Halogen-Halogen Nonbonded Interactions

Halogen-halogen nonbonded interactions were studied for methyl halides and phenyl halides using both B3LYP and MP2 along with 6-311+G* and aug-cc-pVTZ. With the methyl halides, the linear approach was found to lead to little stabilization, whereas the "90°" approach gave 1-2 kcal/mol. This modest stabilization was due to long-range electron correlation effects. The lowest-energy arrangement had the molecules side-by-side, with the major stabilization being derived from halogen-hydrogen interactions. The results for methyl bromide were quite similar. Chlorobenzene dimer with the 90° orientation gave a small stabilization energy, but the best arrangement had the two benzene rings oriented over each other. The meta orientation of the chlorines had a lower energy than ortho or para. The dimerization energy was larger than that for two benzene rings sitting directly above each other, suggesting that whereas Cl···Cl interaction is not very important, the effect of the halogen on the electron distribution does have an effect. This suggests that much of the crystallographic results for these compounds may not be due to halogen-halogen interactions but rather the interaction between the substituted benzene rings along with crystal forces.

Re-Examination of Some Carbocations. Structures, Energies, and Charge Distributions

Chemists have had a long-standing interest in reactive intermediates such as carbenes, carbon radicals, carbanions, and carbocations. Carbocations are an interesting part of this group because of their tendency to undergo rearrangement, sometimes forming bridged ions, as well as their ability in many cases of spreading out the positive charge over several atoms. We have re-examined some of these cases using high-level compound procedures, W1BD and G4, as well as by considering the charge distributions making use of the Hirshfeld method that has been shown to uniquely correlate with several types of experimental data.

Total Synthesis of (±)-Phyllantidine: Development and Mechanistic Evaluation of a Ring Expansion for Installation of Embedded Nitrogen-Oxygen Bonds

The development of a concise total synthesis of (±)-phyllantidine (1), a member of the securinega family of alkaloids containing an unusual oxazabicyclo[3.3.1]nonane core, is described. The synthesis employs a unique synthetic strategy featuring the ring expansion of a substituted cyclopentanone to a cyclic hydroxamic acid as a key step that allows facile installation of the embedded nitrogen-oxygen (N-O) bond. The optimization of this sequence to effect the desired regiochemical outcome and its mechanistic underpinnings were assessed both computationally and experimentally. This synthetic approach also features an early-stage diastereoselective aldol reaction to assemble the substituted cyclopentanone, a mild reduction of an amide intermediate without N-O bond cleavage, and the rapid assembly of the butenolide found in (1) via use of the Bestmann ylide.

Unrecognized Intramolecular and Intermolecular Attractive Interactions between Fluorine-Containing Motifs and Ether, Carbonyl, and Amino Moieties

In order to elucidate to what extent Coulombic and other interactions contribute to the origins of contrasteric phenomena, we have identified a significant, previously unrecognized interaction between fluorine-containing motifs and groups or molecules containing main-group heteroatoms. The axial conformers of both 2-methoxy- and 2-trifluoromethoxytetrahydropyrans preferentially adopt a rotameric arrangement in which the OCH₃ and the OCF₃ groups are gauche to the ring oxygen. Given that one would expect a repulsive Columbic interaction to exist between the electronegative fluorines of the CF₃ group and the ring oxygen in this rotomeric orientation, this surprising result suggests that an attractive interaction exists between the CF₃ group and the oxygen of the ring. The generality and origin of this interaction was examined using nonpolar CF₄ to probe intermolecular interactions with systems such as dimethyl ether, trimethylamine, trimethylphosphine, and acetone. In each case there was an attractive interaction leading to formation of a complex. The attraction is not due to van der Waals forces. Rather, the fluorine lone pairs of the CF₄ often act as an electron donor in these complexes leading to a transfer of charge between the reactants and formation of the complex. These previously unrecognized fluorine-heteroatom interactions likely play a significant role in the context of understanding the binding interactions of medicinally relevant molecules or pharmaceuticals possessing fluorine-containing pharmacophores with their targets.

Atomic Charges

The problem of deriving atomic charges from the results of ab Initio MO calculations has been studied by the use of several reported methods: Mulliken population analysis, the minimal basis set (MBS) procedure, the natural population analysis (NPA), two electrostatic potential fitting methods, M-K and ChelpG, the Hirshfeld population analysis, and CM5 (charge model 5), which is related to the Hirshfeld method. The first set of studies were concerned with hydrogen charges. It was found that the MBS charges were linearly related to the Hirshfeld charges. The Hirshfeld, CM5, and MBS methods, but not the others, provided an excellent correlation for H atomic charge with the H-C-H bond angle, and with calculated gas-phase acidity. The two methods that were linearly related and gave hydrogen charges in agreement with an experimental study of partially deuterated methanes are MBS and Hirshfeld. In order to see which of the two methods is the more satisfactory, the methanol dimer was examined. The calculated H bond energy was 6.2 kcal/mol, which was in good agreement with studies of hydrogen bonds. The Coulombic interaction for the O···H bond was estimated using the MBS and Hirshfeld charges. The latter gave a calculated energy of 3-6 kcal/mol, whereas MBS gave an energy of ∼35 kcal/mol. Clearly, the Hirshfeld method is more satisfactory and should be the method of choice.

Effect of remote aryl substituents on the conformational equilibria of 2,2-diaryl-1,3-dioxanes: importance of electrostatic interactions

The conformational preference of a variety of 2,2-diaryl-1,3-dioxanes bearing remote substituents on the phenyl rings has been studied via equilibration of configurationally isomeric 2,2-diaryl-cis-4,6-dimethyl-1,3-dioxane epimers, X-ray crystallography, (1)H NOESY analysis, and B3LYP/6-311+G* calculations. When the aryl ring bears a remote electron-withdrawing substituent, the isomer having both the higher dipole moment and the electron-withdrawing group in the equatorial phenyl ring and/or an electron-donating group in the axial ring has the lower energy. These results differ from the conclusions reported in a previous study of similar systems. The conformational energy differences of para-substituted 2,2-diaryl-1,3-dioxanes are linearly related to the Hammett σ values with a slope (ρ) of 0.6. In addition, there is a trend toward longer bond lengths between the C(2) ketal center and the aryl ring as the electron-withdrawing nature of the para-substituent is increased. Electrostatic interactions, rather than a hyperconjugative anomeric effect, appear to be responsible for the conformational behavior of such molecules.

Proton donor acidity controls selectivity in nonaromatic nitrogen heterocycle synthesis

Piperidines are prevalent in natural products and pharmaceutical agents and are important synthetic targets for drug discovery and development. We report on a methodology that provides highly substituted piperidine derivatives with regiochemistry selectively tunable by varying the strength of acid used in the reaction. Readily available starting materials are first converted to dihydropyridines via a cascade reaction initiated by rhodium-catalyzed carbon-hydrogen bond activation. Subsequent divergent regio- and diastereoselective protonation of the dihydropyridines under either kinetic or thermodynamic control provides two distinct iminium ion intermediates that then undergo highly diastereoselective nucleophilic additions. X-ray structural characterization of both the kinetically and thermodynamically favored iminium ions along with density functional theory calculations provide a theoretical underpinning for the high selectivities achieved for the reaction sequences.

Substituent effects on O-H bond dissociation enthalpies: a computational study

Bond dissociation enthalpies (BDEs) can exhibit dramatic variations resulting from substituent effects. The remarkable range of experimental OH bond dissociation enthalpies have been reproduced using CBS-APNO calculations with very good accuracy, so we have employed these calculations to extend the available BDE data. The effect on these BDEs of lone pairs on the atom adjacent to oxygen shows that conjugation in the product radicals is the most important interaction leading to the wide range of values. The BDE's were found to be linearly related to both the spin density at the radical center and to the change in X-O bond order in going from X-O-H to X-O·.

Effect of basis set on electron populations calculated by using Bader's criterion for partitioning electron density between atoms

The effect of basis set on Bader's criterion for partitioning electron density between atoms is examined. The major effect is on the position of minimum electron density along the bond of interest. The 6-31G(**) basis leads to the more satisfactory results. The effect of including electron correlation was examined via the use of generalized valence bond wavefunctions. The change in the electron populations was small. The partitioning of electron density is useful in examining the way in which substituents interact with hydrocarbon groups.

Disparate behavior of carbonyl and thiocarbonyl compounds: acyl chlorides vs thiocarbonyl chlorides and isocyanates vs isothiocyanates

The reaction of benzoyl chloride with methanol catalyzed by pyridine is 9 times more rapid than is the same reaction with thiobenzoyl chloride. The difference in reactivity, as well as the dealkylation reactions that occur when the reaction of thiobenzoyl chloride is catalyzed by bases such as Et(3)N, can be understood in terms of the charge distributions in the intermediate acylammonium ions. The reaction of PhNCO with ethanol occurs at a much higher rate (4.8 x 10(4)) than that of PhNCS, corresponding to a difference in activation free energies for the additions of 6 kcal/mol. Transition states for each of these reactions were located, and each involves two alcohol molecules in a hydrogen bonded six-membered ring arrangement. Information concerning differences in reactivity was derived from analysis of Hirshfeld atomic charge distributions and calculated hydrogenolysis reaction energies.