Organometallic Chemistry of Transition Metal Alkylidyne Complexes Centered at Metathesis Reactions.

Transition metals form a variety of alkylidyne complexes with either a d0 metal center (high-valent) or a non-d0 metal center (low-valent). One of the most interesting properties of alkylidyne complexes is that they can undergo or mediate metathesis reactions. The most well-studied metathesis reactions are alkyne metathesis involving high-valent alkylidynes. High-valent alkylidynes can also undergo metathesis reactions with heterotriple bonded species such as N≡CR, P≡CR, and N≡NR+. Metathesis reactions involving low-valent alkylidynes are less known. Highly efficient alkyne metathesis catalysts have been developed based on Mo(VI) and W(VI) alkylidynes. Catalytic cross-metathesis of nitriles with alkynes has also been achieved with M(VI) (M = W, Mo) alkylidyne or nitrido complexes. The metathesis activity of alkylidyne complexes is sensitively dependent on metals, supporting ligands and substituents of alkylidynes. Beyond metathesis, metal alkylidynes can also promote other reactions including alkyne polymerization. The remaining shortcomings and opportunities in the field are assessed.

Alkyne Metathesis with d2 Re(V) Alkylidyne Complexes Supported by Phosphino-Phenolates: Ligand Effect on Catalytic Activity and Applications in Ring-Closing Alkyne Metathesis.

A family of d2 Re(V) alkylidyne complexes bearing two decorated phosphino-phenolates (POs) and a labile pyridine ligand were prepared that can efficiently promote alkyne metathesis reactions in toluene. The relative activity of these complexes varies with the PO ligands. Complexes with an electron-rich metal center have a higher activity. Ligand exchange experiments suggest that the pyridine ligands of the Re(V) alkylidynes with more electron-donating PO ligands are more labile and are more easily released to generate catalytically active species. However, complexes with electron-withdrawing PO ligands are more air-stable than those with electron-donating PO ligands. These Re(V) alkylidyne catalysts can promote the homometathesis of functionalized internal alkyl- and aryl-alkynes, as well as ring-closing alkyne metathesis (RCAM) of methyl-capped diynes, forming macrocycles with a ring size ≥12 efficiently for concentrations ≤5 mM. These reactions represent the first examples of RCAM mediated by non-d0 alkylidyne complexes. The Re(V) alkylidyne catalysts tolerate a wide range of functional groups including ethers, esters, ketones, aldehydes, alcohols, phenols, amines, amides, and heterocycles. Moreover, the catalytic RCAM reactions promoted by robust Re(V) alkylidyne catalysts could also proceed normally in wet toluene.

Robust Alkyne Metathesis Catalyzed by Air Stable d2 Re(V) Alkylidyne Complexes.

We report in this communication the first example of catalytic alkyne metathesis reactions mediated by well-defined non-d0 alkylidyne complexes. The air-stable d2 Re(V) alkylidyne complex Re4, bearing two PO-chelating ligands and a labile pyridine ligand, could catalyze homometathesis of internal alkynes with a broad substrate scope, including alcohols, amines, and even carboxylic acids. The catalyst can tolerate heating, air, and moisture in both solid and solution states, and the catalytic metathesis reactions could proceed normally in wet solvents.

Exploring efficient and air-stable d2 Re(v) alkylidyne catalysts: toward room temperature alkyne metathesis.

Transition metal-catalyzed alkyne metathesis has become a useful tool in synthetic chemistry. Well-defined alkyne metathesis catalysts comprise alkylidyne complexes of tungsten, molybdenum and rhenium. Non-d0 Re(v) alkylidyne catalysts exhibit advantages such as remarkable tolerance to air and moisture as well as excellent functional group compatibility. However, the known Re(v) alkylidynes with a pyridine leaving ligand require harsh conditions for activation, resulting in lower catalytic efficiency compared to d0 Mo(vi) and W(vi) alkylidynes. Herein, we report the first non-d0 alkylidyne complex capable of mediating alkyne metathesis at room temperature, namely, the Re(v) aqua alkylidyne complex Re([triple bond, length as m-dash]CCH2Ph)( Ph PO)2(H2O) (14). The aqua complex readily dissociates a water ligand in solution, confirmed by ligand substitution reactions with other σ-donor ligands. The aqua complex can be readily prepared on a large scale, and is stable to air and moisture in the solid state and compatible with a variety of functional groups. The versatile ability of the catalyst has been demonstrated through examples of alkyne cross-metathesis (ACM), ring-closing alkyne metathesis (RCAM), and acyclic diyne metathesis macrocyclization (ADIMAC) reactions. All in all, this work presents a solution for an efficient and air-stable alkyne metathesis catalytic system based on d2 Re(v)-alkylidynes.

Protein-Bound ß-glucan from Coriolus Versicolor has Potential for Use Against Obesity.

SCOPE: The prevalence of obesity and related disorders has vastly increased throughout the world and prevention of such circumstances thus represents a major challenge. Here, it has been shown that one protein-bound ß-glucan (PBG) from the edible mushroom Coriolus versicolor can be a potent anti-obesity component. METHODS AND RESULTS: PBG can reduce obesity and metabolic inflammation in mice fed with a high-fat diet (HFD). Gut microbiota analysis reveals that PBG markedly increases the abundance of Akkermansia muciniphila, although it does not rescue HFD-induced change in the Firmicutes to Bacteroidetes ratio. It appears that PBG alters host physiology and creates an intestinal microenvironment favorable for A. muciniphila colonization. Fecal transplants from PBG-treated animals in part reduce obesity in recipient HFD-fed mice. Further, PBG is shown to upregulate expression of a set of genes related to host metabolism in microbiota-depleted mice. CONCLUSION: The data highlight that PBG may exert its anti-obesity effects through a mirobiota-dependent (richness of specific microbiota) and -independent (modulation of host metabolism) manner. The fact that C. versicolor PBGs are approved oral immune boosters in cancers and chronic hepatitis with well-established safety profiles may accelerate PBG as a novel use for obesity treatment.

Chemistry of Metallacyclobutadienes.

Metallacyclobutadienes are analogues of cyclobutadienes in which one of the cyclobutadiene CR groups has been formally replaced by a transition-metal fragment. These metallacycles are interesting because they can play an important role in catalysis and can serve as starting materials for the syntheses of organometallic compounds such as metallabenzene, η5 -cyclopentadienyl, and η3 -cyclopropenyl complexes. Unlike cyclobutadienes, metallacyclobutadienes can be significantly more stable. A number of metallacyclobutadienes have now been isolated and thoroughly characterized, especially for those that contain transition metals of groups 5-9. Their properties have also been actively investigated. This article highlights the chemistry of metallacyclobutadienes with reference to their syntheses, reactivity, and structural properties.

A tanshinone I derivative enhances the activities of antibiotics against Staphylococcus aureus in vitro and in vivo.

Infections caused by Staphylococcus aureus are prevalent. The dramatically reduced discovery of new antibiotics, as well as the persistent emergence of resistant bacteria, represents a major health problem in both hospital and community settings. Using antibiotic enhancers to rescue existing classes of antibiotics is an attractive strategy. In this study, 16-aldehyde tanshinone I (ALT) was synthesized and bacteriostatic activity was explored. In addition, synergistic or additive activity between ALT and aminoglycoside antibiotics or ß-lactam antibiotics in vitro was identified. Moreover, ALT was documented to augment clearance of streptomycin (STR) and ampicillin (AMP) against S. aureus in a murine infection model. Primary mechanistic insight indicated that ALT could damage the bacterial cell membrane, leading to accumulation of antibiotics inside bacterial cells. This finding might be useful for treating infections caused by S. aureus and expand the scope of application of tanshinones.