Robust Olefin Metathesis Catalyst Bearing a Tridentate Hemilabile NHC Ligand.

An olefin metathesis catalyst bearing a tridentate hemilabile N-heterocyclic carbene (NHC) ligand was synthesized and characterized. The solid-state crystal structure reveals coordination from all three donation sites of the NHC ligand, giving rise to a stable 18-electron complex. Catalytic activity in three standard metathesis reactions was demonstrated, revealing our catalyst to be particularly long lived and highly selective in the self-metathesis of 1-decene. Although the catalyst in this work initiates more slowly than its second-generation counterparts, it was shown to have high thermal stability, yielding peak performances at higher temperatures. The unique ligand framework of this catalyst may serve as a template for the synthesis of analogous catalysts with improved efficiencies.

Olefin Metathesis Catalyst Supported by a Hemilabile NHC Ligand Bearing Polyether Arms: Structure, Activity, and Decomposition.

An olefin metathesis catalyst (8) bearing a hemilabile N-heterocyclic carbene (NHC) ligand with four methoxyethoxy arms was synthesized and analyzed by X-ray crystallography and NMR spectroscopy. The solid-state structure of the catalyst reveals that one of the aromatic rings is rotated toward the plane of the NHC heterocycle, which enables one ortho-O→Ru coordination. Ring-closing metathesis (RCM) activity was tested at low catalyst loadings. Catalyst 8 was found to display a significant rate enhancement relative to the commercial catalyst 3 but offered no improvement in turnover numbers (TON). The solid-state structure of the catalyst's major degradation product (9) in the presence of 1-hexene reveals a dinuclear compound containing a bridging methylidene (Ru-CH2-Ru), indicating that the methoxyethoxy arms may not be strong enough to stabilize the (NHC)RuCl2(=CH2) catalyst resting state.

Activating Ru nanoparticles on oxide supports for ring-opening metathesis polymerization.

Nano-sized particles mounted on heterogeneous oxide supports such as silica have altered reactivity when compared to their homogeneous analogs. In particular, catalyzed olefin metathesis using supported Ru nanoparticles has shown great promise and various methods have been employed to develop functional heterogeneous Ru catalysts. This article reports a method for synthesizing Ru nanoparticles supported on silica and titania. The nanoparticles were characterized using XPS showing Ru(0) dominates when not exposed to air. TEM showed Ru nanoparticle sizes of 3.1 ± 0.8 nm for Ru supported on SiO2 and 5.6 ± 1.3 nm for Ru supported on TiO2. The materials demonstrated modest activity in ring-opening metathesis reactions via diazo activation and nanoparticle-embedded polymers of norbornene and norbornadiene were synthesized using dry box and Schlenk line techniques. The Ru/support/polymer composite materials were characterized using proton NMR, XPS, and SEM/EDS.

Synthesis and Characterization of Iron Complexes based on Bis-Phosphinite PONOP and Bis-Phosphite PONOP Pincer Ligands.

A series of bis-phosphinite and bis-phosphite PONOP iron complexes were prepared and characterized by NMR and IR spectroscopy. Bis-phosphinite PONOP iron dichloride complexes (RPONOP)FeCl2 (RPONOP = 2,6-(R2PO)2(C5H3N) and R = iPr, tBu) were prepared through complexation of the free ligands with FeCl2 and their solid-state structures were determined. Bis-phosphite PONOP iron complexes (OEtPONOP)Fe(PMe3)2 and (CatPONOP)Fe(PMe3)2 (Cat = catechol) were synthesized through complexation of the free ligands to Fe(PMe3)4. Carbonyl complexes of both bis-phosphinite and bis-phosphite PONOP were prepared and characterized by IR. The monocarbonyl (iPrPONOP)Fe(CO)Cl2 was accessed through exposure of (iPrPONOP)FeCl2 to an atmosphere of CO and the CO stretching frequency was observed at 1969 cm-1. Dicarbonyl complexes (iPrPONOP)Fe(CO)2 and (OEtPONOP)Fe(CO)2 were accessed through reduction of the corresponding chloride complexes with sodium amalgam under a CO atmosphere. Carbonyl stretching frequencies for (iPrPONOP)Fe(CO)2 and (OEtPONOPFe)(CO)2 were observed at 1824 and 1876 cm-1, and at 1871 and 1927 cm-1 respectively. The bis-phosphite PONOP complexes exhibit a less electron rich metal center than the bis-phosphinite PONOP complexes, as would be expected based on the stronger π-acceptor character of these ligands. The electronic properties of the bis-phosphinite PONOP and bis-phosphite PONOP iron complexes are intermediate between previously reported PNP and PDI iron complexes, with the PONOP ligands exhibiting stronger electron donating ability than PDI ligands, but promoting a less electron rich metal center than found in analogous PNP iron complexes.

Reactivation of a Ruthenium-Based Olefin Metathesis Catalyst.

1(st) Generation Hoveyda-Grubbs olefin metathesis catalyst was purposely decomposed in the presence of ethylene yielding inorganic species that are inactive in the ring-closing metathesis (RCM) of benchmark substrate diethyldiallyl malonate (DEDAM). The decomposed catalyst was treated with 1-(3,5-diisopropoxyphenyl)-1-phenylprop-2-yn-1-ol (3) to generate an olefin metathesis active ruthenium indenylidene-ether complex in 43 % yield. This complex was also prepared independently by reacting RuCl(2)(p-cymene)(PCy(3)) with organic precursor 3. The activity of the isolated reactivated catalyst in the RCM of DEDAM is similar to that of the independently prepared complex.

Use of bacteria for rapid, pH-neutral, hydrolysis of the model hydrophobic carboxylic acid ester p-nitrophenyl picolinate.

Caulobacter crescentus, Escherichia coli and Bacillus subtilis cultures promote the hydrolysis of the model ester p-nitrophenyl picolinate (PNPP) at neutral pH with high efficiency. Hydrolysis is related to cell concentration, while the interaction of PNPP with both bacterial cells and their extracellular molecules is required for a maximum rate of PNPP hydrolysis in C. crescentus cultures. Furthermore, C. crescentus cultures hydrolyze PNPP at concentrations useful in synthetic chemistry.

Development of a method for the preparation of ruthenium indenylidene-ether olefin metathesis catalysts.

The reactions between several derivatives of 1-(3,5-dimethoxyphenyl)-prop-2-yn-1-ol and different ruthenium starting materials [i.e., RuCl2(PPh3)3 and RuCl2(p-cymene)(L), where L is tricyclohexylphosphine di-t-butylmethylphosphine, dicyclohexylphenylphosphine, triisobutylphosphine, triisopropylphosphine, or tri-n-propylphosphine] are described. Several of these reactions allow for the easy, in-situ and atom-economic preparation of olefin metathesis catalysts. Organic precursor 1-(3,5-dimethoxyphenyl)-1-phenyl-prop-2-yn-1-ol led to the formation of active ruthenium indenylidene-ether complexes, while 1-(3,5-dimethoxyphenyl)-prop-2-yn-1-ol and 1-(3,5-dimethoxyphenyl)-1-methyl-prop-2-yn-1-ol did not. It was also found that a bulky and strong σ-donor phosphine ligand was required to impart good catalytic activity to the new ruthenium complexes.

Kinetic Selectivity of Olefin Metathesis Catalysts Bearing Cyclic (Alkyl)(Amino)Carbenes.

The evaluation of ruthenium olefin metathesis catalysts 4-6 bearing cyclic (alkyl)(amino)carbenes (CAACs) in the cross-metathesis of cis-1,4-diacetoxy-2-butene (7) with allylbenzene (8) and the ethenolysis of methyl oleate (11) is reported. Relative to most NHC-substituted complexes, CAAC-substituted catalysts exhibit lower E/Z ratios (3:1 at 70% conversion) in the cross-metathesis of 7 and 8. Additionally, complexes 4-6 demonstrate good selectivity for the formation of terminal olefins versus internal olefins in the ethenolysis of 11. Indeed, complex 6 achieved 35 000 TONs, the highest recorded to date. CAAC-substituted complexes exhibit markedly different kinetic selectivity than most NHC-substituted complexes.

Highly efficient ruthenium catalysts for the formation of tetrasubstituted olefins via ring-closing metathesis.

[reaction: see text] A series of ruthenium-based metathesis catalysts with N-heterocyclic carbene (NHC) ligands have been prepared in which the N-aryl groups have been changed from mesityl to mono-ortho-substituted phenyl (e.g., tolyl). These new catalysts offer an exceptional increase in activity for the formation of tetrasubstituted olefins via ring-closing metathesis (RCM), while maintaining high levels of activity in ring-closing metathesis (RCM) reactions that generate di- and trisubstituted olefins.