Al^III–Calix[4]arene Catalysts for Asymmetric Meerwein–Ponndorf–Verley Reduction

Chiral Al^III-calixarene complexes were investigated as catalysts for the asymmetric Meerwein–Ponndorf–Verley (MPV) reduction reaction when using chiral and achiral secondary alcohols as reductants. The most enantioselective catalyst consisted of a new axially chiral vaulted-hemispherical calix[4]­arene phosphite ligand, which attained an enantioselective excess of 99%. This ligand consists of two lower-rim hydroxyl groups, with the remaining two lower-rim oxygens directly connected to the phosphorus of the phosphite, which is derived from a chiral diol. The results emphasize the importance of the rigid calix[4]­arene lower-rim substituents and point to a possible role of a lower-rim chiral pocket and Lewis-basic phosphorus lone pairs in enhancing asymmetric hydride transfer

Silica-Supported Phosphonic Acids as Thermally and Oxidatively Stable Organic Acid Sites

Organic–inorganic materials consisting of organophosphonic-acid-supported-on-silica materials <b>C3</b>/<b>SiO₂</b> and <b>C4/SiO₂</b> are described, where <b>C3</b> is propane-1,2,3-triphosphonic acid and <b>C4</b> is butane-1,2,3,4-tetraphosphonic acid. Solid-state structures of both of these phosphonic acids are analyzed using single-crystal X-ray diffraction, and these data reveal extensive intermolecular hydrogen bonding and no intramolecular hydrogen bonds. Thermogravimetric analysis/mass spectroscopy (TGA/MS) data show a lack of combustion for these materials in air at temperatures below 400 °C, and only release of water corresponding to reversible organophosphonic acid condensation below 150 °C. A comparative series of silica-supported materials were synthesized, consisting of organophosphonic acid <b>CX8</b>, which represents a calixarene macrocycle that is decorated with a high density of organophosphonic-acid substituents on both the lower and upper rim, as well as polyvinylphosphoric acid (<b>PVPA</b>). Material <b>CX8</b>/<b>SiO₂</b> possesses a significantly lower thermal stability and lower combustion temperature of 300 °C in air, whereas <b>PVPA</b> demonstrates comparable thermal stability as observed with <b>C3</b> and <b>C4</b>. TGA coupled with base-probe titration was used to determine the Brønsted acid site density of all silica-supported phosphonic acids at various coverages and temperatures. Material <b>C4/SiO</b>_<b>2</b><b>-37%</b> (corresponding to 37% (by mass) loading and half-monolayer coverage on silica) exhibited the highest Brønsted acid-site density of all materials, corresponding to 0.84 mmol/g at 150 °C, and 0.62 mmol/g at 300 °C. All supported phosphonic acids treated with pyridine at room temperature were strong enough acids to protonate pyridine at room temperature as exhibited by a distinct pyridinium cation band in the infrared spectrum; however, in contrast to much stronger acid sites in silica-supported phosphoric acid materials, almost all adsorbed pyridine was lost by 150 °C. Use of a stronger base for acid-site titration consisting of diisopropylamine (DIPA) demonstrates acid sites in all materials up to 300 °C, at which temperature the acid site was too weak to adsorb DIPA. Thus, these oxidatively stable materials are deemed to be useful in applications requiring weak Brønsted acid sites, while exhibiting high-temperature oxidative stability up to 400 °C

Role of N‑Heterocyclic Carbenes as Ligands in Iridium Carbonyl Clusters

The low-energy isomers of Ir_<i>x</i>(CO)_<i>y</i>(NHC)_<i>z</i> (<i>x</i> = 1, 2, 4) are investigated with density functional theory (DFT) and correlated molecular orbital theory at the coupled cluster CCSD­(T) level. The structures, relative energies, ligand dissociation energies, and natural charges are calculated. The energies of tetrairidium cluster are predicted at the CAM-B3LYP level that best fit the CCSD­(T) results compared with the other four functionals in the benchmark calculations. The NHC’s behave as stronger σ donors compared with CO’s and have higher ligand dissociation energies (LDEs). For smaller isomers, the increase in the LDEs of the CO’s and the decrease in the LDEs of the NHC’s as more NHC’s are substituted for CO’s are due to π-back-bonding and electron repulsion, whereas the trend of how the LDEs change for larger isomers is not obvious. We demonstrate a μ₃-CO resulting from the high electron density of the metal centers in these complexes, as the bridging CO’s and the μ₃-CO’s can carry more negative charge and stabilize the isomers. Comparison of calculations for a mixed tetrairidum cluster consisting of two calixarene-phosphine ligands and a single calixarene-NHC ligand in the basal plane demonstrated good agreement in terms of both the ligand substitution symmetry (<i>C</i>_3<i>v</i> derived), as well as the infrared spectra. Similar comparisons were also performed between calculations and experiment for novel monosubstituted calixarene-NHC tetrairidium clusters

Outer-Sphere Control of Catalysis on Surfaces: A Comparative Study of Ti(IV) Single-Sites Grafted on Amorphous versus Crystalline Silicates for Alkene Epoxidation

The effect of outer-sphere environment on alkene epoxidation catalysis using an organic hydroperoxide oxidant is demonstrated for calix[4]­arene-Ti^IV single-sites grafted on amorphous vs crystalline delaminated zeotype (UCB-4) silicates as supports. A chelating calix[4]­arene macrocyclic ligand helps enforce a constant Ti^IV inner-sphere, as characterized by UV–visible and X-ray absorption spectroscopies, thus enabling the rigorous comparison of outer-sphere environments across different siliceous supports. These outer-sphere environments are characterized by solid-state ¹H NMR spectroscopy to comprise proximally organized silanols confined within 12 membered-ring cups in crystalline UCB-4, and are responsible for up to 5-fold enhancements in rates of epoxidation by Ti^IV centers