A Missing Relative: A Hoveyda–Grubbs Metathesis Catalyst Bearing a Peri-Substituted Naphthalene Framework

Molecular scaffolds of polycyclic aromatic hydrocarbons can serve as unique tools to control the molecular and electronic structure of coordination compounds. Herein, we report the synthesis and properties of a Hoveyda–Grubbs metathesis catalyst bearing a chelating benzylidene ligand assembled on peri-substituted naphthalene. In contrast to other reported naphthalene-based complexes (Barbasiewicz, M.; Grela, K. Chem. Eur. J. 2008, 14, 9330−9337), it exhibits a very fast initiation behavior, attributed to a distorted molecular structure and reduced π-electron delocalization within the chelate ring

Synthesis and Properties of Bimetallic Hoveyda–Grubbs Metathesis Catalysts

The catalytic activity of ruthenium Hoveyda–Grubbs complexes in olefin metathesis is a function of complex steric and electronic effects acting on initiation and propagation steps. In order to study the π-electron factors influencing the initiation process, we attempted syntheses of bimetallic complexes with common organic ligands bearing two chelate rings. While most of the studied ligand exchange reactions of the isomeric bis-chelating benzene derivatives gave mixtures of unstable complexes, a homodinuclear derivative of 1,4-dimethoxy-2,5-divinylbenzene was sparingly soluble and precipitated from the reaction mixture in a pure form. The complex was studied with spectroscopic and X-ray methods, which confirmed the symmetrical bimetallic structure. However, in model metathesis reactions the catalyst displayed activity very comparable to the related monometallic complexes. This suggests that in the bimetallic system two consecutive initiation processes of the metathesis catalyst (first, bimetallic complex + olefin → monometallic complex + propagating species; second, monometallic complex + olefin → styrene + propagating species) proceed at similar rates and, thus, no cooperativity between the two steps is displayed. Properties of the family of bimetallic complexes were probed with NMR studies, and π-electronic effects operating in the systems were discussed

Synthesis and Properties of Bimetallic Hoveyda–Grubbs Metathesis Catalysts

The catalytic activity of ruthenium Hoveyda–Grubbs complexes in olefin metathesis is a function of complex steric and electronic effects acting on initiation and propagation steps. In order to study the π-electron factors influencing the initiation process, we attempted syntheses of bimetallic complexes with common organic ligands bearing two chelate rings. While most of the studied ligand exchange reactions of the isomeric bis-chelating benzene derivatives gave mixtures of unstable complexes, a homodinuclear derivative of 1,4-dimethoxy-2,5-divinylbenzene was sparingly soluble and precipitated from the reaction mixture in a pure form. The complex was studied with spectroscopic and X-ray methods, which confirmed the symmetrical bimetallic structure. However, in model metathesis reactions the catalyst displayed activity very comparable to the related monometallic complexes. This suggests that in the bimetallic system two consecutive initiation processes of the metathesis catalyst (first, bimetallic complex + olefin → monometallic complex + propagating species; second, monometallic complex + olefin → styrene + propagating species) proceed at similar rates and, thus, no cooperativity between the two steps is displayed. Properties of the family of bimetallic complexes were probed with NMR studies, and π-electronic effects operating in the systems were discussed

A Missing Relative: A Hoveyda–Grubbs Metathesis Catalyst Bearing a Peri-Substituted Naphthalene Framework

Molecular scaffolds of polycyclic aromatic hydrocarbons can serve as unique tools to control the molecular and electronic structure of coordination compounds. Herein, we report the synthesis and properties of a Hoveyda–Grubbs metathesis catalyst bearing a chelating benzylidene ligand assembled on peri-substituted naphthalene. In contrast to other reported naphthalene-based complexes (Barbasiewicz, M.; Grela, K. Chem. Eur. J. 2008, 14, 9330−9337), it exhibits a very fast initiation behavior, attributed to a distorted molecular structure and reduced π-electron delocalization within the chelate ring

Effect of Vitamin D Conformation on Interactions and Packing in the Crystal Lattice

The crystal and molecular structures of a series of structurally related analogues of 1,25-dihydroxyvitamin D₂ and of the first analogue with all hydroxyl groups protected were established with single crystal X-ray structural analysis. With the use of the new structural data, we proposed that the A-ring conformation depends on hydrogen bonding of the hydroxyl groups of the A-ring. The A-ring of the 1α-hydroxylated vitamin D analogues exists in the solid state in a preferred chair β-conformation induced by direct hydrogen bonds between the 1-OH and 3-OH hydroxyl groups. In the same A-ring conformation, the vitamin D analogue interacts with the vitamin D receptor. Indirect hydrogen bonds between the A-ring hydroxyl groups, such as the ones through the water molecule, or hydrogen bonds between the A-ring hydroxyl groups and side-chain hydroxyl groups, induce the α-conformation. Theoretical calculations performed in vacuo showed that the β-form has a slightly lower energy than the α-form. Not only the hydroxyl groups but also the exocyclic methylene highly influences intermolecular interactions including the hydrogen bond pattern in the crystal lattices

Rational and Then Serendipitous Formation of Aza Analogues of Hoveyda-Type Catalysts Containing a Chelating Ester Group Leading to a Polymerization Catalyst Family

Analogues of the well-known Hoveyda–Grubbs catalyst bearing both a chelating ester function and a chelating nitrogen atom were obtained. These complexes behave differently depending on the character of the chelating amine. Complexes containing a secondary amine underwent unexpected spontaneous oxidation of the amine group, leading to the Schiff base analogues. In contrast, complexes containing a tertiary amine were prone to intramolecular cyclization in the presence of a base (Et₃N). Probing the activity of such (pre)­catalysts in ring-closing metathesis reactions (RCMs) revealed their dormant character and excellent thermo-switchability. In particular, complexes bearing an iminium nitrogen fragment were found to be very useful in a delayed ring-opening metathesis polymerization (ROMP) and were therefore commercialized

Structural and Energetic Analysis of Molecular Assemblies in a Series of Nicotinamide and Pyrazinamide Cocrystals with Dihydroxybenzoic Acids

Four new cocrystals of pharmaceutically active N-donor compounds, pyrazinamide (<b>P</b>) and nicotinamide (<b>N</b>), with a series of dihydroxybenzoic acids, i.e., 2,3-dihydroxybenzoic acid (<b>23DHB</b>), 2,4-dihydroxybenzoic acid (<b>24DHB</b>), and 2,6-dihydroxybenzoic acid (<b>26DHB</b>), were synthesized and structurally evaluated in order to study basic recognition patterns and crystal lattice energetic features. The literature-reported structures of this kind, i.e., <b>N:24DHB</b>, <b>N:25DHB</b> and <b>N:26DHB</b> (the last two were crystallized and remeasured by us at 100 K) and <b>P:25DHB</b>, completed the series. The analysis of interaction networks in the examined cocrystals reflects the relative affinity of the COOH and OH groups toward N-donor compounds. A major factor that governs the primary synthon formation is the basic character of the proton acceptors in the heterocyclic compounds. In a crystal lattice, the more rigid pyrazinamide tends to form its primary structural motifs, and hence is less influenced by the molecular surrounding than nicotinamide. Consequently, crystal lattice stabilization energy values for the cocrystals of nicotinamide are more advantageous, whereas the patterns created by pyrazinamide are more predictable. Nicotinamide cocrystals are also characterized by crystal lattices being more energetically uniform in all directions than the pyrazinamide equivalents. Importantly, cocrystal cohesive energies are more favorable than that of the respective single component crystal structures, which supports the cocrystal formation when both coformers are dissolved and mixed together. Although classical hydrogen bonds are majorly responsible for synthon formation, weak dispersive forces cannot be neglected either as far as the structure stabilization is concerned