Synthesis and Conformational Dynamics of Selenanthrene (Oxides): Establishing an Energetic Flexibility Index for Scaffolds.

The use of new dynamic scaffolds for constructing inorganic and organometallic complexes with enhanced reactivities is an important new research direction. Toward this fundamental aim, an improved synthesis of the dynamic scaffold selenanthrene, along with its monoxide, trans-dioxide and the previously unknown trioxide, is reported. A discussion of the potential reaction mechanism for selenanthrene is provided, and all products were characterized using 1H, 13C, and 77Se nuclear magnetic resonance (NMR) spectroscopy and single-crystal X-ray crystallography. The dynamic ring inversion processes (i.e., "butterfly motion") for selenanthrene and its oxides were investigated using variable-temperature 1H NMR and density functional theory calculations. The findings suggest that selenanthrene possesses a roughly equal barrier to inversion as its sulfur analogue, thianthrene. However, selenanthrene oxides evidently possess larger inversion barriers as compared to their sulfur analogues due to the enhanced electrostatic intramolecular interactions inherent between the highly polar selenium-oxygen bond and adjacent C-H moieties. Finally, we propose a quantitative "flexibility index" in deg/(kcal/mol) for various tricyclic scaffolds to provide researchers with a comparative scale of dynamic motion across many different systems.

Copper(II) as a Platform for Probing the Steric Demand of Bulky ß-Diketonates.

The synthesis and characterization of a series of copper bis(ß-diketonate) complexes, functionalized with sterically hindered o-biphenyl and m-terphenyl functional groups, are reported. X-ray structural analysis reveals that the ligands exhibit several modes of flexibility in order to accommodate the steric demand. Increased steric bulk of the ligands influences the CuII/I electrochemical reduction, which is likely due to inhibited ligand rotation. Chemical reduction of CuII forms CuI, which disproportionates to Cu0 and CuII. The CuI species could be quantitatively trapped using triphenylphosphine to form Cu(ß-diketonate)(PPh3)2 (7), which is also characterized. The catalytic ability of these complexes, along with several common precatalysts, was determined for the reaction of bromobenzene and 2-naphthol, an Ullmann-type C-O bond coupling reaction. Control experiments in toluene show no catalytic ability in the absence of ß-diketonates, suggesting involvement of the ligand in catalytic turnover.

Synthesis of Sterically Hindered ß-Diketones via Condensation of Acid Chlorides with Enolates.

Bulky ß-diketones have rarely exceeded dipivaloylmethane (DPM) in steric demand, largely due to synthetic limitations of the Claisen condensation. This work demonstrates hindered acid chlorides to be selective electrophiles in noncoordinating solvents for condensations with enolates. An improved synthesis of DPM is described (90% yield), and crowded ß-diketones featuring bulky o-biphenyl or m-terphenyl fragments were prepared in good to excellent yields. These compounds are anticipated to have a steric profile far greater than that of DPM. General reaction conditions and mechanistic considerations are included.