2,2'-bipyridyl are minor. The variation in kf with ligand is discussed in terms of the steric requirements of the ligands, the bulkiest ligand, 2,9-dimethyl-1,l0-phenanthroline, reacting most slowly. For the ligands 1,l0-phenanthroline and 2,2'-bipyridyl in the more polar solvents, successive displacement of the thiophosphate ligands follows the initial addition step.

Elimination and addition reactions. Part XXV. Addition–elimination reactions of phenoxyvinyl sulphones

Reactions of cis- and trans-2-phenoxyvinyl p-tolyl sulphones with alcoholic alkoxides have been studied. Product analyses and kinetic studies show that in each case a two-stage addition–elimination pathway is followed. The elimination step is assigned the (E1cB)R mechanism on the basis of measurements carried out in deuteriated solvents. The rate constants and activation parameters for both addition and elimination stages match closely the rate constants and activation parameters obtained for models which simulate the two stages individually. The results are discussed against the background of earlier work on elimination of poor leaving groups from activated saturated substrates, and of good leaving groups, particularly halide, from activated, unsaturated substrates. It is now shown that in an activated, unsaturated system with a poor leaving group, elimination with formation of acetylene does not compete effectively with an initial addition step.