Geometry dependence of the ring-opening E2 reaction in lactones.

Gas phase bimolecular elimination reactions (E2) of lactones, with and without alkyl substitution on the omega carbon, have been studied. The comparison between the substituted and corresponding unsubstituted lactones reveals, without complications from ring strain, the effect of geometric distortion from the periplanar geometry preferred for E2 reactions. The four- and five-membered ring lactones show a significant effect from the distortion, while the six- and seven-membered rings show relatively little.

Steric and solvation effects in ionic S(N)2 reactions.

This paper explores the contribution of solvation to the overall steric effects of S(N)2 reactions observed in solution. The reactions of chloride ion with a series of alkyl chloronitriles, RCH(CN)Cl (R = methyl, ethyl, isopropyl, tert-butyl) were investigated both experimentally and theoretically. These reactions serve as a model system for the parent reactions, Cl(-) + RCH(2)Cl, which are too slow to measure.

Hydrogen bonding lowers intrinsic nucleophilicity of solvated nucleophiles.

The relationship between nucleophilicity and the structure/environment of the nucleophile is of fundamental importance in organic chemistry. In this work, we have measured nucleophilicities of a series of substituted alkoxides in the gas phase. The functional group substitutions affect the nucleophiles through ion-dipole, ion-induced dipole interactions and through hydrogen bonding whenever structurally possible.

Vinyl hydrogen acidities of two stereoisomers.

The gas-phase acidities of the vinyl hydrogens of cis- and trans-2-butene were measured by the silane kinetic method in a Fourier-transform ion cyclotron resonance spectrometer. The acidities of ethene and the secondary vinyl hydrogen of propene were measured by the same method. The method was calibrated using the known acidities of methane and benzene.

Molecular rotations and dipole-bound state lifetimes.

A computational model based on classical molecular rotation provides insight into the observability of dipole-bound states. The observability is related to the lifetime of the state prior to rotational autodetachment of the electron. The model tracks an ensemble of dipole-bound states.

Enolate structure and electron affinity.

Photodetachment cross sections for a series of cyclic enolates were measured using a continuous wave (CW) ion cyclotron resonance instrument to generate and detect the ions. We report electron affinities for the radicals corresponding to the removal of the extra electron from the following anions: 2-methylcyclopent-1-enolate, 3-methylcyclopent-1-enolate, 4-methylcyclopent-1-enolate, 5-methylcyclopent-1-enolate, 2-methylcyclohex-1-enolate, 3-methylcyclohex-1-enolate, 4-methylcyclohex-1-enolate, 4-ethylcyclohex-1-enolate, 5-methylcyclohex-1-enolate, and 6-methylcyclohex-1-enolate. Some of these anions are mixed with their tautomers, derived from deprotonation of the parent ketone; the consequences of this are analyzed.

Proton exchange and transesterification reactions of acetate enolates with alcohols in the gas phase.

Transesterification reactions and proton exchange reactions between acetate enolates and alcohols were studied both separately and together. Kinetic analysis shows that transesterification and proton exchange happen in a single collision event. The transesterification reaction is best viewed as an endothermic proton transfer, followed by an exchange of alkoxide and an exothermic proton transfer.

Acidities in cyclohexanediols enhanced by intramolecular hydrogen bonds.

Equilibrium gas-phase acidities of the six isomeric cyclohexanediols were measured in a Fourier transform ion cyclotron resonance mass spectrometer. Although all six cyclohexanediols have the same functional groups and similar structures, the acidities vary over 11 kcal/mol. This large difference is due mostly to the balance between hydrogen bonding and geometric strain.

Radical autoxidation and autogenous O2 evolution in manganese-porphyrin catalyzed alkane oxidations with chlorite.

A manganese porphyrin catalyst employing chlorite (ClO(2)(-)) as a "shunt" oxidant displays remarkable activity in alkane oxidation, oxidizing cyclohexane to cyclohexanol and cyclohexanone with >800 turnover numbers. The ketone is apparently formed without the intermediacy of alcohol and accounts for an unusually large fraction of the product ( approximately 40%). Radical scavenging experiments indicate that the alkane oxidation mechanism involves both carbon-centered and oxygen-centered radicals.

Steric retardation of SN2 reactions in the gas phase and solution.

The gas-phase S(N)2 reactions of chloride with ethyl and neopentyl chlorides and their alpha-cyano derivatives have been explored with B3LYP, CBS-QB3, and PDDG/PM3 calculations. Calculations predict that the steric effect of the tert-butyl group raises the activation energy by about 6 kcal/mol relative to methyl in both cases. Solvent effects have been computed with QM/MM Monte Carlo simulations for DMSO, methanol, and water, as well as with a polarizable continuum model, CPCM.

Donor ligand effect on the nature of the oxygenating species in Mn(III)(salen)-catalyzed epoxidation of olefins: experimental evidence for multiple active oxidants.

The stereoselectivity of olefin epoxidation catalyzed by Mn(III)(salen)X (1a, X = Cl(-); 1b, X = BF(4)(-)) complexes is examined in the presence of neutral donor ligands, employing various iodosylarenes (ArIO: PhIO, C(6)F(5)IO, and MesIO) as the oxygen atom source. The cis/trans ratios of stilbene oxides and the enantiomeric excesses of styrene oxide and 1,2-dihydronaphthalene oxide are found to be strongly dependent on the nature of the iodosylarenes under certain conditions. In other cases, olefin epoxidation is shown to proceed with essentially identical diastereoselectivities or enantioselectivities, regardless of the oxygen atom source used.

Dynamics of competitive reactions: endothermic proton transfer and exothermic substitution.

Dynamics of an endothermic proton-transfer reaction, F(-) with dimethyl sulfoxide, and an endothermic proton-transfer reaction with a competing exothermic substitution (S(N)2) channel, F(-) with borane-methyl sulfide complex, were investigated using a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR) and kinetic modeling. The two proton-transfer reactions have slightly positive and a small negative overall free energy changes, respectively. Energy-dependent rate constants were measured as a function of F(-) ion translational energy, and the resulting kinetics were modeled with the RRKM (Rice-Ramsperger-Kassel-Marcus) theory.

Conformation-dependent reaction thermochemistry: study of lactones and lactone enolates in the gas phase.

Gas-phase acidities (Delta H degrees (acid)) of lactones with ring sizes from four to seven have been measured on a Fourier transform ion cyclotron resonance mass spectrometer. Electron affinities (EAs) of the corresponding lactone enolate radicals were measured on a continuous-wave ion cyclotron resonance mass spectrometer, and the bond dissociation energies (BDEs) of the alpha C-H bonds were derived. In order of increasing ring size, Delta H degrees (acid) = 368.

Steric effects and solvent effects in ionic reactions.

Rates of SN2 reactions of chloride ion with methyl- and tert-butyl-substituted chloroacetonitrile were measured by using Fourier transform-ion cyclotron resonance spectrometry to follow the isotopic exchange reaction. Barrier heights for these reactions indicate that steric effects in the gas phase are diminished relative to apparent steric effects in solution. We attribute the increased barrier in solution to a solvation effect.

Catalytic Activation of H(2) and C-H Bonds by Electron-Deficient Ruthenium(II) Porphyrins.

The Ru(2) and RuNi derivatives of 1,8-bis(10,15,20-trimesityl-5-porphyrinato)anthracene-a recently reported cofacial diporphyrin ligand comprising two hindered porphyrins spanned by an anthracene bridge-have been synthesized. Both Ru(2)(DPAHM) and RuNi(DPAHM) are extremely reactive species that apparently contain 14-electron Ru(II) centers and, as is the case for their monoporphyrin analog, (5,10,15,20-tetramesitylporphyrinato)ruthenium [Ru(TMP)], must be rigorously protected from oxygen, nitrogen, and other ligating agents. In addition, these electron-deficient Ru(II) porphyrins all appear to bind aromatic solvents such as benzene and toluene, the weakest ligating solvents in which these Ru(II) porphyrins have been found soluble.