The synthesis of macrocycles is severely impeded by concomitant oligomer formation. Here, we present a biomimetic approach that utilizes spatial confinement to increase macrocyclization selectivity in the ring-closing metathesis of various dienes at elevated substrate concentration up to 25 mM using an olefin metathesis catalyst selectively immobilized inside ordered mesoporous silicas with defined pore diameters. By this approach, the ratio between macro(mono)cyclization (MMC) product and all undesired oligomerization products (O) resulting from acyclic diene metathesis polymerization was increased from 0.55, corresponding to 35% MMC product obtained with the homogeneous catalyst, up to 1.49, corresponding to 60% MMC product. A correlation between the MMC/O ratio and the substrate-to-pore-size ratio was successfully established. Modification of the inner pore surface with dimethoxydimethylsilane allowed fine-tuning the effective pore size and reversing surface polarity, which resulted in a further increase of the MMC/O ratio up to 2.2, corresponding to >68% MMC product. Molecular-level simulations in model pore geometries help to rationalize the complex interplay between spatial confinement, specific (substrate and product) interaction with the pore surface, and diffusive transport. These effects can be synergistically adjusted for optimum selectivity by suitable surface modification.

γ-Al2O3 and γ-Al2O3 impregnated with phosphorus and/or molybdenum, amorphous AlPO4, and Al2(MoO4)3 have been studied by the recently introduced multiple-quantum magic angle spinning (MQMAS) NMR and off-resonance nutation NMR. Average quadrupolar coupling parameters of the resonances in the compounds were determined with 27Al off-resonance nutation spectroscopy. The MQMAS NMR experiment was used to increase the resolution of the spectra in order to gain insight in the distribution of the quadrupole parameters and to resolve overlapping lines from surface species on γ-Al2O3 supports. This combined use of advanced NMR techniques provided information about the bulk and surface structure of γ-Al2O3- and γ-Al2O3-supported catalyst precursors. When phosphorus and molybdenum loadings below “monolayer” coverage are employed, the single pulse spectra did not reveal the formation of new aluminum-containing compounds. At higher phosphorus loading the formation of a new phase was observed that from the MQMAS experiment could be clearly assigned to amorphous AlPO4. In a calcined sample containing both molybdenum and phosphorus, 27Al NMR showed that Al2(MoO4)3 and some AlPO4 had been formed, which could not be detected by XRD and hardly by single-pulse-excitation (SPE) MAS NMR. The fact that the observed distribution of aluminum atoms over octahedral and tetrahedral positions did not change with the loading of the support (except when AlPO4 was formed) led to the conclusion that aluminum atoms with both types of coordination are found not only in the bulk of γ-Al2O3 but also on its surface in about the same ratio.

Solid state double resonance NMR experiments on phosphorus-impregnated y-ALO} and amorphous AIPO4 have been conducted to investigate the interaction between the impregnated phosphorus and the y-A l20 3 surface. The 31P -27A1 REDOR and TRAPDOR experiments have shown that most phosphorus is in close contact with aluminum, thereby excluding the possibility that stacked phosphate layers or bulk phosphates are formed when more phosphorus is impregnated than is necessary to cover the entire y~ AI2O3 surface. The 27A 1 -31P experiments enabled the 27A1 spectrum of the aluminum atoms which interact with phosphorus to be separated from the spectrum of the bulk y-Al20 3. These double resonance experiments have shown that . a layer of AIPO4 is indeed formed on the y-Al20 3 surface and that the structure of this layer, although similar, has a slightly higher degree of ordering than

For entropic reasons, the synthesis of macrocycles via olefin ring-closing metathesis (RCM) is impeded by competing acyclic diene metathesis (ADMET) oligomerization. With cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes confined in tailored ordered mesoporous silica, RCM can be run with macrocyclization selectivities up to 98% and high substrate concentrations up to 0.1 M. Molecular dynamics simulations show that the high conversions are a direct result of the proximity between the surface-bound catalyst, proven by extended X-ray absorption spectroscopy, and the surface-located substrates. Back-diffusion of the macrocycles decreases with decreasing pore diameter of the silica and is responsible for the high macrocyclization efficiency. Also, Z-selectivity increases with decreasing pore diameter and increasing Tolman electronic parameter of the NHC. Running reactions at different concentrations allows for identifying the optimum substrate concentration for each material and substrate combination.

The influence of surface area, paramagnetic impurities, and spinning speed on the 27Al MAS NMR visibility in a number of γ- and η-Al2O3 samples was investigated. It is shown that magic angle spinning (MAS) under typical conditions (∼10 kHz spinning speed at a 9.4 T field) is insufficient to resolve all aluminium sites in the samples. Since, in general, the number of strongly distorted sites in aluminas increases with the surface area, the 27Al MAS NMR signal intensities at 10−15 kHz spinning decrease. Differences between γ- and η-Al2O3 intensities are explained by their structural differences, causing weaker distortions in η- than in γ-Al2O3. At spinning speeds up to 29 kHz all aluminium is accounted for in the MAS spectra. Paramagnetic iron impurities up to 1440 ppm do not have a significant influence on the 27Al MAS NMR visibility of the investigated samples.