Rapid Detection of Salmonella enterica via Directional Emission from Carbohydrate-Functionalized Dynamic Double Emulsions.

Reliable early-stage detection of foodborne pathogens is a global public health challenge that requires new and improved sensing strategies. Here, we demonstrate that dynamically reconfigurable fluorescent double emulsions can function as highly responsive optical sensors for the rapid detection of carbohydrates fructose, glucose, mannose, and mannan, which are involved in many biological and pathogenic phenomena. The proposed detection strategy relies on reversible reactions between boronic acid surfactants and carbohydrates at the hydrocarbon/water interface leading to a dynamic reconfiguration of the droplet morphology, which alters the angular distribution of the droplet's fluorescent light emission. We exploit this unique chemical-morphological-optical coupling to detect Salmonella enterica, a type of bacteria with a well-known binding affinity for mannose. We further demonstrate an oriented immobilization of antibodies at the droplet interface to permit higher selectivity. Our demonstrations yield a new, inexpensive, robust, and generalizable sensing strategy that can help to facilitate the early detection of foodborne pathogens.

Z-selective cross metathesis with ruthenium catalysts: synthetic applications and mechanistic implications.

Olefin cross metathesis is a particularly powerful transformation that has been exploited extensively for the formation of complex products. Until recently, however, constructing Z-olefins using this methodology was not possible. With the discovery and development of three families of ruthenium-based Z-selective catalysts, the formation of Z-olefins using metathesis is now not only possible but becoming increasingly prevalent in the literature. In particular, ruthenium complexes containing cyclometalated NHC architectures developed in our group have been shown to catalyze various cross metathesis reactions with high activity and, in most cases, near perfect selectivity for the Z-isomer. The types of cross metathesis reactions investigated thus far are presented here and explored in depth.

Ru-Based Z-Selective Metathesis Catalysts with Modified Cyclometalated Carbene Ligands.

A series of cyclometalated Z-selective ruthenium olefin metathesis catalysts with alterations to the N-heterocyclic carbene (NHC) ligand were prepared. X-Ray crystal structures of several new catalysts were obtained, elucidating the structural features of this class of cyclometalated complexes. The metathesis activity of each stable complex was evaluated, and one catalyst, bearing geminal dimethyl backbone substitution, was found to be comparable to our best Z-selective metathesis catalyst to date.

Palladium-Catalyzed Decarbonylative Dehydration of Fatty Acids for the Production of Linear Alpha Olefins.

A highly efficient palladium-catalyzed decarbonylative dehydration reaction of carboxylic acids is reported. This method transforms abundant and renewable even-numbered natural fatty acids into valuable and expensive odd-numbered alpha olefins. Additionally, the chemistry displays a high functional group tolerance. The process employs low loading of palladium catalyst and proceeds under solvent-free and relatively mild conditions.

Investigations into Ruthenium Metathesis Catalysts with Six-Membered Chelating NHC Ligands: Relationship between Catalyst Structure and Stereoselectivity.

A series of ruthenium catalysts bearing five-membered chelating NHC architectures that exhibit very high Z-selectivity in a variety of metathesis reactions have recently been reported. It was envisioned that catalysts possessing sixmembered chelates could similarly exhibit high Z-selectivity and address limitations of this methodology. We thus prepared a number of new catalysts and systematically investigated the impact of the NHC and anionic ligand on their stereoselectivity. In standard metathesis assays, only catalysts containing six-membered chelated NHC structures and η2-bound anionic ligands favored the Z-olefin products compared to traditional ruthenium catalysts. In addition, substitution with bulkier N-aryl groups led to improved Z-selectivity. The effect of ligand structure on stereoselectivity discovered in this study will be useful in the future design of highly active and Z-selective ruthenium catalysts.

Z-Selective ethenolysis with a ruthenium metathesis catalyst: experiment and theory.

The Z-selective ethenolysis activity of chelated ruthenium metathesis catalysts was investigated with experiment and theory. A five-membered chelated catalyst that was successfully employed in Z-selective cross metathesis reactions has now been found to be highly active for Z-selective ethenolysis at low ethylene pressures, while tolerating a wide variety of functional groups. This phenomenon also affects its activity in cross metathesis reactions and prohibits crossover reactions of internal olefins via trisubstituted ruthenacyclobutane intermediates. In contrast, a related catalyst containing a six-membered chelated architecture is not active for ethenolysis and seems to react through different pathways more reminiscent of previous generations of ruthenium catalysts. Computational investigations of the effects of substitution on relevant transition states and ruthenacyclobutane intermediates revealed that the differences of activities are attributed to the steric repulsions of the anionic ligand with the chelating groups.

Highly active ruthenium metathesis catalysts exhibiting unprecedented activity and Z-selectivity.

A novel chelated ruthenium-based metathesis catalyst bearing an N-2,6-diisopropylphenyl group is reported and displays near-perfect selectivity for the Z-olefin (>95%), as well as unparalleled TONs of up to 7400, in a variety of homodimerization and industrially relevant metathesis reactions. This derivative and other new catalytically active species were synthesized using an improved method employing sodium carboxylates to induce the salt metathesis and C-H activation of these chelated complexes. All of these new ruthenium-based catalysts are highly Z-selective in the homodimerization of terminal olefins.

Concise syntheses of insect pheromones using Z-selective cross metathesis.

Very short synthetic routes to nine cis-olefin-containing pheromones containing a variety of functionality, including an unconjugated (E,Z) diene, are reported. These lepidopteran pheromones are used extensively for pest control, and were easily prepared using ruthenium-based Z-selective cross metathesis, highlighting the advantages of this method over less efficient ways to form Z olefins.

Stereoselective access to Z and E macrocycles by ruthenium-catalyzed Z-selective ring-closing metathesis and ethenolysis.

The first report of Z-selective macrocyclizations using a ruthenium-based metathesis catalyst is described. The selectivity for Z macrocycles is consistently high for a diverse set of substrates with a variety of functional groups and ring sizes. The same catalyst was also employed for the Z-selective ethenolysis of a mixture of E and Z macrocycles, providing the pure E isomer. Notably, an ethylene pressure of only 1 atm was required. These methodologies were successfully applied to the construction of several olfactory macrocycles as well as the formal total synthesis of the cytotoxic alkaloid motuporamine C.

Decomposition pathways of Z-selective ruthenium metathesis catalysts.

The decomposition of a Z-selective ruthenium metathesis catalyst and structurally similar analogues has been investigated utilizing X-ray crystallography and density functional theory. Isolated X-ray crystal structures suggest that recently reported C-H activated catalysts undergo decomposition via insertion of the alkylidene moiety into the chelating ruthenium-carbon bond followed by hydride elimination, which is supported by theoretical calculations. The resulting ruthenium hydride intermediates have been implicated in previously observed olefin migration, and thus lead to unwanted byproducts in cross metathesis reactions. Preventing these decomposition modes will be essential in the design of more active and selective Z-selective catalysts.

Z-Selectivity in olefin metathesis with chelated Ru catalysts: computational studies of mechanism and selectivity.

The mechanism and origins of Z-selectivity in olefin metathesis with chelated Ru catalysts were explored using density functional theory. The olefin approaches from the "side" position of the chelated Ru catalysts, in contrast to reactions with previous unchelated Ru catalysts that favor the bottom-bound pathway. Steric repulsions between the substituents on the olefin and the N-substituent on the N-heterocyclic carbene ligand lead to highly selective formation of the Z product.

Improved ruthenium catalysts for Z-selective olefin metathesis.

Several new C-H-activated ruthenium catalysts for Z-selective olefin metathesis have been synthesized. Both the carboxylate ligand and the aryl group of the N-heterocyclic carbene have been altered and the resulting catalysts evaluated using a range of metathesis reactions. Substitution of bidentate with monodentate X-type ligands led to a severe attenuation of metathesis activity and selectivity, while minor differences were observed between bidentate ligands within the same family (e.g., carboxylates). The use of nitrato-type ligands in place of carboxylates afforded a significant improvement in metathesis activity and selectivity. With these catalysts, turnover numbers approaching 1000 were possible for a variety of cross-metathesis reactions, including the synthesis of industrially relevant products.

Z-selective homodimerization of terminal olefins with a ruthenium metathesis catalyst.

The cross-metathesis of terminal olefins using a novel ruthenium catalyst results in excellent selectivity for the Z-olefin homodimer. The reaction was found to tolerate a large number of functional groups, solvents, and temperatures while maintaining excellent Z-selectivity, even at high reaction conversions.