Jean M. Standard

X-Ray crystallographic and 13C nuclear magnetic resonance studies of 3,4,5,6-tetrahydro-2H-1,3,4-oxadiazin-2-ones derived from ephedrine and pseudoephedrine ☆

Abstract 3,4,5,6-Tetrahydro-2H-1,3,4-oxadiazin-2-ones derived from (1R,2S)-ephedrine and (1S,2S)-pseudoephedrine have been synthesized and their conformational properties have been examined. The ephedrine heterocycles 5–7a appear to favor one set of equilibrating conformers while the pseudoephedrine heterocycles 5–7b exist as multiple conformers at room temperature. The observed conformational behavior of these heterocycles is attributed to allylic strain and a gauche effect arising from the torsional energy barrier between the lone pair electrons of the N3- and N4-nitrogens.

β-Amino Alcohol Derived β-Hydroxy- and β-(o-Diphenylphosphino)benzoyloxy(o-diphenylphosphino)benzamides: An Ester−Amide Ligand Structural Model for the Palladium-Catalyzed Allylic Alkylation Reaction

A commercially available collection of beta-amino alcohols have been converted to their corresponding beta-hydroxy- and beta-(o-diphenylphosphino)benzoyloxy(o-diphenylphosphino)benzamides 11a-f and 12a-f and have been employed in the Tsuji-Trost asymmetric alkylation reaction with 1,3-diphenylpropenyl acetate. With the exception of ligands 11b and 11f, the beta-hydroxybenzoyloxy(o-diphenylphosphino)benzamide ligands 11a-f primarily afforded the (R)-enantiomer of the product. In contrast, the bis(phosphine) ligands 12a-f consistently afforded the (S)-enantiomer. The best ligand (12c) was derived from cis-(1R,2S)-2-amino-1,2-diphenyl-1-ethanol, and when applied in the asymmetric allylic alkylation reaction, it yielded the product in an enantiomeric ratio of 97.8.22 favoring the (S)-enantiomer. A computational study was conducted on the conformation that this ligand might adopt in the palladium-catalyzed alkylation reaction as compared to that of the Trost ligand 1a.

Density functional calculations exploring the methylene-water interaction in the gas phase and in solution

Abstract Results are presented from density functional calculations which investigate the interaction between water and singlet methylene. Geometries, energies, and vibrational frequencies were characterized for a number of local minima and transition states on the water-methylene potential energy surface in the gas phase and in solution. The solution phase studies were carried out using the Onsager reaction field model with a range of dielectric constants from 2 to 80. Calculations were performed using density functional theory with the B3LYP functional. The 6-31G∗ and 6-311++G∗∗ basis sets were employed for all calculations. When results are compared to MP2 and QCISD studies, it is found that density functional theory overestimates the binding energies of the ylides formed between methylene and water. In addition, density functional theory poorly describes the energetics of the ylide rearrangement via a 1,2-hydrogen shift mechanism to form methanol. The barrier for rearrangement is underestimated by density functional theory when compared to MP2 or QCISD results. However, density functional theory qualitatively predicts the stabilization of the methylene-water ylides and the increase in barrier height for rearrangement in solution.

Theoretical studies of the interactions of singlet carbenes with water and alcohols

Abstract Results are presented from ab initio calculations which explore the interactions of singlet carbenes with water and alcohols. The carbenes investigated include methylene and carbomethoxycarbene. A number of different minima on the potential energy surfaces have been located and characterized. These minima correspond to oxonium ylide-type structures as well as to hydrogen-bonded structures. Equilibrium geometries, binding energies, and vibrational frequencies have been determined. We also report on the results of preliminary studies of the geometries and energetics of transition states for rearrangement of the oxonium ylides to alcohols or ethers. Calculations were performed at levels of theory ranging from HF/3-21G to MP2/6-311G ∗∗ , and include zero-point energy corrections, as well as basis set superposition error corrections.

Modeling image potential surface states on silver and gold surfaces covered with self-assembled monolayers of alkanethiols and alkaneselenols

A one-dimensional potential based on nearly free electron (NFE) theory and the method of images is used to describe image potential surfaces states on roughened Ag and Au surfaces covered with self-assembled monolayers of alkanethiols and alkaneselenols. NFE is applied to produce the model potential in both the substrate and headgroup portion of the monolayer, and calculated image state energies are in excellent quantitative agreement with experimental energies. The image plane is located beyond the headgroup layer that is chemisorbed onto step edges, and the electrons move perpendicular to step edges and parallel to terraces.

Effects of Solvation and Hydrogen Bond Formation on Singlet and Triplet Alkyl or Aryl Carbenes

Formation of hydrogen-bonded complexes involving singlet and triplet alkyl or aryl carbenes and the impacts of solvation and hydrogen bonding upon the carbene singlet-triplet gaps have been investigated using computational methods. Single-point CCSD(T)-F12 and MRCI+Q methodologies have been employed with aug-cc-pVDZ and aug-cc-pVTZ basis sets to determine accurate singlet-triplet gaps of carbenes and hydrogen-bonded complexes involving carbenes, with geometries and vibrational frequencies obtained at the B3LYP-D3/aug-cc-pVTZ level. Using the PCM continuum solvent method and density functional theory (B3LYP/aug-cc-pVTZ), the singlet-triplet gaps of the carbenes are found to exhibit significant solvent effects; due its higher polarity, the singlet carbene is stabilized to a greater degree than the corresponding triplet carbene, impacting the singlet-triplet gap by as much as 4.4 kcal/mol. In addition, water and methanol, acting as hydrogen bond donors, form hydrogen bonds with all the singlet and triplet carbenes studied in this work. Singlet carbenes form relatively strong hydrogen bonds with binding energies in the range 3-9 kcal/mol; triplet carbenes form weaker hydrogen bonds with binding energies in the range 1-4 kcal/mol. NBO analysis demonstrates that the singlet carbene hydrogen bonds are stabilized in typical fashion, through donation of electron density from the lone pair orbital on carbon into the O-H antibonding orbital. This stabilizing interaction also is present in triplet carbene hydrogen bonds; however, a back-donation from the O-H bonding orbital into the carbon lone pair orbitals also is observed, which leads to reduced charge transfer in the triplet carbene hydrogen-bonded complexes. With the exception of methylene, hydrogen bond formation is strong enough to reverse the ordering of the singlet and triplet states for the carbenes possessing triplet ground states.